《通信工程专业英语（第二版）》课件Unit 8

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,legal,military,andothersensitiveinformationTextWordsNotesDiscussionHome3.WhatarethelimitationsofFSOlasercom?Formanycasesofpracticalinterest,theconstantpresenceofopticalturbulenceintheatmosphericchannelisthelimitingfactorinreliablefree-spacelasercommunicationperformance.Otheratmosphericeffectsthatcandegradeperformanceincludeabsorptionandscatteringofthelaserbeam.Anotherlimitationoflightbeamcommunicationsistherequirementofaclearline-of-sightpathbetweenthelasertransmitterandthereceiver,sincelightcannotpenetratetrees,hills,buildings,etc.Afurtherlimitation,sometimesoverlooked,appliestothepositionofthesunrelativetothelasertransmitterandreceiver,toavoidhavingthereceiverlookintothesun.TextWordsNotesDiscussionHome4.Whatisthefunctionofamodulatorinatransmitter?Thetransmitterconsistsbasicallyofalaser,amodulator(withaselectedmodulationscheme),alaserdriver(toprovidenecessarypowersupply),andatelescope.Dependingonthemodulationformat,themodulatorconvertsbitsofinformationintoanelectricalsignalandmodulatesthelasertogenerateanopticalsignal.TextWordsNotesDiscussionHomeThankyou!Unit8TextBWhatisVisibleLightCommunication(VLC)?WhatisVisibleLightCommunication(VLC)?TextTextWordsNotesDiscussionHome1.TheBackgroundofVLCWiththegrowthoftheInternetofThingsandtherapiddevelopmentofartificialintelligence,humanityisheadedtowardsanintelligenteracharacterizedby“all-pervasiveperception,all-pervasiveconnectedness,andall-pervasiveintelligence.”Thelargevolumeofdatageneratedintheintelligentageplacesincreasingdemandsoncommunicationsystems'highspeedandlowlatency,puttingtraditionalcommunicationaccessnetworktechnologytothetest.Visiblelightcommunication(VLC)[1]isanovelmethodofcommunicationthatwasdesignatedoneofTimeMagazine'stop50scientificandtechnologicalinventionsintheworldin2011.2.OverviewofVLCIntermsofdependability,powerconsumption,andluminousenergyefficiency,LEDs[2]haveexceededincandescentbulbs,andtheimportanceoflightinghasexpandeddramatically.Theideaofemployinglightasatransmissionchannel,ontheotherhand,isnotnew.Itdatesbackto1792inFrancewhensignallightsmountedontowersallowedforlight-basedcommunicationbetweencities.TextTextWordsNotesDiscussionHomeThe“Heliograph,”whichemployedmirrorstoreflectsunlightbyinterruptinglightorspinningthemirror,wasinventedbytheUSmilitaryattheturnofthenineteenthcentury.AlexanderGrahamBell(thetelephone'sinventor)inventedthephotoelectricphonein1880,whichsendsvoicesignalsthroughabeamofvisiblelight.TextWordsNotesDiscussionHomeThevisiblelight430-790THzofthe380-750nmrangeisusedinmodernVLCsystems.Communicationcanbeformedwithoutrelyingonradiofrequencytechnologybyalteringthelightofexistinglamps.TextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeThehugeexpansioninmobiledatausingradiofrequencyinthemodernworldhasresultedinanextremelycongestedspectrum,whichcancauseinterference.VLCistypicallyutilizedinside,wherethemostsignificantsourceofinterference—high-intensitylightsourcessuchasthesun—doesn'texist.Visiblelightisalsoanunlicensedtransmissionfrequencyband,whichmeansthere'snoneedtoapplyforalicenseandit'snotaffectedbyneighboringsystemsusingthesamefrequencyrange.Fromasafetystandpoint,radiofrequencywavescaneasilypenetratewalls,allowingthesignaltobeintercepted,andlong-termexposuretoradiofrequencywavesathighpowerlevelsmaybedetrimentaltohumanhealth.NoadditionalpoweramplifiersorsignalchainsarerequiredinVLCsystemswithlowtransmissionspeeds,suchasindoorpositioning,loweringsystempowerconsumption,anddesigncomplexity.TextTextWordsNotesDiscussionHomeLight-basedwirelesscommunications,vehicle-to-vehicle(V2V)communications,andunderwaterapplicationsareallpossibleapplicationsforVLC.VLCisespeciallyusefulinsomeprofessionalapplicationswhereregularradiofrequencycommunicationsareharmful,suchasminingoroilandgasplatforms.Italsoprovidesforinterference-freecommunicationinsettingslikehospitalsandairlineswheresensitiveequipmentispresent.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHome3.WhatisVLC?(1)DefinitionofVLCSo,whatexactlydoesvisiblelightcommunicationentail?ItistoallowtheLEDtosendandreceivesignalswhilemaintainingtheLEDilluminationfunction.Toputitanotherway,that'sthesameasputtingaWIFIrouterintoanLEDlight.Doesn'tthisseemlikesomethingoutofasciencefictionnovel?Thistypeofintegratedthinkingcanbeseeninmanyareas,suchaswhenacameraisstuffedintoaphoneoracomputerisstuffedintoaniPad.It'sjustthatitcan'tbenamedWIFIatthemoment,butLIFI[3],becausethewirelessWhasbeenreplacedwithalightL,andthefrequencyoftheelectromagneticwavehasbeenalteredfromGHz(1000000000)toTHz(1000000000000).TheFigure8-2depictsahypotheticalfuturescenario.Peoplearesmilingastheyuselaptops,andabeamoflightfromabovehitsthelaptops.Thislampisremarkableinthatitnotonlybrightensthespacebutalsoservesasarouterforthenotebook'snetwork.TextTextWordsNotesDiscussionHomeTextWordsNotesDiscussionHomeFigure8-2(2)AdvantagesofVLC1)Visiblelightoffersalotofspectrumresourcescomparedtotraditionalwirelesscommunication,andthespectrumbandwidthisaround400THz,whichisablankregiontoresearch.TextTextWordsNotesDiscussionHome2)Lighting,communication,andcontrolplacementareallfunctionsofvisiblelightcommunication.It'ssimpletointegratewithcurrentbasiclightingsystemsandadherestothecountry'senergy-savingandemission-reductiongoals.TextTextWordsNotesDiscussionHome3)Visiblelightcommunicationhastheadvantagesofbeingfreeofradiointerference,electromagneticradiation,andhighconfidentialityinelectromagneticsensitiveareas(nuclearpowerplants,mines,gasstations,etc.)andspecialplaceswithstrongelectromagneticenvironments(suchassubstations,modernmilitarybattlefields,etc.).TextTextWordsNotesDiscussionHome4)Themostsignificantbenefitofvisiblelightcommunicationisitsrapidity.Inthefutureintelligentera,itwillbeanessentialwirelesscommunicationtechniqueforB5G/6Gultra-high-speedubiquitousopticalnetworking.TextTextWordsNotesDiscussionHome(3)DisadvantageofVLCThedesignismorechallenging,andthestabilityandpracticabilityarepoor.TextTextWordsNotesDiscussionHome4.SystemStructureofVLCThevisiblelightcommunicationsystem'sbasicstructure,whichincludesthevisiblelightsignalbroadcastingend,thevisiblelightsignaltransmissionchannel,andthevisiblelightsignalreceivingend,isdepictedinFigure8-3.