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Final Paper

  1. 1. Free Space Optical (FSO) Communication Systems E E 421 FinalProject ThisdocumentcontainsinformationonFSOcommunication as well assome of the challengesmetwhendesigningsuch systems. Furthermore,thisdocumentproposestoaddress the atmosphericattenuationproblemforthe Space Photonic proprietaryLaserFire FSOcommunicationsystem. Chad Weiss JosephCourtright 5/2/2016
  2. 2. 1 Abstract: Free space optical (FSO) communicationisaformof communicationthathasbeeninuse since the dawn of earlyhumancivilization. Fire signalswereusedinordertosendalertmessagesorsignalsof warover verylongdistanceswithverylittletime oreffortinvolved. Now,communicationhasevolvedtoits presentdayform;i.e.a meshnetworkof interconnecteddevicesenablinglongdistance communication overguided/unguidedmedia. Although presentdayFSOcommunicationtechnologyallowsforeffortlesscommunication,designing such systemsisanythingbuttrivial. Engineersare facedwiththe huge problemthatisatmospheric attenuationwhendesigningFSOcommunicationsystems. ManufacturerslikeSpace Photonicshave beenable toachieve wirelessoptical communicationatdistancesupto5 km but remainvulnerable to adverse weatherconditionslike fog,rainorsnow. Thispaperseeksto addressthe vulnerabilitiesthatSpace Photonicsface regardingFSOcommunication systems. Furthermore,thispaperproposesasolutiontothe problemthatisatmosphericattenuation on the LaserFire FSOcommunicationsystem(Space Photonicsproprietary). Finally,the paperwill conclude withashort discussionabout the future of FSOcommunication.
  3. 3. 2 Table of Contents Section Page i.Abstract 1 ii.Contents 2 I. Introduction 3 II.Advantages 3 III.Last Mile Problem 4 IV.How itWorks 4 V.SystemSolution 5 VI.Proposal 8 VII.Future 8 VIII.WorksCited 10 List of Tables Table Page Table 1: MitigationTechniques 7
  4. 4. 3 Introduction Free space optical communication(FSOCommunication) islaserbasedcommunicationsystem. Similar to fiberopticscommunication, FSOcommunication hasaveryhighdata rate whencomparedwithradio transmissionorcopperlines. Since complicatedcableinstallationsare unnecessaryforFSOsystems,the systemsare much cheaper.Itisthe combinationof highspeedandlow costwhichmakesFSO technologysouseful. Optical signalsare one of the earliestformsof longdistance communication.In800 BC and perhaps earlier,fire beaconswouldbe flashedbythe Romanstosendmessagesoverlongdistances. OnJune 3, 1880 AlexanderGrahamBell testedthe Photophone.The Photophonemodulatedvoice signalswitha mirrorusedto direct sunlight.The systemhada range of about 700 feet,butfailedtoworkwhenitwas cloudyor at night. Free space optical communicationsystemsintheirmodernformoriginatedinthe late 60s,withthe inventionof laser. Dr.Erhard Kube is consideredone of the founders of fibersopticnetworks.InJune of 1968, Dr. Kube published"Informationtransmissionbylightbeams throughthe atmosphere"the first journal article proposingoptical transmissionwith lasersystems [1] [2]. Early applicationsinvolvedmilitarysatellitesandNASA.Thesespace basedapplications rallied communicationsbetweensatellites. Because atmosphericinterference isthe primaryobstacle toFSO systems, space providesanexcellent environmentforapplication. FSOhas beenwidelyreplacingother formsof wirelesscommunication.Somebusinesseshave found FSOsystems tobe a suitable replacementforEthernetnetworkingcables. Furthermore, FSOsystems are beingusedtotransmitlive television andcell signals.Ashigherresolutionsof videobecome more commonplace,the needto switchto higherspeedscausesgroupstorely more andmore on optical communicationsystemssuchas free space optics. Advantages One of the primaryadvantagesof the FSO communicationsystem isthe highspeed atwhichthe system transmitsdata. All FSOsystems are guaranteedtohave a data rate of at least2.5 Gbps.Some newer systemsevenhave speedsof upto160 Gbps [1] [3]. Comparatively, radiowaves tendtofunctionata maximumof only15Kbps. Signal modulation isusedto encode the datawithinthe waves.Dependingon the type of modulation,the numberof bit sentina single wave canbe varied.However, datarate is alwaysinverselyproportional tothe wavelength. FSOprimarily usesinfraredlightasa carrierwhichhas a smallerwavelengththanRadioorTV waves. Anotherimportantadvantage of FSOsystemsiscost.Fiberopticcablesare able to rival FSOwithspeed. However, fiberopticscable networkscanbe expensive.Anysignificantinfrastructure of fiberoptic cableswill require timeandmanpowertoinstall. Howeverfree space optical systems onlyrequiresa transmitteranda receiver.Finallyradioandtelevisionwavesare regulatedandspecifictransmission frequenciesare owned.Because infraredtransmissionsare unregulatedone wouldnotneedtoowna sectionof the radiospectrum[3].
