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Wireless Optical Communications

Wireless OpticalCommunications

Olivier Bouchet

Series EditorPierre-Noeumll Favennec

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley amp Sons Inc

Apart from any fair dealing for the purposes of research or private study or criticism or review aspermitted under the Copyright Designs and Patents Act 1988 this publication may only be reproducedstored or transmitted in any form or by any means with the prior permission in writing of the publishersor in the case of reprographic reproduction in accordance with the terms and licenses issued by theCLA Enquiries concerning reproduction outside these terms should be sent to the publishers at theundermentioned address

ISTE Ltd John Wiley amp Sons Inc27-37 St Georgersquos Road 111 River StreetLondon SW19 4EU Hoboken NJ 07030UK USA

wwwistecouk wwwwileycom

copy ISTE Ltd 2012

The rights of Olivier Bouchet to be identified as the author of this work have been asserted by him inaccordance with the Copyright Designs and Patents Act 1988____________________________________________________________________________________

Library of Congress Cataloging-in-Publication Data

Bouchet OlivierWireless optical telecommunications Olivier Bouchetp cmIncludes bibliographical references and indexISBN 978-1-84821-316-61 Wireless communication systems 2 Optical communications I TitleTK51032B69 20126213827--dc23

2012005891

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-316-6

Printed and bound in Great Britain by CPI Group (UK) Ltd Croydon Surrey CR0 4YY

Table of Contents

Foreword xiPierre-Noeumll FAVENNEC

Acronyms xiii

Introduction xix

Chapter 1 Light 1

Chapter 2 History of Optical Telecommunications 7

21 Some definitions 7211 Communicate 7212 Telecommunication 7213 Optical telecommunication 8214 Radio frequency or Hertzian waves 8

22 The prehistory of telecommunications 823 The optical aerial telegraph 1124 The code 1425 The optical telegraph 18251 The heliograph or solar telegraph 18252 The night and day optical telegraph 19

26 Alexander Graham Bellrsquos photophone 20

Chapter 3 The Contemporary and the Everyday Life of WirelessOptical Communication 25

31 Basic principles 25311 Operating principle 263111 Block diagram 26

312 The optical propagation 27

vi Wireless Optical Communications

3121 Line of sight propagation ndash LOS 273122 Wide line of sight ndash WLOS 293123 Diffusion propagation (DIF) and controlled diffusion 30

313 Elements of electromagnetics 313131 Maxwellrsquos equations in an unspecified medium 323132 Propagation of electromagnetic waves in anisotropic medium 343133 Energy associated to a wave 363134 Propagation of a wave in a non-homogeneous medium 383135 Coherent and incoherent waves 383136 Relations between electromagnetism andgeometrical optics 403137 The electromagnetic spectrum 433138 Units and scales 433139 Examples of sources in the visible and near visible light 4731310 Conclusion 49

314 Models for data exchange 503141 The OSI model 503142 The DoD model 52

32 Wireless optical communication 53321 Outdoor wireless optical communication 533211 Earth-satellite wireless optical communication 533212 Intersatellite wireless optical communication 543213 Free-space optic 55

322 Indoor wireless optical communication 553221 The remote controller 563222 The visible light communication 573223 The IrDA solutions 573224 The indoor wireless optical network (WON) 57

323 The institutional and technical ecosystem 59

Chapter 4 Propagation Model 63

41 Introduction 6342 Baseband equivalent model 63421 Radio propagation model 64422 Model of free-space optical propagation 66423 The signal-to-noise ratio 71

43 Diffuse propagation link budget in a confined environment 73431 Intersymbol interference 73432 Reflection models 764321 Specular reflection 764322 Diffuse reflection 764323 Lambertrsquos model 77

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Wireless Optical Communications

Wireless OpticalCommunications

Olivier Bouchet

Series EditorPierre-Noeumll Favennec

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley amp Sons Inc

Apart from any fair dealing for the purposes of research or private study or criticism or review aspermitted under the Copyright Designs and Patents Act 1988 this publication may only be reproducedstored or transmitted in any form or by any means with the prior permission in writing of the publishersor in the case of reprographic reproduction in accordance with the terms and licenses issued by theCLA Enquiries concerning reproduction outside these terms should be sent to the publishers at theundermentioned address

ISTE Ltd John Wiley amp Sons Inc27-37 St Georgersquos Road 111 River StreetLondon SW19 4EU Hoboken NJ 07030UK USA

wwwistecouk wwwwileycom

copy ISTE Ltd 2012

The rights of Olivier Bouchet to be identified as the author of this work have been asserted by him inaccordance with the Copyright Designs and Patents Act 1988____________________________________________________________________________________

Library of Congress Cataloging-in-Publication Data

Bouchet OlivierWireless optical telecommunications Olivier Bouchetp cmIncludes bibliographical references and indexISBN 978-1-84821-316-61 Wireless communication systems 2 Optical communications I TitleTK51032B69 20126213827--dc23

2012005891

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-316-6

Printed and bound in Great Britain by CPI Group (UK) Ltd Croydon Surrey CR0 4YY

Table of Contents

Foreword xiPierre-Noeumll FAVENNEC

Acronyms xiii

Introduction xix

Chapter 1 Light 1

Chapter 2 History of Optical Telecommunications 7

21 Some definitions 7211 Communicate 7212 Telecommunication 7213 Optical telecommunication 8214 Radio frequency or Hertzian waves 8

22 The prehistory of telecommunications 823 The optical aerial telegraph 1124 The code 1425 The optical telegraph 18251 The heliograph or solar telegraph 18252 The night and day optical telegraph 19

26 Alexander Graham Bellrsquos photophone 20

Chapter 3 The Contemporary and the Everyday Life of WirelessOptical Communication 25

31 Basic principles 25311 Operating principle 263111 Block diagram 26

312 The optical propagation 27

vi Wireless Optical Communications

3121 Line of sight propagation ndash LOS 273122 Wide line of sight ndash WLOS 293123 Diffusion propagation (DIF) and controlled diffusion 30

313 Elements of electromagnetics 313131 Maxwellrsquos equations in an unspecified medium 323132 Propagation of electromagnetic waves in anisotropic medium 343133 Energy associated to a wave 363134 Propagation of a wave in a non-homogeneous medium 383135 Coherent and incoherent waves 383136 Relations between electromagnetism andgeometrical optics 403137 The electromagnetic spectrum 433138 Units and scales 433139 Examples of sources in the visible and near visible light 4731310 Conclusion 49