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeFigure8-3Theoriginalbinarysignalisfirsttransformedthroughencoding,modulation,andpre-equalization,andtheresultingpre-processedsignalisthenconvertedtodigitalandusedtocontrolthelightintensityofalightemitter(suchasanLED),resultingintheconversionofelectricalsignalstoopticalsignals.Additionally,afterthelighttransmitter,anopticallensandacondensercupcanbeaddedtoimprovethesignalstrengthatthereceivingend,henceexpandingthetransmissiondistance.TextTextWordsNotesDiscussionHomeThemodulatedvisiblelightsignaltravelsthroughopenspacechannelsliketheatmosphereortheoceantoreachthevisiblelightsignalreceiver.PhotodetectorssuchasthephotodiodePINandtheavalanchephotodiodeAPDarecommonlyusedtodetectopticalimpulsesandconvertthemtoelectricalsignals.Theoriginaltransmissionsignalisrestoredafterthereceivedelectricalsignalhasundergonedigitalsignalprocessingsuchaspost-equalization,demodulation,anddecoding.TextTextWordsNotesDiscussionHome5.VLC-BasedPositioningManyapplicationscanbenefitfromindoorplacement.Itcanbeusedtosteerautomatedguidedvehicles(AGV)throughfactoriesandwarehousesortoallowhumanstonavigatevastpublicstructureslikeairports,hospitals,andstadiumsinindustrialapplications.TextTextWordsNotesDiscussionHomeAlthoughtheGlobalLocationSystem(GPS)[4]isextremelyaccurateforoutdoorplacement,itisineffectiveforpreciseindoorpositioning.Manystructuresmakeitharderforasignaltopenetrate.Evenifitgetsthrough,thesignal'smultipathpropagationasitbouncesoffwallsandobjectswillseverelylimititsperformance.TextTextWordsNotesDiscussionHomeIndoorlocationaccuracyoftensofcentimeterscanbeachievedwithVLC,easilyoutperformingRF-basedmethods.Thelight-basedpositioningsystemestablishesapositioningsystemindoorsbyutilizingthecurrentlightingsystem.EachLEDballasthasauniqueidentifierthatisembeddedintothesignalthatdrivesthelamp,allowingthelighttobevariedwhiletheidentifieristransmittedcontinually.TextTextWordsNotesDiscussionHomeThesequickpulsesareundetectabletohumansduetothepersistenceofvision,buttheymaybecapturedanddetectedbyequipmentlikesmartphones.Whenthemobilephoneiswithinrangeofseveral(3ormore)lamps,eachofwhichhasitsownuniquecode,asimpletriangulationmethodmaybeusedtocomputethedistancebetweeneachlampandthentheposition.TextTextWordsNotesDiscussionHome6.HowtoUseVLC?Thefirstisthat,aspreviouslysaid,itcanenablenetworkroutinginspecificconfidentiallocations.Second,becausewaterabsorbsverylittleblue-violetlight(thusthebluecolorofseawater),bluelightcanbeemployedaslightwavesforunderwatercommunication.TextTextWordsNotesDiscussionHomeInaddition,visiblelightcommunicationcan