  5. 5. 4 The beam divergenceisproportionaltowavelength asindicatedbythe equationbelow. θdiv = 2.44λ Dr FSO useswavelengthsbetween700nmand 1600nm whichissignificantlysmallerthanthe 30mm to 3m whichisusedinRF communications.The FSOsignal propagatesata much smallerangle. The signal takeslesspowertotransmitovera longer distances,because the powerismore focused overasmaller area at the same distance. Generally,wirelesstechnologyisasecurityrisk,because itpropagateswithinalargerfield.However, FSO propagatesata muchsmallerangle thanradioor televisionwaves. Thissmall fieldof propagation makesitnearlyimpossible totapintoa communicationwithoutalsocuttingthe communication. Additionally,FSOsignalscannotbe detectedusing aspectrumanalyzerorRF meter. Finally,FSOsystemsare notsubjecttointerference fromotherFSOsystems. FSOoperatesona narrow beamunlike RF.Itusesa highlydirectionalbeamandtherefore manyFSOsystemscanoperate onthe same frequencywithinclose proximityandnothave to worryabout interferingwitheachother. Last Mile Problem Since the 1960s the worldhasbuiltup a massive networkof highspeedfiberopticcablesbetweencities and majorcenters;however,installingfiberopticcables, especiallyinurbanareas,posesmany problems. There are several obstacles regardingthe costsandpermits required toinstall fiberoptic cables. Inthe UnitedStatesonlyaboutfive percentof all buildings have adirectconnectiontofiber opticsinternet.Whileatthe same time 75% of all businessesexistwithinone mileof one of these centers[4].Many of these businessesare connectedwith slowerEthernetcables. The speedof any connectionisonlyasfast as the slowestpart,sothese corporationsdonotreceive the speedbenefits associatedwithfiberopticsandinsteadrelyonmuchslowerconnectionspeeds;thisisknownare the lastmile problem. One of the majorapplications of FSOsystemsistosolve the lastmile problem, because theymaintainahighspeedconnectionwithoutthe needforexpensive,highcostinstallations. By creatinga FSO network, otherbusinessesinthe regioncanbenefit fromhighspeedconnections withoutneedingtobuildupexpensive infrastructure. How It Works Like manyfiberopticssystemsFSOusespulsesof lighttocarrydata. However,unlikefiberoptics systems, FSOsystemseliminate the needfora guidingmedium suchthatthe lightsignal canpass safely fromthe transmittertothe receiver.Instead,the message isdirectedthroughthe openairbyusing unguided line of sighttoreachthe receiver. The data transmissioninfree space opticssystemsisdone entirelybylaser. The energyof electronsis quantized,meaningthatitcan onlyexistatparticularevery level. A laseriscreatedbygettingan electrontoabsorb an electriccharge movingupto a higherenergylevel. Ina shortperiodof time the electronwill rerelease the energyasa wave of lightat a specificfrequency. The frequencycanbe found
  6. 6. 5 by the RydbergequationshownbelowwhereRisthe Rydbergconstantand n1 and n2 are propertiesof the atom. f = c × R( 1 n1 2 + n2 2) The lasercan be modulatedbycontrollingthe amountof currentusedto create the laser.By usinga largercurrent,the laserwill be more intense.However,since thisrelationshipisnon-lineardigital modulationispreferred [5]. The wavelength of the lightisanimportantfactorin determininghow much isscatter or absorbedinthe atmosphere. The lightfromthe laseristo be focusedontoa beamusingsurroundingmirrorsandlenses. Thenthe selectionof the lenswill determine the diversionof the beam andthe laser‘sfocuslength.A more focusedbeam hasa higherpowerconcentration andthe focuslengthcanbe carefullychosenbasedon howfar the lightisto travel. The unguided laserlightwillneedtobe aimed atthe receiveroverthe openair. Inthe openair the signal can be scattered,absorbedorblocked. Once atthe receiveranotherlenswillfocusinontoa photodetector. The selectionof the