外文翻译--移动通信.doc -- 5 元

英文资料及中文翻译MobileCommunicationCellularTelephoneSystemsAcellulartelephonesystemprovidesawirelessconnectiontothePSTNforanyuserlocationwithintheradiorangeofthesystem.Cellularsystemsaccommodatealargenumberofusersoveralargegeographicarea,withinalimitedfrequencyspectrum.Cellularradiosystemsprovidehighqualityservicethatisoftencomparabletothatofthelandlinetelephonesystems.Highcapacityisachievedbylimitingthecoverageofeachbasestationtransmittertoasmallgeographicareacalledacellsothatthesameradiochannelsmaybereusedbyanotherbasestationlocatedsomedistanceaway.Asophisticatedswitchingtechniquecalledahandoffenablesahandoffenablesacalltoproceeduninterruptedwhentheusermovesfromonecelltoanother.Abasiccellularsystemconsistsofmobilestations,basestationsandamobileswitchingcenterMSC.TheMobileSwitchingCenterissometimescalledamobiletelephoneswitchingofficeMTSO,sinceitisresponsibleforconnectingallmobilestothePSTNinacellularsystem.Eachmobilecommunicatesviaradiowithoneofthebasestationsandmaybeheadedofftoanynumberofbasestationsthroughoutthedurationofacall.Themobilestationcontainsatransceiver,anantenna,andcontrolcircuitry,andmaybemountedinacuticleorusedasaportablehandheldunit.Thebasestationsofseveraltransmittersandreceiverswhichsimultaneouslyhandlefullduplexcommunicationsandgenerallyhavetowerswhichsupportseveraltransmittingandreceivingantennas.ThebasestationservesasabridgebetweenallmobileusersinthecellandconnectsthesimultaneousmobilecallsvistelephonelinesormicrowavelinkstotheMSC.TheMSCcoordinatesofallofthebasestationsandconnectstheentirecellularsystemtothePSTN.AtypicalMSChandles100000cellularsubscribersand5000simultaneousconversationsatatime,andaccommodatesallbillingandsystemmaintenancefunctions,aswell.Inlargecities,severalMSCsareusedbyasinglecarrier.CordlessTelephoneSystemsCordlesstelephonesystemsarefullduplexcommunicationsystemsthatuseradiotoconnectaportablehandsettoadedicatedbasestation,whichisthenconnectedtoadedicatedtelephonelinewithaspecifictelephonenumberonthepubicswitchedtelephonenetworkPSTN.Infirstgenerationcordlesstelephonesystemsmanufacturedinthe1980s,theportableunitcommunicatesonlytothededicatedbaseunitandonlyoverdistancesofafewtensofmeters.EarlycordlesstelephonesoperatesolelyasextensiontelephonestoatransceiverconnectedtoasubscriberlineonthePSTNandareprimarilyforinhomeuse.SecondgenerationscordlesstelephoneshaverecentlybeenintroducedwhichallowsubscriberstousetheirhandsetsatmanyoutdoorlocationswithinurbancenterssuchasLondonorHongKong.Moderncordlesstelephonesaresometimescombinedwithpagingreceiverssothatasubscribermayfirstbepagedandthenrespondtothepageusingthecordlesstelephone.Cordlesstelephonesystemsprovidetheuserwithlimitedrangeandmobility,asitisusuallynotpossibletomaintainacalliftheusertravelsoutsidetherangeofthebasestation.Typicalsecondgenerationbasestationsprovidecoveragerangesuptoafewhundredmeters.BasicKnowledgeofCommunicationCommunicationSystemAgeneralizedcommunicationsystemhasthefollowingcomponentsaInformationSource.Thisproducesamessagewhichmaybewrittenorspokenwords,orsomeformofdata.bTransmitter.Thetransmitterconvertsthemessageintoasignal,theformofwhichissuitablefortransmissionoverthecommunicationchannel.cCommunicationChannel.Thecommunicationchannelisthemediumusedtransmitthesignal,fromthetransmittertothereceiver.Thechannelmaybearadiolinkoradirectwireconnection.dReceiver.Thereceivercanbethoughtofastheinverseofthetransmitter.Itchangesthereceivedsignalbackintoamessageandpassesthemessageontoitsdestinationwhichmaybealoudspeaker,telepromptersorcomputerdatabank.Oncethisnewbaseboardsignal,agroupof4channels,hasbeenformeditismovedaroundthetrunknetworkasasingleunit.Ahierarchycanbesetupwithseveralchannelsformingagroup,severalgroupsasupergroupandseveralsupergroupeitheramastergrouporhypergroup.Groupsorsupergroupsaremovedaroundassingleunitsbythecommunicationsequipmentanditisnotnecessaryfortheradiostoknowhowmanychannelsareinvolved.Aradiocanhandleasupergroupprovidedsufficientbandwidthisavailable.Thesizeofthegroupsisacompromiseastreatingeachchannelindividuallyinvolvesfarmoreequipmentbecauseseparatefilters,modulatorsandoscillatorsarerequiredforeverychannelratherthanforeachgroup.Howeverthefailureofonemodulewillloseallofthechannelsassociatedwithagroup.TimeDivisionMultiplexingItispossible,withpulsemodulationsystems,tousethebetweensamplestotransmitsignalsfromothercircuits.ThetechniqueisknownastimedivisionmultiplexingTDM.Todothisitisnecessarytoemploysynchronizedswitchesateachendofthecommunicationlinktoenablesamplestobetransmittedinturn,fromeachofseveralcircuits.Thusseveralsubscribersappeartousethelinksimultaneously.Althougheachuseronlyhasperiodicshorttimeslots,theoriginalanalogsignalsbetweensamplescanbereconstitutedatthereceiver.PulseCodeModulationInanalogmodulation,thesignalwasusedtomodulatetheamplitudeorfrequencyofacarrier,directly.Howeverindigitalmodulationastreamofpulses,representingtheoriginal,iscreated.Thisstreamisthenusedtomodulateacarrieroralternativelyistransmitteddirectlyoveracable.PulseCodeModulationPCMisoneofthetwotechniquescommonlyused.Allpulsesystemsdependontheanalogwaveformbeingsampledatregularintervals.Thesignalcreatedbysamplingouranalogspeechinputisknownaspulseamplitudemodulations.ItisnotveryusefulinpracticebutisusedasanintermediatestagetowardsformingaPCMsignal.Itwillbeseenlaterthatmostoftheadvantagesofdigitalmodulationcomefromthetransmittedpulseshavingtwolevelsonly,thisbeingknownasabinarysystem.InPCMtheheightofeachsampleisconvertedintoabinarynumber.TherearethreestepsintheprocessofPCMsampling,quarteringandcoding.OpticalFiberCommunicationsCommunicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistanceacommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiorequitesaswellasmicrowaveandmillimeterwavefrequencies.TypicalopticalfibercommunicationssystemisshowninFig.13.Inthiscasetheinformationsourceprovidesanelectricalsignaltoatransmittercomprisinganelectricalstagewhichdrivesanopticalsourcetogivemodulationofthelightwavecarrier.TheopticalsourcewhichprovidestheelectricalopticalconversionmaybeeitherasemiconductorlaserorlightemittingdiodeLED.Thetransmissionmediumconsistsofanopticalfibercableandthereceiverconsistsofanopticaldetectorwhichdrivesafurtherelectricalstageandhenceprovidesdemodulationoftheopticalcarrier.PhotodiodesPN,PINoravalancheand,insomeinstances,phototransistorandphotoconductorsareutilizedforthedetectionoftheopticalsignalandtheopticalelectricalconversion.ThusthereisarequirementforelectricalinterfacingateitherendoftheopticallinkandatpresentthesignalprocessingisusuallyTheopticalcarriermaybemodulatedbyusingeitherananalogordigitalinformationsignal..Analogmodulationinvolvesthevariationofthelightemittedfromtheopticalsourceinacontinuousmanner.Withdigitalmodulation,however,discretechangesinthelengthintensityareobtainedi.e.onoffpulses.Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.Also,thelinearityneededforanalogmodulationismotalwaysprovidedbysemiconductoropticalsource,especiallyathighmodulationfrequencies.Forthesereasons,analogopticalfibercommunicationslinkaregenerallylimitedtoshorterdistancesandlowerbandwidthsthandigitallinks.Initially,theinputdigitalsignalfromtheinformationsourceissuitablyencodedforopticaltransmission.Thelaserdrivecircuitdirectlymodulatestheintensityofthesemiconductorlastewiththeencodeddigitalsignal.Henceadigitalopticalsignalislaunchedintotheopticalfibercable.TheavalanchephotodiodedetectorAPDisfollowedbyafrontendamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.Finally,thesignalobtainedisdecodedtogivetheoriginaldigitalinformation.BroadbandCommunicationAscanbeinferredfromtheexamplesofvideophoneandHDTV,theevolutionof