prof. brandt-pearce lecture 1 introduction
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Optical Wireless Communications. Prof. Brandt-Pearce Lecture 1 Introduction. Course Outline. Introduction Definition of free-space optical communications Why wireless optical communications? Basic block diagram Optical Sources Challenges - PowerPoint PPT PresentationTRANSCRIPT
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Prof. Brandt-Pearce
Lecture 1Introduction
Optical Wireless Communications
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Course Outline
1. Introduction Definition of free-space optical communications Why wireless optical communications? Basic block diagram Optical Sources Challenges Alignment, acquisition, pointing, and tracking (APT) Modulation techniques and noise
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Course Outline
2. Channel Modeling Attenuation Beam Wander Turbulence (Scintillation/ Fading) Turbidity (rain, fog, snow) Cloud-free line of sight
3. Modulation and Coding4. Visible Light Communications5. Non-line-of-sight (NLOS) Ultraviolet (UV) Communications6. Satellite Optical Communications7. Underwater Optical Communications8. Radio Frequency (RF)/FSO Hybrid Networks
According to the Internet Society, over
80% of the world will be connected to the
Internet by 2020.
Mobile and application services are the
future of the Internet.
3G: 2 Mb/s
4G: designed for 1Gb/s
4G speed in ATT and Verizon is 10 Mb/s
Demand for High-speedCommunications
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Demand for High-speedCommunications
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Optical Communications:The Backbone of Telecommunications
Optical fibers around the world
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Free Space Optical (FSO) Communications
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History of FSO Communications
Has been used for thousands of years in various forms Around 800 BC, ancients Greeks and Romans used fire beacons for signaling In 1880 Alexander Graham Bell created the Photophone by modulating the sun radiation with voice signal German troops used Heliograph telegraphy transmitters to send optical Morse signals for distances of up to 4 km at daylight (up to 8 km at night) during the 1904/05 The invention of lasers in the 1960s revolutionized FSO communicationsTransmission of television signal over a 30-mile using GaAs LED by researchers working in the MIT Lincolns Laboratory in 1962 The first laser link to handle commercial traffic was built in Japan by Nippon Electric Company (NEC) around 1970
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History of FSO Communications
Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
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Spectrum is scarce and low bandwidth Spectrum is regulated Suffers from multi-path fading Susceptible to eavesdropping Large components
Why Free Space Optics (FSO)?FSO vs Radio-Frequency (RF)
RF
A single FSO channel can offers Tb/s throughput Spectrum is large and license free (very dense reuse) Small components Secure Transmission range limited by weather condition Are very difficult to intercept
FSO
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High cost Requires permits for digging
(Rights of Way) Trenching Time consuming installation Mobility impossible
FSO vs Fiber Optic
No permits (especially through the window) No digging No fees Faster installation Mobility/reconfigurability possible
Fiber Optic
FSO
Why Free Space Optics (FSO)?
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Access Network Bottleneck
Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
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Bandwidth capabilities for a range of optical and RF technologies
Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
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1010
1010
DATA
IN
LE
D/L
DD
RIV
ER
PHO
TOD
ET
EC
TOR
SIGN
AL
PRO
CE
SSOR
DA
TA O
UTATMOSPHERIC CHANNEL
TRANSMITTER RECEIVER
FSO Block-Diagram
1 Network traffic converted into pulses of invisible light representing 1’s and 0’s
2 Transmitter projects the carefully aimed light pulses into the air
5 Reverse direction data transported the same way.• Full duplex
3 A receiver at the other end of the link collects the light using lenses and/or mirrors
4 Received signal converted back into fiber or copper and connected to the network
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ChallengesSunlight
Building Motion
Alignment
WindowAttenuation
Fog
Scintillation
RangeObstructions
Low Clouds
16850 nm 1550 nm
Challenges
Visible range
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Power Spectra of Ambient Light Sources
Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
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AbsorptionDiffractionRayleigh scattering (atmospheric gases molecules)Mie scattering (aerosol particles)Atmospheric (refractive) turbulence:
ScintillationBeam wander
Channel Effects
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• Uncoated glass attenuates 4% per surface due to reflection• Tinted or insulated windows can have much greater attenuation• Possible to trade high altitude rooftop weather losses vs. window attenuation
Window Attenuation
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Small Angles - Divergence and Spot Size
1 mrad
1 km
1 m
Small angle approximation:
Angle (in milliradians) * Range (km)= Spot Size (m)
Divergence Range Spot Diameter0.5 mrad 2.0 km ~1 m (~40 in)2.0 mrad 1.0 km ~2.0 m (~6.5 ft)
4.0 mrad (~ ¼ deg) 1.0 km ~4.0 m (~13.0 ft)
1° ≈ 17 mrad → 1 mrad ≈ 0.0573°
Alignment
Building MotionType Cause(s) Magnitude Frequency
Tip/tilt Thermal expansion High Once per day
Sway Wind Medium Once every several seconds
Vibration Equipment, door slamming, etc.
Low Many times per second
Building Motion Due to the Thermal Expansion• 15% of buildings move more than 4 mrad• 5% of buildings move more than 6 mrad• 1% of buildings move more than 10 mrad
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Alignment Challenges
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1. Automatic Pointing and Tracking– Allows narrow divergence beams for greater link margin– System is always optimally aligned for maximum link margin– Additional cost and complexity
2. Large Divergence and Field of View– Beam spread is larger than expected building motion– Reduces link margin due to reduced energy density– Low cost
Compensating for Building Motion – Two Methods
0.2 – 1 mrad divergence
= 0.2 to 1 meter spread at 1 km
2 – 10 mrad divergence
=2 to 10 meter spread at 1 km
Alignment
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Modulation Method
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Noise in FSO Systems
Background Radiation (e.g. sun light)
Shot Noise (Poisson distributed)
Thermal Noise (Gaussian distributed)
Scintillation Noise
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Applications of FSO Communications
Infra-red (IR) communications (remote control applications)
Visible light communications (VLC) for indoor applications
Non-line-of-sight (NLOS) ultraviolet (UV) communications
Inter-satellite communications
Underwater communications
Terrestrial optical communications
Hybrid RF/FSO communications
Optical quantum communications