lenscandeterminethe acceptance angle intoreceiver.The combinationof the acceptance angle andthe size of the aperture openingdeterminethe amountof lightwhichentersthe photoreceptor. The photoreceptorthenconvertsthe signal backintoan electronicsignal andsendsthe communication. System Solution The primarychallenge whendesigningaFSOcommunicationsystemisthe abilitytocompensate for atmosphericattenuationsandatmosphericdisturbances,especiallywhendesigningforlongrange and highspeed. Toaccomplishthis,one musttake intoaccount the effectsof atmosphericabsorption, scatteringandturbulence,whichleadtoahigherattenuationfactor. Due tothe interactionbetween lightandmatter,atmosphericabsorptionandscattering canbe minimizedbychoosingasource of propagatinglightwithanappropriate wavelength. Furthermore,itisimperative tochoose agoodlight source,suchas a laserdiode andnota poorsource such as a LED. Otherfactors that affectthe overall performance of FSOsystemsinclude:atmosphericattenuation,scintillation,buildingalignment, vibrations,solarinterference andline-of-sightobstructions. In orderto minimize the scatteringandabsorptioneffectof the atmosphere hasonthe propagatinglight signal,one mustselectthe wavelengthwisely. If the wavelengthistooshort,toomuch of the signal will scatter duringtransmission,causingthe biterrorrate (BER) to become increasinglydetrimentaltothe fidelityof the signal. Asforthe absorption,one mustchoose awavelengththatfallswithinthe appropriate atmospherictransmissionwindow,i.e.the range of wavelengthvaluesthatwill minimize absorption. The absorptionvaluesfordifferentwavelengthscanbe foundindatabaseslike HITRAN (HighResolutionTransmission) orwithcomputerprogramslike MODTRAN (Moderate Resolution AtmosphericTransmission). MODTRAN simulatesvariouswavelengthsof the electromagneticspectrum as theypropagate throughthe atmosphere undersetatmospheric conditionssuchasfog,rain or snow.
  7. 7. 6 These simulationsprovidesystemengineerswiththe informationtomake decisionsaboutthe overall design. Itisimperative thatone alsoconsidersthe probabilityandstatisticsof weatherphenomena withinthe natural environmentwhere the FSOcommunicationwilloperate. Aside fromatmosphericattenuation,additionallyone mustconsideratmosphericturbulence asamajor systeminterrupt. Turbulentcells,alsoknownaseddies,ultimatelyensuesconstructive and deconstructive interferencesonthe propagatinglightthatpassesthroughthem. Due tocorrelations betweenpressure,temperature andindex of refraction,highertemperaturesandgreaterpressures pocketshave a higherindex of refractionthanlowerpressure andlowertemperature pockets. Asthe propagatinglightpassesthroughthe non-uniformmedium,constructiveordeconstructiveinterference may occur. The resultof thisinterference isaredirectionof the propagatingsignalwhichleadsto fluctuations inintensity,alsoknownasscintillation. Turbulence inducedbeamwanderingand turbulence inducedbeamspreadingalsooccurs. Engineersare facedwithahuge dilemmabecause the effectsof turbulence cannotbe avoided. Turbulence inducedbeamspreadingoccurswhenthe beam size islargerthan the eddies,whenthe beamsizeissmallerthanthe eddiesandwhenthe beamsize is approximatelythe same size of the eddies. The last three thingsthatcan affectthe overall performance of FSOcommunication systemsare:Beam divergence,ambientlightandmisalignment/buildingsway. Beamdivergence occurswhenthe length of the linkisverylarge incomparisontothe aperture of the receiver. Divergence occursatthe receiving endwhichleadstosome geometriclossof light. Again,thislossincreasesasthe lengthof the link increasesanddecreasesasthe size of the receiveraperture isincreased. Ambientlightreferstothe sun or moonlightthatinterfereswiththe signal byaddingsomethingcalledshotnoise. Shotnoise canbe avoidedif the