Presentation on theme: "Optical Wireless Communications"— Presentation transcript:
1Optical Wireless Communications Prof. Brandt-PearceLecture 1Introduction
2Course Outline Introduction Definition of free-space optical communicationsWhy wireless optical communications?Basic block diagramOptical SourcesChallengesAlignment, acquisition, pointing, and tracking (APT)Modulation techniques and noise
3Course Outline Channel Modeling Modulation and Coding AttenuationBeam WanderTurbulence (Scintillation/ Fading)Turbidity (rain, fog, snow)Cloud-free line of sightModulation and CodingVisible Light CommunicationsNon-line-of-sight (NLOS) Ultraviolet (UV) CommunicationsSatellite Optical CommunicationsUnderwater Optical CommunicationsRadio Frequency (RF)/FSO Hybrid Networks
4Demand 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/s4G: designed for 1Gb/s4G speed in ATT and Verizon is 10 Mb/s
8History of FSO Communications Has been used for thousands of years in various formsAround 800 BC, ancients Greeks and Romans used fire beacons for signalingIn 1880 Alexander Graham Bell created the Photophone by modulating the sun radiation with voice signalGerman 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 1904/05The 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 1962The first laser link to handle commercial traffic was built in Japan by Nippon Electric Company (NEC) around 1970
9History of FSO Communications Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
10Why Free Space Optics (FSO)? FSO vs Radio-Frequency (RF) Spectrum is scarce and low bandwidthSpectrum is regulatedSuffers from multi-path fadingSusceptible to eavesdroppingLarge componentsFSOA single FSO channel can offers Tb/s throughputSpectrum is large and license free (very dense reuse)Small componentsSecureTransmission range limited by weather conditionAre very difficult to intercept
11Why Free Space Optics (FSO)? FSO vs Fiber OpticFiber OpticHigh costRequires permits for digging(Rights of Way)TrenchingTime consuming installationMobility impossibleFSONo permits (especially through the window)No diggingNo feesFaster installationMobility/reconfigurability possible
12Access Network Bottleneck Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
13Bandwidth capabilities for a range of optical and RF technologies Chapter 1, “Optical Wireless Communication Systems: Channel Modelling with MATLAB”, Z.Ghassemlooy.
14FSO Block-Diagram1010TRANSMITTERRECEIVERPROCESSORSIGNALDATAINDRIVERLED/LDOUTDATAATMOSPHERIC CHANNELDETECTORPHOTO10102Transmitter projects the carefully aimed light pulses into the air3A receiver at the other end of the link collects the light using lenses and/or mirrors5Reverse direction data transported the same way.Full duplex4Received signal converted back into fiber or copper and connected to the network1Network traffic converted into pulses of invisible light representing 1’s and 0’s
15Challenges Sunlight Window Attenuation Fog Building Motion Alignment ScintillationRangeObstructionsLow Clouds
19Window AttenuationUncoated 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
20Small Angles - Divergence and Spot Size AlignmentSmall Angles - Divergence and Spot Size1° ≈ 17 mrad → 1 mrad ≈ °Small angle approximation:Angle (in milliradians) * Range (km)= Spot Size (m)1 mrad1 m1 kmDivergenceRangeSpot Diameter0.5 mrad2.0 km~1 m (~40 in)2.0 mrad1.0 km~2.0 m (~6.5 ft)4.0 mrad (~ ¼ deg)~4.0 m (~13.0 ft)
21Building Motion Alignment Challenges TypeCause(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 mrad
22Compensating for Building Motion – Two Methods AlignmentCompensating for Building Motion – Two MethodsAutomatic Pointing and TrackingAllows narrow divergence beams for greater link marginSystem is always optimally aligned for maximum link marginAdditional cost and complexityLarge Divergence and Field of ViewBeam spread is larger than expected building motionReduces link margin due to reduced energy densityLow cost0.2 – 1 mrad divergence= 0.2 to 1 meter spread at 1 km2 – 10 mrad divergence=2 to 10 meter spread at 1 km