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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,IFYOUDONOTKNOW“DIGITALCOMMUNICATION”OR“DIGITALSIGNAL”,ITISSEEMTOBEOUTDATED,EVENWOULDBELAUGHEDAS“AIGNORANTMEN”WELLTHEN,WHATISTHE“DIGITALCOMMUNICATION”ANDWHATISTHE“DIGITALSIGNAL”THISTEXTISTODOANGENERALIZEINTHEWAYOFBRIEFANDPLAIN1、WHYDIGITALWHYARECOMMUNICATIONSYSTEM,MILITARYANDCOMMERCIALALIKE,“GOINGDIGITAL”THEPRIMARYADVANTAGEISTHEEASEWITHWHICHDIGITALSIGNALS,COMPAREDWITHANALOGSIGNALS,AREREGENERATEDFIGURE1ILLUSTRATESANIDEALBINARYDIGITALPULSEPROPAGATINGALONGATRANSMISSIONLINETHESHAPEOFTHEWAVEFORMISAFFECTEDBYTWOBASICMECHANISMS1ASALLTRANSMISSIONLINESANDCIRCUITSHAVESOMENONIDEALFREQUENCETRANSFERFUNCTION,THEREISADISTORTINGEFFECTONTHEIDEALANDPULSE;AND2UNWANTEDELECTRICALNOISEOROTHERINTERFERENCEFURTHERDISTORTSTHEPULSEWAVEFORMBOTHOFTHESEMECHANISMSCAUSETHEPULSESHAPETODEGRADEASAFUNCTIONOFLINELENGE,ASSHOWNINFIGURE1DURINGTHETIMETHATTHETRANSMITTEDPULSECANSTILLBERELIABLYIDENTIFIEDBEFOREITISDEGRADEDTOANAMBIGUOUSSTATE,THEPULSEISAMPLIFIEDBYADIGITALAMPLIFIERTHATRECOVERSITSORIGINALIDEALSHAPETHEPULSEISTHUS“REBOR”ORREGENERATEDCIRCUITSTHATPERFORMTHISFUNCTIONATREGULARINTERVALSALONGATRANSMISSIONSYSTEMARECALLEDREGENERATIVEREPEATERSDISTANCE1DISTANCE2DANCE3STANCE4DISTANCE5ORIGINALSOMESIGNALDEGRADEDSIGNALISBADLYAMPLIFICATIONDSTORTIUONDSTORTIUONDSTORTIUONDEGRADEDREGENERATEPULSEPROPAGATIONDISTANCEFIGURE1PULSEDEGRADATIONANDREGENERATIONDIGITALCIRCUITARELESSSUBJECTTODISTORTIONANDINTERFERNCETHANANALOGCIRCUITBECANSEBINARYCIRCUITSOPOERATEINONEOFTWOSTATESFULLYONFULLYOFTOBEMEANINGFUL,ADISTURBANCEMUSTBELARGEENOUGHTOCHANGETHECIRCUITOOPERATINGPOINTFROMONESTATETOTHEOTHERSUCHTWOSTATEOPERATIONFACILITATESSIGNALREGENERATIONANDTHUSPREVENTSNOISEANDOTHERDISTURBANCESFROMACCUMULATINGINTRANSMISSIONANALOGSIGNALS,HOWEVER,ARENOTTWOSTATESIGNALS;THEYCANTAKEANINFINITEVARIETYOFSHAPESWITHANALOGCIRCUITS,EVENASMALLDISTURBANCECANRENDERTHEREPRODUCEDWAVEFORMUNACCEPTABLYDISTORTEDONCETHEANALOGSIGNALISDISTORTED,THEDISTORTIONCANNOTBEREMOVEDBYAMPLIFICATIONBECAUSEACCUMULATEDNOISEISIRREVOCABLYBOUNDTOANALOGSIGNALS,THEYCANNOTBEPERFECTLYREGENERATEDWITHDIGITALTECHNIQUES,EXTREMELYLOWERRORRATESPRODURESHIGHSIGNALFIDELITYAREPOSSIBLETHROUGHERRORDETECTIONANDCORRECTIONBUTSIMILARPROCEDURESARENOTAVAILABLEWITHANALOGTHEREAREOTHERIMPORTANTADVANTAGESTODIGITALCOMMUNICATIONSDIGITALCIRCUITAREMORERELIABLEANDCANBEPRODUCEDATALOWERCOSTTHANANALOGCIRCUITALSO,DIGITALHARDWAVELENDSITSELFTOMOREFLEXBLEIMPLEMENTATIONTHANANALOGHARDWAVEEG,MICROPROCESSOR,DIGITALSWITCHING,ANDLARGESCALEINTEGRATEDLSICIRCUITTHECOMBINGOFDIGITALSIGNALSUSINGTIMEDIVISIONMULTIPLEXINGTDMISSIMPLERTHANTHECOMBINGOFANALOGSIGNALSUSINGFREQUCENCYDIVISIONMULTIPLEXINGFDMDIFFERENTTYPESOFDIGITALSIGNALSDATA,TELEGRAPH,TELEPHONE,TELEVISIONCANBETREATEDASIDENTICALSIGNALSINTRANSMISSIONANDSWITCHINGABITISABITALSO,FORCONVENIENTSWITCHING,DIGITALMESSAGESCANBEHANDLEDINAUTONOMOUSGROUPSCALLEDPACKETSDIGITALTECHNIQUESLENDTHEMSELVESNATURALLYTOSIGNALPROCESSINGFUNCTIONSTHATPROJECTAGAINSTINTERFERENEANDJAMMING,ORTHATPROVIDEENCRYPTIONANDPRIVACYALSOMUCHDATACOMMUNICATIONISFROMCOMPUTERTOCOMPUTER,ORFROMDIGITALINSTRUMENTSORTERMINALTOCOMPUTERSUCHDIGITALTERMINATIONSARENATURALLYBESTSERVEDBYDIGITALCOMMUNICATIONLINKSWHATARETHECOSTSASSOCIATEDWITHTHEBENEFICIALATTRIBUTESOFDIGITALCIMMUNICATIONSYSTEMDIGITALSYSTEMTENDTOBEVERYSIGNALPROCESSINGINTENSIVECOMPAREDWITHANALOGALSO,DIGITALSYSTEMNEEDTOALLOCATEASIGNIFICANTSHAREOFTHEIRRESOURCESTOTHETASKOFSYNCHRONIZTIONATVARIOUSLEVELSWITHANALOGSYSTEM,ONTHEOTHERHAND,SYNCHRONIZTIONOFTENISACCOMPLISHEDMOREEASILYONEDISADVANTAGEOFADIGITALCOMMUNICATIONSYSTEMISNONGRACEFULDEGRADATIONWHENTHESIGNALTONOISERATIODROPSBELOWACERTAINTHRESHOLD,THEQUALITYOFSERVICECANCHANGESUDDENLYFROMVERYPOORINCINTRAST,MOSTCOMMUNICATIONAYSTEMDEGRADEMOREGRACEFULLY2、TYPICALBLOOKDIAGRAMANDTRANSFORMATIONSTHEFUNCTIONBLOCKDIAGRAMSHOWNINFIGURE2ILLUSTRATESTHESIGNALFLOWANDTHESIGNALPROCESSINGSTEPSTHROUGHATYPICALDIGITALCOMMUNICATIONSYSTEMDCSTHISFIGURECANSERVEASAKINDOFROADMAP,GUIDINGTHEREADERTHROUGHTHECHAPTERTHEUPPERBLOCKSFORMAT,SOURCEENCODE,ENCRYPT,CHANNELENCODE,MULTIPLEX,PULSEMODULATE,BANDPASSMODULATE,FREQUENCYSPREAD,ANDMULTIPLEACCESSDENOTESIGNALTRANSFORMATIONSFROMTHESOURCETOTHETRANSMITTERTHELOWERBLOCKDENOTESIGNALTRANSFORMATIONSFROMTHERECEIVERTOTHESINK,ESSENTIALLYREVERSINGTHESIGNALPROCESSINGSTEPSPERFORMEDBYTHEUPPERBLOCKSTHEMODULATEANDDEMODULATE/DETECTBLOCKSTOGETHERARECALLEDAMODERNTHETERM“MODERN”OFTENENCOMPASSESSEVERALOFTHESIGNALPROCESSINGSTEPSSHOWNINFIGURE2;WHENTHISISTHECASE,THEMODERNCANBETHOUGHOFASTHE“BRAIN”OFTHESYSTEMTHETRANSMITTERANDRECEIVERCANBETHOUGHOFASTHE“MUSCLES”OFTHESYSTEMFORWIRELESSAPPLICATIONS,THETRANSMITTERANDCONSISTSOFAFREQUENCYUPCONVERSIONSTAGETOARADIOFREQUENCYRF,AHIGHPOWERAMPLIFIERLNAFREQUENCYDOWNCONVERSIONISPERFORMEDINTHEFORNTENDOFTHERECEIVERAND/ORTHEDEMODULATORFIGURE2ILLUSTRATESAKINDOFRECIPROCITYBETWEENTHEBLOCKINTHEUPPERTRANSMITTERPARTOFTHEFIGUREANDTHOSEINTHELOWERRECEIVERPARTTHESIGNALPROCESSINGSTEPSTHATTAKEPLACEINTHETRANSMITTERARE,FORTHEMOSTPART,REVERSEDINTHEREVEIVERINFIGURE2,THEINPUTINFORMATIONSOURCEISCONVERTEDTOBINARYDIGITSBIS;THEBITSARETHENGROUPEDTOFROMDIGITALMESSAGEORMESSAGEEACHSUCHSYMBOLMI,WHEREI1,2,3,,MCANBEREGARDEDASAMEMERBEROFFINITEALPHABETSETCONTAININGMMEMBERSTHUS,FORM2,THEMESSAGESYMBOLMIISBINARYMEANINGTHATITCONSTITUTESJUSTASIGNALBITEVENTHOUGHBINARYSYMBOLFALLWITHINTHEGENERALDEFINITIONOFMARY,NEVERTHELESSTHENAMEMARYISUSUALLYAPPLIEDTOTHOSECASESWHEREM2;HENCE,SUCHSYMBOLAREEACHMADEUPOFASEQUENCEOFTWOORMOREBITSCOMPARESUCHAFINITEALPHABETINADCSWITHANANALOGSYSTEM,WHERETHEMESSAGEWAVEFORMISTYPICALLYAMEMBEROFANINFINITESETOFPOSSIBLEWAVEFORMFORSYSTEMTHATUSECHANNELCODINGERRORCORRECTIONCODING,ASEQUENCEOFMESSAGESYMBOLISDENOTEDUIBECAUSEAMESSAGESYMBOLORACHANNELSYMBOLCANCONSISTOFASINGLEBITORAGROUPOFBITS,ASEQUENCEOFSUCHSYMBOLISALSODESCRIBEDASABITSTREAM,ASSHOWNINFIGURE2CONSIDERTHEKEYSIGNALPROCESSINGBLOCKSHOWNINFIGURE2,ONLYFORMATTING,MODULATION,DEMODULATION/DETECTION,ANDSYNCHRONIZATIONAREESSENTIALFORADCSFORMATTING,TRANSFORMTHESOURCEINFORMATIONINTOBITS,THUSASSURINGCOMPATIBILITYBETWEENTHEINFORMATIONANDTHESIGNALPROCESSINGWITHINTHEDCSFROMTHISPOINTINTHEFIGUREUPTOTHEPULSEMODULATIONBLOCK,THEINFORMATIONREMAINSINTHEFORMOFABITSTREAMMODULATIONISTHEPROCESSBYWHICHMESSAGESYMBOLSORCHANNELSYMBOLSWHENCHANNELCODINGISUSEDARECONVERTEDTOWAVEFORMSTHATARECOMPATIBLEWITHTHEREQUIREMENTSIMPOSEDBYTHETRANSMISSIONCHANNELPULSEMODULATIONISANESSENTIALSTEPBECAUSEEACHSYMBOLTOBETRANSMITTEDMUSTFIRSTBETRANSFORMEDFROMABINARYREPRESENTATIONVOLTAGELEVELSREPRESENTINGBINARYONESANDZEROSTOABASEBANDWAVEFORMTHETERMBASEBANDREFERSTOASIGNALWHOSESPECTRUMEXTENDSFROMORNEARDCUPTOSOMEFINITEVALUE,USUALLYLESSTHANAFEWMEGAHERTZTHEPULSEMODULATIONBLOCKUSUALLYINCLUDESFILTERINGFORMINIMIZINGTHETRANSMISSIONBANDWIDTHWHENPULSEMODULATIONISAPPLIEDTOBINARYSYMSITFROMOTHERSOURCESDIGITALINPUTMIUIGITMTUTZTTOOTHERDESTINATIONRTBANDPASSMODULATEINFORMATIONSOURCEMESSAGESYMBOLSINFORMATIONSINKMESSAGESYMBOLSFORMATSOURCEENCODEENCRYPTCHANNELENCODEMULTIPLEXPULSEMODULATESYNCHRONIZATIONDIGITALBASEBANDWAVEFORMFREQUENCYSPREADMULTIPLEACESSXMTRCVDIGITALBASEPASSWAVEFORMFORMATSOURCEDECODEDECRYPTCHANNELDECODEMULTIPLEXDETECTDEMODULATESAMPLEFREQUENCYDESREADMULTIPLEACESSDIGITALBASEPASSWAVEFORM

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