modulation in central station of a radio a thesis...

MODULATION IN CENTRAL STATION OF A RADIO

OVER FIBER DOWNLINK SYSTEM FOR GENERATING

DUAL FREQUENCY MM-WAVE SIGNAL

A Thesis

Submitted in partial fulfilment of the

Requirements for the award of the degree of

MASTERS OF ENGINEERING

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

BY

SOUMYADEEP DAS

(ME/ECE/10021/2013)

Department of Electronics and Communication Enginee ring

Birla Institute of Technology

Mesra-835215, Ranchi

2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI i i

DECLARATION CERTIFICATE

This is to certify that the work presented in the thesis entitled

“Modulation in central station of a Radio Over Fiber Downlink

System For Generating a Dual Frequency mm-Wave Sign al” in

the partial fulfilment of the requirement for the award of Degree of

Masters of Engineering in Electronics and Communica tion of

Birla Institute of Technology Mesra, Ranchi is an authentic work

carried out under my supervision and guidance.

To the best of my knowledge, the content of this thesis does not

form a basis for the award of any previous Degree to anyone else.

DATE:

Head,

Dept. of Electronics and Communication Engineering

Birla Institute of Technology Mesra, Ranchi-835215

Dr. S.K.Ghorai

Professor Dept. of Electronics and Communication Engineering


Dean

(Post Graduate Studies)


DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI ii ii

CERTIFICATE OF APPROVAL

The foregoing thesis entitled “Modulation in central station of a

Radio Over Fiber Downlink System For Generating a D ual

Frequency mm-Wave Signal” , is hereby approved as a

creditable study of research topic and has been presented in

satisfactory manner to warrant its acceptance as prerequisite to

the degree for which it has been submitted.

It is understood that by this approval, the undersigned do not

necessarily endorse any conclusion drawn or opinion expressed

therein, but approve the thesis for the purpose for which it is

submitted.

(Internal Examiner) (External Examiner)

(Chairperson)

Head of the Department

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI iii iii

ACKNOWLEDGEMENT

I would like to take this opportunity to express my profound gratitude and indebtness

to my guide Dr. S.K.Ghorai, Professor, Department of Electronics and

communication Engineering, B.I.T, Mesra for his outstanding guidance, excellent

support, complete belief and inspiring suggestions throughout the course of this

investigation. His encouragement and motivation have been invaluable during my

studies at B.I.T, Mesra.

I would like to thank our Head of the Department, Dr. (Prof.) V.R.Gupta, for

providing all the departmental facilities required and extending kind co-operation

during the entire study period.

I am immensely thankful to Dr. M.K.Mishra, Vice Chancellor, B.I.T, Mesra, Ranchi

for providing all the institutional facilities and infrastructure during M.E. course.

It gives me immense pleasure to acknowledge Dr. N. Gupta, Dr. N. Chattoraj and

my entire respected faculty members of ECE department, for their interest and direct

or indirect co-operation throughout my project.

I also like to thank my senior Mainak Basu for his encouragement and suggestions

during this investigation.

I wish to thank all non-teaching staff of the department for their direct and indirect

help extended during my M.E. course.

I would like to acknowledge AICTE for providing me fellowship during M.E.

studies.

I would like to express my sincere gratitude and love to my parents ,Soma Das and

Dipak Kumar Das, along with my entire family for their patience, belief and

consistent motivation.

I am glad to express my love and gratitude to all of my friends with a special mention

of Parikhit Dutta, Aakash and Rajmita Chakraborty for their enthusiastic presence

and encouragement throughout my M.E. studies as well as entire life.

I would like to dedicate my thesis to my aunt Late Sumita Das for her contributions

in my life and my studies.

Date: SOUMYADEEP DAS

(ME/ECE/10021/2013)

Abstract 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI iv iv

ABSTRACT

The sheer need for higher data rates and also high channel capacity has introduced

various new technologies in wireless domain. Micro and pico-cellular architecture are

redefining communication system. It is true that increase in the carrier frequency

increases bandwidth but it results in to complicated end user units. Optical fibres on

the other hand have an infinity bandwidth theoretically and have advantages of

immunity to electromagnetic interference (EMI) and reduced attenuation of signals

propagating through an optical fibre. Utilizing the mobility of wireless technology and

bandwidth advantages of optical communication, Radio-over-Fibre (RoF) promises to

be a redefining technology. RoF systems also result in simplified Remote Antenna

Units. In this project work, a novel, cost effective and simplified Central Station (CS)

for a dual frequency RoF downlink system is proposed and designed. The various

modulation schemes for central station has been studied and compared. External

modulation using Mach-Zehnder Modulator is described in detail. Apart from

communication another aim of this proposed architecture is to generate dual

frequency millimeter wave carrier signal for baseband data and to transmit it into

wireless medium to the end user units. Optical to Electrical re-conversion at the Base

Station (BS) is obtained by means of coherent photo detection resulting in frequency

multiplication. Due to this we can go in the range of mm wave transmission, having

advantages of low atmospheric attenuation beyond 60 GHz. Thus integrating optical,

microwave and millimetre wave technology to design a RoF system and study the

performance of this system is aimed at.

TABLE OF CONTENTS 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI v v

TABLE OF CONTENTS

CHAPTER 1 ............................................................................................................................. 1

INTRODUCTION .................................................................................................................... 1

1.1 Problem Statement ........................................................................................................... 1

1.2 Motivation ........................................................................................................................ 1

1.3 Objective .......................................................................................................................... 1

1.4 Background and overview ............................................................................................... 2

1.5 Scope of the Project ......................................................................................................... 6

1.6 Organization of Thesis ..................................................................................................... 6

CHAPTER 2 .............................................................................................................................8

LITERATURE REVIEW........................................................................................................8

CHAPTER 3 ........................................................................................................................... 11

RADIO OVER FIBER .......................................................................................................... 11

3.1 Microwave Photonics .................................................................................................... 11

3.2 Generalized Study of ROF ............................................................................................. 13

3.2.1 Basic architecture .................................................................................................... 14

3.2.2 Advantages and Disadvantages (RoF general system) ........................................... 15

3.3 Central Station ............................................................................................................... 16

3.3.1 Optical – Electrical (RF signal) Modulation ........................................................... 16

3.3.2 Optical Interleaving ................................................................................................ 20

3.3.3 Baseband Modulation ............................................................................................. 22

3.4 Millimeter Wave Generation ......................................................................................... 25

CHAPTER 4 ........................................................................................................................... 28

PROPOSED SYSTEM .......................................................................................................... 28

4.1 Proposed Central Station ................................................................................................ 28

4.2 Component Analysis ...................................................................................................... 29

4.2.1 Mach Zehnder Modulator ....................................................................................... 29

4.2.2 Optical Interleaver .................................................................................................. 30

4.2.3 Fiber Brag Grating .................................................................................................. 31

CHAPTER 5 ........................................................................................................................... 33

THEORETICAL MODELLING ......................................................................................... 33

TABLE OF CONTENTS 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI vi vi

5.1 Frequency Domain Analysis .......................................................................................... 33

5.2 Bessel Function .............................................................................................................. 35

5.3 Mathematical Modelling of Proposed Central Station ................................................... 40

CHAPTER 6 ........................................................................................................................... 47

RESULTS AND DISCUSSION ............................................................................................ 47

6.1 Analysis of Central Station ............................................................................................ 47

6.1.1 Mach Zehdner Modulator ....................................................................................... 48

6.1.2 Optical Interleaver .................................................................................................. 49

6.1.3 Baseband Data Modulation ..................................................................................... 50

6.1.4 Optical Coupler ....................................................................................................... 51

6.2 Analysis of Laser Source ............................................................................................... 52

6.2.1 Variation of Linewidth ............................................................................................ 52

6.3 Variation of Baseband Data ........................................................................................... 54

6.3.1 Type of Pulses ......................................................................................................... 54

6.3.2 Variation of Data Rate ............................................................................................ 56

6.4 Millimeter Wave Generation ......................................................................................... 57

6.4.1 Experimental Setup ................................................................................................. 57

6.4.2 Simulation Analysis ................................................................................................ 60

CHAPTER 7 ........................................................................................................................... 62

CONCLUSION AND FUTURE SCOPE ............................................................................. 62

7.1 Conclusion ..................................................................................................................... 62

7.2 Flexibility of dual frequency mm wave ......................................................................... 62

7.3 Future Scope .................................................................................................................. 63

LIST OF FIGURES 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI vii vii

LIST OF FIGURES

Fig. 1.1: ROF technology used for domestic purposes 3

Fig. 1.2: Basic Block Diagram of ROF 5

Fig: 3.1. Schematic diagram of a simple RoF downlink system 14

Fig 3.2: Left: absorption of a semiconductor as a function of wavelength with and

without an external applied electric field. Right: typical loss versus applied voltage

curves for an electro-absorption modulator 18

Fig. 3.3: Principle of operation of a Mach-Zehnder modulator. 19

Fig. 3.4: (a) optical interleaver technique; (b) optical de-interleaver technique 21

Fig. 3.5: NRZ and RZ data stream 23

Fig. 3.6: Single Drive Mach-Zehdner Modulator 25

Fig. 3.7: Attenuation for microwave and millimeter wave range (1GHz to 300GHz

27

Fig. 4.1: Block Diagram of the proposed ROF downlink Central Station 28

Fig. 4.2: Unbalanced Mach Zehnder Modulator (MZM) 30

Fig. 4.3 Working principle of a Fibre Bragg Grating[36] 32

Fig. 5.1: Basic representation of Fourier Transform 35

Fig 5.2: Plot of Bessel function of the first kind, Jα(x), for integer orders α = 0, 1, 2

37

Fig. 5.3: Plot of Bessel function of the second kind, Yα(x), for integer orders α = 0, 1,

2. 38

Fig. 5.4: Modulated signal after MZM1 and MZM2 43

Fig. 5.5: PRBS (NRZ) data; Baseband data 44

Fig. 5.6: Response after FBG (Baseband data modulates the freq component) 45

Fig. 5.7: Output of the Central Station (towards base stations) 46

Fig. 6.1: Design of the Central station of ROF downlink in Optiwave 13.0 48

Fig. 6.2: (a) Output of MZM1; suppressing the even order sidebands; (b) Output of

MZM2; suppressing the odd order components along with the carrier 48

Fig. 6.3: (a) Filtered output of MZM1; (b) Filtered output of MZM2 49

Fig. 6.4: (a)-(b) Combination of selected order of sidebands after interleaver setup

50

Fig. 6.5: NRZ PRBS data sequence with bitrate 2.5Gbps 50

Fig. 6.6: Baseband data, modulated with (a) +4 component; (b) -4 component 51

LIST OF FIGURES 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI viii viii

Fig. 6.7: Final output signal of Central Station transmitted towards (a) BS1 and

(b) BS2 52

Fig. 6.8: Output signal of ROF Central Station with various Laser Linewidth; 53

Fig. 6.9: (a) data pattern of NRZ; (b) data pattern of RZ; Frequency domain

representation (c) NRZ; (d) RZ 54

Fig. 6.10: Fourth order sideband being modulated by (a) NRZ data; (b) RZ data; The

final output of the CS when the baseband data pulses are (c) NRZ & (d) RZ 55

Fig. 6.11: Baseband modulation done with various datarate; (a)2.5Gbps; (b) 5Gbps;

