外文翻译--移动通信.doc

英文资料及中文翻译MobileCommunicationCellularTelephoneSystemsAcellulartelephonesystemprovidesawirelessconnectiontothePSTNforanyuserlocationwithintheradiorangeofthesystem.Cellularsystemsaccommodatealargenumberofusersoveralargegeographicarea,withinalimitedfrequencyspectrum.Cellularradiosystemsprovidehighqualityservicethatisoftencomparabletothatofthelandlinetelephonesystems.Highcapacityisachievedbylimitingthecoverageofeachbasestationtransmittertoasmallgeographicareacalledacellsothatthesameradiochannelsmaybereusedbyanotherbasestationlocatedsomedistanceaway.Asophisticatedswitchingtechniquecalledahandoffenablesahandoffenablesacalltoproceeduninterruptedwhentheusermovesfromonecelltoanother.Abasiccellularsystemconsistsofmobilestations,basestationsandamobileswitchingcenter(MSC).TheMobileSwitchingCenterissometimescalledamobiletelephoneswitchingoffice(MTSO),sinceitisresponsibleforconnectingallmobilestothePSTNinacellularsystem.Eachmobilecommunicatesviaradiowithoneofthebasestationsandmaybeheaded-offtoanynumberofbasestationsthroughoutthedurationofacall.Themobilestationcontainsatransceiver,anantenna,andcontrolcircuitry,andmaybemountedinacuticleorusedasaportablehand-heldunit.Thebasestationsofseveraltransmittersandreceiverswhichsimultaneouslyhandlefullduplexcommunicationsandgenerallyhavetowerswhichsupportseveraltransmittingandreceivingantennas.ThebasestationservesasabridgebetweenallmobileusersinthecellandconnectsthesimultaneousmobilecallsvistelephonelinesormicrowavelinkstotheMSC.TheMSCcoordinatesofallofthebasestationsandconnectstheentirecellularsystemtothePSTN.AtypicalMSChandles100000cellularsubscribersand5000simultaneousconversationsatatime,andaccommodatesallbillingandsystemmaintenancefunctions,aswell.Inlargecities,severalMSCsareusedbyasinglecarrier.CordlessTelephoneSystemsCordlesstelephonesystemsarefullduplexcommunicationsystemsthatuseradiotoconnectaportablehandsettoadedicatedbasestation,whichisthenconnectedtoadedicatedtelephonelinewithaspecifictelephonenumberonthepubicswitchedtelephonenetwork(PSTN).Infirstgenerationcordlesstelephonesystems(manufacturedinthe1980s),theportableunitcommunicatesonlytothededicatedbaseunitandonlyoverdistancesofafewtensofmeters.EarlycordlesstelephonesoperatesolelyasextensiontelephonestoatransceiverconnectedtoasubscriberlineonthePSTNandareprimarilyforin-homeuse.SecondgenerationscordlesstelephoneshaverecentlybeenintroducedwhichallowsubscriberstousetheirhandsetsatmanyoutdoorlocationswithinurbancenterssuchasLondonorHongKong.Moderncordlesstelephonesaresometimescombinedwithpagingreceiverssothatasubscribermayfirstbepagedandthenrespondtothepageusingthecordlesstelephone.Cordlesstelephonesystemsprovidetheuserwithlimitedrangeandmobility,asitisusuallynotpossibletomaintainacalliftheusertravelsoutsidetherangeofthebasestation.Typicalsecondgenerationbasestationsprovidecoveragerangesuptoafewhundredmeters.BasicKnowledgeofCommunicationCommunicationSystemAgeneralizedcommunicationsystemhasthefollowingcomponents:(a)InformationSource.Thisproducesamessagewhichmaybewrittenorspokenwords,orsomeformofdata.(b)Transmitter.Thetransmitterconvertsthemessageintoasignal,theformofwhichissuitablefortransmissionoverthecommunicationchannel.(c)CommunicationChannel.Thecommunicationchannelisthemediumusedtransmitthesignal,fromthetransmittertothereceiver.Thechannelmaybearadiolinkoradirectwireconnection.