314 Models for data exchange 503141 The OSI model 503142 The DoD model 52

32 Wireless optical communication 53321 Outdoor wireless optical communication 533211 Earth-satellite wireless optical communication 533212 Intersatellite wireless optical communication 543213 Free-space optic 55

322 Indoor wireless optical communication 553221 The remote controller 563222 The visible light communication 573223 The IrDA solutions 573224 The indoor wireless optical network (WON) 57

323 The institutional and technical ecosystem 59

Chapter 4 Propagation Model 63

41 Introduction 6342 Baseband equivalent model 63421 Radio propagation model 64422 Model of free-space optical propagation 66423 The signal-to-noise ratio 71

43 Diffuse propagation link budget in a confined environment 73431 Intersymbol interference 73432 Reflection models 764321 Specular reflection 764322 Diffuse reflection 764323 Lambertrsquos model 77

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Wireless OpticalCommunications

Olivier Bouchet

Series EditorPierre-Noeumll Favennec

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley amp Sons Inc

Apart from any fair dealing for the purposes of research or private study or criticism or review aspermitted under the Copyright Designs and Patents Act 1988 this publication may only be reproducedstored or transmitted in any form or by any means with the prior permission in writing of the publishersor in the case of reprographic reproduction in accordance with the terms and licenses issued by theCLA Enquiries concerning reproduction outside these terms should be sent to the publishers at theundermentioned address

ISTE Ltd John Wiley amp Sons Inc27-37 St Georgersquos Road 111 River StreetLondon SW19 4EU Hoboken NJ 07030UK USA

wwwistecouk wwwwileycom

copy ISTE Ltd 2012

The rights of Olivier Bouchet to be identified as the author of this work have been asserted by him inaccordance with the Copyright Designs and Patents Act 1988____________________________________________________________________________________

Library of Congress Cataloging-in-Publication Data

Bouchet OlivierWireless optical telecommunications Olivier Bouchetp cmIncludes bibliographical references and indexISBN 978-1-84821-316-61 Wireless communication systems 2 Optical communications I TitleTK51032B69 20126213827--dc23

2012005891

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-316-6

Printed and bound in Great Britain by CPI Group (UK) Ltd Croydon Surrey CR0 4YY

Table of Contents

Foreword xiPierre-Noeumll FAVENNEC

Acronyms xiii

Introduction xix

Chapter 1 Light 1

Chapter 2 History of Optical Telecommunications 7

21 Some definitions 7211 Communicate 7212 Telecommunication 7213 Optical telecommunication 8214 Radio frequency or Hertzian waves 8

22 The prehistory of telecommunications 823 The optical aerial telegraph 1124 The code 1425 The optical telegraph 18251 The heliograph or solar telegraph 18252 The night and day optical telegraph 19

26 Alexander Graham Bellrsquos photophone 20

Chapter 3 The Contemporary and the Everyday Life of WirelessOptical Communication 25

31 Basic principles 25311 Operating principle 263111 Block diagram 26

312 The optical propagation 27

vi Wireless Optical Communications

3121 Line of sight propagation ndash LOS 273122 Wide line of sight ndash WLOS 293123 Diffusion propagation (DIF) and controlled diffusion 30

313 Elements of electromagnetics 313131 Maxwellrsquos equations in an unspecified medium 323132 Propagation of electromagnetic waves in anisotropic medium 343133 Energy associated to a wave 363134 Propagation of a wave in a non-homogeneous medium 383135 Coherent and incoherent waves 383136 Relations between electromagnetism andgeometrical optics 403137 The electromagnetic spectrum 433138 Units and scales 433139 Examples of sources in the visible and near visible light 4731310 Conclusion 49

314 Models for data exchange 503141 The OSI model 503142 The DoD model 52

32 Wireless optical communication 53321 Outdoor wireless optical communication 533211 Earth-satellite wireless optical communication 533212 Intersatellite wireless optical communication 543213 Free-space optic 55

322 Indoor wireless optical communication 553221 The remote controller 563222 The visible light communication 573223 The IrDA solutions 573224 The indoor wireless optical network (WON) 57

323 The institutional and technical ecosystem 59

Chapter 4 Propagation Model 63

41 Introduction 6342 Baseband equivalent model 63421 Radio propagation model 64422 Model of free-space optical propagation 66423 The signal-to-noise ratio 71

43 Diffuse propagation link budget in a confined environment 73431 Intersymbol interference 73432 Reflection models 764321 Specular reflection 764322 Diffuse reflection 764323 Lambertrsquos model 77

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

First published 2012 in Great Britain and the United States by ISTE Ltd and John Wiley amp Sons Inc

Apart from any fair dealing for the purposes of research or private study or criticism or review aspermitted under the Copyright Designs and Patents Act 1988 this publication may only be reproducedstored or transmitted in any form or by any means with the prior permission in writing of the publishersor in the case of reprographic reproduction in accordance with the terms and licenses issued by theCLA Enquiries concerning reproduction outside these terms should be sent to the publishers at theundermentioned address

ISTE Ltd John Wiley amp Sons Inc27-37 St Georgersquos Road 111 River StreetLondon SW19 4EU Hoboken NJ 07030UK USA

wwwistecouk wwwwileycom

copy ISTE Ltd 2012

The rights of Olivier Bouchet to be identified as the author of this work have been asserted by him inaccordance with the Copyright Designs and Patents Act 1988____________________________________________________________________________________

Library of Congress Cataloging-in-Publication Data

Bouchet OlivierWireless optical telecommunications Olivier Bouchetp cmIncludes bibliographical references and indexISBN 978-1-84821-316-61 Wireless communication systems 2 Optical communications I TitleTK51032B69 20126213827--dc23

2012005891

British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-84821-316-6

Printed and bound in Great Britain by CPI Group (UK) Ltd Croydon Surrey CR0 4YY

Table of Contents

Foreword xiPierre-Noeumll FAVENNEC

Acronyms xiii

Introduction xix

Chapter 1 Light 1

Chapter 2 History of Optical Telecommunications 7

21 Some definitions 7211 Communicate 7212 Telecommunication 7213 Optical telecommunication 8214 Radio frequency or Hertzian waves 8

22 The prehistory of telecommunications 823 The optical aerial telegraph 1124 The code 1425 The optical telegraph 18251 The heliograph or solar telegraph 18252 The night and day optical telegraph 19

26 Alexander Graham Bellrsquos photophone 20

Chapter 3 The Contemporary and the Everyday Life of WirelessOptical Communication 25