(c) 25Gbps; (d) 40Gbps 56

Fig. 6.12: WDM setup with Laser source and internal PIN detector and Digital

Storage Oscilloscope 58

Fig. 6.13: Tunable Laser source 58

Fig. 6.14: OSA shows the optical spectrum of both the laser sources after combining

using a 3dB coupler 58

Fig. 6.15: Setup of the central station with TLS and OSA for mm wave generation

59

Fig. 6.16: Setup of the CS continued… with FBG and optical couplers. 59

Fig. 6.17: Final Frequency spectrum of the mm wave signal after BS 59

Fig. 6.18: Simulation data for the single Laser and the combined Lasers after the 3dB

coupler with the laboratory specifications 60

Fig. 6.19: Simulated frequency profile of the generated mm wave with laboratory

specifications 60

Fig. 6.20: Combining two laser sources with narrower linewidth 61

Fig. 6.21: Simulation result for the generated mm wave signal with a frequency of

37.5GHz 61

LIST OF TABLES Table 6.1: Important design parameters for downlink system 47

Table. 6.2: Observed signal power at the end of the CS 55

Table. 6.3: Components used for experiments and their specifications 57

LIST OF SYMBOLS 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI ix ix

LIST OF SYMBOLS

Eg Electron Band Gap

Ν Frequency

Λ Wavelength

Η Refractive Index

Ø(t) Phase shift

Vπ Half Wave Voltage

f0 Optical Carrier Frequency

ω0 Optical Carrier Frequency (angular)

fRF RF signal frequency

ωRF RF signal frequency (angular)

Jn(m) nth order Bessel Function of first kind

A0 Amplitude of the optical field

M Modulation index

ABBREVIATIONS 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI x

ABBREVIATIONS

CS Central Station

BS Base Station

FBG Fiber Bragg Grating

RF Radio Frequency

HAN Home Area Network

HDMI High Definition Multimedia Interface

EDFA Erbium Doped Fiber Amplifier

SSMF Standard Single Mode Fiber

HFC Hybrid Fiber Coaxial

FTTH Fiber To The Home

PON Passive Optical Network

MMW Millimeter Wave

QPSK Quadrature Phase Shift Keying

M-QAM M-ary Quadrature Amplitude Modulation

OFDM Orthogonal Frequency Division Multiplexing

ROF Radio Over Fiber

MWP Microwave Photonics

EVM Error Vector Magnitude

MZM Mach-Zehnder Modulator

TW Travelling wave

PD Pin Diode

NF Noise Figure

DR Dynamic Range

MMF Multi Mode Fiber

DML Direct Modulating Laser

EAM Electro Absorption Modulator

DWDM Dense Wavelength Division Multiplexing

IM Intensity Modulator

ABBREVIATIONS 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI xi

NRZ Non Return to Zero

RZ Return to Zero

ASK Amplitude Shift Keying

EHF Extreme High Frequency

DSB Double Side Band

SSB Single Side Band

OCS Optical Carrier Supression

CW Continuous Wave

OSA Optical Spectrum Analyzer

DSO Digital Storage Oscilloscope

TLS Tuneable Laser Source

CHAPTER 1: INTRODUCTION 2015

DEPT. OF ELCTRONICS AND COMMUNICATION B.I.T. MESRA, RANCHI 1

CHAPTER 1

INTRODUCTION

1.1 Problem Statement Communication is one of the most interesting and important field in modern

day electronics. It defines day to day life and each and every step of our progress.

Starting from the telephonic conversations to video conferences, from weather

predictions to live telecast of important incidents communication has a key role to

play. It is difficult to priorities the type of communication, say wired or wireless.

Each of them has its own pros and cons. Hence it is essential for the future

communication to integrate these two methods of communication to gain

maximum benefit for the mankind. Hence wireless communication in Radio

Frequency domain may be integrated with Optical Fiber Communication to create

architecture of efficient, cost effective, secure communication as well as to

generate high frequency millimeter wave signals with ease.

1.2 Motivation Disadvantages of the current fiber- wireless communication are the limitation

of frequency, complex and bulky architecture, limitation of data rate of the

baseband data, etc. In this thesis we wish to overcome these drawbacks by

investigating the actual cause and proposing the possible solutions.

In this thesis work Radio Frequency is being used to enhance the frequency

range. Also we use frequency multiplication and frequency reuse to make the

system compact and also flexible to the end users. We also generate the millimeter

wave signal in a cost effective and efficient way.

1.3 Objective In this work, the design and performance of a dual frequency full duplex RoF

system Central Station is attempted. The main objective is to design a simple and

cost effective CS. External modulation at the CS is used to achieve a dual

frequency signal containing 72 and 84 GHz (mm wave) frequency components at

base station (BS).



The objectives are

• To modulate RF signal with optical carrier frequency efficiently. Here

Mach-zehnder modulator is used.

• To extract sideband harmonics using Fiber Bragg Grating (FBG)

techniques.

• Frequency multiplication (six times and seven times in our case) of RF

signals to be done at the Base Stations.

• To connect multiple (at least two) base stations to a single central station.

• To reuse some frequency components for uplink, so that base stations

remain mobile and less complicated and no separate laser source is

needed.

• To design the interleaver setup using add-drop multiplexing.

• Experimentally generate millimeter (mm) wave from optical source.

1.4 Background and overview Since the late 1990s, wireless technologies have been developed to replace

the wires installed in the Home Area Networks (HAN). One of the most successful

interfaces is the IEEE 802.11 standards also known as Wi-Fi. The current

generation can achieve theoretically up to 600Mbps, but in the coming years higher

data rates will be needed. Indeed, the ability to deliver high bit rate services

through the optical access networks and the increasing needs in terms of device

interconnection inside the home will boost the throughput requirements. And this

tendency will keep growing as new usage models arrive continuously: thus, the

instantaneous data rate in the HAN could reach 10Gbps. As a result, these last

years, new wireless technologies appeared.

The current Wi-Fi technologies exploit the 2.4 or the 5 GHz unlicensed

bands, but because these frequencies know a huge success, they tend to become

very saturated. To preempt this and increase the connection speeds, the industrials

are going towards the unlicensed millimeter-wave band, from 57 to 66 GHz. This



band is widely available around the world, it is not yet massively used, it offers

large channels of 2.16GHz bandwidth with high authorized emitted powers and it

allows advanced integration into radio terminals thanks to miniaturization at such

frequencies. Thus, wireless systems able to achieve data rate up to 7Gbitps and

dedicated to HAN are emerging or are under development. With this speed, users

can really start to replace every domestic cable they use, even High Definition

Multimedia Interface (HDMI) cables that convey uncompressed High Definition

(HD) video signals.

Fig. 1.1: ROF technology used for domestic purposes

The society as we know today has been greatly influenced by the

development of telecommunications technology and the invention of internet and

its multimedia rich applications like YouTube, HULU, Facebook, and others. Now

it goes beyond the HAN. The Internet has led to the exponential growth in data

transmission [1] in telecommunication networks, which in turn has led to

development of new technologies, and vice versa. The majority of

telecommunication networks are based on fiber optic communications, which with

the invention of Erbium-doped fiber amplifiers (EDFA) [2] has become the

favourite technology for long haul data transport [3, 4]. Optical communication

systems are widely deployed all over the world, mainly for long haul (> 1000 km)

and regional/metro communication links. Until recently, the last mile user

connectivity was provided mostly by cable TV, or through twisted pair (ADSL),

which have a very limited bandwidth distance product (around 10 Mb/s*km). The

most recent access technology, the hybrid fiber coaxial (HFC) can only provide a



few tens of Mb/s to each user, and all these technologies do not meet the

bandwidth requirements of the future. Recently, fiber optics is entering the last

mile segment with the passive optical network (PON) technology also called as

fiber to the home (FTTH), where 2.5 Gbps connectivity using a time-division-

multiplexed PONs [5] are already under implementation. The current state of the

art standard for optical access network is a 10 Gbps Ethernet based connectivity

called 10GEPON [6], and optical access networks with 40 Gbps [7] are being

investigated. This sudden increase in the access networks' capacity is well

supported by the advances in metro and core segment where systems for 100 Gbps

[8-12] or more [13] are being investigated, with a few of them in deployment.

With the rapid growth in optical access networks, there is a digital divide

emerging where remote areas, and emerging isolated residential complexes and it

hard to have the last mile fiber for either cost or logistical reasons. In such cases,

wireless access has always played a crucial role in expanding the reach of access

networks in a cost-effective manner. On the other hand, wireless access through

wireless LANs, WiMAX, LTE, etc are evolving rapidly, and capacity of 100 Mbps

per user are already under consideration. But for the wireless technology to support

the current optical access, data rates up to 10 Gbps or more are currently needed,

for extending the reach of the optical access networks. Some other applications of

such high capacity wireless links are inter-building gigabit connectivity,

emergency service deployment, redundancy links, etc.

To provide multi-Gbps wireless links, new frequency bands like the

millimetre wave (mmW) bands have to be considered [14]. The advantages of the

mmW bands are the availability of a few GHz of bandwidth, which may provide

wireless links with Gbps capacity. Another advantage is the low beam-width of the

higher mmW bands, which in combination with high gain antennas enable higher

spatial multiplexing and frequency reuse [15]. Of the mmW band spectrum, the 60

GHz (57-64 GHz) band, and the 70/80 GHz (71-76/81-86 GHz) are mostly

considered by various wireless system developers. The frequency bands of 120

GHz or 140 GHz (both unregulated) are under investigation for very high capacity

wireless. To provide 10 Gbps wireless links in these frequency bands, spectral

efficient modulation formats like quadrature phase shift keying (QPSK),m-ary



quadrature amplitude modulation (M-QAM), or even orthogonal frequency

division multiplexing (OFDM) based modulation must be considered especially in

a bi-directional link scenario.

Recently, in 2009, the IEEE 802.11ad group has been created dealing with

60GHz components for IEEE 802.11 family systems. It should lead the

competition that occurs between industrials thanks to Wi-Fi certification.

Additionally, the first Wi-Fi 60GHz chipsets are expected at the second half of

2013, and by 2016 they could account for more than 40% of the total Wi-Fi market

[16]. However, these wireless systems have a coverage limited to a single room

and to a small indoor open area due to the high propagation attenuation at 60GHz

and to non-propagation of millimeter-waves across the walls. Therefore, the main

challenge to be resolved by this thesis consists in finding solutions to extend the

radio coverage to the entire HAN, and to interconnect wireless devices located in

different rooms.

Fig. 1.2: Basic Block Diagram of ROF



1.5 Scope of the Project Radio-Over-Fiber (R-o-F) technology gives the solution for the needs of the

future generation communication system. By designing a full duplex R-o-F system

architecture we may achieve the required data rate of the transmitted data as well

as the solution of the problem of last mile fiber. All the frequency domain analysis

by Fourier Transform is being done to get the frequency components. Generation

of millimeter wave signal can also be done with this architecture. Dual frequency

millimeter wave carrier signal gives the flexibility to the Base Stations. In the due

course of the project the designing of the optical interleaver system using add-drop

multiplexers adds to the options of various interleaver systems. This R-o-F system

is aiming to provide a high data rate up to 40Gbps or even more.

1.6 Organization of Thesis This following thesis describes the executed work:

Chapter 1: Introduction: It gives the overview and introduction of the project

along with the problem statement.

Chapter 2: Literature Review: Brief discussions of the works, already done by

several researchers and authors, have been included in this chapter.

Chapter 3: Radio Over Fiber: This chapter focuses on the basic concept of

Microwave Photonics and also the detailed study on Radio Over Fiber technique. It

gives stress on the modulation at the Central Station (CS) of the R-o-F system.

Generation of millimeter wave is also being described in this chapter.

Chapter 4: Proposed System: In this chapter we discuss about our proposed

system architecture. Based on Radio Over Fiber technology we are introducing

the novel architecture of a downlink central station for generating a dual

frequency mm wave at the end of Base Station.

