通信及基础工程 1

Unit8TextAFree-SpaceLaserCommunicationsFree-SpaceLaserCommunicationsRecentrapidprogressininformationandcommunicationtechnologieshasexceededourexpectationsformeetingtherequirementsofmultimediasocietyinthe21stcentury.Free-spaceopticalcommunication[1]isconsideredtobeoneofthekeytechnologiesforrealizingvery-high-speedmulti-gigabit-per-second(multi-Gb/s)large-capacityaerospacecommunications.Usinglasersassignalcarriers,FSOlasercomcanprovidealine-of-sight,wireless,high-bandwidth,communicationlinkbetweenremotesites.RapidlygrowinguseoftheInternetandmultimediaserviceshascreatedcongestioninthetelecommunicationsnetworksandplacedmanynewrequirementsoncarriers.TextTextWordsNotesDiscussionHomeLasertransmittersofferanintermediate,low-riskmeanstointroducedesirednetworkfunctionalitieswithextremelyhighbandwidth.ThewirelessaspectofFSOlasercomcanbeacrucialadvantage,particularlyinlocalareanetworks(LANs)andmetropolitanareanetworks(MANs)whereincitiesthelayingoffiberopticcablesisexpensive.WhentheWorldTradeCenterscollapsedonSeptemberllth,2001,sotoodidthenetworksofallofthecorporationslocatedwithinthebuildings.Hundredsofthousandsofdollarswerelostduetothedown-timethatthesecompaniesexperienced.WiththehelpofafewwirelessbroadbandprovidersusingFSOtechnology,thesecompanieswereabletogettheirdatanetworksupandrunninginafractionofthetimeandconsiderablylessmoneythatitwouldhavecostthemtoreinstalltheirwirelines,beitafiberorcablenetwork.TextTextWordsNotesDiscussionHomeFSOlasercomofferssubstantialadvantagesoverconventionalRFwirelesscommunicationstechnology,includinghigherdatarates,lowprobabilityofintercept,lowpowerrequirements,andmuchsmallerpackaging.FSOlasercomsystemshaveproventobeaviablealternativetoopticalfiberbasedsystemsinseveralapplications,asthetechnologycomescloserandclosertoprovidingthe5-nines(99.999%)servicethatmanycorporationsrequireoftheirdatanetworksTextWordsNotesDiscussionHome1.BriefHistoryCommunicationsusinglightisnotarecentscience.OldRomanrecordsindicatethatpolishedmetalplateswereoccasionallyusedasmirrorstoreflectsunlightforlongrangesignaling.TheU.S.militaryusedsimilarsunlight-powereddevicestosendtelegraphinformationfrommountaintoptomountaintopintheearly1800's.Blinkinglightshavebeenusedtosendmessagesfromoneshiptoanotherformanycenturies.AlexanderGrahamBellperformedanexperimentwithhis"Photophone"whereheusedsunlightreflectedoffofavibratingmirrorandaseleniumphotocelltosendtelephone-likesignalsoverarangeof600feet.TextTextWordsNotesDiscussionHomeIn1895,SirJ.C.BoseofCalcutta,Indiagavehisfirstpublicdemonstrationofradiocommunication,usingelectromagneticwavestoringabellremotelyandtoexplodesomegunpowder.AsreportedbytheDailyChronicleofEngland:"Theinventor(J.C.Bose)hastransmittedsignalstoadistanceofnearlyamileandhereinliesthefirstandobviousandexceedinglyvaluableapplicationofthisnewtheoreticalmarvel."InvitedbyLordRayleigh,Bosereportedonhismicrowave/millimeter-waveexperimentstotheRoyalInstitutionandothersocietiesinEngland.ThefirstsuccessfulwirelesssignalingexperimentbyMarconiwasnotuntil1897.Thus,theinventorofRadioCommunicationwasnotMarconibutSirJ.C.Bose.TheauthorappreciatesthisopportunitytoacknowledgeafellowscientistfromCalcuttaforhispioneeringworkleadingtothedevelopmentofmodemwirelesslasercommunications.TextTextWordsNotesDiscussionHomeDuringtheyears1930-1950,increasedcarrierfrequencieswereconsideredtoachievegreaterinformationcapacity/bandwidth:Microwavesof3-30GHz,andMillimeterWavesof30-300GHz.Followingthefirstdemonstrationofaworkinglaserin1960attheHughesResearchLaboratoriesinMalibu,California,thefirstseriousdevelopmentofFSOlasercomwascalledthe"Lightphone".NASAandtheAirForcewereespeciallyinterestedandforcedsomenewtechnologicaldevelopments.Duringtheboomperiodofopticalfiberinstallation,civilFSOlasercomtechnologylaydormant;butinmilitaryandspacelaboratoriesthedevelopmentneverreallystopped.TextTextWordsNotesDiscussionHomeDuringthelastthirtyyears,greatadvanceshavebeenmadeinelectro-opticsandopto-electronics,andincorporatedintotoday'slasercommunicationssystems,mostlyfordefenseapplications.Theaerospace/defenseactivityestablishedastrongfoundationuponwhichtoday'scommercialopticalwirelesssystemsarebased.Systemshavebeendevelopedforground-to-ground,ground-to-aircraft,ground-to-satellite,satelliteto-satellite,andevensatellite-to-submarineapplications.TextTextWordsNotesDiscussionHome2.ApplicationsSometypicalscenarioswhereFSOlasercomcanbeusedare•LAN-LANconnectionsoncampusesatFast-EthernetorGigabit-EthernetSpeeds•LAN-to-LANconnectioninacity•Speedyservicedeliveryofhigh-bandwidthaccesstofibernetworks•Temporarynetworkinstallation(forspecialeventsorotherpurposes)•Re-establishinghigh-speedconnectionquickly(DisasterRecovery/EmergencyResponse)•Communicationsbetweengroundandspacecraft,orbetweenspacecrafts,includingelementsofasatelliteconstellation•Interstellarcommunication[2]•Ship-to-shipcommunicationswithhighdataratesprovidingcompletesecurityTextTextWordsNotesDiscussionHomeFigure8-1illustratesfree-spacelasercommunicationsarchitecturesforground-toground,ground-to-space,space-to-groundandspace-to-spacelinks.Theselinksmayinclude,forexample,ametropolitannetworkusingair-to-airlasercommunicationbetweenofficebuildings,along-distanceopticallinkbetweenanopticalgroundstation(withadaptiveoptics)andabuildingorsatelliteplatform,oraninter-satellitelink.Anotherareaofinterestisthebroadbandwirelesssolutionforclosingtheso-calledlastmileconnectivitygapthroughoutmetropolitannetworks.TextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeFigure8-13.AdvantagesandChallenges•Highdatarates•Hightransmissionsecurity•NoFederalCommunicationsCommission(FCC)licenseorfrequencyallocationrequired•Lightweight,smallvolume,andlowerpowerconsumption,providingapotentialedgeoverRFcommunication•Portabilityandquickdeployment•Increasedsecurityduetothelaser'snarrowbeam-idealforthewirelesstransferoffinancial,legal,military,andothersensitiveinformationTextTextWordsNotesDiscussionHome4.LimitationsFormanycasesofpracticalinterest,theconstantpresenceofopticalturbulenceintheatmosphericchannelisthelimitingfactorinreliablefree-spacelasercommunicationperformance.Otheratmosphericeffectsthatcandegradeperformanceincludeabsorptionandscatteringofthelaserbeam.Anotherlimitationoflightbeamcommunicationsistherequirementofaclearline-of-sightpathbetweenthelasertransmitterandthereceiver,sincelightcannotpenetratetrees,hills,buildings,etc.Afurtherlimitation,sometimesoverlooked,appliestothepositionofthesunrelativetothelasertransmitterandreceiver,toavoidhavingthereceiverlookintothesun.TextTextWordsNotesDiscussionHome5.BasicsofOperationTheobjectofthecommunicationsystemistotransferdataorinformationfromatransmitteratonepointtoareceiveratanotherthroughtheinterveningatmosphericchannelwithanacceptableerrorratewhileprovidinghighreliability.Thetransmitterconsistsbasicallyofalaser,amodulator(withaselectedmodulationscheme),alaserdriver(toprovidenecessarypowersupply),andatelescope.Dependingonthemodulationformat,themodulatorconvertsbitsofinformationintoanelectricalsignalandmodulatesthelasertogenerateanopticalsignal.Thetelescopeexpandstheopticalbeamtoreducediffractiveeffectsthatspreadthebeamout.TextTextWordsNotesDiscussionHomeTheopticalsignalpropagatesthroughtheatmosphericchannelandiscollectedbythereceivertorecoverthedata.Thereceiverconsistsofatelescope,adetector,andadecodertogetthesignalout.Thetelescopecollectstheopticalsignalandcontractsthebeamsizesmallenoughtofitontothedetector.Thedetectorconvertstheopticalsignalbacktoanelectricalsignal.Generallyforadirectdetectionsystem,thereceiverconsistsofanavalanchephotodiode(APD)[3]photodetector,amatchedfilterimplementedintheformofamovingintegrator,andaclockedcomparator.Theclockedcomparatorperformsathresholdtestatbitintervalboundaries.TextTextWordsNotesDiscussionHomefraction[ˈfrækʃn]n.分数，小数；小部分，微量selenium[səˈliːniəm]n.[化学]硒chronicle[ˈkrɒnɪk(ə)l]n.编年史，大事记；记事，叙述millimeter-wave毫米波dormant[ˈdɔːmənt]adj.（似）在深睡的；（植物或芽）休眠的interstellar[ˌɪntəˈstelə(r)]adj.星际的，恒星之间的WordsWordsTextWordsNotesDiscussionHometurbulence[ˈtɜːbjələns]n.骚乱，动荡；（空气或水的）湍流，紊流photodiode[ˌfəʊtəʊˈdaɪəʊd]n.[电子]光电二极管WordsTextWordsNotesDiscussionHome[1]FreeSpaceOpticalCommunications的缩写是FSO，意为自由空间光通信，是指以光波为载体，在真空或大气中传递信息的通信技术。可分为大气光通信、卫星间光通信和星地光通信

空间激光通信系统是指以激光光波作为载波，大气作为传输介质的光通信系统。自由空间激光通信结合了光纤通信与微波通信的优点，既具有大通信容量、高速传输的优点，又不需要铺设光纤，因此各技术强国在空间激光通信领域投入大量人力物力，并取得了很大进展。TextWordsNotesDiscussionNotesHome[2]Interstellarcommunication（星际通信）指星系与星系之间进行数据的交换或数据的传达。远距离通讯是人类探索太阳系外世界的最大障碍之一。人类科技还无法做到星际通信。TextWordsNotesDiscussionHome[3]APD：雪崩光电二极管，指的是在激光通信中使用的光敏元件。在以硅或锗为材料制成的光电二极管的P-N结上加上反向偏压后，射入的光被P-N结吸收后会形成光电流。加大反向偏压会产生“雪崩”（即光电流成倍地激增）的现象，因此这种二极管被称为“雪崩光电二极管”。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.WhatarethescenariosthatFSOlasercomcanbeapplied?2.WhataretheadvantagesandChallengesofFSOlasercom?3.WhatarethelimitationsofFSOlasercom?4.Whatisthefunctionofamodulatorinatransmitter?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.WhatarethescenariosthatFSOlasercomcanbeapplied?•LAN-LANconnectionsoncampusesatFast-EthernetorGigabit-EthernetSpeeds•LAN-to-LANconnectioninacity•Speedyservicedeliveryofhigh-bandwidthaccesstofibernetworks•Temporarynetworkinstallation(forspecialeventsorotherpurposes)•Re-establishinghigh-speedconnectionquickly(DisasterRecovery/EmergencyResponse)•Communicationsbetweengroundandspacecraft,orbetweenspacecrafts,includingelementsofasatelliteconstellation•Interstellarcommunication•Ship-to-shipcommunicationswithhighdataratesprovidingcompletesecurityTextWordsNotesDiscussionHome2.WhataretheadvantagesandChallengesofFSOlasercom?•Highdatarates•Hightransmissionsecurity•NoFederalCommunicationsCommission(FCC)licenseorfrequencyallocationrequired•Lightweight,smallvolume,andlowerpowerconsumption,providingapotentialedgeoverRFcommunication•Portabilityandquickdeployment•Increasedsecurityduetothelaser'snarrowbeam-idealforthewirelesstransferoffinancial,