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英文资料及中文翻译DigitalCommunicationSystemFormodernpeople,ifyoudonotknow“DigitalCommunication”or“DigitalSignal”,itisseemtobeoutdated,evenwouldbelaughedas“aignorantmen”.wellthen,whatisthe“DigitalCommunication”andwhatisthe“DigitalSignal”?Thistextistodoangeneralizeinthewayofbriefandplain.1、WhyDigital?Whyarecommunicationsystem,militaryandcommercialalike,“goingdigital”?Theprimaryadvantageistheeasewithwhichdigitalsignals,comparedwithanalogsignals,areregenerated.Figure1illustratesanidealbinarydigitalpulsepropagatingalongatransmissionline.Theshapeofthewaveformisaffectedbytwobasicmechanisms:(1)asalltransmissionlinesandcircuitshavesomenonidealfrequencetransferfunction,thereisadistortingeffectontheidealandpulse;and(2)unwantedelectricalnoiseorotherinterferencefurtherdistortsthepulsewaveform.Bothofthesemechanismscausethepulseshapetodegradeasafunctionoflinelenge,asshowninFigure1.Duringthetimethatthetransmittedpulsecanstillbereliablyidentifiedbeforeitisdegradedtoanambiguousstate,thepulseisamplifiedbyadigitalamplifierthatrecoversitsoriginalidealshape.Thepulseisthus“rebor”orregenerated.Circuitsthatperformthisfunctionatregularintervalsalongatransmissionsystemarecalledregenerativerepeaters.Distance1Distance2Dance3stance4Distance5OriginalSomesignalDegradedSignalisbadlyAmplificationdstortiuondstortiuondstortiuondegradedregeneratepulsePropagationdistanceFigure1PulsedegradationandregenerationDigitalcircuitarelesssubjecttodistortionandinterferncethananalogcircuit.Becansebinarycircuitsopoerateinoneoftwostatesfullyonfullyoftobemeaningful,adisturbancemustbelargeenoughtochangethecircuitooperatingpointfromonestatetotheother.Suchtwo-stateoperationfacilitatessignalregenerationandthuspreventsnoiseandotherdisturbancesfromaccumulatingintransmission.Analogsignals,however,arenottwo-statesignals;theycantakeaninfinitevarietyofshapes.Withanalogcircuits,evenasmalldisturbancecanrenderthereproducedwaveformunacceptablydistorted.Oncetheanalogsignalisdistorted,thedistortioncannotberemovedbyamplification.Becauseaccumulatednoiseisirrevocablyboundtoanalogsignals,theycannotbeperfectlyregenerated.Withdigitaltechniques,extremelylowerrorratesprodureshighsignalfidelityarepossiblethrougherrordetectionandcorrectionbutsimilarproceduresarenotavailablewithanalog.Thereareotherimportantadvantagestodigitalcommunications.Digitalcircuitaremorereliableandcanbeproducedatalowercostthananalogcircuit.Also,digitalhardwavelendsitselftomoreflexbleimplementationthananaloghardwave(e.g.,microprocessor,digitalswitching,andlarge-scaleintegrated(LSI)circuit).Thecombingofdigitalsignalsusingtime-divisionmultiplexing(TDM)issimplerthanthecombingofanalogsignalsusingfrequcency-divisionmultiplexing(FDM).Differenttypesofdigitalsignals(data,telegraph,telephone,television)canbetreatedasidenticalsignalsintransmissionandswitching-abitisabit.Also,forconvenientswitching,digitalmessagescanbehandledinautonomousgroupscalledpackets.Digitaltechniqueslendthemselvesnaturallytosignalprocessingfunctionsthatprojectagainstinterfereneandjamming,orthatprovideencryptionandprivacy.Alsomuchdatacommunicationisfromcomputertocomputer,orfromdigitalinstrumentsorterminaltocomputer.Suchdigitalterminationsarenaturallybestservedbydigitalcommunicationlinks.Whatarethecostsassociatedwiththebeneficialattributesofdigitalcimmunicationsystem?Digitalsystemtendtobeverysignal-processingintensivecomparedwithanalog.Also,digitalsystemneedtoallocateasignificantshareoftheirresourcestothetaskofsynchroniztionatvariouslevels.Withanalogsystem,ontheotherhand,synchroniztionoftenisaccomplishedmoreeasily.Onedisadvantageofadigitalcommunicationsystemisnon-gracefuldegradation.Whenthesignal-to-noiseratiodropsbelowacertainthreshold,thequalityofservicecanchangesuddenlyfromverypoor.Incintrast,mostcommunicationaystemdegrademoregracefully.2、TypicalBlookDiagramandTransformationsThefunctionblockdiagramshowninFigure2illustratesthesignalflowandthesignal-processingstepsthroughatypicaldigitalcommunicationsystem(DCS).Thisfigurecanserveasakindofroadmap,guidingthereaderthroughthechapter.Theupperblocks-format,sourceencode,encrypt,channelencode,multiplex,pulsemodulate,bandpassmodulate,frequencyspread,andmultipleaccess-denotesignaltransformationsfromthesourcetothetransmitter.Thelowerblockdenotesignaltransformationsfromthereceivertothesink,essentiallyreversingthesignalprocessingstepsperformedbytheupperblocks.Themodulateanddemodulate/detectblockstogetherarecalledamodern.Theterm“modern”oftenencompassesseveralofthesignalprocessingstepsshowninFigure2;Whenthisisthecase,themoderncanbethoughofasthe“brain”ofthesystem.Thetransmitterandreceivercanbethoughofasthe“muscles”ofthesystem.Forwirelessapplications,thetransmitterandconsistsofafrequencyup-conversionstagetoaradiofrequency(RF),ahigh-poweramplifier(LNA).Frequencydown-conversionisperformedintheforntendofthereceiverand/orthedemodulator.Figure2illustratesakindofreciprocitybetweentheblockintheuppertransmitterpartofthefigureandthoseinthelowerreceiverpart.Thesignalprocessingstepsthattakeplaceinthetransmitterare,forthemostpart,reversedinthereveiver.InFigure2,theinputinformationsourceisconvertedtobinarydigits(bis);thebitsarethengroupedtofromdigitalmessageormessage.Eachsuchsymbol(mi,wherei=1,2,3,M)canberegardedasamemerberoffinitealphabetsetcontainingMmembers.Thus,forM=2,themessagesymbolmiisbinary(meaningthatitconstitutesjustasignalbit).EventhoughbinarysymbolfallwithinthegeneraldefinitionofM-ary,neverthelessthenameM-aryisusuallyappliedtothosecaseswhereM2;hence,suchsymbolareeachmadeupofasequenceoftwoormorebits.(ComparesuchafinitealphabetinaDCSwithananalogsystem,wherethemessagewaveformistypicallyamemberofaninfinitesetofpossiblewaveform).Forsystemthatusechannelcoding(errorcorrectioncoding),asequenceofmessagesymbolisdenotedui.Becauseamessagesymbolorachannelsymbolcanconsistofasinglebitoragroupofbits,asequenceofsuchsymbolisalsodescribedasabitstream,asshowninFigure2.ConsiderthekeysignalprocessingblockshowninFigure2,Onlyformatting,modulation,demodulation/detection,andsynchronizationareessentialforaDCS.Formatting,transformthesourceinformationintobits,thusassuringcompatibilitybetweentheinformationandthesignalprocessingwithintheDCS.Fromthispointinthefigureuptothepulse-modulationblock,theinformationremainsintheformofabitstream.Modulationistheprocessbywhichmessagesymbolsorchannelsymbols(whenchannelcodingisused)areconvertedtowaveformsthatarecompatiblewiththerequirementsimposedbythetransmissionchannel.Pulsemodulationisanessentialstepbecauseeachsymboltobetransmittedmustfirstbetransformedfromabinaryrepresentation(voltagelevelsrepr
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