Chapter 5: Theoretical Modeling: The main focus of this chapter is the detailed

modeling of the Radio Over Fiber technology along with the mathematical tools



and functions we required. It gives idea of Fourier Transformation and Bessel

Function. It also models the R-o-F central station mathematically.

Chapter 6: Results and Discussion: Results are obtained and analyzed in this

chapter. Both the simulation and the experimental setup are discussed in this

chapter.

Chapter 7: Conclusion and Future Scope: This chapter covers conclusions and

future scopes. The concluding remarks are made for the project. Suggestions for

further improvements in analysis and simulation results are included in this

chapter.

CHAPTER 2: LITERATURE REVIEW 2015


CHAPTER 2 LITERATURE REVIEW

Radio-Over-Fiber technology comes under the broader area of Microwave

Photonics. The very term Microwave Photonics (MWP) was coined in 1991. Since

then this area of research has been witnessing regular progress and development in

architecture.

Researcher named Jäger. D described novel and innovative components

based upon the interaction of travelling optical waves and microwave signals [17-

18]. He used Microwave Photonics as a tool. It introduced a new era in the field of

already known Non-Linear photonics.

In the year of 1992 Polifko, D. and Ogawa, H. had foreseen that the merging

of microwave and photonic technologies could gain real commercial interest,

where the RF signal is optically transmitted along extremely low-loss glass fibers

[19].

In the last decade MWP added a new dimension to the multi and

interdisciplinary researches. Seeds, A.J. , Vilcot, A. et al. and Yao, J. are

developing their researches based on MWP and developing the less known theory

of the field also [20-23].

Hong Wen, et al. came up with a new idea called radio-over-fiber technology

full duplex system using frequency reuse. The modulate the laser directly with the

help of RF signal and tried to achieve less than 2 dB power penalty for a 2.5Gbps

dta over a 40km standard Optical Fiber Cable (OFC). [24]

One year later in 2009 J.He et al. overcome the limitations of direct

modulation of the laser and introduced external modulation using Mach-Zehdner

Modulator (MZM). [25] They also generated millimeter wave signal by

quadrupling the RF signal frequency at the Base Stations (BS).

This external modulation opened up the field of ROF for the researchers as

well as commercial manufacturers. In 2010 to till now various techniques of



modulation and frequency multiplication are being used. Someone does a octupling

or a dual quadrupling [26] of the frequency; some researchers observe a decupling

[27]. Also Single side band modulation [28] and double sideband modulation for

MZM [29] have been carried out to obtain the most suitable combination for ROF

technology.

The most recent paper that we have studied is authored by Guangming

Cheng, Banghong Guo, Songhao Liu and Weijin Fang. In their 2014 Optik paper

they mentioned about frequency octupling to achieve W-band range mm wave

signal. Also they successfully obtained a less than 0.6dB power penalty for a

5Gbps data sequence over a 60km standard single mode fiber (SSMF).[30]

Vishal Sharma, Amarpal Singh and Ajay K. Sharma studied the challenges

and how to mitigate those for a better performance of the ROF system. [31, 32]

Zaid Al-Husseini et al. makes 60GHz ROF system for future generation

wireless system. They measured the phase noise of the system and most

importantly the effect of residual carrier signal. [33]

B. Cabon, F. Brendel and J. Poette investigate the performance of various

possible architectures, built for 60GHz transmission channel. They concluded that

Error Vector Magnitude (EVM) is more or less same for all architectures. But,

conversion gain depends on the architecture. Best conversion gain is obtained with

mode-locked laser diode and external modulation, while the worst value is

obtained for a cascade of external modulators. [34]

Laser Phase noise and linewidth affect the ROF transmission due to

incoherency of the laser source. For efficient communication these to aspects of

laser sources should be taken care of. Linewidth should be narrow for efficient

transmission and generation of mm wave. [35-39]

For any kind of optical communication including ROF technology one of the

main components is Fiber Bragg Grating (FBG). Kenneth. O. Hill and Gerald

Meltz gave an overview of FBG technology in their 1997 paper. [40]



Chung-Ting Lin et al. studied and observed the effects of nonlinear transfer

function of MZM on optical up-conversion. They also consider imperfect splitting

ratio of MZM. They used double sideband carrier suppression modulation. [41]

Millimeter wave generation is one of the main aspects of ROF technology.

MM wave can be generated efficiently usinf ROF technology and frequency

multiplication. Mohmoud Mohamed et al. designed three architectures to analyse

the performance. It was observed the frequency doubling with dual drive MZM

may have got the maximum power transmission but not good for mm wave

generation. [42]

Po-Tsung Shih et. Al, compared the FFTx communication with ROF. They

used WDM for modulating MZM.[43]

CHAPTER 3: RADIO OVER FIBER 2015


CHAPTER 3 RADIO OVER FIBER

3.1 Microwave Photonics Within the last two decades, the field of optic–microwave interactions and

the attractive integration of the best concepts of both domains have attracted

growing interest worldwide. The specific term of “microwave photonics” was first

introduced in 1991 and used to describe novel and innovative optoelectronic

components based upon the interaction of traveling optical and microwaves [1,2], a

concept already known from nonlinear optics or traveling wave (TW) tubes in

radio frequency (RF) electronics. In the following, it was foreseen that the merging

of microwave and photonic technologies could gain real commercial interest, and

would develop and become a new approach for future phased array or fiber radio

communication systems, as two examples of major interest, where the RF signal is

optically transmitted along extremely low-loss glass fibers [3]. Since then, the field

of RF optoelectronics and microwave photonics (MWP) rapidly broadened.

Today, MWP is already a most innovative and commercially interesting new

multi- and interdisciplinary field combining and transferring different beneficial

technologies [4–7]. This means that microwave technologies are used and

employed in photonics and, vice versa, photonic technologies are utilized in

microwave techniques. Most important, however, is that in specific areas novel

synergistic concepts have been developed by merging both technologies, which

particularly holds for the field of optoelectronics and its devices as their interface.

Even more and as a result of ever-increasing frequencies (see e.g., the field of

wireless communications), the term microwave stands here not only for GHz and

millimeter waves but also for THz frequencies in the frequency domain and,

equivalently, for picosecond or femtosecond timescales in the time domain [7].

The general field of optic–microwave interactions, initially called RF

optoelectronics, goes back to the study of high-speed photonic devices operating

at microwave or millimeter-wave frequencies [2] and their corresponding use in

RF, microwave, millimeter wave, THz, or photonic systems. This

multidisciplinary field is located at the interface between microwave techniques,



ultrafast electronics, and photonic technologies, and typical investigations include,

for example, high-speed and microwave signal generation, signal processing, and

conversion as well as the distribution and transmission of microwave signals via

broadband optical wired or wireless links such as glass or polymer fibers or even

free space. From first and pioneering ideas and experiments in the 1970s, the field

of MWP has dramatically broadened and paved the way for an enabling novel

technology with a number of innovative and, nowadays, more and more

commercially important applications.

This chapter is intended to give an exemplary overview together with some

recent typical results in this multidisciplinary field of MWP, ranging from basic

concepts, devices, and technologies to systems under investigation and

applications of worldwide interest. In particular, the following topics will be

addressed by way of characteristic examples showing the synergetic mixture of

the two technologies involved:

• Monolithic integration technologies and packaging aspects

• Ultrafast photonic components such as optical modulators,

photodetectors (PDs), photonic mixers, and transceivers with special

emphasis on TW devices using the distributed interaction between

traveling microwave and optical signals in order to avoid otherwise

limiting RC time constant.

• Concepts and examples of microwave signal generation and processing

by way of using photonic technologies

• Broadband and analog optical links for high-speed interconnects

• Microwave photonic systems based on the merging of microwave and

optical technologies. Examples here are optical links for wireless

communication techniques such as cellular radio systems or field

sensor applications and optically controlled phased array antenna

systems

Recent results, especially in this field of high-speed optical interconnects,

clearly demonstrate the synergetic mixture and advantages of the different

technologies involved in MWP.



Generation and transmission of radio signals was previously performed using

electrical technology, where the bandwidth and transmission distances were not in

comparison with today's requirements. With the invention of the laser [44], and

hence the birth of optical communications, the bandwidth and the distances started

to increase drastically. In parallel, the use of optical communications for

transmission of radio wireless signals started to gain a lot of interest [45-48] owing

to the great advantages of photonic technologies like high bandwidth, flexibility,

and low transmission losses. This field of using photonic technologies for

microwave applications is named as Microwave Photonics (MWP) [49-52] and its

applications for transmission and distribution of radio signal is referred to as

Radio-over-Fiber (RoF). Some of the main applications of MWP include high

performance analog links [53, 54], cellular/ mobile links [55-57], RADAR, Cable

TV [58, 59] etc. All these applications are possible because of the advantages of

MWP technology like low cost, small form factor, low attenuation, higher

operational frequencies, etc [51]. Another application of MWP technology is Gbps

wireless, which will be introduced in the next section.

3.2 Generalized Study of ROF

Radio over Fiber (RoF) refers to a technology whereby light is modulated by

a radio signal and transmitted over an optical Fiber link to facilitate wireless

access, such as 3G and WiFi simultaneous from the same antenna. In other words,

radio signals are carried over Fiber-optic cable. Thus, a single antenna can receive

any and all radio signals (3G, Wifi, cell, etc..) carried over a single-Fiber cable to a

central location where equipment then converts the signals; this is opposed to the

traditional way where each protocol type (3G, WiFi, cell) requires separate

equipment at the location of the antenna.

In RF-over-Fiber architecture, a data-carrying RF (radio frequency) signal

with a high frequency is imposed on a lightwave signal before being transported

over the optical link. Therefore, wireless signals are optically distributed to base

stations directly at high frequencies and converted from the optical to electrical

domain at the base stations before being amplified and radiated by an antenna. As a

result, no frequency up–down conversion is required at the various base stations,

CHAPTER 3: RADIO OVER FIBER

DEPT. OF ELCTRONICS AND COMMUNICATION

thereby resulting in simple and r

the base stations.

Fig: 3.1. Schematic diagram of a simple RoF downlink system

3.2.1 Basic architecture Figure 3.1 shows a typical RF signal (modulated by analog or digital

modulation techniques)

RF signal may be baseband data, modulated IF, or the actual modulated RF

signal to be distributed. The RF signal is used to modulate the optical source

in transmitter. The resulting optical signal is lau

At the other end of the fiber, we need an optical receiver that converts the

optical signal to RF again. The generated electrical signal must meet the

specifications required by the wireless application be it GSM, UMTS,

wireless LAN, WiMax or other. By delivering the radio signals directly, the

optical fiber link avoids the necessity to generate high frequency radio

carriers at the antenna site. Since antenna sites are usually remote from easy

access, there is a lot to gain from

can carry information in one direction only (simplex) which means that we

usually require two Fibers for bidirectional (duplex) communication.

However, recent progress in wavelength division multiplexing makes i

possible to use the same Fiber for duplex communication using different

wavelengths. WDM can be use to combine several wavelengths together to

send them through a Fiber optic network, greatly increasing the use of the

available Fiber bandwidth and maximi

to meet future wireless bandwidth requirements.

CHAPTER 3: RADIO OVER FIBER

DEPT. OF ELCTRONICS AND COMMUNICATION 14

thereby resulting in simple and rather cost-effective implementation is enabled at


3.2.1 Basic architecture Figure 3.1 shows a typical RF signal (modulated by analog or digital

modulation techniques) being transported by an analog fiber optic link. The



in transmitter. The resulting optical signal is launched into an optical fiber.




s LAN, WiMax or other. By delivering the radio signals directly, the



access, there is a lot to gain from such an arrangement. Usually a single fiber



However, recent progress in wavelength division multiplexing makes i




available Fiber bandwidth and maximizing total data throughput that in order

to meet future wireless bandwidth requirements.

