外文翻译数字通信系统文档格式.docx
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外文翻译数字通信系统文档格式.docx
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DigitalCommunicationSystem
Formodernpeople,ifyoudonotknowDigitalCommunication”orDigitalSignal”itisseemtobeoutdated,evenwouldbelaughedasaignorantmer”.wellthen,whatistheDigitalCommunication”andwhastheDigitalSignal”Thistextistodoangeneralizeinthewayofbriefandplain.
1、WhyDigital?
Whyarecommunicationsystem,militaryandcommercialalike,“goingdigital?
Theprimaryadvantageistheeasewithwhichdigitalsignals,comparedwithanalogsignals,areregenerated.Figure1illustratesanidealbinarydigitalpulsepropagatingalongatransmissionline.Theshapeofthewaveformisaffectedbytwobasicmechanisms:
(1)asalltransmissionlinesandcircuitshavesomenonidealfrequencetransferfunction,thereisadistortingeffectontheidealandpulse;
and
(2)unwantedelectricalnoiseorotherinterferencefurtherdistortsthepulsewaveform.BothofthesemechanismscausethepulseshapetodegradeasafunctionoflinelengasshowninFigure1.Duringthetimethatthetransmittedpulsecanstillbereliablyidentifiedbeforeitisdegradedtoanambiguousstate,thepulseisamplifiedbyadigitalamplifierthatrecoversitsoriginalidealshape.Thepulseisthus“rebororregenerated.Circuitsthatperformthisfunctionatregularintervalsalongatransmissionsystemarecalledregenerativerepeaters.
Distance1Distance2Dance3stance4Distance5
OriginalSomesignalDegradedSignalisbadlyAmplification
dstortiuondstortiuondstortiuondegradedregeneratepulse
Figure1Pulsedegradationandregeneration
Digitalcircuitarelesssubjecttodistortionandinterferncethananalogcircuit.
Becansebinarycircuitsopoerateinoneoftwostatesfullyonfullyof—tobe
meaningful,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、TypicalBlookDiagramandTransformations
ThefunctionblockdiagramshowninFigure2illustratesthesignalflowand
thesignal-processingstepsthroughatypicaldigitalcommunicationsystem(DCS).Thisfigurecanserveasakindofroadmap,guidingthereaderthroughthechapter.Theupperblocks---format,sourceencode,encrypt,channelencode,multiplex,pulsemodulate,bandpassmodulate,frequencyspread,andmultipleaccess-denotesignaltransformationsfromthesourcetothetransmitter.Thelowerblockdenotesignaltransformationsfromther
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