31 Basic principles 25311 Operating principle 263111 Block diagram 26

312 The optical propagation 27

vi Wireless Optical Communications

3121 Line of sight propagation ndash LOS 273122 Wide line of sight ndash WLOS 293123 Diffusion propagation (DIF) and controlled diffusion 30

313 Elements of electromagnetics 313131 Maxwellrsquos equations in an unspecified medium 323132 Propagation of electromagnetic waves in anisotropic medium 343133 Energy associated to a wave 363134 Propagation of a wave in a non-homogeneous medium 383135 Coherent and incoherent waves 383136 Relations between electromagnetism andgeometrical optics 403137 The electromagnetic spectrum 433138 Units and scales 433139 Examples of sources in the visible and near visible light 4731310 Conclusion 49

314 Models for data exchange 503141 The OSI model 503142 The DoD model 52

32 Wireless optical communication 53321 Outdoor wireless optical communication 533211 Earth-satellite wireless optical communication 533212 Intersatellite wireless optical communication 543213 Free-space optic 55

322 Indoor wireless optical communication 553221 The remote controller 563222 The visible light communication 573223 The IrDA solutions 573224 The indoor wireless optical network (WON) 57

323 The institutional and technical ecosystem 59

Chapter 4 Propagation Model 63

41 Introduction 6342 Baseband equivalent model 63421 Radio propagation model 64422 Model of free-space optical propagation 66423 The signal-to-noise ratio 71

43 Diffuse propagation link budget in a confined environment 73431 Intersymbol interference 73432 Reflection models 764321 Specular reflection 764322 Diffuse reflection 764323 Lambertrsquos model 77

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Table of Contents

Foreword xiPierre-Noeumll FAVENNEC

Acronyms xiii

Introduction xix

Chapter 1 Light 1

Chapter 2 History of Optical Telecommunications 7

21 Some definitions 7211 Communicate 7212 Telecommunication 7213 Optical telecommunication 8214 Radio frequency or Hertzian waves 8

22 The prehistory of telecommunications 823 The optical aerial telegraph 1124 The code 1425 The optical telegraph 18251 The heliograph or solar telegraph 18252 The night and day optical telegraph 19

26 Alexander Graham Bellrsquos photophone 20

Chapter 3 The Contemporary and the Everyday Life of WirelessOptical Communication 25

31 Basic principles 25311 Operating principle 263111 Block diagram 26

312 The optical propagation 27

vi Wireless Optical Communications

3121 Line of sight propagation ndash LOS 273122 Wide line of sight ndash WLOS 293123 Diffusion propagation (DIF) and controlled diffusion 30

313 Elements of electromagnetics 313131 Maxwellrsquos equations in an unspecified medium 323132 Propagation of electromagnetic waves in anisotropic medium 343133 Energy associated to a wave 363134 Propagation of a wave in a non-homogeneous medium 383135 Coherent and incoherent waves 383136 Relations between electromagnetism andgeometrical optics 403137 The electromagnetic spectrum 433138 Units and scales 433139 Examples of sources in the visible and near visible light 4731310 Conclusion 49

314 Models for data exchange 503141 The OSI model 503142 The DoD model 52

32 Wireless optical communication 53321 Outdoor wireless optical communication 533211 Earth-satellite wireless optical communication 533212 Intersatellite wireless optical communication 543213 Free-space optic 55

322 Indoor wireless optical communication 553221 The remote controller 563222 The visible light communication 573223 The IrDA solutions 573224 The indoor wireless optical network (WON) 57

323 The institutional and technical ecosystem 59

Chapter 4 Propagation Model 63

41 Introduction 6342 Baseband equivalent model 63421 Radio propagation model 64422 Model of free-space optical propagation 66423 The signal-to-noise ratio 71

43 Diffuse propagation link budget in a confined environment 73431 Intersymbol interference 73432 Reflection models 764321 Specular reflection 764322 Diffuse reflection 764323 Lambertrsquos model 77

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

vi Wireless Optical Communications

3121 Line of sight propagation ndash LOS 273122 Wide line of sight ndash WLOS 293123 Diffusion propagation (DIF) and controlled diffusion 30

313 Elements of electromagnetics 313131 Maxwellrsquos equations in an unspecified medium 323132 Propagation of electromagnetic waves in anisotropic medium 343133 Energy associated to a wave 363134 Propagation of a wave in a non-homogeneous medium 383135 Coherent and incoherent waves 383136 Relations between electromagnetism andgeometrical optics 403137 The electromagnetic spectrum 433138 Units and scales 433139 Examples of sources in the visible and near visible light 4731310 Conclusion 49

314 Models for data exchange 503141 The OSI model 503142 The DoD model 52

32 Wireless optical communication 53321 Outdoor wireless optical communication 533211 Earth-satellite wireless optical communication 533212 Intersatellite wireless optical communication 543213 Free-space optic 55

322 Indoor wireless optical communication 553221 The remote controller 563222 The visible light communication 573223 The IrDA solutions 573224 The indoor wireless optical network (WON) 57

323 The institutional and technical ecosystem 59

Chapter 4 Propagation Model 63

41 Introduction 6342 Baseband equivalent model 63421 Radio propagation model 64422 Model of free-space optical propagation 66423 The signal-to-noise ratio 71

43 Diffuse propagation link budget in a confined environment 73431 Intersymbol interference 73432 Reflection models 764321 Specular reflection 764322 Diffuse reflection 764323 Lambertrsquos model 77

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Table of Contents vii

4324 Phongrsquos model 79433 Modeling 81

Chapter 5 Propagation in the Atmosphere 85

51 Introduction 8552 The atmosphere 86521 The atmospheric gaseous composition 86522 Aerosols 87

53 The propagation of light in the atmosphere 87531 Molecular absorption 89532 Molecular scattering 89533 Aerosol absorption 90534 Aerosol scattering 91

54 Models 93541 Kruse and Kim models 93542 Bataillersquos model 945421 Molecular extinction 945422 Aerosol extinction 95

543 Al Naboulsirsquos model 95544 Rain attenuation 96545 Snow attenuation 97546 Scintillation 98

55 Experimental set-up 10356 Experimental results 104561 Comparaison with Kruse and Kim model (850 nm) 105562 Comparaison with Al Naboulsirsquos model 105

57 Fog haze and mist 10758 The runway visual range (RVR) 108581 The visibility 108582 Measuring instruments 1105821 The transmissometer 1105822 The scatterometer 112