2015

B.I.T. MESRA, RANCHI

effective implementation is enabled at


Figure 3.1 shows a typical RF signal (modulated by analog or digital

being transported by an analog fiber optic link. The



nched into an optical fiber.




s LAN, WiMax or other. By delivering the radio signals directly, the



such an arrangement. Usually a single fiber



However, recent progress in wavelength division multiplexing makes it




zing total data throughput that in order



3.2.2 Advantages and Disadvantages (RoF general system) 3.2.2.1 Benefits of ROF

The transmission of radio signal over Fiber is preferable because

the enormous bandwidth that offered by optical Fiber, which is up to

THz. Many data formats such as videos, data, telephony and etc. could

be transferred via a single Fiber link. The high optical bandwidth also

enables high speed signal processing to be implemented. This might be

more difficult or impossible to be done electronically. Furthermore,

optical Fiber has low loss transmission (0.3 dB/km at 1550 nm and

0.5dB/km at 1310 nm wavelengths) which is very beneficial to

distribute wireless data transmission. The minimum losses occurred at

the three wavelengths, which are 850 nm, 1310 nm and 1550 nm. The

combination of bandwidth from these three regions is huge that made

the mm wave as a viable solution. RoF also immune to radio frequency

interference since the transmission of signal is in the form of light. This

also will provide the much needed privacy and security measures. At

the hardware implementation, complex and expensive equipments are

kept at the CS. Simpler, smaller and lighter remote antenna units are

located at the BSs. This contributes to easy installation and

maintenance processes. These easier tasks reduce the system cost. At

the management level, centralized control of the resources is possible.

Therefore, resources such as bandwidth can be dynamically allocated

depending on the demands and priorities.

So in summarized form the advantages are,

• A large transmission bandwidth

• Immunity to electromagnetic interference and other atmospheric

impairments

• A simplified BS ensures reduced cost

• The losses are minimized

• Extremely high data rates



3.2.2.2 Limitations of ROF

Since RoF is fundamentally an analog transmission, the

limitations could be caused by noise and distortion. It is resulted from

the nonlinear condition of the devices used in the link such as laser .

This limits the noise figure (NF) and dynamic range (DR) of radio

signal transmission. These two parameters are very important in radio

transmission. A method to suppress noise and distortion must be

properly applied in order to have an improved NF and DR. Besides

that, Fiber link itself has limitations. Chromatic dipersion in SMF

limits the Fiber link length. It may also cause the de-correlation leading

to increased RF carrier phase noise. In MMF, modal dispersion limits

the available bandwidth and distance.

So in summarized form the disadvantages are,

• A bottleneck between the optical and electrical components

• Chromatic dispersion in optical fibers limit transmission

distances.

3.3 Central Station Central Station (CS) of a ROF downlink system comprises of Laser source,

External Modulator, Optical Interleaver system and Optical couplers. For a

downlink system, CS to BS communication, the CS does the electrical to optical

conversion and sends the optical-mm wave signal over the optical fiber medium

towards the Base Station. A Central Station also does the processing of the

message data or baseband data. This baseband data is modulated with that

generated optical-mm wave signal.

In this thesis we will discuss the Central Station of a RoF downlink system in

details.

3.3.1 Optical – Electrical (RF signal) Modulation In optical-electrical modulation a laser source can be modulated

directly by using a RF signal. This kind of laser is called Directly Modulated

Laser (DML) sources. The ability of directly modulating a laser is an



attractive cost-effective solution for RoF links. DMLs were extensively

studied for their use in optical communication systems for metro applications

[60, 61]. The limiting factors of a DML are its modulation bandwidth,

extinction ratio, chirp and linearity, but for baseband data modulation these

parameters do not greatly affect the performance in a typical RoF link. In

directly modulated semiconductor lasers when the carrier density is changed

due to the changes in the modulation current, changes the refractive index in

the active area resulting in a frequency deviation also called as frequency

chirp [62]. The effect of chirp in an optical communication system is spectral

broadening due to dispersion in a SMF, which limits the total transmission

distance [63].

When data rates were in the low gigabit range and transmission

distances were less than 100 km or so, most fiber optic transmitters used

directly modulated lasers. However, as data rates and span lengths grew,

waveguide chirp, caused by turning a laser on and off, limited data rates.

Dispersion problems resulted when the wavelength chirp widened the

effective spectral width of the laser. A laser source with no wavelength chirp

and a narrow linewidth provide one solution to the problem. This solution

took the form of external modulation which allows the laser to be turned on

continuously; the modulation is accomplished outside of the laser cavity.

This modulation technique is called external modulation. There are mainly

two types of external modulation technique is available depending on the

nature of modulator materials.

• Electro- absorption modulation

• Electro- optic modulation

Electro Absorption Modulation (EAM) technique is based on the

change of the absorption coefficient of the material used as modulator. This

type of modulator relies on the fact that the effective bandgap Eg of a

semiconductor material decreases when an external voltage is applied.

Consequently, if the frequency ν of an incoming lightwave is chosen so that

its energy E = hν is smaller than the bandgap when no voltage is applied, the

material will be transparent. On the other hand, when an external voltage is

applied, the effective bandgap will be reduced, meaning that the lightwave



will be absorbed by the material when E > Eg. Such a shift of the absorption

edge of a semiconductor under the influence of an external voltage is

represented in Fig. 3.2. By properly selecting the signal wavelength so that it

experiences a significant change in absorption when the voltage is applied, it

thus becomes possible to achieve optical modulation controlled by an

electrical signal. A typical absorption versus applied voltage transfer function

for an electro-absorption modulator is also shown in Fig. 3.2.

Fig 3.2: Left: absorption of a semiconductor as a function of wavelength with and without an external applied electric field (adapted from [64]). Right: typical loss versus applied voltage

curves for an electro-absorption modulator (adapted from [65]).

Electro-optic modulators are based on the change of refractive index of

the material used for the modulation. The refractive index of some materials

can be modified by applying an external electric field to them through the

linear electro-optic effect. Since the phase shift experienced by a lightwave

of wavelength λ propagating through a length L of a medium with refractive

index η is

∅ = �� (3.1)

A straightforward application is the realization of phase modulators

made from an electro-optic waveguide subjected to a time dependent electric

field. The applied voltage will modulate the refractive index of the

waveguide material, hence the phase shift experienced by a lightwave

propagating along the waveguide. However, legacy optical communication

systems typically rely on intensity modulation of light. This can be achieved

by transforming phase modulation induced by the electro-optic effect to

intensity modulation using an interferometric structure.



Fig. 3.3: Principle of operation of a Mach-Zehnder modulator.

In order to illustrate the principle, we consider the simple

interferometric structure represented in Fig. 3.3. It is based on a Mach-

Zehnder interferometer including one electro-optic material in one of the

arms. In practice, this interferometer is realized by implanting waveguides

into an electro-optic crystal, typically lithium-niobate (LiNbO3). Assuming a

power splitting and combining ratio of 1:2 for the input and output couplers

of the Mach-Zehnder interferometer, the power at the output of the

interferometer depends on the phase shift difference, ∆∅ = ∅�� −∅�experienced by the light propagating in the upper and lower arms of the

structure according to,

�� = �� cos� ∆∅2 3.2� The phase shift induced in the upper arm of the interferometer depends

on its refractive index, which itself depends on the applied external electric

field through the electro-optic effect. If a time-dependent voltage V (t) is

applied to the upper waveguide of the modulator, its refractive index will

become time-dependent and, in turn the transmission of the Mach-Zehnder

interferometer will also depend on time. If a continuous optical wave is

applied to the input of the modulator, the output power will thus be

modulated according to the electrical data V (t). The value of the phase shift

created by an applied external voltage depends upon many parameters,

including the choice of the electro-optic material, the orientation of the

crystal with respect to the external electric field, as well as to the polarisation

of the incoming lightwave, the geometry and dimensions of the waveguide.



In any case it is possible to make abstraction of the actual physical

implementation of the modulator and describe the ability of the material and

chosen configuration to respond to an applied voltage by introducing a

quantity known as the half-wave voltage Vπ. Applying a voltage of Vπ to the

electrode of an electro-optic waveguide will result in a voltage-induced phase

shift of π. The voltage-induced phase shift ϕ (t) can therefore be related to the

applied voltage V (t) according to

�� = � �� 3.3�

Through eq. (3.2) and (3.3), it is then possible to calculate the transfer

function Pout/Pin of the modulator as a function of the applied voltage. Such

a transfer function, where the applied voltage has been normalised to the

half-wave voltage, is also represented in Fig. 3.3. It can easily be shown that,

if the Mach-Zehnder configuration of Fig. 3.3 is used, the optical modulated

signal will be chirped. The problem can be solved by applying two

complementary modulating signals to the two arms of the Mach-Zehnder

modulator. If one arm is driven with a voltage corresponding to the data to be

transmitted d(t), while the second arm is driven with a voltage corresponding

to the complementary data d(t), then it can be shown that the chirp can be

suppressed. Mach-Zehnder modulators are usually employed this way, a

technique known as push-pull modulation, either by driving the two arms of

the interferometer with complementary signals, or by creating phase shifts of

opposite signs in each arm by a proper configuration of the crystal and

electrodes.

3.3.2 Optical Interleaving

An optical interleaver is a 3-port passive fiber-optic device that is used

to combine two sets of dense wavelength-division multiplexing (DWDM)

channels (odd and even channels) into a composite signal stream in an

interleaving way. For example, optical interleaver takes two multiplexed

signals with 100 GHz spacing and interleaves them, creating a denser

DWDM signal with channels spaced 50 GHz apart. The process can be



repeated, creating even denser composite signals with 25 GHz or 12.5 GHz

spacing.

The device can be used in a reverse direction, forming an optical de-

interleaver that separates a denser DWDM signal into odd channels and even

channels. See schematic diagram.

For example, in most DWDM equipment, the standard channel spacing

is 100 GHz. But spacing the signal-carrying frequencies every 50 or even 25

GHz can double or even quadruple the number of channels per fiber. Thus,

optical interleaver can expand the number of channels per fiber, and devices

and/or networks can be upgraded without requiring that all devices be

upgraded.

(a)

(b)

Fig. 3.4: (a) optical interleaver technique; (b) optical de-interleaver technique

Optical interleaver is based on multiple-beam interference. Currently,

there are two approaches to building optical interleaver: 1) Step-phase

Michelson interferometer, and 2) Birefringent crystal networks. The former

is based on Michelson interferometer combined with Gires-Tournois

interferometer.



In this thesis we use a third technique to realize the optical interleaver.

Add-drop multiplexing is used for this purpose. WDM drop module

differentiates all the frequency components and the WDM add module adds

them up according to the required set of frequencies.

3.3.3 Baseband Modulation Baseband data is the message signal which contains the entire

information which we wish to transmit through our communication channel.

It is an electric pulse as we are considering the baseband data as digital data

stream. Now the medium of transmission is optical fiber and so we have to

generate optical pulses corresponding to the baseband data pulses. The

stringent requirements on various aspects of pulses, and their respective

modulation formats, have forced research groups across the world to look

into many different ways of generating optical pulses that are: first of all

short enough to enable very high repetition rates; and secondly capable of

providing conditioned pulses with a good extinction ratio, both temporally

and spectrally, exhibiting low timing jitter, and preferably transform limited

characteristics. Two low-cost ways of generating short optical pulses suitable

for optical telecommunication are studied in this chapter: gain switching of a

semiconductor laser, and pulse carving with a Mach-Zehdner modulator,

working as an intensity modulator (IM).

