Optical Wireless
Communications
Prof. Brandt-Pearce
Lecture 1
Introduction
1
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
2
Course Outline
2.
Channel Modeling
 Attenuation
 Beam Wander
 Turbulence (Scintillation/ Fading)
 Turbidity (rain, fog, snow)
 Cloud-free line of sight
3.
4.
5.
6.
7.
8.
Modulation and Coding
Visible Light Communications
Non-line-of-sight (NLOS) Ultraviolet (UV) Communications
Satellite Optical Communications
Underwater Optical Communications
Radio Frequency (RF)/FSO Hybrid Networks
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Demand for High-speed
Communications
 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-speed
Communications
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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 communications
Transmission 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
8
History of FSO Communications
Chapter 1, “Optical Wireless Communication Systems: Channel
Modelling with MATLAB”, Z.Ghassemlooy.
9
Why Free Space Optics (FSO)?
FSO vs Radio-Frequency (RF)
RF





Spectrum is scarce and low bandwidth
Spectrum is regulated
Suffers from multi-path fading
Susceptible to eavesdropping
Large components
FSO
 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
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Why Free Space Optics (FSO)?
FSO vs Fiber Optic
Fiber Optic
 High cost
 Requires permits for digging
(Rights of Way)
 Trenching
 Time consuming installation
 Mobility impossible
FSO
 No permits (especially through the window)
 No digging
 No fees
 Faster installation
 Mobility/reconfigurability possible
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Access Network Bottleneck
Chapter 1, “Optical Wireless Communication Systems: Channel
Modelling with MATLAB”, Z.Ghassemlooy.
12
Bandwidth capabilities for a range
of optical and RF technologies
Chapter 1, “Optical Wireless Communication Systems: Channel
Modelling with MATLAB”, Z.Ghassemlooy.
13
FSO Block-Diagram
TRANSMITTER
1010
RECEIVER
DATA
OUT
SIGNAL
PROCESSO
R
PHOTO
DETECTOR
LED/LD
DRIVER
DATA
IN
ATMOSPHERIC CHANNEL
1010
2
Transmitter projects the carefully
aimed light pulses into the air
3 A receiver at the other end of the link collects
the light using lenses and/or mirrors
5 Reverse direction data transported the same way.
• Full duplex
1
Network traffic converted into
pulses of invisible light
representing 1’s and 0’s
4
Received signal converted back
into fiber or copper and
connected to the network
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Challenges
Sunlight
Window
Attenuation
Fog
Building
Motion
Alignment
Scintillation
Obstructions
Range
Low Clouds
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Challenges
Visible range
850 nm 1550 nm
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Power Spectra of Ambient Light Sources
Chapter 1, “Optical Wireless Communication Systems: Channel
Modelling with MATLAB”, Z.Ghassemlooy.
17
Channel Effects
 Absorption
 Diffraction
 Rayleigh scattering (atmospheric gases molecules)
 Mie scattering (aerosol particles)
 Atmospheric (refractive) turbulence:
Scintillation
Beam wander
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Window Attenuation
• 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
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Alignment
Small Angles - Divergence and Spot Size
1° ≈ 17 mrad → 1 mrad ≈ 0.0573°
Small angle approximation:
Angle (in milliradians) * Range (km)= Spot Size (m)
1m
1 mrad
1 km
Divergence
Range
Spot Diameter
0.5 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)
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Alignment Challenges
Building Motion
Type
Cause(s)
Tip/tilt
Thermal expansion
Sway
Wind
Vibration Equipment, door slamming, etc.
Magnitude
High
Medium
Low
Frequency
Once per day
Once every several seconds
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
Compensating for Building Motion – Two Methods
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
0.2 – 1 mrad divergence
= 0.2 to 1 meter spread at 1 km
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
2 – 10 mrad divergence
=2 to 10 meter spread at 1 km
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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
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