59 Calculating process of an FSO link availability 114510 Conclusion 116

Chapter 6 Indoor Optic Link Budget 119

61 Emission and reception parameters 119611 Transmission device parameters 121612 Reception device 125

62 Link budget for line of sight communication 128621 Geometrical attenuation 128622 Optical margin 130

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

viii Wireless Optical Communications

623 Coverage 130624 Reciprocity and not reciprocity of the channel 131

63 Link budget for communication with retroreflectors 132631 Principle of operation 132632 Optical budget 133

64 Examples of optical budget and signal-to-noise ratio (SNR) 135641 Examples of optical budget 136642 Examples of SNR and BER 139

Chapter 7 Immunity Safety Energy and Legislation 141

71 Immunity 141711 International references 141712 Type of laser classes 143713 Method for calculation 146

72 The confidentiality of communication 149721 Physical confidentiality 149722 Numerical solution 1507221 Cryptography 1507222 Public and secret key cryptography 1517223 Quantum cryptography 1517224 Quantum telecommunications in free space 1527225 Non-encrypted connections in confined space 153

73 Energy 15374 Legislation 154741 Organization of regulation activities 154742 Regulation of wireless optical equipment 155

Chapter 8 Optics and Optronics 157

81 Overview 15782 Optronics transmitters and receivers 157821 Overviews on materials and structures 157822 Light sources 1608221 Light-emitting diodes (LEDs) and spontaneous emission 1618222 White LEDs or visible light communication (VLC) LED 1628223 The semiconductor laser structure 1638224 Synthesis 165

823 Optronics receivers 1668231 Photovoltaic cells 1678232 PIN photodiode 1688233 Avalanche photodiode 1698234 Metalndashsemiconductorndashmetal (MSM) structure 170

83 Optics 170

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Table of Contents ix

831 Transmitter optical device 170832 Receiver optical device 171833 Optical filtering 1748331 Spatial filter or diaphragm 1748332 Wavelength filters or attenuators 174

834 Summary 176

Chapter 9 Data Processing 177

91 Introduction 17792 Modulation 178921 On-off keying (OOK) modulation 178922 The pulse position modulation 180923 The orthogonal frequency-division multiplexing (OFDM) 181924 The diversity MIMO 182925 Summary 184

93 The coding 184931 Principle and definitions 1849311 Principle 1849312 Definitions 185

932 Example of coding 1869321 Basic codes 1869322 Block codes 1879323 Convolutional codes 191

933 Summary 194

Chapter 10 Data Transmission 197

101 Introduction 1971011 Definition 1971012 The access methods 19810121 Time division multiple access 19810122 Frequency division multiple access 19910123 Code division multiple access 19910124 Carrier sense multiple access 19910125 Wavelength division multiple access 19910126 Space division multiple access 200

1013 Quality of service parameters 200102 Point-to-point link 2011021 The remote control 2011022 Infrared Data Association 2031023 Visible light communication consortium 206

103 Point-to-multipoint data link 2061031 IEEE 80211 IR 206

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

x Wireless Optical Communications

1032 ICSA ndash STB50 (IEEE 8023 ndash Ethernet) 2081033 IEEE 802153 2091034 IEEE 802157 2091035 Optical wireless media access control 210

104 Summary 212

Chapter 11 Installation and System Engineering 213

111 Free-space optic system engineering and installation 2131111 Principle of operation 2131112 Characteristics 21411121 Principal parameters 21511122 Secondary parameters 21611123 Examples of FSO systems 216

1113 Implementation recommendations 2171114 Optic link budget 21811141 Geometrical attenuation concept 21911142 Link margin concept 219

1115 FSO link availability 22011151 Characteristics 22011152 Results 223

1116 Summary 225112 Wireless optical system installation engineering in limited space 2251121 Habitat structure 2251122 Statistical analysis and coverage area 2261123 Optical link budget 2301124 Optimization of indoor wireless optical system 234

Chapter 12 Conclusion 237

APPENDICES 241

Appendix 1 Geometrical Optics Photometry and Energy Elements 243

Appendix 2 The Decibel Unit (dB) 257

Bibliography 261

List of Figures 273

List of Tables 277

List of Equations 279

Index 283

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Foreword

Modern telecommunication at least in the vicinity of terminals (TV receiverscomputers recorders smartphones network games consoles e-books etc) will beldquowirelessrdquo and high speed the physical link will not be a copper wire or made fromfiber silica or other but an electromagnetic wave propagating in free space betweenone transmitterndashreceiver and another transmitterndashreceiver

The most common physical wireless link is the use of radio an electromagneticwave in the range of radio spectrum It is a well-developed technology but wecan see the limitations in terms of speed (bits per second) frequency powerelectromagnetic compatibility and electromagnetic pollution among othersRegarding transmission of information we know that the higher the frequency of theelectromagnetic transmitted wave the higher the speed Hence current laboratorystudies are looking at communication systems operating at frequencies of gigahertz(GHz) to terahertz (THz) and above For frequencies beyond terahertz andparticularly in the ranges corresponding to optical waves infrared or visible light(100ndash1000 THz) a communication speed in the range of terabits per second can beachieved

Because of the laser (invented in 1960) and silica fiber (the potential of silica fiberfor telecom applications was demonstrated in 1961) optical telecommunicationstogether with the fantastic progress made in the manufacturing technology oflasers and optoelectronic systems in parallel to those of silica fibers have enabledthe irreversible development of optical fiber telecommunication These opticalcommunications have generated intercontinental telecommunications and broadbandinternet From basic-oriented research they have an obvious important societal impact

Wireless optical communications use the atmosphere as a transmission mediumThe ambient atmosphere is much more complex than the fibrous silica in terms ofcomposition uniformity and reproducibility But taking advantage of advanced

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xii Wireless Optical Communications

technologies useful for fiber telecommunications it gives excellent results forbroadband transmitted over short distances and even allows us a glimpse of wirelessoptical communication with terabits per second even though today (in 2011) we areusing gigabit to the terminal (GTTT) in a limited confined environment

The atmospheric optical links are always subject to environmental variations(dust fog rain etc) which can cause temporary performance degradation of thetelecommunications system The propagation properties of optical beams in thisenvironment must provide a good quality of service as in the model of Al Naboulsiet al [NAB 04] based on visibility the setting that characterizes the opacity ofthe atmosphere Using components (LED laser photodetectors etc) at wavelengthsthat are non-ionizing photons whose technologies are now mature in free-spacecommunication over short distances especially indoors (in rooms) has great potentialThe book Wireless Optical Communications follows a previous book Free-SpaceOptics ndash Propagation and Communication [BOU 06] that presented the physics andfoundations useful for communications in free space and in limited spaces Sincethe last book great progress has been made on all issues related to a realtelecommunications system incorporating channel properties propagation models linkbudgets and the data processing including coding modulation standards and safety

This book is designed as an excellent tool for any engineer wanting to learnabout wireless optical communications or who is involved in the implementation ofreal complete systems Students will find lots of information and useful conceptssuch as those relating to propagation optics and photometry as well as thenecessary information on safety

This book is written with as an overview of a useful technology fortelecommunications The ideas developed allow us a glimpse of the applications in thefield of communication devices by photons Since the early work of Gfeller in 1979 onoptical wireless limited space [GFE 79] or the work of Kintzig et al [KIN 02]published in 2002 who suggested solutions for optical wireless communicationdevices we can now glimpse totally secure wireless optical communication from ldquonrdquoobjects to ldquomrdquo objects and very high data rates (up to THz soon) limiting itself to thewalls of a room

Optical wireless telecommunications also allow absolute security incommunications subject to having transmitters in a single reliable and reproduciblephoton These free-space quanta in free space will certainly find useful applicationsfor those who want absolute security in their information exchange

Pierre-Noeumll FAVENNECURSI-FranceMarch 2012

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Acronyms

A AmpereAAC Automatic attenuation controlAc Area coverACG Automatic control gainAEL Accessible emission limitAIR Advanced infraredAP Access pointAPD Avalanche photodiodeAPPM Amplitude pulse position modulationARIB Association of Radio Industries and BusinessesARP Address resolution protocolASCII American standard code for information interchangeASK Amplitude shift keyingATM Asynchronous transfer modeATPC Automatic transmit power controlAWGN Additive white Gaussian noiseBCH BosendashChaudhuryndashHocquenghem codeBCJR BahlndashCockendashJelinekndashRaviv codeBER Binary error rateBPM Beam propagation method in time domainBT British TelecomCAO Concentrateur amplificateur optique (Fireball)BC Conduction bandCC Convolutional codeCCD Charge coupled deviceCCETT Centre Commun drsquoEtudes de Teacuteleacutevision et de

TeacuteleacutecommunicationsCD Compact discCDMA Code division multiple access

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xiv Wireless Optical Communications

CEI Commission Electrotechnique InternationaleCEPT Confeacuterence Europeacuteenne des Postes et TeacuteleacutecommunicationsCIR Channel impulse responseCNES Centre National drsquoEtudes SpatialesCNET Centre National drsquoEtudes des TeacuteleacutecommunicationsCOFDM Coded orthogonal frequency division multiplexCPG Conference Preparatory GroupCQI Color quality indicationCRC Cyclic redundancy checkCSI Channel state informationCSMA Carrier sense multiple accessCSMACA Carrier sense multiple access with collision avoidanceCSMACD Carrier sense multiple access with collision detectionDARPA Defense Advanced Research Projects AgencyDC Direct currentDD Direct detectionDFB Distributed feedbackDIF DiffusionDIV DivergenceDLR Deutsch Land Radio German Spatial agencyDMT Discrete multitone modulationDPIM Digital pulse interval modulationDPPM Differential pulse position modulationDRM Digital Radio MondialeDSL Digital subscriber lineDSSS Direct sequence spread spectrumDVD Digital versatile discECC Error corrector codeECMA European Computer Manufacturers AssociationEDFA Erbium-doped fiber amplifiersEDRS European Data Relay SatelliteEEL Edge emitting laserEFIR Extremely fast infrared communicationEHF Extremely high frequencyEN European Norm (Euronorm)ERO European Radiocommunication OfficeESA European Space AgencyEthernet LAN packet protocolFCS Frame check sequenceFDD Frequency division duplexFDDI Fiber distributed data interfaceFDMA Frequency division multiple accessFDTD Finite difference time domain

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Acronyms xv

FET Field effect transistorFFT Fast Fourier transformFIR Fast infraredFOV Field of viewFSO Free-space opticFTTx Fiber to the Home BusinesshellipFTTH Fiber to the homeGSM Global system for mobile communicationsGUI Graphical user interfaceHAP High-altitude platformHF High frequencyHHH HirtndashHassnerndashHeise codeHP Optical transmitted half-power angleHTTP Hypertext transfer protocolIBM International Business MachinesICSA Infrared Communication Systems AssociationICT Information and Communication TechnologiesId Dark currentIdP Indoor positioningIEC International Electrotechnical CommissionIEE Institution of Electrical EngineersIEEE Institute of Electrical and Electronics EngineersIIS Interference intersymbolIM Intensity modulationIMDD Intensity modulationdirect detectionInGaAs Indium gallium arsenideIP Internet protocolIPv6 Internet protocol version 6IR InfraredIRC Infrared communicationIrDA Infrared Data AssociationIrLAP Infrared link access protocolIrLMP Infrared link management protocolISCA Infrared communication Systems AssociationISI Intersymbol interferenceISO International Standards for OrganizationITS Intelligent transport systemITU International Telecommunication UnionITU-R International Telecommunication Union Radiocommunication sectorJVC Japan Victor CompanyKDDI Japanese telecommunication operatorLAP Link access protocolLASER Light amplification by stimulated emission of radiation

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xvi Wireless Optical Communications

LD Laser diodeLCD Liquid crystal displayLCR Line clock recoveryLDPC Low-density parity check codeLED Light-emitting diodeLEOT Laser electro-optics technologyLLC Logical link controlLMP Link management protocolLOS Line of sightLRC Longitudinal redundancy checkMAC Medium access controlMIMO Multiple-input multiple-outputMPDU MAC protocol data unitMPE Maximum permissible exposureMPEG Moving Picture Experts GroupMRR Modulating retroreflectorMS MultispotMSD Multispot diffuseMSDU MSMAC service data unitMSM Metal-semiconductor-metal photodiodeMozilla Code name for the web Netscape NavigatorNASA National Aeronautical and Space AdministrationNEC Nippon Electric Company LimitedNFIRE Near-field infrared experimentNLOS Non-line of sightNRZ Non-return to zeroNTT Nippon Telegraph and Telephone CorporationOBEX Object Exchange (IrDA exchange protocol)OFDM Orthogonal frequency division multiplexOMEGA HOME Gigabit AccessOOK Onndashoff keyingOPPM Overlap pulse position modulationOQAM Offset quadrature amplitude modulationOSI Open systems interconnectionOWMAC Optical wireless media access controlPC Personal computerPER Packet error ratePD PhotodiodePDA Personal digital assistantPDU Protocol data unitPHY OSI physical layerPIN Positive intrinsic negative diodePLC Power line communication