There are two basic ways of using the time slot for transmitting data

through intensity modulation. The first one is return to zero (RZ), in which

the signal always returns to a rest state (i.e. zero) during a port ion of the bit

period. The second possibility is non-return to zero (NRZ), where the signal

level can only make a transit ion at the borders of the bit period when the bit

value changes. For a given bit period, the bandwidth of an RZ signal will

thus be broader than that of an NRZ signal. However, RZ pulses are

advantageous as they allow for passive optical interleaving to higher

aggregate bit rates. RZ also becomes less sensitive to nonlinearities and

dispersion than NRZ for bit rates of 40 Gbps and higher [66 - 68].



3.3.3.1 Data Format and Modulation Scheme

The Non-Return to Zero (NRZ) format is a normal and

commonly used pulse coding modulation format in optical

communication systems as shown in Figure 2. This format is sending a

signal during the bit period for a '1' and null signal during a '0'. NRZ

has potential pulse spreading problems. It also does not efficiency on

using bandwidth. The Return to zero (RZ) of Figure 2 has half of the

bit period for a '1' when the pulse signal is The Non-Return to Zero

(NRZ) format is a normal and commonly used pulse coding

modulation format in optical communication systems as shown in

Figure 3.5. This format is sending a signal during the bit period for a '1'

and null signal during a '0'. NRZ has potential pulse spreading

problems. It also does not efficiency on using bandwidth. The Return

to zero (RZ) of Figure 3.5 has half of the bit period for a '1' when the

pulse signal is high and null signal for a '0' and is more tolerant towards

pulse dispersion. The bandwidth required by RZ is twice larger than

that of NRZ; therefore it only requires half of NRZ power in

transmission.

Fig. 3.5: NRZ and RZ data stream

The random binary data is a binary data signal in which the pulse

is either 1 or 0 shaped (like square wave). However this random binary

pulses data requires a wider bandwidth. It is used to modulate the

carrier signal. Amplitude Shift Keying (ASK) is a well known



modulation technique that allows digital signal m(t) multiply with a

carrier wave fc. It can be mathematically defined by,

!� = "#!�$% &'($!,(%*� < � < ,3.4�

where A is an amplitude envelop, m(t) is the binary signal state of

“1 or 0”, fc is the optical carrier frequency and T is the bit period. The

frequency spectrum of an ASK signal can be represented by the

baseband spectrum of the modulation m(t) but shifted to the carrier

frequency fc.

3.3.3.2 Modulation Technique

Modern optical communication systems employ two types of

external modulators for ASK modulation formats, the interferometric

or electro-absorption (EA) types. However the most popular one is the

LiNbO3 MZIM waveguide modulator configuration because of its low

optical loss and high electro-optic coefficient while the EA

semiconductor waveguide modulator does not offer the flexibility in

modulation and linearity [1]. The structure of the single drive MZIM is

shown in Figure 3.6 in which the incoming lightwaves is equally split

into the two waveguide arms forming an optical inteferometer. An

external voltage is applied onto one electrode generating a change in

the refractive index of the optical waveguide branch via the electro-

optic effect. Depending on the orientation of the crystal substrate chirp

or chirp free optical modulation can be achieved.

For lithium niobate the X-cut Z-propagating is usually preferred

to obtain chirp free modulation. When a phase difference between two

optical arms is 180 degree the lightwave paths combine destructively

and hence the output is at its minimum point or “0-state”. When the

lightwaves from the upper and lower arms recombine at the output of

MZI modulator, if the carriers of two arms are in phase, a maximum

constructively output is formed. In the case of dual electrode drive,

voltages can be applied to both paths and hence one could inject a

phase bias to generate either double or SSB property of the lightwaves

in the frequency domain.



Fig. 3.6: Single Drive Mach-Zehdner Modulator

Fiber chromatic dispersion accumulates along the fiber

propagation path. Thence pulse spreading is influence by the sideband

spectrum components of the modulated signals. Therefore a reduction

of the total bandwidth of a carrier modulated wideband signals is

critical. In additional, with DSB transmission system, the data signals

contained in the upper sideband is identical to the information

contained in the lower sideband. Thence in order to minimize the

effects of the fiber chromatic dispersion and to increase the optical

bandwidth efficiency, the optical SSB modulation can be used [3]

provided that complexity in implementation at ultra-speed operation is

not the constraint.

3.4 Millimeter Wave Generation

Millimeter wave signals are also defined as Extremely High Frequency

(EHF). It is the ITU designation for the band of radio frequencies in the

electromagnetic spectrum from 30 to 300 gigahertz, above which electromagnetic

radiation is considered to be low (or far) infrared light, also referred to as terahertz

radiation. Radio waves in this band have wavelengths from hundred to one

millimeter.

Compared to lower bands, radio waves in this band have high atmospheric

attenuation; they are absorbed by the gases in the atmosphere. Therefore they have

a short range and can only be used for terrestrial communication over about a

kilometer. In particular, signals in the 57–64 GHz region are subject to a resonance



of the oxygen molecule and are severely attenuated. Even over relatively short

distances, rain fade is a serious problem, caused when absorption by rain reduces

signal strength. In climates other than deserts absorption due to humidity also has

an impact on propagation. While this absorption limits potential communications

range, it also allows for smaller frequency reuse distances than lower frequencies.

The short wavelength allows modest size antennas to have a small beam width,

further increasing frequency reuse potential.

One of the main facades of ROF is mm wave generation. Due to complex

technique and expense of electrical components – mixers, amplifiers – in high

frequency, say above 40GHz [30], an effective technique was required for mm

wave generation. ROF technology gives a solution to it. Broadly speaking about

this technology, we modulate a laser source by an intensity as well as phase

modulator. Modulating frequency is a low range RF signal (10 to 15 GHz

approximately). Generated mm wave at the BS will depend on this modulating

frequency and the modulation technique only. This external modulation includes

the techniques like Double side band (DSB), Single side band (SSB) and Optical

carrier Suppression (OCS) [30]. The order of these sidebands decide the factor,

after multiplying with which the modulating RF signal generates the high

frequency mm wave at the BS.

On the wireless side, the broadband wireless technology has an interest in

the use of mm-wave, specifically radio frequency of71–76 GHz and 81–86 GHz

[33,34]. Due to wireless systems operating in the 71–76 GHz and 81–86 GHz

bands owing a narrower radiation pattern of antennas, mm-wave technology could

provide minimal interference to other adjacent links except those lying directly

along the intended path. At conventional microwave frequencies up to 20 GHz,

atmospheric absorption is reasonably low. However, when microwave frequencies

rise to around 60 GHz, the absorption by oxygen molecules results in a peak

attenuation of 15 dB/km. As a consequence, conventional microwave system is

seriously limited by radio transmission distances. After 60 GHz, a large frequency

window opens, where the attenuation drops to less than 1 dB/km (effectively

negligible) before rising again due to other molecular effects. Thus, the spectrum

band from around 70 GHz to around 120 GHz exhibits low atmospheric

attenuation and other advantages for broadband wireless application.



Fig. 3.7: Attenuation for microwave and millimeter wave range (1GHz to 300GHz)

Hence we are concentrating on this window. There are two targeted band to

be worked on. Those bands are 70-76GHz and 81-88GHz. Hence we choose the

two frequencies of our aimed dual frequency mm wave as 72GHz and 84GHz.



CHAPTER 4 PROPOSED SYSTEM

4.1 Proposed Central Station Here, my proposed system is a downlink system in RoF communication

consisting one Central station (CS) and two Base Stations (BS). Downlink in RoF

system refers to the flow of data from CS to BS. In my system, a 1550nm Laser

source is being modulated by the RF frequency of 12GHz. A local oscillator is

used for this RF signal. This modulation is done by external modulator. In this case

the external modulator is Mach Zhender Modulator (MZM). We take one

unbalanced MZM to modulate 193.548THz (1550 nm wavelength) optical signal

with 12GHz RF signal. The modulation is basically an intensity modulation as well

as phase modulation. After modulation various sideband harmonic components is

available and we use one such component to be modulated with the baseband data,

generated by CS. This RF modulated optical signal acts as carrier and carry the

baseband NRZ data towards the specific BS. This transmission is done via optical

fibre cable. In a BS , the obtained data modulated optical signal is fed to a photo

detector. The output of the photo detector would be the NRZ baseband data

modulated RF signal at certain frequency. This frequency will be determined by

the sideband components allowed to pass. Using a filter the required frequency RF

signal can be taken and transmitted via antenna over a wireless medium.

Fig. 4.1: Block Diagram of the proposed ROF downlink Central Station

CHAPTER 4: PROPOSED SYSTEM 2015


4.2 Component Analysis Here we analyse the proposed system block wise. There are four distinct

blocks in the proposed Central station of this RoF downlink system. These blocks

are,

• Modulator (MZM)

• Interleaver

• Fibre Bragg Grating

• Coupler

We may discuss each block separately with its mathematical interpretation.

Relevant block diagrams also be given.

4.2.1 Mach Zehnder Modulator In a central station electrical signal is converted into optical signal.

External modulator MZM modulates the 1550nm Laser light signal with

12GHz RF signal. Local oscillator generates this RF signal. 12GHz RF

signal is fed to unbalanced MZM. This MZM has a unique property to

suppress either odd or even order sideband harmonics.

Let the optical signal frequency be denoted by ω and RF signal

frequency is ωRF.

So an MZM will give an output containing side bands ω±n*ωRF ;

Where n is either odd integer or even integer, depending on the transfer

function of that particular MZM. An unbalanced MZM modulates an optical

signal with a RF signal when subjected to bias voltage. This bias voltage

governs the interference pattern of Mach Zhender Interferometer (MZI), thus

controls the phase modulation of the signal. In this way in an unbalance

MZM both the intensity and phase modulation are taken place.



Fig. 4.2: Unbalanced Mach Zehnder Modulator (MZM)

The most common modulator used in RoF systems is the

Mach{Zehnder modulator, which is based on a Mach Zehnder

interferometric structure. In a MZM an electrical voltage induces phase shift

between the two arms of the interferometer and depending upon the quantity

of phase shift, constructive or destructive interference occurs. This

interference results in intensity modulation of the optical signal by the

electrical voltage applied.

Figure 4.2 shows the schematic of a MZM. The electrical voltage

induced optical phase shift can be expressed as:

∅t� = �� .

�/012�� + �45

�� . 6782�9:;��< (4.1)

where Vп is the voltage required to induce a п phase shift. The

parameter Vп is an intrinsic property of the modulator which depends on

various design parameter and the material used. Considering an unbalanced

MZ modulator with a finite extinction ratio ε the optical field at the output of

the MZM can be written as:

=�� = =�� cos>∅��? cos2�9@�� (4.2)

Now after replacing ∅t� in the above equation we get;

=�� = =�� cos A�� .�/012�� + �45

�� . 6782�9:;��<B cos2�9@�� (4.3)

4.2.2 Optical Interleaver After MZM the modulated signal is passed through an interleaver

setup. Basic work of this setup is to first drop some desired component from



the main signal and then combined those dropped components to a separate

channel. Interleaver in this case is a 1X2 port device. It means one signal

input is processed and divided into two channels of output.

Interleaver setup acts as add-drop multiplexer. Here we take the input

in a single channel with 2*ωRF GHz separation of the sideband components.

