Определение
Acommunicationmethodinwhichamessageistransmittedindigitalformoracarriersignalismodulatedindigitalformbeforetransmission.Bothconventionaltelephonesandtelevisionsbelongtoanalogcommunications.Aftertheanalogsignaloftelephoneandtelevisionisdigitized,thedigitalsignalismodulatedandtransmitted,whichiscalleddigitaltelephoneanddigitaltelevision.Themutualdatacommunicationwithcomputersasterminalsbelongstodigitalcommunicationbecausethesignalitselfisindigitalform.Multi-channelcommunicationusingtimedivisionorcodedivisioninsatellitecommunicationalsobelongstodigitalcommunication.
ThemodelofthedigitalcommunicationsystemisshowninFigure1.InFigure1,thesourceoutputisananalogsignal,whichisconvertedintoadigitalsignalbythesourceencoderofthedigitalterminal,andthedigitalsignaloutputbytheterminalisencodedbythechannel.Aftertheconverterbecomesadigitalsignalsuitableforchanneltransmission,themodulatormodulatesthedigitalsignalontothedigitalchannelusedbythesystem,andthentransmitsittothereceivingend,andfinallysendsittothesinkafterreverseconversion.
Ролята на всяка част от цифровата комуникационна система
1.Source:Transformtheoriginalinformationintotheoriginalelectricalsignal.
2. Кодиране на източника:
①RealizethedigitaltransmissionofanalogsignalsandcompletetheA/Dchange.
②Improvetheeffectivenessofsignaltransmission.Thatis,undertheconditionofensuringcertaintransmissionquality,useasfewdigitalpulsesaspossibletorepresenttheinformationgeneratedbythesource.Sourcecodingisalsocalledbandcompressioncodingordatacompressioncoding.
3.Канално кодиране:
①Thepurposeofchannelcoding:Channelcodingmainlysolvesthereliabilityproblemofdigitalcommunication.
②Theprincipleofchannelcoding:addsomeredundantcodes(supervisioncodes)tothetransmittedinformationsymbolsaccordingtocertainrulestoformnewcodewords,andthereceivingendperformserrordetectionorevenerrordetectionaccordingtotheagreedrulesErrorcorrection.
③Channelcodingisalsocallederrorcontrolcoding,anti-interferencecoding,anderrorcorrectioncoding.
4. Цифрова модулация
①Theconceptofdigitalmodulationtechnology:movethefrequencyspectrumofthedigitalbasebandsignaltoahighfrequencytoformafrequencybandsignalsuitablefortransmissioninthechannel.
②Themainfunctionofdigitalmodulation:improvetheefficiencyofsignaltransmissiononthechannel,andachievethepurposeoflong-distancesignaltransmission.
③Basicdigitalmodulationmethods:amplitudekeyingASK,frequencyshiftkeyingFSK,phaseshiftkeyingPSK.
5.Синхронизация
①Theconceptofsynchronization:referstothatthereceivingandsendingpartiesofthecommunicationsystemhaveaunifiedtimestandard,sothattheirworkis"instep."
②Theroleofsynchronization:itisessentialfordigitalcommunication.Ifthereisanerrororlossofsynchronization,alargenumberoferrorswilloccurduringthecommunicationprocess,whichwillcausetheentirecommunicationsystemtofail.
6.Канал:
Channelisthegeneraltermforsignaltransmissionmedia.Therearetwotypesoftransmissionchannels:wiredchannel(suchascable,opticalfiber)andwirelesschannel(suchasfreespace).
7.Източник на шум:
Inherentinvariousequipmentandchannelsinthecommunicationsystem,fortheconvenienceofanalysis,thenoisesourceisregardedastheconcentratedexpressionofnoiseeverywhereandabstractlyaddedTothechannel.
История на развитието
Theearlyhistoryofdigitalcommunicationisconnectedwiththedevelopmentoftelegraph.
През 1937 г. британецът A.H.Reeves предложи импулсно-кодова модулация (PCM), която насърчи цифровизацията на аналоговите сигнали.
През 1946 г. французинът E.M.Deloraine изобретява делтамодулацията.
In1950C.C.Cutlerproposeddifferencecoding.In1947,AmericanBellLabsdevelopeda24-channelelectronictubepulsecodemodulationdeviceforexperiments,whichprovedthefeasibilityofrealizingPCM.
In1953,afeedback-comparativeencoderwithoutanencodingtubewasinvented,whichexpandedthedynamicrangeoftheinputsignal.