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Acronyms xvii

PLCP Physical layer convergence procedurePmP Point-to-multipoint communicationPtP Point-to-point communicationPPDU PLCP protocol data unitPPM Pulse position modulationPSDU Physical service data unitPSK Phase-shift keyingQAM Quadrature amplitude modulationQKD Quantum key distributionQOFI Qualiteacute Optique sans Fil IndoorQOS Quality of serviceRC5 Philips IRDA remote control protocolRGB Red green blueRLL Run length limited encodingRR Radio regulationRS ReedndashSalomon codeRSA RivestndashShamirndashAdleman codeRSV Association of ReedndashSalomon and Viterbi codeRS232 Universal data interfaceRTSP Real-time streaming protocolRVR Runway visual rangeSAP Service access pointSDMA Space division multiple accessSEI Space Exploration InitiativeSFD Start frame delimiterSFTF Spaceborne flight test systemSHF Super high frequencySILEX Semiconductor intersatellite link experimentSIMO Single-input multiple-outputSIR Serial infraredSIRSC Sony IrDA data transmission protocolSISO Single-input single-outputSMTP Simple mail transfer protocolSNR Signal-to-noise ratioSPIE Society of Photo-optical Instrumentation EngineersSWO Smart wireless opticTIA Transimpedance amplifierTFTP Trivial file transfer protocolTCP Transmission control protocolTCPIP Transmission control protocolinternet protocolTDD Time division duplexTDMA Time division multiple accessTG Task group

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xviii Wireless Optical Communications

UDP User datagram protocolUFIR Ultrafast infraredUHF Ultrahigh frequencyUSB Universal serial busUV UltravioletVB Valence bandVCSEL Vertical external-cavity surface-emitting laserVFIR Very fast infraredVISPLAN Infrared wireless LAN systems WLAN system which combine IR

technology (Ethernet 100 Mbps) and LAN mobilityVLC Visible light communicationVLCC Visible Light Communication ConsortiumVoIP Voice over IPVRC Vertical redundancy checkW WattWDAN Wireless domestic area networksWDD Wavelength division duplexWDM Wavelength division multiplexingWDMA Wavelength division multiple accessWIFI Wireless communication protocols governed by IEEE 80211 normsWLAN Wireless local area networksWPAN Wireless personal area networksWLOS Wide line of sightWON Wireless optical networkWS Weapons systemWWRF Wireless World Research ForumWWW World wide web

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Introduction

Telecom operators are finding themselves confronted by a growing demand fora higher volume of information to be transmitted (voice data pictures etc)The increasing frequency in the systems used is a solution because it is able tooffer higher bandwidth and allow higher flow rates In the field of wirelesscommunications the use of links in the range of optical wavelengths visibleultraviolet and infrared constitutes a form of wireless transmission of a few kilobitsper second to hundreds of gigabits per second They can be implemented either overshort distances limited to one room (office living room car airplane cabin etc) orover medium distances (a few tens of meters to several kilometers) outside(atmospheric optical links or free-space optics ndash FSO) or over large distances inspace (high-altitude platform ndash HAP planes drones intersatellite etc)

This technique is not new Over thousands of years well before the work of theAbbot Claude Chappe communication processes although very primitive wereimplementing optical transmission But the amount of information providedremained low Optical communications over long distances did not really start untilthe late 18th Century with the optical telegraph But the quality of service (QoS) waslow the transmitters and receivers men and materialsrsquo lack of reproducibility andreliability and the transmission medium the air was changeable

Soon electricity (electrical charges) and copper replaced the optical (photons)and air Transporting information through a copper line allows relatively high flowrates At the beginning of the third millennium these connections with copper as themedium are still widely used For very large distances for many decades copperwas the base material it has covered the planet with a vast network of informationtransmission

The invention of the laser in 1960 paved the way for an alternative solution ndashthat of fiber optic telecommunication ndash offering a virtually unlimited transmission

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xx Wireless Optical Communications

capacity In 1970ndash1971 the almost simultaneous development of low-loss fiber opticsand a semiconductor laser emitting in continuous operation at room temperature ledto the explosion in wire optical communication Glass is the medium for transmissionof photons and glass fibers may have lengths of several thousand kilometersThe optical wires were therefore unchallenged in underwater transmissionstransmissions over long distances and interurban transmissions It is the essentialelement of the information superhighway

Since the liberalization of the telecommunications sector motivation for thetransmission of digital signals by the laser beam in free space is apparent Severalfactors condition the renewal of this technology First regulatory reasons there is noneed for frequency authorizations or a special license to operate such links incontrast to a large number of radio links Second economic reasons the deploymentof a wireless link is easier faster and less expensive for an operator than theengineering of optical cables Finally in the race for speed the optical flow is thewinner over the radio (even for millimeter wave) for desirable rates of severalgigabits per second In addition the availability of components (lasers receiversmodulators etc) widely used in optical fiber telecommunications technologypotentially reduces equipment costs The global market for digital wireless datatransmission today is based primarily on radio wireless technologies However theyhave limitations and cannot be absorbed on their own with a limited spectral widthdevelopment increases the need for higher speed

The main applications of optical wireless focus on wireless telephonyinformation networks and high-definition TV

The objective of this book is to present the FSO that is currently used for theexchange of information but because of its many benefits (speed rates low costmobility equipment safety etc) it will explode as a telecommunications techniqueover the next decade and even become indispensable in computer architectures onshort- medium- and long-range telecommunications

From a didactic point of view the book is organized into 12 chapterssupplemented by two Appendices

Chapter 1 discusses the basic concepts relating to light the symbolism of thehistory the different theories (wave particle) the propagation and its various laws(reflection transmission refraction diffusion diffraction etc) interference speedspectral composition emission etc That ends in 1960 with the laser inventionwhich opened up the way for many applications CD DVD printers computerdisks optical fibers welding surgery etc

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Introduction xxi

Chapter 2 after some definitions related to telecommunications reviews thevarious phases of the development of wireless optical communications over thecenturies (smoke signals light signals movement of torches etc) And then inthe 18th Century after many tests we review the appearance of Chappersquos opticaltelegraph the solar telegraph or heliograph and the photophone of Graham BellTheir principles (mechanism code etc) are detailed and applications are described