This input is nothing but the output of the unbalanced MZM. Interleaver first

drops the frequency components with a separation of 4*ωRF, which is double

the separation of the input channel. Now the dropped components are added

to an output channel, say channel 1. The remaining frequency components of

that modulated signal will pass through the output channel 2.

Hence in this interleaver the separation bandwidth of the frequency

components of the input channel is say ω GHz and that of two output channel

is twice of ω GHz. In this way we separate out frequency components from

the same modulated signal for two different Base Stations (BS).

4.2.3 Fiber Brag Grating Fiber Bragg grating (FBG) is a periodic perturbation of the refractive

index along the Fibre length which is formed by exposure of the core to an

intense optical interference pattern.

Now in the particular system proposed, some modifications are done to

achieve a dual frequency signal at a single BS. The baseband data can be

transmitted with both the carrier RF signal. To achieve this kind of signal

some changes in the CS design are incorporated.

It is important to know the term “uniform Fibre Bragg grating”. A

grating is a device that periodically modifies the phase or the intensity of a

wave reflected on, or transmitted through, it [35]. The propagating wave is

reflected, if its wavelength equals Bragg resonance wavelength, λBragg

, in the

other case is transmitted. The uniform means that the grating period, Λ, and

the refractive index change, δη, are constant over whole length of the grating.

The equation relating the grating spatial periodicity and the Bragg resonance

wavelength is given by [35, 36]:

CHAPTER 4: PROPOSED SYSTEM


where ηeff is effective mode index.

A typical uniform bragg grating can be explained in the diagram below;

Fig. 4

Here in our system Both the say Interleaver ‘b’.

FBGs separate the fourth order sideband harmonic components and fed those signal to respective intensity modulators (MZMs in this case).

4.1.1 Optical Coupler

A Fibre optic coupler

or more input Fibres and one or several output Fibres.

Fibre can appear at one or more outputs and its

depending on the

fabricated in different ways, for example by thermally fusing Fibres so that

their cores get into intimat

(supporting only a single mode per polarization direction for a given

wavelength), there are certain physical restrictions on the performance of the

coupler. In particular, it is not possible to combine two

same optical frequency into one single

significant excess losses.

CHAPTER 4: PROPOSED SYSTEM


CDEFGG = 2�HIIΛeffective mode index.

A typical uniform bragg grating can be explained in the diagram

Fig. 4.3 Working principle of a Fibre Bragg Grating[36]

Here in our system Both the FBGs are connected to the 2say Interleaver ‘b’.


Optical Coupler

Fibre optic coupler is a device used in optical Fibre

or more input Fibres and one or several output Fibres. Light

Fibre can appear at one or more outputs and its power distribution

depending on the wavelength and polarization. Such couplers can be


their cores get into intimate contact. If all involved Fibres are single



coupler. In particular, it is not possible to combine two or more inputs of the

same optical frequency into one single-polarization output without

significant excess losses.

2015


(4.4)

A typical uniform bragg grating can be explained in the diagram

Working principle of a Fibre Bragg Grating[36]

FBGs are connected to the 2nd interleaver,


optical Fibre systems with one

Light entering an input

power distribution potentially

. Such couplers can be


e contact. If all involved Fibres are single-mode



or more inputs of the

polarization output without

CHAPTER 5: THEORETICAL MODELLING 2015


CHAPTER 5 THEORETICAL MODELLING

5.1 Frequency Domain Analysis

In electronics, control systems engineering, and statistics, the frequency

domain refers to the analysis of mathematical functions or signals with respect to

frequency, rather than time. Put simply, a time-domain graph shows how a signal

changes over time, whereas a frequency-domain graph shows how much of the

signal lies within each given frequency band over a range of frequencies. A

frequency-domain representation can also include information on the phase shift

that must be applied to each sinusoid in order to be able to recombine the

frequency components to recover the original time signal.

A given function or signal can be converted between the time and frequency

domains with a pair of mathematical operators called a transform. An example is

the Fourier transform, which converts the time function into a sum of sine waves of

different frequencies, each of which represents a frequency component. The

'spectrum' of frequency components is the frequency domain representation of the

signal. The inverse Fourier transform converts the frequency domain function back

to a time function. A spectrum analyzer is the tool commonly used to visualize

real-world signals in the frequency domain.

The Fourier transform decomposes a function of time (a signal) into the

frequencies that make it up, similarly to how a musical chord can be expressed as

the amplitude (or loudness) of its constituent notes. The Fourier transform of a

function of time itself is a complex-valued function of frequency, whose absolute

value represents the amount of that frequency present in the original function, and

whose complex argument is the phase offset of the basic sinusoid in that

frequency. The Fourier transform is called the frequency domain representation of

the original signal. The term Fourier Transform refers to both the frequency

domain representation and the mathematical operation that associates the



frequency domain representation to a function of time. The Fourier transform is not

limited to functions of time, but in order to have a unified language, the domain of

the original function is commonly referred to as the time domain. For many

functions of practical interest one can define an operation that reverses this: the

inverse Fourier transformation, also called Fourier synthesis, of a frequency

domain representation combines the contributions of all the different frequencies to

recover the original function of time.

Linear operations performed in one domain (time or frequency) have

corresponding operations in the other domain, which are sometimes easier to

perform. The operation of differentiation in the time domain corresponds to

multiplication by the frequency, so some differential equations are easier to

analyze in the frequency domain. Also, convolution in the time domain

corresponds to ordinary multiplication in the frequency domain. Concretely, this

means that any linear time-invariant system, such as a filter applied to a signal, can

be expressed relatively simply as an operation on frequencies. After performing the

desired operations, transformation of the result can be made back to the time

domain. Harmonic analysis is the systematic study of the relationship between the

frequency and time domains, including the kinds of functions or operations that are

"simpler" in one or the other, and has deep connections to almost all areas of

modern mathematics. The spectrum of the signal x(t) can be obtained using fourier

transform. We can define the Fourier Transform as,

KLM� = N O��PQRS�T�UQU (5.1)

The time domain signal can be retrieved using Inverse Fourier Transform. It

can be formulated as,

O�� = 12�W KLM�PRS�TM5.2�

U

QU



Fig. 5.1: Basic representation of Fourier Transform

5.2 Bessel Function There are mainly two types of Bessel Function.

• Bessel Function of first kind

• Bessel Function of second kind

The third kind of Bessel Function is known as Henkel Function. It is not an independent function. It is a mathematical combination of Bessel Function of first kind and second kind.

Bessel functions, first defined by the mathematician Daniel Bernoulli and

generalized by Friedrich Bessel, are the canonical solutions y(x) of Bessel's

differential equation,

O� T�Y

TO� + OTYTO + O� − Z�� = 05.3�

for an arbitrary complex number α (the order of the Bessel function). Although α

and −α produce the same differential equation for real α, it is conventional to

define different Bessel functions for these two values in such a way that the Bessel

functions are mostly smooth functions of α.

The most important cases [73] are for α, an integer or half-integer. Bessel

functions for integer α are also known as cylinder functions or the cylindrical

harmonics because they appear in the solution to Laplace's equation in cylindrical

coordinates. Spherical Bessel functions with half-integer α, are obtained when the

Helmholtz equation is solved in spherical coordinates.



Bessel's equation arises when finding separable solutions to Laplace's

equation and the Helmholtz equation in cylindrical or spherical coordinates. Bessel

functions are therefore especially important for many problems of wave

propagation and static potentials. In solving problems in cylindrical coordinate

systems, one obtains Bessel functions of integer order (α = n); in spherical

problems, one obtains half-integer orders (α = n+1/2).

For example:

• Electromagnetic waves in a cylindrical waveguide

• Pressure amplitudes of inviscid rotational flows

• Heat conduction in a cylindrical object

• Modes of vibration of a thin circular (or annular) artificial membrane

(such as a drum or other membranophone)

• Diffusion problems on a lattice

• Solutions to the radial Schrödinger equation (in spherical and cylindrical

coordinates) for a free particle

• Solving for patterns of acoustical radiation

• Frequency-dependent friction in circular pipelines

• Dynamics of floating bodies

• Angular resolution

Bessel functions of the first kind, denoted as Jα(x), are solutions of Bessel's

differential equation that are finite at the origin (x = 0) for integer or positive α, and

diverge as x approaches zero for negative non-integer α. It is possible to define the

function by its series expansion around x = 0, which can be found by applying the

Frobenius method to Bessel's equation;[74]

\]O� = ^ −1�_`!b` + Z + 1� c

O2d

�_e] U

[email protected]�

where Γ(z) is the gamma function, a shifted generalization of the factorial function

to non-integer values. The Bessel function of the first kind is an entire function if α

is an integer, otherwise it is a multi-valued function with singularity at zero. The

graphs of Bessel functions look roughly like oscillating sine or cosine functions

CHAPTER 5: THEORETICAL MODELLING


that decay proportionally to 1/

their roots are not generally periodic, except asymptotically for large

indicates that −J1(x) is the derivative of

cos(x); more generally, the d

by the identities below

Fig 5.2: Plot of Bessel function of the first kind,

For non-integer

are therefore the two solutions of the differential equation. On the other hand, for

integer order α, the following relationship is valid (note that the Gamm

has simple poles at each of the non

This means that the two solutions are no longer linearly independent. In this

case, the second linearly independent solution is then found to be the Bessel

function of the second kind, as discussed below

Another definition of the Bessel function, for integ

using an integral representation:

Another integral representation is:



that decay proportionally to 1/√x (see also their asymptotic forms below), although

their roots are not generally periodic, except asymptotically for large

) is the derivative of J0(x), much like -sin(x) is the derivative of

); more generally, the derivative of Jn(x) can be expressed in terms of

below.)

: Plot of Bessel function of the first kind, Jα(x), for integer orders α

integer α, the functions Jα(x) and J−α(x) are linearly independent, and


, the following relationship is valid (note that the Gamm

has simple poles at each of the non-positive integers):[75]

\Q�O� = −1��\�O�



second kind, as discussed below

Another definition of the Bessel function, for integer values of

using an integral representation:[76]

\�O� = g� N cosh�i − O8jki�lTi�

@

Another integral representation is:[76]

\�O� = g�� N P�mnQo pqrn��Ti�

Q�

2015


(see also their asymptotic forms below), although

their roots are not generally periodic, except asymptotically for large x. (The series

) is the derivative of

) can be expressed in terms of Jn±1(x)

orders α = 0, 1, 2

) are linearly independent, and


, the following relationship is valid (note that the Gamma function

5.5�



er values of n, is possible

5.6�

5.7�



This was the approach that Bessel used, and from this definition he derived

several properties of the function. The definition may be extended to non

orders by (for Re(x

\]O� � 1�W�

@

The Bessel functions of the second kind, denoted by

denoted instead by

have a singularity at the origin (

called Weber functions as they were introduced by

Neumann functions after

Fig. 5.3: Plot of Bessel function of the second kind,

For non-integer α

In the case of integer order

non-integer α tends to




several properties of the function. The definition may be extended to non

x) > 0), one of Schläfli's integrals:[76]

W cosZi O8jki� Ti sinZ�� W PQo pqrwU

@

The Bessel functions of the second kind, denoted by Y

denoted instead by Nα(x), are solutions of the Bessel differential equation that

have a singularity at the origin (x = 0) and are multivalued. These are sometimes

called Weber functions as they were introduced by H. Weber

Neumann functions after Carl Neumann.

