Prof. Brandt-Pearce Lecture 1 Introduction
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Lecture 1IntroductionOptical Wireless Communications2Course OutlineIntroductionDefinition of free-space optical communicationsWhy wireless optical communications? Basic block diagramOptical SourcesChallengesAlignment, acquisition, pointing, and tracking (APT)Modulation techniques and noise3Course OutlineChannel ModelingAttenuationBeam WanderTurbulence (Scintillation/ Fading)Turbidity (rain, fog, snow)Cloud-free line of sight
Modulation and CodingVisible Light CommunicationsNon-line-of-sight (NLOS) Ultraviolet (UV) CommunicationsSatellite Optical CommunicationsUnderwater Optical CommunicationsRadio 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-speedCommunications5
Demand for High-speedCommunications
6Optical Communications:The Backbone of Telecommunications
Optical fibers around the world 7
Free Space Optical (FSO) Communications
8History 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
History of FSO Communications Chapter 1, Optical Wireless Communication Systems: Channel Modelling with MATLAB, Z.Ghassemlooy.
10Spectrum is scarce and low bandwidthSpectrum is regulatedSuffers from multi-path fadingSusceptible to eavesdropping Large componentsWhy 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 interceptFSO11High costRequires permits for digging (Rights of Way)TrenchingTime consuming installationMobility impossibleFSO vs Fiber OpticNo permits (especially through the window) No digging No fees Faster installationMobility/reconfigurability possibleFiber OpticFSOWhy Free Space Optics (FSO)?
Access Network BottleneckChapter 1, Optical Wireless Communication Systems: Channel Modelling with MATLAB, Z.Ghassemlooy.
13Bandwidth capabilities for a range of optical and RF technologiesChapter 1, Optical Wireless Communication Systems: Channel Modelling with MATLAB, Z.Ghassemlooy.
1410101010DATA IN LED/LDDRIVERPHOTODETECTORSIGNALPROCESSORDATA OUTATMOSPHERIC CHANNELTRANSMITTERRECEIVERFSO Block-Diagram
1Network traffic converted into pulses of invisible light representing 1s and 0s2Transmitter projects the carefully aimed light pulses into the air
5Reverse direction data transported the same way.Full duplex 3A receiver at the other end of the link collects the light using lenses and/or mirrors4Received signal converted back into fiber or copper and connected to the network
850 nm1550 nmChallengesVisible range17Power Spectra of Ambient Light SourcesChapter 1, Optical Wireless Communication Systems: Channel Modelling with MATLAB, Z.Ghassemlooy.
18AbsorptionDiffractionRayleigh scattering (atmospheric gases molecules)Mie scattering (aerosol particles)Atmospheric (refractive) turbulence:ScintillationBeam wander
Channel Effects19Uncoated glass attenuates 4% per surface due to reflectionTinted or insulated windows can have much greater attenuationPossible to trade high altitude rooftop weather losses vs. window attenuation
Small Angles - Divergence and Spot Size1 mrad1 km1 mSmall angle approximation:Angle (in milliradians) * Range (km)= Spot Size (m)DivergenceRangeSpot Diameter0.5 mrad2.0 km~1 m (~40 in)2.0 mrad1.0 km~2.0 m (~6.5 ft)4.0 mrad (~ deg)1.0 km~4.0 m (~13.0 ft)1 17 mrad 1 mrad 0.0573 AlignmentBuilding MotionTypeCause(s)MagnitudeFrequencyTip/tiltThermal expansionHighOnce per daySwayWindMediumOnce every several secondsVibrationEquipment, door slamming, etc.LowMany times per secondBuilding Motion Due to the Thermal Expansion15% of buildings move more than 4 mrad5% of buildings move more than 6 mrad1% of buildings move more than 10 mrad21Alignment Challenges22Automatic Pointing and TrackingAllows narrow divergence beams for greater link marginSystem is always optimally aligned for maximum link marginAdditional cost and complexity
Large Divergence and Field of ViewBeam spread is larger than expected building motionReduces link margin due to reduced energy densityLow 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 kmAlignment23Modulation Method
24Noise in FSO Systems Background Radiation (e.g. sun light) Shot Noise (Poisson distributed) Thermal Noise (Gaussian distributed) Scintillation Noise25Applications 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