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外文翻译--数字通信系统.doc

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外文翻译--数字通信系统.doc

英文资料及中文翻译DigitalCommunicationSystemFormodernpeople,ifyoudonotknowDigitalCommunicationorDigitalSignal,itisseemtobeoutdated,evenwouldbelaughedasaignorantmen.wellthen,whatistheDigitalCommunicationandwhatistheDigitalSignalThistextistodoangeneralizeinthewayofbriefandplain.1、WhyDigitalWhyarecommunicationsystem,militaryandcommercialalike,goingdigitalTheprimaryadvantageistheeasewithwhichdigitalsignals,comparedwithanalogsignals,areregenerated.Figure1illustratesanidealbinarydigitalpulsepropagatingalongatransmissionline.Theshapeofthewaveformisaffectedbytwobasicmechanisms1asalltransmissionlinesandcircuitshavesomenonidealfrequencetransferfunction,thereisadistortingeffectontheidealandpulseand2unwantedelectricalnoiseorotherinterferencefurtherdistortsthepulsewaveform.Bothofthesemechanismscausethepulseshapetodegradeasafunctionoflinelenge,asshowninFigure1.Duringthetimethatthetransmittedpulsecanstillbereliablyidentifiedbeforeitisdegradedtoanambiguousstate,thepulseisamplifiedbyadigitalamplifierthatrecoversitsoriginalidealshape.Thepulseisthusrebororregenerated.Circuitsthatperformthisfunctionatregularintervalsalongatransmissionsystemarecalledregenerativerepeaters.Distance1Distance2Dance3stance4Distance5OriginalSomesignalDegradedSignalisbadlyAmplificationdstortiuondstortiuondstortiuondegradedregeneratepulsePropagationdistanceFigure1PulsedegradationandregenerationDigitalcircuitarelesssubjecttodistortionandinterferncethananalogcircuit.Becansebinarycircuitsopoerateinoneoftwostatesfullyonfullyoftobemeaningful,adisturbancemustbelargeenoughtochangethecircuitooperatingpointfromonestatetotheother.Suchtwostateoperationfacilitatessignalregenerationandthuspreventsnoiseandotherdisturbancesfromaccumulatingintransmission.Analogsignals,however,arenottwostatesignalstheycantakeaninfinitevarietyofshapes.Withanalogcircuits,evenasmalldisturbancecanrenderthereproducedwaveformunacceptablydistorted.Oncetheanalogsignalisdistorted,thedistortioncannotberemovedbyamplification.Becauseaccumulatednoiseisirrevocablyboundtoanalogsignals,theycannotbeperfectlyregenerated.Withdigitaltechniques,extremelylowerrorratesprodureshighsignalfidelityarepossiblethrougherrordetectionandcorrectionbutsimilarproceduresarenotavailablewithanalog.Thereareotherimportantadvantagestodigitalcommunications.Digitalcircuitaremorereliableandcanbeproducedatalowercostthananalogcircuit.Also,digitalhardwavelendsitselftomoreflexbleimplementationthananaloghardwavee.g.,microprocessor,digitalswitching,andlargescaleintegratedLSIcircuit.ThecombingofdigitalsignalsusingtimedivisionmultiplexingTDMissimplerthanthecombingofanalogsignalsusingfrequcencydivisionmultiplexingFDM.Differenttypesofdigitalsignalsdata,telegraph,telephone,televisioncanbetreatedasidenticalsignalsintransmissionandswitchingabitisabit.Also,forconvenientswitching,digitalmessagescanbehandledinautonomousgroupscalledpackets.Digitaltechniqueslendthemselvesnaturallytosignalprocessingfunctionsthatprojectagainstinterfereneandjamming,orthatprovideencryptionandprivacy.Alsomuchdatacommunicationisfromcomputertocomputer,orfromdigitalinstrumentsorterminaltocomputer.Suchdigitalterminationsarenaturallybestservedbydigitalcommunicationlinks.WhatarethecostsassociatedwiththebeneficialattributesofdigitalcimmunicationsystemDigitalsystemtendtobeverysignalprocessingintensivecomparedwithanalog.Also,digitalsystemneedtoallocateasignificantshareoftheirresourcestothetaskofsynchroniztionatvariouslevels.Withanalogsystem,ontheotherhand,synchroniztionoftenisaccomplishedmoreeasily.Onedisadvantageofadigitalcommunicationsystemisnongracefuldegradation.Whenthesignaltonoiseratiodropsbelowacertainthreshold,thequalityofservicecanchangesuddenlyfromverypoor.Incintrast,mostcommunicationaystemdegrademoregracefully.2、TypicalBlookDiagramandTransformationsThefunctionblockdiagramshowninFigure2illustratesthesignalflowandthesignalprocessingstepsthroughatypicaldigitalcommunicationsystemDCS.Thisfigurecanserveasakindofroadmap,guidingthereaderthroughthechapter.Theupperblocksformat,sourceencode,encrypt,channelencode,multiplex,pulsemodulate,bandpassmodulate,frequencyspread,andmultipleaccessdenotesignaltransformationsfromthesourcetothetransmitter.Thelowerblockdenotesignaltransformationsfromthereceivertothesink,essentiallyreversingthesignalprocessingstepsperformedbytheupperblocks.Themodulateanddemodulate/detectblockstogetherarecalledamodern.ThetermmodernoftenencompassesseveralofthesignalprocessingstepsshowninFigure2Whenthisisthecase,themoderncanbethoughofasthebrainofthesystem.Thetransmitterandreceivercanbethoughofasthemusclesofthesystem.Forwirelessapplications,thetransmitterandconsistsofafrequencyupconversionstagetoaradiofrequencyRF,ahighpoweramplifierLNA.Frequencydownconversionisperformedintheforntendofthereceiverand/orthedemodulator.Figure2illustratesakindofreciprocitybetweentheblockintheuppertransmitterpartofthefigureandthoseinthelowerreceiverpart.Thesignalprocessingstepsthattakeplaceinthetransmitterare,forthemostpart,reversedinthereveiver.InFigure2,theinputinformationsourceisconvertedtobinarydigitsbisthebitsarethengroupedtofromdigitalmessageormessage.Eachsuchsymbolmi,wherei1,2,3,,McanberegardedasamemerberoffinitealphabetsetcontainingMmembers.Thus,forM2,themessagesymbolmiisbinarymeaningthatitconstitutesjustasignalbit.EventhoughbinarysymbolfallwithinthegeneraldefinitionofMary,neverthelessthenameMaryisusuallyappliedtothosecaseswhereM2hence,suchsymbolareeachmadeupofasequenceoftwoormorebits.ComparesuchafinitealphabetinaDCSwithananalogsystem,wherethemessagewaveformistypicallyamemberofaninfinitesetofpossiblewaveform.Forsystemthatusechannelcodingerrorcorrectioncoding,asequenceofmessagesymbolisdenotedui.Becauseamessagesymbolorachannelsymbolcanconsistofasinglebitoragroupofbits,asequenceofsuchsymbolisalsodescribedasabitstream,asshowninFigure2.ConsiderthekeysignalprocessingblockshowninFigure2,Onlyformatting,modulation,demodulation/detection,andsynchronizationareessentialforaDCS.Formatting,transformthesourceinformationintobits,thusassuringcompatibilitybetweentheinformationandthesignalprocessingwithintheDCS.Fromthispointinthefigureuptothepulsemodulationblock,theinformationremainsintheformofabitstream.Modulationistheprocessbywhichmessagesymbolsorchannelsymbolswhenchannelcodingisusedareconvertedtowaveformsthatarecompatiblewiththerequirementsimposedbythetransmissionchannel.Pulsemodulationisanessentialstepbecauseeachsymboltobetransmittedmustfirstbetransformedfromabinaryrepresentationvoltagelevelsrepresentingbinaryonesandzerostoabasebandwaveform.Thetermbasebandreferstoasignalwhosespectrumextendsfromorneardcuptosomefinitevalue,usuallylessthanafewmegahertz.Thepulsemodulationblockusuallyincludesfilteringforminimizingthetransmissionbandwidth.WhenpulsemodulationisappliedtobinarysymsitFromothersourcesDigitalinputmiuigitmtutZtTootherdestinationrtBandpassmodulateInformationsourceMessagesymbolsInformationsinkMessagesymbolsFormatSourceencodeEncryptChannelencodeMultiplexPulseModulateSynchronizationDigitalBasebandwaveformFrequencyspreadMultipleacessXMTRCVDigitalBasepasswaveformFormatSourcedecodeDecryptChanneldecodeMultiplexDetectDemodulateSampleFrequencydesreadMultipleacessDigitalBasepasswaveform

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