wavelengthisincreased. Increasingthe wavelengthreducesthe probabilitythatnoise will be addedto the system. Lastly,there isbuildingswayormisalignment. Thiscancause serious problems,especiallyif the beamcannotsee the receiver. Buildingswaycanoccur formany reasons. Thermal linearexpansionof materialsinhightemperature environmentsmaycause the buildingto expandorcontract in coldweatherenvironments. Also,tremorsandearthquakes maycause vibrations inthe superstructure of the buildingwhichwouldultimatelyrattle the signal transmitter. Additionally, there isa chance that highwindspeedscouldshake the FSOcommunicationsystemanddisruptthe link also. Systemengineershave manyobstaclestoovercome whendesigningaFSOcommunicationsystemand despite all the effortstomaintainaviable linkbetweentwolocations,thereisalwaysthe chance that something,whetheritisabird,a plane orSuperman,will obstructthe signal pathbyblockingthe line- of-sight(LOS). Engineershave tocome upwitha way to mitigate the effectsof all these nuances. Moreover,itisimperative todevelopasystemthatisadaptable toany situationimaginable;otherwise the system’sexecution isextremelylikelytofail. Thatiswhymuch thoughtisinvolvedwhendesigninga FSO system. FSOcommunicationsystemsare veryvulnerableandneedtobe as strongas Mother Nature itself. Some of the designparametersconsideredwhendesigningaFSO communicationsysteminclude the beamdivergence,transmitterpower,operatingwavelength,andtransceiverfield-of-view (FOV). For
  8. 8. 7 Space Photonics,anadvancedtracking,acquisitionandpointingsystem(TAP) isusedtocompensate for all the adverse weathereffectsandlongdistance challenges. The subjectof interestisthe LaserFire FSO communicationsystem. Thissystemisahighlysecure communicationsystemthatprovides uninterrupted,secure wirelesscommunicationwithultra-highwirelessbandwidth. Ithas an automatic tracking,acquisitionandpointingsystemwhichallowsforsmallerspotsize transmissionbeamswhich make it virtuallyimpossibletodetectorintercept. The LaserFire systemoperatesinthe nearinfrared spectrumandprovidesafast continuouslinksynchronizationthatcorrectsforatmosphericturbulence and beamfading. Forextrasecurity,if the beamisblockedforany reason,the transmitterwill automaticallystopsendingdataas to ensure informationsecurity. The highfidelitysystemcanprovide up to 1.0 Gbpsat distancesupto5 km; furthermore,the systemhasbeendesignedtobe compatible withcommerciallyavailable wavelengthdivisionmultiplexing(WDM) componentsformuchhigher aggregate bandwidths. LaserFirehasa low mass, low powerrequirementandcanbe deployedrelatively easyinpoint-to-pointandmultimode configurations. Itusesa small,highlycollimated,infraredbeam and GUI to assure a reliable connection. Despite all of the greatthingsaboutLaserFire,itisstill mildlysusceptible toadverse weatherconditions such as snow,heavyrain,fogor dust. Much researchhasbeendone totry and mitigate the effectsof such impairments. Developmentsonthe actual physical layeraswell assome of the upper layersinthe system,like the linkornetworklayerare pushingthe limitsforFSOcommunicationsystems. Here is a table thatincludessome of the alreadyexistingmethodsforaddressingFSOcommunication systemdesignchallenges:(Redreferstophysical layertechniques,bluereferstothe upperTCP layers) Table 1: Mitigation Techniques MitigationTechnique DesignChallenge Justification Aperture Averaging AtmosphericTurbulence Increasingthe receiveraperture createsa largeracceptance angle whichcan helpaverage out the fastfluctuations,or scintillations,causedbyeddies, thusreducingchannel fading. Adaptive Optics AtmosphericTurbulence A closedloopfeedbacksystem compensatesforthe beam misalignmentdue toturbulence. Adaptive Thresholding AtmosphericTurbulence Codingandmodulation techniquescanbe usedto simplifythe detectionof a1 or 0 at the receiver. If dispersionor scintillationwere tooccur, adaptive thresholdingcould cleanup the BER of the system. BackgroundNoise Rejection SolarInterference Spatial filters,polarizingfilters and modulatingtechniqueswith highpeak-to-average power