Chapter 3 presents the contemporary and the everyday life of wireless opticalcommunications the basic principles the elements of electromagnetism theelectromagnetic spectrum the propagation modes (line of sight wide line of sightdiffusion etc) the different layers of OSI model and the standardization aspects(VLC IEEE 802157 ECMA IrDA) Then contemporary and daily applicationsof wireless optical communication are described indoor (limited space) outdoor(free-space optic) or spatial (links to aircraft drones HAP intersatellitecommunications etc)

Chapter 4 is dedicated to the modeling of the propagation channel It outlines theoptical channel baseband and different types of modulation (on-off key (OOK)intensity modulation (IM) pulse position modulation (PPM) etc) A comparison ofthe radio model is presented The noise disturbance (thermal noise periodic noise(artificial light) shot noise etc) is described The signal-to-noise ratio compares theperformance of different systems based on different technologies of digitalcommunication The channel is multipath (direct reflected diffused etc) thedifferent paths are combined together Intersymbol interference may occur Thedifferent models of reflection (specular and diffuse (Lambert Phong)) are presentedReflection occurs when the wave encounters a surface on which the dimensions arelarge compared to the wavelength (floor wall ceiling furniture etc) The reflectioncharacteristics depend on the material surface the wavelength and the angle ofincidence Emphasis is then placed on the different models of diffusion

Chapter 5 deals with propagation in the atmosphere Atmospheric effects onpropagation such as absorption and diffusion (molecular and aerosol particles) thescintillations due to the change in the index of air under the influence of temperaturevariations and attenuation by hydrometeors (rain snow) and their different models(Kruse Kim Bataille Al Nabulsi Carbonneau etc) are presented along withexperimental results The experiment implemented to characterize the channeloptical propagation in the presence of various weather conditions (rain hail snowfog mist etc) is presented Fog whose presence is most detrimental to opticaland infrared wave propagation is explained (definition formation characteristicsand development) Visibility the parameter that characterizes the opacity of theatmosphere is defined Measuring instruments for this characterization aredescribed (transmissometer scatterometer) The features of the ldquoFSO Predictionrdquosoftware simulating an atmospheric optical link in terms of probability of

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xxii Wireless Optical Communications

availability or interruption are described It is a tool designed to help supportdecisions for the development of atmospheric optical links at high speeds over point-to-point links on short and medium distances

Chapter 6 discusses the optical link budget in limited space which is animportant step in establishing a link Knowing the sensitivity of the receiver thegoal is to calculate the power to implement at the emitter to enable taking intoaccount the losses in the optical channel These various losses are identified andevaluated geometric loss optical loss pointing loss molecular loss etc Differentcases are considered a line of sight system and an optical system with reflectionThe knowledge of the signal-to-noise ratio is then used to determine the error rate Itis connected to the different attenuations or disruptions of the transmitted signal inthe channel

Chapter 7 deals with immunity and standardsrsquo aspects as well as security andenergy issues For safety reasons care must be taken to transmit power Standardswere developed by the International Electrotechnical Commission They list theoptical sources in seven different classes according to their level of dangerousnessCommunication security is provided either in hardware or in software (encryption)The energy consumption of systems is an important parameter in choosing atechnology Finally a presentation of the legislative aspect ends this chapter

Chapter 8 entitled ldquoOptics and Optronicsrdquo addresses the analog physical part ofan optical device Optical devices for transmission and reception and optical filteringare presented The issue of optronics is then developed the operating principle ofthe device and optronics emitters (white LEDs infrared LEDs laser etc) andreceivers (photovoltaic cell PIN photodiode avalanche photodiode (APD) MSMphotodiode etc)

Chapter 9 deals with data processing before the digitalanalog conversion at theemission and after the analogdigital conversion at the reception The dataprocessing includes operations such as filtering compression analysis predictionmodulation and coding Only modulation and coding parts in a specificconfiguration to optical wireless are described Other items not directly related to theoptical wireless are described elsewhere in the literature Different modulations areexplored OOK NRZ ASK QAM PPM OFDM and MIMO techniques arediscussed Coding aspects are detailed principle definition performance and manyexamples are mentioned parity checks cyclic redundancy check block codes BCHRS convolutional etc

Chapter 10 presents the ldquodata linkrdquo layer the second layer of the OSI systemThe protocols of this layer handle service requests from the network layer andperform a solicitation of requests for services to the physical layer (downlink

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Introduction xxiii

direction) and vice versa (upward direction) Access methods (TDMA FDMACDMA CSMA WDMA and SDMA) are described The QoS parameters arementioned The various protocols used in wireless optical communications arepresented for different types of data links point-to-point (remote control IrDAVLC) point-to-multipoint (IEEE 80211 IR IEEE 8023 Ethernet (ISCA-STB50)IEEE 802153 IEEE 802157 OWMAC)

Chapter 11 is dedicated to engineering of the installation of wireless opticalcommunication in free space and limited space In the area of free space (FSO) firstthere is a description of the principles of operation before turning to thecharacteristics of the equipment and recommendations for implementation Opticalbudget calculations are detailed and examples of the availability of links for variousFrench cities are presented In the area of limited space the habitat structure is firstdescribed the distribution of areas of different rooms and the population percentageof a communication covered area In the architecture of a wireless optical systemthere is at least one optical wireless transmissionreception system per room calledbase station (BS)

Each BS communicates with the terminals present in every room via a wirelessoptical communication Finally these terminals are connected or integrated tomultimedia communication equipment (PC monitor PDA etc) Different simulationsof optical system installations are carried out with a free software tool called ldquoQOFIrdquoand the link budget prepared the base station is located in the middle of the ceiling(case A) above the door (case B) or on a socket (telephone Ethernet PLC (case C))the terminal is installed in the lower opposite corner of the room (case 1) at a heightequivalent to the top of a door (loud speaker motion detector) (case 2) or on theground in the middle of the room (case 3)

The aspects of the system are then discussed (the production of optoelectronicsmodules suitable for optical wireless taking into account the safety aspect by usinga diffuser at the emitter obtaining an optical gain reception by setting in place anoptical device called ldquofisheyerdquo or processes such as equalization and OFDM etc)

Chapter 12 discusses the future of wireless optical communications in free andlimited space at a home or an office In each case the advantages of this medium areunderlined The home and office potential are evaluated and faced with theeconomic and commercial realities

Appendices remind the reader of various concepts related to optical geometric(refractive index Snellrsquos law sources definition image focus etc) photometry(steradian solid angle etc) and energy (light intensity luminous flux illuminance