(d)Receiver.Thereceivercanbethoughtofastheinverseofthetransmitter.Itchangesthereceivedsignalbackintoamessageandpassesthemessageontoitsdestinationwhichmaybealoudspeaker,telepromptersorcomputerdatabank.Oncethisnewbaseboardsignal,a“group”of4channels,hasbeenformeditismovedaroundthetrunknetworkasasingleunit.Ahierarchycanbesetupwithseveralchannelsforminga“group”,severalgroupsa“supergroup”andseveral“supergroup”eithera“mastergroup”or“hypergroup”.Groupsorsupergroupsaremovedaroundassingleunitsbythecommunicationsequipmentanditisnotnecessaryfortheradiostoknowhowmanychannelsareinvolved.Aradiocanhandleasupergroupprovidedsufficientbandwidthisavailable.Thesizeofthegroupsisacompromiseastreatingeachchannelindividuallyinvolvesfarmoreequipmentbecauseseparatefilters,modulatorsandoscillatorsarerequiredforeverychannelratherthanforeachgroup.Howeverthefailureofonemodulewillloseallofthechannelsassociatedwithagroup.TimeDivisionMultiplexingItispossible,withpulsemodulationsystems,tousethebetweensamplestotransmitsignalsfromothercircuits.Thetechniqueisknownastimedivisionmultiplexing(TDM).Todothisitisnecessarytoemploysynchronizedswitchesateachendofthecommunicationlinktoenablesamplestobetransmittedinturn,fromeachofseveralcircuits.Thusseveralsubscribersappeartousethelinksimultaneously.Althougheachuseronlyhasperiodicshorttimeslots,theoriginalanalogsignalsbetweensamplescanbereconstitutedatthereceiver.PulseCodeModulationInanalogmodulation,thesignalwasusedtomodulatetheamplitudeorfrequencyofacarrier,directly.Howeverindigitalmodulationastreamofpulses,representingtheoriginal,iscreated.Thisstreamisthenusedtomodulateacarrieroralternativelyistransmitteddirectlyoveracable.PulseCodeModulation(PCM)isoneofthetwotechniquescommonlyused.Allpulsesystemsdependontheanalogwaveformbeingsampledatregularintervals.Thesignalcreatedbysamplingouranalogspeechinputisknownaspulseamplitudemodulations.ItisnotveryusefulinpracticebutisusedasanintermediatestagetowardsformingaPCMsignal.Itwillbeseenlaterthatmostoftheadvantagesofdigitalmodulationcomefromthetransmittedpulseshavingtwolevelsonly,thisbeingknownasabinarysystem.InPCMtheheightofeachsampleisconvertedintoabinarynumber.TherearethreestepsintheprocessofPCM:sampling,quarteringandcoding.OpticalFiberCommunicationsCommunicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistanceacommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiorequitesaswellasmicrowaveandmillimeterwavefrequencies.TypicalopticalfibercommunicationssystemisshowninFig.1-3.Inthiscasetheinformationsourceprovidesanelectricalsignaltoatransmittercomprisinganelectricalstagewhichdrivesanopticalsourcetogivemodulationofthelightwavecarrier.Theopticalsourcewhichprovidestheelectrical-opticalconversionmaybeeitherasemiconductorlaserorlightemittingdiode(LED).Thetransmissionmediumconsistsofanopticalfibercableandthereceiverconsistsofanopticaldetectorwhichdrivesafurtherelectricalstageandhenceprovidesdemodulationoftheopticalcarrier.Photodiodes(P-N,P-I-Noravalanche)and,insomeinstances,phototransistorandphotoconductorsareutilizedforthedetectionoftheopticalsignalandtheoptical-electricalconversion.ThusthereisarequirementforelectricalinterfacingateitherendoftheopticallinkandatpresentthesignalprocessingisusuallyTheopticalcarriermaybemodulatedbyusingeitherananalogordigitalinformationsignal.Analogmodulationinvolvesthevariationofthelightemittedfromtheopticalsourceinacontinuousmanner.Withdigitalmodulation,however,discretechangesinthelengthintensityareobtained(i.e.on-offpulses).Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.Also,thelinearityneededforanalogmodulationismotalwaysprovidedbysemiconductoropticalsource,especiallyathighmodulationfrequencies.Forthesereasons,analogopticalfibercommunicationslinkaregenerallylimitedtoshorterdistancesandlowerbandwidthsthandigitallinks.Initially,theinputdigitalsignalfromtheinformationsourceissuitablyencodedforopticaltransmission.Thelaserdrivecircuitdirectlymodulatestheintensityofthesemiconductorlastewiththeencodeddigitalsignal.Henceadigitalopticalsignalislaunchedintotheopticalfibercable.Theavalanchephotodiodedetector(APD)isfollowedbyafront-endamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.Finally,thesignalobtainedisdecodedtogivetheoriginaldigitalinformation.BroadbandCommunicationAscanbeinferredfromtheexamplesofvideophoneandHDTV,theevolutionof