Definition
Acommunicationmethodinwhichamessageistransmittedindigitalformoracarriersignalismodulatedindigitalformbeforetransmission.Bothconventionaltelephonesandtelevisionsbelongtoanalogcommunications.Aftertheanalogsignaloftelephoneandtelevisionisdigitized,thedigitalsignalismodulatedandtransmitted,whichiscalleddigitaltelephoneanddigitaltelevision.Themutualdatacommunicationwithcomputersasterminalsbelongstodigitalcommunicationbecausethesignalitselfisindigitalform.Multi-channelcommunicationusingtimedivisionorcodedivisioninsatellitecommunicationalsobelongstodigitalcommunication.
ThemodelofthedigitalcommunicationsystemisshowninFigure1.InFigure1,thesourceoutputisananalogsignal,whichisconvertedintoadigitalsignalbythesourceencoderofthedigitalterminal,andthedigitalsignaloutputbytheterminalisencodedbythechannel.Aftertheconverterbecomesadigitalsignalsuitableforchanneltransmission,themodulatormodulatesthedigitalsignalontothedigitalchannelusedbythesystem,andthentransmitsittothereceivingend,andfinallysendsittothesinkafterreverseconversion.
Theroleofeachpartofthedigitalcommunicationsystem
1.Source:Transformtheoriginalinformationintotheoriginalelectricalsignal.
2.Sourcecoding:
①RealizethedigitaltransmissionofanalogsignalsandcompletetheA/Dchange.
②Improvetheeffectivenessofsignaltransmission.Thatis,undertheconditionofensuringcertaintransmissionquality,useasfewdigitalpulsesaspossibletorepresenttheinformationgeneratedbythesource.Sourcecodingisalsocalledbandcompressioncodingordatacompressioncoding.
3.Channelcoding:
①Thepurposeofchannelcoding:Channelcodingmainlysolvesthereliabilityproblemofdigitalcommunication.
②Theprincipleofchannelcoding:addsomeredundantcodes(supervisioncodes)tothetransmittedinformationsymbolsaccordingtocertainrulestoformnewcodewords,andthereceivingendperformserrordetectionorevenerrordetectionaccordingtotheagreedrulesErrorcorrection.
③Channelcodingisalsocallederrorcontrolcoding,anti-interferencecoding,anderrorcorrectioncoding.
4.Digitalmodulation
①Theconceptofdigitalmodulationtechnology:movethefrequencyspectrumofthedigitalbasebandsignaltoahighfrequencytoformafrequencybandsignalsuitablefortransmissioninthechannel.
②Themainfunctionofdigitalmodulation:improvetheefficiencyofsignaltransmissiononthechannel,andachievethepurposeoflong-distancesignaltransmission.
③Basicdigitalmodulationmethods:amplitudekeyingASK,frequencyshiftkeyingFSK,phaseshiftkeyingPSK.
5.Synchronization
①Theconceptofsynchronization:referstothatthereceivingandsendingpartiesofthecommunicationsystemhaveaunifiedtimestandard,sothattheirworkis"instep."
②Theroleofsynchronization:itisessentialfordigitalcommunication.Ifthereisanerrororlossofsynchronization,alargenumberoferrorswilloccurduringthecommunicationprocess,whichwillcausetheentirecommunicationsystemtofail.
6.Channel:
Channelisthegeneraltermforsignaltransmissionmedia.Therearetwotypesoftransmissionchannels:wiredchannel(suchascable,opticalfiber)andwirelesschannel(suchasfreespace).
7.Noisesource:
Inherentinvariousequipmentandchannelsinthecommunicationsystem,fortheconvenienceofanalysis,thenoisesourceisregardedastheconcentratedexpressionofnoiseeverywhereandabstractlyaddedTothechannel.
Developmenthistory
Theearlyhistoryofdigitalcommunicationisconnectedwiththedevelopmentoftelegraph.
In1937,theBritishA.H.Reevesproposedpulsecodemodulation(PCM),whichpromotedthedigitizationofanalogsignals.
In1946,theFrenchmanE.M.Deloraineinventeddeltamodulation.
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.
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Advantagesofdigitalcommunicationsystem
1.Stronganti-interferenceability
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.Errorcontrollable
Theerrors(errors)thatoccurduringthetransmissionofdigitalsignalscanbecontrolledbyerrorcorrectioncodingtechnologytoimprovethereliabilityoftransmission.
3.Easytoencrypt
Comparedwithanalogsignals,digitalsignalsareeasiertoencryptanddecrypt.Therefore,theconfidentialityofdigitalcommunicationisgood.
4.Easytointegratewithmoderntechnology
Duetotherapiddevelopmentofmoderntechnologiessuchascomputertechnology,digitalstoragetechnology,digitalexchangetechnology,anddigitalprocessingtechnology,manydevicesandterminalinterfacesareItisadigitalsignal,soitisveryeasytoconnectwithadigitalcommunicationsystem.
Disadvantagesofthedigitalcommunicationsystem
1.Thefrequencybandutilizationrateisnothigh
Thefrequencybandutilizationrateofthesystem,theavailablesystemallowsthemaximumtransmissionbandwidth(bandwidthofthechannel)Comparedwiththeeffectivebandwidthofeachsignal,thefrequencybandwidthoccupiedbydigitalsignalsindigitalcommunication.Taketelephoneasanexample.Ananalogphoneusuallyonlyoccupies4kHzbandwidth,butadigitalphonewiththesamevoicequalitymayoccupy20-60kHz.Bandwidth.Therefore,ifthesystemtransmissionbandwidthisfixed,thefrequencybandutilizationrateofanalogphonesshouldbe5to15timeshigherthanthatofdigitalphones.
2.Systemequipmentismorecomplicated
Indigitalcommunication,toaccuratelyrestorethesignal,thereceivingendneedsastrictsynchronizationsystemtokeepthestrictrhythmandgroupingofthereceivingendandthesendingend.Unanimous.Therefore,digitalcommunicationsystemsandequipmentaregenerallymorecomplexandbulky.
However,withtheadoptionofnewbroadbandtransmissionchannels(suchasopticalfiber),narrowbandmodulationtechnologyandthedevelopmentofverylarge-scaleintegratedcircuits,theseshortcomingsofdigitalcommunicationhavebeenweakened.Withtherapiddevelopmentandwideapplicationofmicroelectronictechnologyandcomputertechnology,digitalcommunicationwillgraduallyreplaceanalogcommunicationandoccupyadominantpositioninfuturecommunicationmethods.
Classificationofdigitalcommunicationsystems
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.
Digitalbasebandtransmissioncommunicationsystem
Correspondingtothefrequencybandtransmissionsystem,wecallthedigitalcommunicationsystemwithoutmodulator/demodulatoradigitalbasebandtransmissioncommunicationsystem,asshowninFigure3.Shown.
ThebasebandsignalgeneratorinFigure3mayincludeanencoder,anencryptor,andawaveformtransform,andthereceivingfiltermayalsoincludeadecoder,adecryptor,etc.
Analogsignaldigitaltransmissioncommunicationsystem
Inthedigitalcommunicationsystemdiscussedabove,thesignalsoutputbythesourcearealldigitalbasebandsignals.Infact,mostofthesignalsindailylife(Suchasvoicesignal)isacontinuouslychanginganalogsignal.Then,torealizethetransmissionofanalogsignalsindigitalsystems,theanalogsignalsmustbedigitizedatthesendingend,thatis,A/Dconversion;atthereceivingend,theoppositeconversion,thatis,D/Aconversionmustbeperformed.ThesystemthatrealizesthedigitaltransmissionofanalogsignalsisshowninFigure4.