In1962,theUnitedStatesdevelopeda24-channel1.544Mbit/spulsecodemodulationdevicefortransistorsanduseditbetweenlocaltelephoneandnetworkoffices.
Digitalcommunicationhasobviousadvantagescomparedwithanalogcommunication.Ithasstronganti-interferenceability,communicationqualityisnotaffectedbydistance,canadapttotherequirementsofvariouscommunicationservices,facilitatestheuseoflarge-scaleintegratedcircuits,andfacilitatessecurecommunicationandcomputermanagement.Thedisadvantageisthattheoccupiedchannelfrequencybandisrelativelywide.
Inthe1990s,digitalcommunicationsdevelopedinthedirectionofultra-high-speed,large-capacity,andlong-distance.High-efficiencycodingtechnologyhasbecomeincreasinglymature,voicecodinghasbecomepractical,andnewdigitalintelligentterminalswillbefurtherdeveloped.
"DigitalCommunication" е основана през 1974 г. След навлизането в новия век, за да се адаптират към развитието на пазарната икономика, бяха позиционирани, пакетирани и подчертани нейната "цифрова" основа; "мобилен" като ядро; Характеристиката на "мобилен телефон" се превърна в първото практично списание за мобилни телефони за широката общественост в Китай.
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Предимства на цифровата комуникационна система
1. Силна анти-интерференция
Becauseindigitalcommunication,thetransmittedsignalamplitudeisdiscrete,takebinarysystemasanexample,Thevalueofthesignalisonlytwo,sothereceivingendonlyneedstodistinguishtwostates.Whenthesignalisdisturbedbynoiseduringtransmission,thewaveformwillinevitablybedistorted,andthereceivingendwillmakeasamplingdecisiononittodistinguishwhichofthetwostatesis.Aslongasthenoiseisnotlargeenoughtoaffectthecorrectnessofthedecision,itcanbereceivedcorrectly(reproduced).Inanalogcommunication,thetransmittedsignalamplitudechangescontinuously.Oncenoiseissuperimposed,itisdifficulttoeliminateitevenifthenoiseissmall.
Digitalcommunicationhasgoodanti-noiseperformance,anditcanalsoeliminatenoiseaccumulationduringmicrowaverelaycommunication.Thisisbecauseaftereachregenerationofthedigitalsignal,aslongasthereisnocodeerror,itisstilllikethesignalsentfromthesource,withoutnoisesuperimposedonit.Therefore,nomatterhowmanyrelaystations,digitalcommunicationstillhasgoodcommunicationquality.Whenanalogcommunicationisrelayed,itcanonlyincreasesignalenergy(amplifythesignal),butcannoteliminatenoise.
2.Контролируема грешка
Theerrors(errors)thatoccurduringthetransmissionofdigitalsignalscanbecontrolledbyerrorcorrectioncodingtechnologytoimprovethereliabilityoftransmission.
3.Лесно криптиране
Comparedwithanalogsignals,digitalsignalsareeasiertoencryptanddecrypt.Therefore,theconfidentialityofdigitalcommunicationisgood.
4. Лесен за интегриране с модерни технологии
Duetotherapiddevelopmentofmoderntechnologiessuchascomputertechnology,digitalstoragetechnology,digitalexchangetechnology,anddigitalprocessingtechnology,manydevicesandterminalinterfacesareItisadigitalsignal,soitisveryeasytoconnectwithadigitalcommunicationsystem.
Недостатъци на цифровата комуникационна система
1. Степента на използване на честотната лента не е висока
Thefrequencybandutilizationrateofthesystem,theavailablesystemallowsthemaximumtransmissionbandwidth(bandwidthofthechannel)Comparedwiththeeffectivebandwidthofeachsignal,thefrequencybandwidthoccupiedbydigitalsignalsindigitalcommunication.Taketelephoneasanexample.Ananalogphoneusuallyonlyoccupies4kHzbandwidth,butadigitalphonewiththesamevoicequalitymayoccupy20-60kHz.Bandwidth.Therefore,ifthesystemtransmissionbandwidthisfixed,thefrequencybandutilizationrateofanalogphonesshouldbe5to15timeshigherthanthatofdigitalphones.
2.Системното оборудване е по-сложно
Indigitalcommunication,toaccuratelyrestorethesignal,thereceivingendneedsastrictsynchronizationsystemtokeepthestrictrhythmandgroupingofthereceivingendandthesendingend.Unanimous.Therefore,digitalcommunicationsystemsandequipmentaregenerallymorecomplexandbulky.