providenetworkandlocationservicesinsomebigindoororenclosedareas,suchasshoppingmalls,hospitals,orhugeenterprises,aswellasundergroundmines.TheGPSsignalwillbequiteweakintheselocations,andthewirelessnetwork'squalitywilldeclineaswell;nevertheless,theselocationsfrequentlyneedtoturnonthelightsforlengthyperiodsoftime,sowhynotusetheselights?TheenergyconsumptionoftheLEDwillonlyincreaseby30%afteritischangedandthecommunicationfeatureisintroduced.TextTextWordsNotesDiscussionHomeBecausethecar'sheadlightsandtaillightsare“natural”visiblelightcommunicationdevices,visiblelightcommunicationhasapplicationprospectsonthe

InternetofVehicles.TextTextWordsNotesDiscussionHomeSo,howdoesitfunction?TheupperandbelowmodulesareLEDlampsandlaptops,respectively,asindicatedinFigure8-4,andtheirinternalstructuresareidentical.AfterthesignalisOFDMcodedandmodulated,theLEDlampiscontrolledbytheLEDcurrentdriver,andthenthevisiblelightLEDsendsthevisiblelightcarryingthecodedinformation,whichiscommunicatedbyanotherlamp.Itisreceivedbythephotodiode(photodiodeasaphotodetector).ThesignalisamplifiedbyanamplifierbeforebeingsentintotheOFDMreceiver,whichdecodesitandsendsitoverEthernet.Inreversetransmission,thesameistrue.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeFigure8-47.ConcludingRemarksSmart-linkedlighting,whichispoweredbylow-energyLEDs,isasignificantgrowthsector.Althoughlongservicelifeandlowpowerconsumptionarethetwomainreasonsforthisachievement,currentsmartlighting'sversatilityisallowingforthedevelopmentofnewapplications.TextTextWordsNotesDiscussionHomeAlthoughdatatransmissionvialightisnotanewconcept,ithasnowarrivedandisgainingpopularitybecauseitisenergy-efficient,stable,dependable,andsafe,anditavoidsmanyoftheproblemsthatRFdoes.Asaresult,moreadvancedapplications,suchasindoorpositioningsystems,becomepossible.Becausetheyarebasedonlight,theycanbeusedinplaceswhereradiofrequencyisnotpermitted,suchashospitalsandmines.TextTextWordsNotesDiscussionHomeluminous[ˈluːmɪnəs]adj.夜光的，发亮的；鲜亮的，鲜艳的；照亮的，明亮的；睿智的，美丽的；光亮的incandescent[ˌɪnkænˈdes(ə)nt]adj.辉耀的；炽热的；发白热光的entail[ɪnˈteɪl]v.使必要，需要stuff[stʌf]v.塞进，填进；填满，装满confidentiality[ˌkɒnfɪˌdenʃiˈæləti]n.保密性，机密性WordsWordsTextWordsNotesDiscussionHomepost-equalization[pəʊstˌiːkwəlaɪˈzeɪʃn]复元multipath[ˌmʌltɪˈpɑːθ]adj.多通道的，多途径的WordsTextWordsNotesDiscussionHome[1]VLC是VisibleLightCommunications的缩写，指可见光通信，是一种无线通讯技术，是利用荧光灯或发光二极管（LED）等物体发出的明暗闪烁信号来实现信息传输的通信技术，可见光的频率介于400THz（波长780nm）至800THz（波长375nm）之间。使用普通的日光灯时，它的传输能力为10kbit/s。使用LED灯时，则可以到达500Mbit/s。传输距离则可以到达1-2公里。它最大的特色是可以结合固态照明技术。可见光通讯可以应用于光照上网、可见光点播电视业务、可见光新型无线广播、可见光精确定位等领域。TextWordsNotesDiscussionNotesHome[2]LED：发光二极管，是一种常用的发光器件，通过电子与空穴复合释放能量发光，它在照明领域应用广泛。发光二极管可高效地将电能转化为光能，在现代社会具有广泛的用途，如照明、平板显示、医疗器件等。TextWordsNotesDiscussionHome[3]LIFI（LightFidelity）光保真技术，又称为可见光无线通信，该技术是一种利用可见光波谱进行数据传输的全新无线传输技术。LIFI通过在LED上植入一个微小的芯片，利用电信号控制发光二极管（LED）发出肉眼看不到的高速闪烁信号来传输信息，这种技术做成的系统能够覆盖室内灯光达到的范围，电脑不需要电线连接只要在室内开启电灯，无需WIFI也可接入互联网。TextWordsNotesDiscussionHome[4]GPS是GlobalPositioningSystem的缩写，意为全球定位系统，是美国研制发射的一种以人造地球卫星为基础的高精度无线电导航的定位系统。GPS在全球任何地方以及近地空间都能够提供准确的地理位置、车行速度及精确的时间信息。GPS自问世以来，就以其高精度、全天候、全球覆盖、方便灵活吸引了众多用户。GPS不仅是汽车的守护神，同时也是物流行业管理的智多星。随着物流业的快速发展，GPS有着举足轻重的作用，成为继汽车市场后的第二大主要消费群体。GPS是美国从20世纪70年代开始研制，历时20年，耗资200亿美元，于1994年全面建成，具有在海、陆、空进行全方位实时三维导航与定位功能的新一代卫星导航与定位系统。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.

WherecanVLCbeapplied?

2.

WhatisthedefinitionofVLC?3.WhataretheadvantagesanddisadvantagesofVLC?4.

HowtoUseVLC?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.WherecanVLCbeapplied?Light-basedwirelesscommunications,vehicle-to-vehicle(V2V)communications,andunderwaterapplicationsareallpossibleapplicationsforVLC.VLCisespeciallyusefulinsomeprofessionala