Plot of Bessel function of the second kind, Yα(x), for integer orders

integer α, Yα(x) is related to Jα(x) by:

x]O� � \]O� cosZ�� \Q]O�sinZ��

In the case of integer order n, the function is defined by taking the limit as a

tends to n:

x�O� � lim]→�x]O�

2015



several properties of the function. The definition may be extended to non-integer

pqrw��Q]�T�5.8� [41-43]

Yα(x), occasionally

Bessel differential equation that

. These are sometimes

H. Weber (1873), and also

), for integer orders α = 0, 1, 2.

5.9�

, the function is defined by taking the limit as a

5.10�



There is also a corresponding integral formula (for Re(x) > 0),[74]

x�O� � 1�W sinO8jk

�

@~ − k~�T~ − 1

�W >P�� + −1�PQ��?PQo��U

@

5.11�

Yα(x) is necessary as the second linearly independent solution of the Bessel's

equation when α is an integer. But Yα(x) has more meaning than that. It can be

considered as a 'natural' partner of Jα(x). See also the subsection on Hankel

functions below.

When α is an integer, moreover, as was similarly the case for the functions of

the first kind, the following relationship is valid:

xQ�O� = −1��x�O�5.12) Both Jα(x) and Yα(x) are holomorphic functions of x on the complex plane cut

along the negative real axis. When α is an integer, the Bessel functions J are entire

functions of x. If x is held fixed at a non-zero value, then the Bessel functions are

entire functions of α.

The Bessel functions of the second kind when α is an integer is an example of

the second kind of solution in Fuchs's theorem.

Another important formulation of the two linearly independent solutions to

Bessel's equation are the Hankel functions of the first and second kind, Hα(1)(x)

and Hα(2)(x), defined by:[75]

�](g)(O) = \](O) + jx](O)(5.13) �](�)(O) = \](O) − jx](O)(5.14)

where i is the imaginary unit. These linear combinations are also known as Bessel

functions of the third kind; they are two linearly independent solutions of Bessel's

differential equation. They are named after Hermann Hankel.

The importance of Hankel functions of the first and second kind lies more in

theoretical development rather than in application. These forms of linear



combination satisfy numerous simple-looking properties, like asymptotic formulae

or integral representations. Here, 'simple' means an appearance of the factor of the

form eif(x). The Bessel function of the second kind then can be thought to naturally

appear as the imaginary part of the Hankel functions.

The Hankel functions are used to express outward- and inward-propagating

cylindrical wave solutions of the cylindrical wave equation, respectively (or vice

versa, depending on the sign convention for the frequency).

Using the previous relationships they can be expressed as:

�]g�O� � \Q]O� PQ]��\]O�jsin(Z�) (5.15)

�](�)(O) = \Q](O) − P]��\](O)−jsin(Z�) (5.16)

If α is an integer, the limit has to be calculated. The following relationships are

valid, whether α is an integer or not:

�Q](g)(O) = P]��](g)(O)(5.17) �Q](�)(O) = PQ]��](�)(O)(5.18)

In particular, if α = m + 1/2 with m a nonnegative integer, the above relations

imply directly that

\Qc_eg�d(O) = (−1)_egx_eg�(O)(5.19) xQc_eg�d(O) = (−1)_\_eg�(O)(5.20)

These are useful in developing the spherical Bessel functions .

5.3 Mathematical Modelling of Proposed Central Station Central Station of a ROF downlink system is being designed here.

Mathematical realization of the central station and the corresponding plotting of

the obtained equation are shown in this chapter.



External modulation of this ROF downlink system is done with two MZMs.

Continuous Wave (CW) laser source gives optical signal of angular frequency ω0.

The field of this signal can be given as E0=A0exp(jω0t). Amplitude of the field is

A0. External modulators modulate the intensity of the lightwave. As we use

balanced MZM we need a driving RF signal. Local oscillator generates this RF

signal with an angular frequency of ωLO. The Rf signal can be represented as

V(t)=VLOcos(ωLOt).

DC bias voltage Vbias is used for the required phase modulation of the MZM,as

it is an intensity as well as phase modulator.

So, we can write the governing equation of MZM as,

=�� =@ cos(ω@t) cos[Φ(V(t))] (5.21)

Where, �(�(�)) = п� .�/012�� + �� . cos(2п9:;�)< (5.22)

As we know Vп is the half wave voltage. That means at Vп bias voltage the

phase difference between two arms of the MZM is п.

Let us assume � = �� . �/012�п

And ̀ = �� . ��п

So, we can write the MZM equation as,

=�� = =�� cos(� + `. cos(M��)) . cos(M@�)

= =��[cos(�) . cos(`. cos(~) − sin(�) . sin(`. cos(~))] . cos(M@�) (5.23)

Where, ~ = M�� We can further simplify the equation as,

=�� = =��678�. cos(`. 678~)�. cos(M@�) − =��sin �. sin(`. 678~)�. cos(M@�) = =��[cos ��cos(`. 678 ~). 678(M@�)�. sin ��sin(`. 678~). cos(M@�)�]

(5.24)



Hence the equation will be,

=�� =�� cos ��\@(`) cosM@�+^[\��(`) cos(M@� + 2kM�� − k�)+ \��(`) cos(M@� − 2kM�� + k�)+ =�� sin b �^\��Qg(`) cos(M@� + (2k − 1)M�� = k�)+ \��Qg(`)cos(M@� − (2k − 1)M�� + k�)�

(5.25)

Jn(m) is the nth order Bessel function of the first kind.

As we have two MZMs for external modulation, the outputs are, say Eout1 and

Eout2 for MZM1 and MZM2 respectively. Now MZM1 suppresses the odd order

sidebands. Hence for MZM1 sin b value should be 0 and the cos b value should be

1.

If,

Cos b=1; b=nπ;

So the output from the MZM1 is,

=��g = =��\@(`) cosM@�+^[\��(`) cos(M@� + 2kM�� − k�)+ \��(`) cos(M@� − 2kM�� + k�)]�

(5.26)

For MZM2, the output will only be the odd order sidebands,

=�� = =��^[\��Qg(`) cos(M@� + (2k − 1)M�� − k�)+ \��Qg(`) cos(M@� − (2k − 1)M�� + k�)]�

(5.27)



Fig. 5.4: Modulated signal after MZM1 and MZM2

Here we are taking an approximation and hence neglecting the higher order

terms. We neglect the sidebands starting from the 5th order. Hence the equation no

(A) and (B) can be written as

=��g �=��[\@`� cos(M@�) − \�(`) cos(M@� + 2M��)+ \�(`) cos(M@� + 4M��) − \�(`) cos(M@� − 2M��)+ \�(`) cos(M@� − M��)]

(5.28)

And,

=�� ==��[−\g(`) cos(M@� + M��) + \�(`) cos(M@� + 3M��)− \g(`) cos(M@� − M��) + \�(`) cos(M@� − 3M��)]

(5.29)

Hence from MZM1 we get the modulated carrier signal along with its 2nd and

4th order sideband frequencies. MZM2 gives only the 1st and the 3rd order

sidebands and suppressing the optical carrier component itself. Outputs from

MZM1 and MZM2 are fed to the interleaver system Int1 and Int2 respectively.

An optical interleaver system can be designed by add drop multiplexers.

With the help of interleaver specific order sidebands can be selected for a single

channel. Before feeding to the Int1 the carrier component of the frequency ω0 is

dropped using an FBG. Two different output channels of the Int1 gives us the



signal containing sideband components of the frequency (ω0+2ωLO; ω0 – 4ωLO )

and (ω0 –2ωLO; ω0 +4ωLO). Similarly the Int2 separates the odd order sideband

components and combines in two output channels having (+1, –3) and (–1, +3)

order of sidebands. Now here we consider two BSs, connected to one CS. So these

8 sideband components (±1,±2,±3,±4) will again coupled to the separate channels

designed for the two BSs. After the interleavers we separate out the ±4th order

components from both the channel using FBG. These 4th order sideband signals are

being modulated by baseband data. This baseband data contains the processed

information for the end user. For this data modulation we use a 1Gbps electrical

signal with Pseudo Random Bit Sequence of the length 212-1. Higher order data

could not be plotted using Matlab due to approximation error.

We denote the data sequence by b(t). It is Non-Return to Zero (NRZ) data

and it modulates the 4th order sidebands in two different simple balanced MZMs.

Fig. 5.5: PRBS (NRZ) data; Baseband data

The general equation of the output of the MZMs can be written as;

=�F�F � \�`�=��678[ø��?. cos(M@ ± 4M��)� (5.30)

Where,

ø(�) = �(�)�� . �2

Hence the output of the MZM can be written as,

=�F�F = \�(`)=�� cos ��(�)�� . �2� cos(M@ ± 4M��)� (5.31)



Fig. 5.6: Response after FBG (Baseband data modulates the freq component)

Optical coupler is used to reconstruct the signal and send those to the BSs via

optical fibre.

For BS1,

=D�g � =��{ \g`� cos(M@� + M��) − \�(`) cos(M@� − 2M��)+ \�(`) cos(M@� − 3M��) + \�(`) cos(M@� + 4M��) cos ��(�)�� . �2�

(5.32)

For BS2,

=D�� = =��−\g(`) cos(M@� − M��) − \�(`) cos(M@� + 2M��)+ \�(`) cos(M@� + 3M��) + \�(`) cos(M@� − 4M��) cos ��(�)�� . �2�

(5.33)



Fig. 5.7: Output of the Central Station (towards base stations)

These two signals EBS1 and EBS2 will be transmitted through optical fibre

towards the respective Base Stations. Here the signal is presented in both the dB

scale and linear scale. Power level of the signal is high here. After the transmission

through optical fiber, attenuation will take place due to fiber losses.

CHAPTER 6: RESULTS AND DISCUSSION 2015


CHAPTER 6 RESULTS AND DISCUSSION

6.1 Analysis of Central Station We simulated our proposed design of Central Station for a downlink ROF

system. For that we used Optisystem 13.0 software and get the results. In software

simulation we consider

• Losses due to laser source

• Splitting ratio and extinction ratio of MZM

• Losses due to passive optical components, such as FBG, add-drop

multiplexer,etc.

• Also the optical fibre length and hence losses due to optical fibre

We took the initial parameters as follows,

Parameter Symbol Values

Wavelength of laser sources λ 1550.388nm

Frequency of laser sources fOP 193.5 THz

Frequency of RF signal fRF 12 GHz

Data Signal: PBRS 231- 1; Bit Rate: 2.5 Gbps

Table 6.1: Important design parameters for downlink system

The design of the proposed Central Station in Optiwave 13.0 is shown below



Fig. 6.1: Design of the Central station of ROF downlink in Optiwave 13.0

6.1.1 Mach Zehdner Modulator This graph Fig. 6.2 (a)-(b) shows the output of both the MZMs (MZM1

and MZM2). For MZM1 even order suppression is applicable. So in the first

part we may see -3,-1,+1,+3 components are there. So we have got peaks at

193.464THz, 193.488THz, 193.512THz and 193.536THz

Second part consists of -4,-2,+2,+4 components along with the carrier

frequency itself. So we have got peaks at 193.452THz, 193.476THz,

193.5THz, 193.524THz and 193.548THz

(a)

(b) Fig. 6.2: (a) Output of MZM1; suppressing the even order sidebands; (b) Output of MZM2;

suppressing the odd order components along with the carrier

Interleaver



The results in this simulation are quite similar with the theoretical graphs,

plotted in MATLAB. But due to extinction ratio and chirp of the MZMs and

also for the linewidth of the laser source we get some noises along with the

desired frequency components. To eliminate those noises we use here optical

rectangular filters. The filters output for MZM1 and MZM2 are in Fig. 6.3.