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

xxiv Wireless Optical Communications

luminance energy flow lighting geometric extent etc) and various items relatingto the use of logarithmic notation (dB dBW dBm etc)

Various elements described in this book contributed to the development of newrecommendations at ITU-R the Radiocommunication Sector of the InternationalTelecommunication Union dedicated to propagation data and prediction methodsrequired for the design of terrestrial free-space optical links and the definition ofassociated systems

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Chapter 1

Light

In the beginning God created the heavens and the earth The earthwas formless and empty darkness was over the surface of the abyssand the spirit of God was hovering over the waters God said lsquoLetthere be lightrsquo and there was light God saw that the light was goodand God divided the light from the darkness God called the light Dayand the darkness he called Night And there was evening and therewas morning it was the first day

ldquoFiat Lux ndash Let there be lightrdquoOld Testament

The Pentateuch ndash Genesis 1Chapter 1

Light has long fascinated man exalted depictions by painters or praise fromwriters with many areas of study for scientists and scholars Figure 11 representsfor example Lady Taperet (22nd Dynasty 10th or 9th Century BC) praying to thesun god Ra-Horakhty The symbolism of light provides an almost unlimited field forcelebration of all kinds in all civilizations past and present

For centuries the only known radiation was light The first written analysisof light seems to date from Greek and Latin civilizations For the Greeks Euclid(325ndash265 BC) and Ptolemy (90ndash168 BC) the light is emitted from our eye and is thevector of an object image On the other hand Epicurus (341ndash270 BC) and the Latinpoet Lucretius (98ndash55 BC) thought that the bright objects sent little pictures ofthemselves into space referred to as ldquosimulacrasrdquo These simulacras were enteringour eyes so we could ldquoseerdquo these objects This latter theory called ldquocorpuscular

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

2 Wireless Optical Communications

theory of lightrdquo would be taken up again in a more abstract manner during the 17thand 18th Centuries

Figure 11 Stele of the Lady Taperet (Louvre museum)

Because of this from the 17th Century the nature of light was a source of debatethat lasted for more than 300 years With the fundamental question ldquoIs light a waveor a stream of particlesrdquo

To explain the laws of reflection and refraction of light rays Rene Descartes(1596ndash1650) evokes particles that bounce off a mirror like a ball in a French game(jeu de paume) whose speed changes when entering a transparent medium (water orglass for example) It is the source of the fundamental SnellndashDescartesrsquo laws Theauthorship of the refraction law is attributed to Willebrord Snell (1580ndash1626) afterChristian Huygens (1629ndash1695) refers to the date of the unpublished work of Snellon the subject Note that the paternity of the discovery of the law of refraction iscurrently attributed to Ibn Sahl (940ndash1000) in 985 Ibn Al-Haytham (965ndash1039)wrote a book on optics (Opticae thesaurus) in which he mentions the phenomenon ofrefraction but he could not develop the mathematical law This discipline was

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

Light 3

originally called ldquodioptricrdquo but later it was called geometrical optics for (or due tothe fact that) the trajectory of light rays is built to geometrical rules

Only a few decades later Isaac Newtown (1643ndash1727) developed his particlemodel of light in 1704 It has a light composed of small ldquoparticlesrdquo emitted byluminous bodies moving very fast in a vacuum and in different transparent mediaHe does not hesitate to complicate the model to make it compatible withobservations such as ldquoNewtonrsquos ringsrdquo This interference phenomenon (Figure 12)is achieved by placing a lens (L) on a flat surface (P) with a light source (L )It is possible to observe a series of concentric rings (A) alternating light and dark[NEW 18] This is now explained by the wave approach

Figure 12 Device and Newtonrsquos rings

During the same period Christian Huygens developed a wave model of light byanalogy with the wave propagation on the surface of the water This model alsoexplains the phenomena of reflection and refraction But with his particular prestigeacquired by his law of universal gravitation Newton turned off the debate andimposed his corpuscular theory of light onto the scientific community at the time

It was not until about a century later that the existence of many known phenomenawas explained by geometrical optics (decomposition of light interference etc)returning to the wave approach with studies of Thomas Young (1773ndash1829) andAugustin Fresnel (1788ndash1827) The ldquowave theory of lightrdquo defines the light as avibration similar to sound vibrating in an invisible environment called ldquoEtherrdquo

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle

4 Wireless Optical Communications

Because measurements were not possible with the instruments of the time aninitial estimate of the propagation speed was 200000ndash300000 kms with a veryimportant frequency of vibration This model is predominant when explaining thephenomena of interference and diffraction

Finally almost half a century later James Clerk Maxwell (1831ndash1879) offeredfour fundamental equations that summarized the knowledge of the time in theelectrical magnetic and electromagnetic fields He succeeded in electromagneticfields by applying what Newton had done in the field of mechanics One of thesethe MaxwellndashAmpere equation defines light as an electromagnetic wave consistingof electrical fields and magnetic fields vibrating transversely with a velocity of300000 kms

This is the electromagnetic wave theory of light and this model faced withmeasures of speed of light dedicates Maxwellrsquos proposal But visible light from redto violet is a special case of those electromagnetic radiations as Maxwell predictedthe existence of other radiation emissions from natural or artificial sources (egcosmic rays or radio transmitters)

In fact in 1887 Heinrich Hertz (1857ndash1894) invented an electromagnetic wavetransmitter whose frequency is below infrared frequencies (below the red) Thesefrequencies known as radio frequencies are the wave bands of radio and televisionThen in 1895 Wilhelm Roumlntgen (1845ndash1923) discovered very high frequencyradiation higher than the ultraviolet frequencies this is X-rays

In 1900 Max Planck (1858ndash1947) made a significant contribution with theexplanation of the spectral composition (color distribution) of emitted light and thequantification of energy exchange between light and matter These energyexchanges are realized by integer multiples of an indivisible base quantity (Figure13) These quanta or quantum of energy are related to a given frequency radiationmultiplied by a constant This new constant of physics is called Planckrsquos constant (h)and is initiated by quantum physics

A few years later in 1905 Albert Einstein (1879ndash1955) hypothesized that lightwas made up of energy (photons) and he proposed a corpuscular theory of light Thelaws of Fresnel and Maxwell are still valid but the energy approach shows that thesame wave transports energy called photons This last point helps to explain suchphenomena as the photoelectric effect (discovered by Hertz in 1887) And in 1909despite reticence from the scientific world at that time to reconcile his theory withthe electromagnetic wave model Einstein concluded that light is both a wave and aparticle