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 Alignment, acquisition, pointing, and tracking (APT) Modulation techniques and noise

Course Outline Channel Modeling Modulation and Coding Attenuation Beam Wander Turbulence (Scintillation/ Fading) Turbidity (rain, fog, snow) Cloud-free line of sight 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

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

Optical Communications: The Backbone of Telecommunications Optical fibers around the world

Free Space Optical (FSO) Communications

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

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

Why Free Space Optics (FSO)? FSO vs Radio-Frequency (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

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

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

Bandwidth capabilities for a range of optical and RF technologies Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.

FSO Block-Diagram 1010 TRANSMITTER RECEIVER PROCESSOR SIGNAL DATA IN DRIVER LED/LD OUT DATA ATMOSPHERIC CHANNEL DETECTOR PHOTO 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 4 Received signal converted back into fiber or copper and connected to the network 1 Network traffic converted into pulses of invisible light representing 1’s and 0’s

Challenges Sunlight Window Attenuation Fog Building Motion Alignment Scintillation Range Obstructions Low Clouds

Challenges Visible range 850 nm 1550 nm

Power Spectra of Ambient Light Sources Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.

Channel Effects Absorption Diffraction Rayleigh scattering (atmospheric gases molecules) Mie scattering (aerosol particles) Atmospheric (refractive) turbulence: Scintillation Beam wander

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

Small Angles - Divergence and Spot Size 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) 1 mrad 1 m 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) ~4.0 m (~13.0 ft)

Building Motion Alignment Challenges 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

Compensating for Building Motion – Two Methods Alignment Compensating for Building Motion – Two Methods 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 Large Divergence and Field of View Beam spread is larger than expected building motion Reduces link margin due to reduced energy density Low cost 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

Modulation Method

Noise in FSO Systems Background Radiation (e.g. sun light) Shot Noise (Poisson distributed) Thermal Noise (Gaussian distributed) Scintillation Noise

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