(a)

(b)

Fig. 6.3: (a) Filtered output of MZM1; (b) Filtered output of MZM2

6.1.2 Optical Interleaver This graph Fig 6.4 represents both the channels 1 and 2 for the interleaver

‘a’ and ‘b’. Here two interleavers separate the entire signal in different

channels. At first it combines (+1,-3); (-1,+3); (-2,+4); (+2,-4) components of

the modulated signal. Then finally it combines to give the (+1,-2,-3) and

(-1,+2,+3) order of sidebands together. The ±4th order sidebands are separated

for baseband data multiplication.



(a)

(b)

Fig. 6.4: (a)-(b) Combination of selected order of sidebands after interleaver

setup

6.1.3 Baseband Data Modulation We modulate the electrical baseband data with the ±4th order sidebands of

the modulated optical-millimeter wave signal. The baseband data is a pseudo

random sequence of Non Return to Zero (NRZ) bits with bit rate of 2.5Gbps.

That means with a Spectral Efficiency of 1 the frequency of the baseband data

is 2.5GHz.

Fig. 6.5: NRZ PRBS data sequence with bitrate 2.5Gbps



Now this baseband data stream modulates the ±4th order sideband of that

optical-mm wave signal.

(a)

(b)

Fig. 6.6: Baseband data, modulated with (a) +4 component; (b) -4 component

6.1.4 Optical Coupler At the end of the central station the fourth order sideband component is

added with other frequency components and those signals are transmitted to

the respective Base Stations via optical fiber cable.

This graph Fig 6.7 shows the output of optical coupler. It’s also the output

of central station. This combined signal will go to base station via OFC.



(a)

(b)

Fig. 6.7: Final output signal of Central Station transmitted towards (a) BS1 and

(b) BS2

6.2 Analysis of Laser Source Continuous Wave (CW) lasers are used as a source for our proposed ROF

system. Every laser has its own linewidth. By varying linewidth of a laser we

might get different results. Also the variation of the initial laser power is analyzed.

6.2.1 Variation of Linewidth The linewidth (or line width) of a laser, typically a single-frequency laser,

is the width (typically the full width at half-maximum, FWHM) of its optical

spectrum. More precisely, it is the width of the power spectral density of the

emitted electric field in terms of frequency, wave number or wavelength.

The spectral coherence depends on the linewidth of the laser. The narrower

the linewidth the more spectrally coherent the laser is. Hence in Optiwave 13.0

CHAPTER 6: RESULTS AND DISCUSSION


we change the linewidth values of both the laser to 1 MHz,

500MHz. We obtained the following results, shown in the figure

Fig. 6.8: Output signal of

CHAPTER 6: RESULTS AND DISCUSSION


the linewidth values of both the laser to 1 MHz,


(a)

(b)

(c)

Fig. 6.8: Output signal of ROF Central Station with various Laser Linewidth;

(a) 1MHz; (b) 100MHz; (c) 500 MHz

2015


the linewidth values of both the laser to 1 MHz, 100MHz and


ROF Central Station with various Laser Linewidth;



As we may infer from the results that the lower the linewidth the more the

coherency of the frequency components. We get sharp peaks for 1MHz

linewidth, which gradually flattens in lasers with the linewidth of 100MHz and

500M

6.3 Variation of Baseband Data As mentioned earlier, we have used PRBS data stream with NRZ pulses. The

bit rate was 2.5Gbps and the order of the PRBS is 231-1. Now we varied both the

bit rate and the type of data pulses to observe the corresponding effect on the

proposed ROF downlink system.

6.3.1 Type of Pulses We initially used the Non Return to Zero (NRZ) data pulses. With the

predefined spectral efficiency 1 the 2.5Gbps data gives us the frequency of

2.5GHz. To observe the differences we replaced the NRZ data pulses with

Return to Zero (RZ) pulses. Observations are as follows.

(a) (b)

(c) (d)

Fig. 6.9: (a) data pattern of NRZ; (b) data pattern of RZ; Frequency domain representation (c) NRZ; (d) RZ



(a) (b)

(c) (d)

Fig. 6.10: Fourth order sideband being modulated by (a) NRZ data; (b) RZ data; The final output of the CS when the baseband data pulses are (c) NRZ & (d) RZ

From these graphs we can say that the RZ data with similar bit rate and

similar spectral efficiency observes almost twice the frequency of the NRZ

data pulses. As it has higher frequency, after modulation with optical-mm

wave it overlaps with frequency components other than fourth order

component. Hence RZ data increases noise as well as power attenuation.

Observed power for RZ data pulses is almost half of the observed power for

NRZ data pulses. We have got the following observation at the end of the

central station.

Type of Pulses Power in mW Power in dBm

NRZ data 18.579 -17.310

RZ data 9.942 -20.025

Table. 6.2: Observed signal power at the end of the CS



6.3.2 Variation of Data Rate Initially we successfully used a data rate of 2.5Gbps. But the future

objective of the proposed ROF system is to achieve data rate of 40Gbps and

more. Hence in simulation we vary the data rate from 2.5Gbps to 40Gbps and

observe the changes. Four distinct values of data rate are given below.

Fig. 6.11: Baseband modulation done with various datarate; (a)2.5Gbps; (b) 5Gbps;

(c) 25Gbps; (d) 40Gbps

(a)

(b)

(c)

(d)



As we can witness the higher the data rate be the more the overlap between

two frequency components is happening. Hence in future to attain such higher

rate through ROF system we have to increase the spectral efficiency, so that the

overlapping doesn’t occur.

6.4 Millimeter Wave Generation One of the important aspects of the ROF technology is to generate mm-wave

signal generation. Though mm-wave is generated at the Base Station, Central

Station provides the required frequency components and suppresses the others.

6.4.1 Experimental Setup In our Fiber Optic Research Lab, BIT, Mesra, we setup an experimental

arrangement to generate mm-wave signal with the help of available Laser

source, FBGs, Optical Couplers, Attenuators, Optical Spectrum Analyser

(OSA) and Digital Storage Oscilloscope (DSO).

Components used Specification Purpose

Tunable Laser Source (TLS) AQ321D Model

Range: 1520nm – 1620nm

Resolution: 0.001nm

Use as a laser source

WDM setup Inbuilt 1550nm Laser source and PIN detector

Use as a laser source and also use for detection

Fiber Bragg Grating (FBG) 1550.4nm with 0.25nm FWHM

Use for filtering the noise

Optical Coupler NeST photonics 2X2 3dB coupler

Use to combine two laser sources

Optical Spectrum Analyser (OSA)

Wavelength range:

600nm -1750nm

Resolution: 0.015nm

To observe the optical spectrum

Digital Storage Oscilloscope ScienTECH 7040M To observe the generated mm wave

Table. 6.3: Components used for experiments and their specifications



The experimental setup is given below.

Fig 6.12: WDM setup with Laser source and internal PIN detector and Digital Storage

Oscilloscope

Fig. 6.13: Tunable Laser source

Fig. 6.14: OSA shows the optical spectrum of both the laser sources after combining

using a 3dB coupler



Fig. 6.15: Setup of the central station with TLS and OSA for mm wave generation

Fig. 6.16: Setup of the CS continued… with FBG and optical couplers.

Now we take the readings from DSO and it is observed in Fig. 6.17.

Fig. 6.17: Final Frequency spectrum of the mm wave signal after BS



6.4.2 Simulation Analysis Experimentally we did not get the exact frequency peak of 20GHz for the

aimed mm-wave. Then we simulate the experimental setup in Optiwave 13.0

to find out exact problem.

Fig. 6.18: Simulation data for the single Laser and the combined Lasers after the 3dB

coupler with the laboratory specifications

Fig. 6.19: Simulated frequency profile of the generated mm wave with laboratory

specifications



Here we can see that the simulation results and the experimental results are

same. Both could not show the frequency peak. Now we simulate the same

setup with a laser source with narrow linewidth.

Fig. 6.20: Combining two laser sources with narrower linewidth

Fig. 6.21: Simulation result for the generated mm wave signal with a frequency of

37.5GHz

Now with the same laser wavelength and narrow laser linewidth we have

got the frequency peak of the mm-wave signal, which was aimed. Hence we

can infer due to very large linewidth of the lasers available in our laboratory

the power got distributed and we could not obtain the exact frequency

component.

CHAPTER 7: CONCLUSION AND FUTURE SCOPE 2015


CHAPTER 7 CONCLUSION AND FUTURE SCOPE

7.1 Conclusion A simplified Central Station in RoF downlink system is designed. It uses

external modulation technique to modulate a laser optical source with RF local

oscillator frequency. This modulated signal itself acts as carrier in RoF technology.

For this carrier to modulate there is a baseband NRZ data stream. This data

generated in central station is transmitted via this modulated signal. We design CS

in such a way, so that we get two frequency components after frequency

multiplication at Base Stations. Central Stations can be a little complicated, but

Base stations are less complicated and mobile. We target to get dual band around

70GHz to 110GHz as in this window the signal attenuation is less. Instead of

unbalanced MZM we may use the LiNbO3 crystal MZM for modulation.

The aim of the downlink system is to provide exact modulation and

interleaving technique so that desired frequency multiplication can be obtained at

the Base Station. Here in the proposed system the initial RF signal frequency is

getting multiplied by factors of 6 and 7 to generate a dual frequency millimeter

wave at the Base Station. A dual frequency mm wave signal having frequency

components at 72 GHz and 84 GHz were generated by means of a PIN diode.

A phase shift in the data signal was observed which occurs due to dispersive

nature of the optical fibre. A good performance of the system for transmitting the

data over 80km of SSMF is aimed at. For the 2.5Gbps baseband data the power

penalty for the downlink system should be approximately less than 0.6 dB for a

BER of 10-9.

7.2 Flexibility of dual frequency mm wave One of the uniqueness of the proposed ROF system is the flexibility in the

range of the generated mm wave. As the generated mm wave signal has dual

frequency components with a separation equal to the initial RF signal in the range

of tens of GHz, the single system can be used for multiple purposes. Data

transmission will be more secure and accurate.

CHAPTER 7: CONCLUSION AND FUTURE SCOPE 2015


Also we can vary the frequency components just changing the value of a

single RF signal which is initially present at CS. Every other components and the

entire technique will be the same. Hence it increases the flexibility of the system.

7.3 Future Scope Broadband wireless links integrated with optical access networks need to

mature before becoming a reality. The integration of wireless and wired services

will be the crucial motivation for implementation of such network architectures.

Photonic generation techniques offer the flexibility, but transparency at higher

network protocol layers must be considered. The optical downlinks carrying these

high capacity wireless links mainly rely on the electro-optic modulator, which

operates at the mm Wave frequency.

Currently modulators up to 100 GHz are available, but when higher

frequency bands are considered, devices working at such high frequencies must be

investigated. Optical coherent detection is maturing and commercial

implementation of such devices will happen in the near future. Coherent detection

in radio over fiber will play a crucial link, especially demonstrating the ability of

direct conversion of optical baseband to RF signal. New algorithms can be

investigated which will provide more robustness to phase noise, and higher

linewidth devices like VCSELS can be used to reduce the cost of the systems.

ROF technology is a relatively new area of research. So there are lot of

detailed aspects which can be explored in recent future. Regarding our proposed

system various scopes of work are there for future.

The future scope of work in our proposed system is;

• To realize the entire system in experimental setup.

• To design and fabricate each components of the system.

• Introducing phase noises and laser fabrication defects into the studies.

• Designing of Dual Band antenna for transmission of the signal from

the Base Station.

• Designing of Wide Band filter or Dual band Notch filter.

• To make our proposed architecture a duplex system.

• To enhance the data rate beyond 40Gbps efficiently.

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