However,withtheadoptionofnewbroadbandtransmissionchannels(suchasopticalfiber),narrowbandmodulationtechnologyandthedevelopmentofverylarge-scaleintegratedcircuits,theseshortcomingsofdigitalcommunicationhavebeenweakened.Withtherapiddevelopmentandwideapplicationofmicroelectronictechnologyandcomputertechnology,digitalcommunicationwillgraduallyreplaceanalogcommunicationandoccupyadominantpositioninfuturecommunicationmethods.
Класификация на цифровите комуникационни системи
Digitalcommunicationsystemscanbefurthersubdividedintodigitalfrequencybandtransmissioncommunicationsystems,digitalbasebandtransmissioncommunicationsystems,andanalogsignaldigitaltransmissioncommunicationsystems.
Digitalfrequencybandtransmissioncommunicationsystem
Thebasicfeatureofdigitalcommunicationisthatitsmessagesorsignalshave"discrete"or"digital"characteristics,sothatdigitalcommunicationhasmanyspecialfeatures.problem.Forexample,thesecondtransformationmentionedaboveemphasizesthelinearcharacteristicofthetransformationinanalogcommunication,thatis,theproportionalcharacteristicbetweentheadjustedparameterandthebasebandsignalrepresentingthemessage;whileindigitalcommunication,itemphasizestheadjustedparameterandtherepresentativeOne-to-onecorrespondencebetweenthedigitalsignalsofthemessage.
Inaddition,thefollowingoutstandingproblemsstillexistindigitalcommunication:First,errorscausedbychannelnoiseorinterferenceduringdigitalsignaltransmissioncanbecontrolledinprinciple.Thisisachievedbyso-callederrorcontrolcoding.Therefore,anencoderneedstobeaddedatthetransmittingend,andadecoderisneededatthereceivingendaccordingly.Second,whenitisnecessarytorealizesecurecommunication,thedigitalbasebandsignalcanbeartificially"jammed"(encrypted),anddecryptionmustbeperformedatthereceivingend.Third,becausedigitalcommunicationtransmitsdigitalsignalsoneafteranotheratacertaintempo,thereceivingendmusthavethesametempoasthesendingend,otherwise,itwillcauseconfusionduetoinconsistentsendingandreceivingsteps.Inaddition,inordertoexpressthecontentofthemessage,thebasebandsignalsaregroupedaccordingtothecharacteristicsofthemessage.Therefore,thecodingrulesofagroupofgroupsmustbeconsistentbetweensendingandreceiving,otherwisethetruecontentofthemessagewillnotberestoredwhenreceiving.Indigitalcommunication,tempoconsistencyiscalled"bitsynchronization"or"symbolsynchronization",andgroupingconsistencyiscalled"groupsynchronization"or"framesynchronization".Therefore,theremustbeanimportantissueof"synchronization"indigitalcommunication.
Tosumup,thepoint-to-pointdigitalcommunicationsystemmodelcangenerallybeshowninFigure2.
Itshouldbenotedthatwhetherthemodulator/demodulator,encryptor/decrypter,encoder/decoderandotherlinksinFigure2arealladoptedinthespecificcommunicationsystemdependsonSpecificdesignconditionsandrequirements.Butinasystem,ifthesenderhasmodulation/encryption/encoding,thereceivingendmusthavedemodulation/decryption/decoding.Adigitalcommunicationsystemwithamodulator/demodulatorisusuallycalledadigitalfrequencybandtransmissioncommunicationsystem.
Комуникационна система за предаване на цифрова основна лента
Correspondingtothefrequencybandtransmissionsystem,wecallthedigitalcommunicationsystemwithoutmodulator/demodulatoradigitalbasebandtransmissioncommunicationsystem,asshowninFigure3.Shown.
ThebasebandsignalgeneratorinFigure3mayincludeanencoder,anencryptor,andawaveformtransform,andthereceivingfiltermayalsoincludeadecoder,adecryptor,etc.
Комуникационна система с аналогов сигнал и цифрово предаване
Inthedigitalcommunicationsystemdiscussedabove,thesignalsoutputbythesourcearealldigitalbasebandsignals.Infact,mostofthesignalsindailylife(Suchasvoicesignal)isacontinuouslychanginganalogsignal.Then,torealizethetransmissionofanalogsignalsindigitalsystems,theanalogsignalsmustbedigitizedatthesendingend,thatis,A/Dconversion;atthereceivingend,theoppositeconversion,thatis,D/Aconversionmustbeperformed.ThesystemthatrealizesthedigitaltransmissionofanalogsignalsisshowninFigure4.