英文資料及中文翻譯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