  9. 9. 8 couldeliminateanynoise enteringthe receiver. HybridRF/FSO AtmosphericAttenuation If weatherconditionsmake it impossible forFSO communication,the system couldswitchto a backupRF. PacketRe-transmission IncreasedBER,single-biterrors and bursterrors Re-transmissionprotocolssuch as automaticrepeatrequest (ARQ) can improve the overall performance of the link. ReconfigurationandRerouting LOS obstructions,faulty equipment Path configurationanddata reroutingisissuedinorderto increase the reliabilityand availabilityof the link. Qualityof Service Control AtmosphericAttenuation, Distortion,Turbulence,Solar Interference,Scattering, Absorption,Hardware Failure, LOS obstructions,FOV limitations,misalignmentand weather Implementationof routing protocolsandcontrol algorithms can be usedtoimprove the overall qualityof the service beingprovidedbymeasuringthe data rate,latency,delayjitter, data loss,energyconsumption, reliabilityandefficiency. Proposal Regardingall of the mitigationtechniqueslistedinTable 1onlyone addressesthe atmospheric attenuationfactorsuch,i.e.the hybridRF/FSOsystem. UponthoroughlyinvestigatingSpace Photonics’ LaserFire,itwasdiscoveredthatthe systemwasvulnerable toadverse weatherconditionslike fog,rain and snowwithabsolutelynobackupavailable. Therefore,itissuggestedtoimplementahybridRF/FSO systeminorderto compensate forthe effectsof atmosphericattenuation. Increasingthe datarate will involve researchinvestigatingthe propertiesof semi-conductormaterialsinrelationtothe fastforward recombinationtimesaswell asmanyothersubjects. Inorderto improve the range of currentFSO systems,largerbeamsandmore advancedtransmitterreceiverswill have tobe researchedand developed. Since the definitionof aFSOcommunicationsystemincludesthe factthatitis an unguided transmissionlink,one wouldhave tocreate avacuum or rarefactioninthe transmissionpath. One mightbe able to accomplishrarefactionusingsoundwavesbut,itisstill very unlikelythatitwill actually increase the range of the FSO system. Sounddoesn’ttravel veryfarbefore attenuatinganditisvery difficulttocontrol. Laserpoweramplifierswouldhave tobe integratedinordertoachieve further distances. Future The future for FSOcommunicationisverypromising. Itprovidesuserswithhigherbandwidths,greater securityandincreaseddataratesfor lessmoney,lesspowerandlesspermissionwhichmakessetup
  10. 10. 9 easyand quick. The applicationsforFSOtechnology are endless;theycanbe usedindevelopingthird worldcountries,establishingsecure communicationincombatsituations,promotingsustainabilityand much more. The main challenge iscontrollingthe atmospherictransmissionchannel. Until weathercan be controlledandlossymediaeradicatedfromthe transmissionchannel,FSOcommunicationsystems will remainlimited.
  11. 11. 10 Works Cited [1] Laseroptronics,"FSOHistoryandTechnology,"Laseroptronics,2016.[Online].Available: http://www.laseroptronics.com/index.cfm/id/57-66.htm.[Accessed16April 2016]. [2] LightPointe,2006.[Online].Available:http://www.freespaceoptics.org/freespaceopticshome.html. [Accessed16 April 2016]. [3] Sona Optical,"FSOGuide,"SonaOptical,[Online]. Available: http://www.fsona.com/technology.php?sec=fso_guide. [Accessed17April 2016]. [4] P. S.J. Rajput,Director, Free SpaceOptical Communication. [Film].EandC Engg Dept.,2015. [5] M. Carter,"LaserBeam Modulation,"October2015. [Online]. Available: http://www.maxmcarter.com/lasrstuf/lasermodulator.html.[Accessed19April 2016].

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