1

Prof. Brandt-Pearce

Lecture 1Introduction

Optical Wireless Communications

2

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

3

Course Outline

2. Channel Modeling Attenuation Beam Wander Turbulence (Scintillation/ Fading) Turbidity (rain, fog, snow) Cloud-free line of sight

3. Modulation and Coding

4. Visible Light Communications

5. Non-line-of-sight (NLOS) Ultraviolet (UV) Communications

6. Satellite Optical Communications

7. Underwater Optical Communications

8. 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

5

Demand for High-speedCommunications

6

Optical Communications:The Backbone of Telecommunications

Optical fibers around the world

7

Free Space Optical (FSO) Communications

8

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

9

History of FSO Communications

Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.

10

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

11

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)?

12

Access Network Bottleneck

Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.

13

Bandwidth capabilities for a range of optical and RF technologies

Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.

14

1010

1010

DA

TA

IN

LE

D/L

DD

RIV

ER

PH

OT

OD

ET

EC

TO

R

SIG

NA

LP

RO

CE

SS

OR

DA

TA

OU

TATMOSPHERIC CHANNEL

TRANSMITTER RECEIVER

FSO Block-Diagram

1 Network traffic converted into pulses of invisible light representing 1’s and 0’s

2Transmitter 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

15

ChallengesSunlight

Building Motion

Alignment

WindowAttenuation

Fog

Scintillation

RangeObstructions

Low Clouds

16

850 nm 1550 nm

Challenges

Visible range

17

Power Spectra of Ambient Light Sources

Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.

18

AbsorptionDiffractionRayleigh scattering (atmospheric gases molecules)Mie scattering (aerosol particles)Atmospheric (refractive) turbulence:

ScintillationBeam wander

Channel Effects

19

• 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

20

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 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)

1° ≈ 17 mrad → 1 mrad ≈ 0.0573°

Alignment

Building Motion

Type 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

21

Alignment Challenges

22

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

23

Modulation Method

24

Noise in FSO Systems

Background Radiation (e.g. sun light)

Shot Noise (Poisson distributed)

Thermal Noise (Gaussian distributed)

Scintillation Noise

25

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