Optical Wireless An OverviewOutlineWhat is Optical Wireless?Slide 4Last Mile problemMore ApplicationsTransmitterSlide 8Eye SafetyReceiverSimplified Transceiver DiagramPoint-to-Multipoint TopologyPoint-to-Point TopologyRing with Spurs TopologyMesh TopologyTypical Topology in a MetroChallengesPhysical ObstructionFree space lossClear Air AbsorptionWeather ConditionsScatteringScintillationScintillation (cont…)Building Sway and Seismic activityEmpirical Design PrinciplesLimitations of FSO TechnologyTopology Control and RoutingSolution StrategySolution Strategy (cont)Basic Rollout AlgorithmBasic Rollout Algorithm (Math)Slide 33Path ComputationBase HeuristicIndex Rollout AlgorithmCommentsConclusionReferencesUniversity at BuffaloOptical Wireless An OverviewChintan [email protected] 9, 2004University at BuffaloOutlineIntroductionWhat is Optical Wireless?ApplicationsTransmitter and ReceiverTopologiesChallenges and LimitationsTopology Control and RoutingConclusionUniversity at BuffaloWhat is Optical Wireless?Optical Wireless a.k.a. Free Space Optics (FSO) refers to the transmission of modulated light beams through the atmosphere to obtain broadband communicationLine-of-sight technologyUses lasers/LEDs to generate coherent light beamsUniversity at BuffaloWhat is Optical Wireless?Data rates of up to 2.5 Gbps at distances of up to 4km available in commercial productsUniversity at BuffaloLast Mile problemConnecting the user directly to the backbone high speed fiber optic network is known as the Last Mile problemFSO as the low cost bridging technologyUniversity at BuffaloMore ApplicationsAllows quick Metro network extensionsInterconnecting local-area network segments spread across separate buildings (Enterprise connectivity)Fiber backupInterconnecting base stations in cellular systemsUniversity at BuffaloTransmitterFSO uses the same transmitter technology as used by Fiber OpticsLaser/LED as coherent light sourceWavelengths centered around 850nm and 1550nm widely used Telescope and lens for aiming light beam to the receiverUniversity at BuffaloSafety while using LasersUniversity at BuffaloEye Safety650 nm (visible)880 nm (infrared)1310 nm(infrared)1550 nm(infrared)Class 1 Up to 0.2 mW Up to 0.5 mW Up to 8.8 mW Up to 10 mWClass 2 0.2-1 mW N/A N/A N/AClass 3A 1-5 mW 0.5-2.5 mW 8.8-45 mW 10-50 mWClass 3B 5-500 mW 2.5-500 mW 45-500 mW 50-500 mWTable1: Laser safety classification for point-source emitterClass 1 eye safety requirement for lasers used indoorsArray of LEDs are usedClass 3B eye safety requirement for laser used outdoors1550 nm lasers are generally chosen for this purposeClassifies light sources depending on the amount of power they emitUniversity at BuffaloReceiverPhotodiode with large active areaNarrowband infrared filters to reduce noise due to ambient lightReceivers with high gainBootstrap receivers using PIN diode and avalanche photodiode (APD) usedUniversity at BuffaloSimplified Transceiver DiagramUniversity at BuffaloPoint-to-Multipoint TopologyUniversity at BuffaloPoint-to-Point TopologyUniversity at BuffaloRing with Spurs TopologyUniversity at BuffaloMesh TopologyUniversity at BuffaloTypical Topology in a MetroUniversity at BuffaloChallengesPhysical ObstructionAtmospheric LossesFree space lossClear air absorptionWeather conditions (Fog, rain, snow, etc.)Scattering ScintillationBuilding Sway and Seismic activityUniversity at BuffaloPhysical ObstructionConstruction crane or flying bird comes in path of light beam temporarilySolution:Receiver can recognize temporary loss of connectionIn packet-switched networks such short-duration interruptions can be handled by higher layers using packet retransmissionUniversity at BuffaloFree space lossProportion of transmitted power arriving at the receiverOccurs due to slightly diverging beamSolution:High receiver gain and large receiver apertureAccurate pointingUniversity at BuffaloClear Air AbsorptionEquivalent to absorption loss in optical fibersWavelength dependentLow-loss at wavelengths ~850nm, ~1300nm and ~1550nmHence these wavelengths are used for transmissionUniversity at BuffaloWeather ConditionsAdverse atmospheric conditions increase Bit Error Rate (BER) of an FSO systemFog causes maximum attenuationWater droplets in fog modify light characteristics or completely hinder the passage of lightAttenuation due to fog is known as Mie scatteringSolution:Increasing transmitter power to maximum allowableShorten link length to be between 200-500mUniversity at BuffaloScatteringCaused by collision of wavelength with particles in atmosphereCauses deviation of light beam Less power at receiverSignificant for long range communicationUniversity at BuffaloScintillationCaused due to different refractive indices of small air pockets at different temperatures along beam pathAir pockets act as prisms and lenses causing refraction of beamOptical signal scatters preferentially by small angles in the direction of propagationDistorts the wavefront of received optical signal causing ‘image dancing’Best observed by the simmering of horizon on a hot dayUniversity at BuffaloScintillation (cont…)Solution:Large receiver diameter to cope with image dancingSpatial diversity: Sending same information from several laser transmitters mounted in same housingNot significant for links < 200m apart, so shorten link lengthUniversity at BuffaloBuilding Sway and Seismic activityMovements of buildings upsets transmitter-receiver alignmentSolution:Use slightly divergent beamDivergence of 3-6 milliradians will have diameter of 3-6 m after traveling 1kmLow costActive trackingFeedback mechanism to continuously align transmitter- receiver lensesFacilitates accelerated installation, but expensiveUniversity at BuffaloEmpirical Design PrinciplesUse lasers ~850 nm for short distances and ~1550 nm for long distance communication with maximum allowable powerSlightly divergent beamLarge receiver aperture Link length between 200-1000m in case of adverse weather conditionsUse multi-beam systemUniversity at BuffaloLimitations of FSO TechnologyRequires line-of-sightLimited range (max ~8km)Unreliable bandwidth availabilityBER depends on weather conditionsAccurate alignment of transmitter- receiver necessaryUniversity at