Definice
Acommunicationmethodinwhichamessageistransmittedindigitalformoracarriersignalismodulatedindigitalformbeforetransmission.Bothconventionaltelephonesandtelevisionsbelongtoanalogcommunications.Aftertheanalogsignaloftelephoneandtelevisionisdigitized,thedigitalsignalismodulatedandtransmitted,whichiscalleddigitaltelephoneanddigitaltelevision.Themutualdatacommunicationwithcomputersasterminalsbelongstodigitalcommunicationbecausethesignalitselfisindigitalform.Multi-channelcommunicationusingtimedivisionorcodedivisioninsatellitecommunicationalsobelongstodigitalcommunication.
ThemodelofthedigitalcommunicationsystemisshowninFigure1.InFigure1,thesourceoutputisananalogsignal,whichisconvertedintoadigitalsignalbythesourceencoderofthedigitalterminal,andthedigitalsignaloutputbytheterminalisencodedbythechannel.Aftertheconverterbecomesadigitalsignalsuitableforchanneltransmission,themodulatormodulatesthedigitalsignalontothedigitalchannelusedbythesystem,andthentransmitsittothereceivingend,andfinallysendsittothesinkafterreverseconversion.
Úloha každé části digitálního komunikačního systému
1.Source:Transformtheoriginalinformationintotheoriginalelectricalsignal.
2. Zdrojové kódování:
①RealizethedigitaltransmissionofanalogsignalsandcompletetheA/Dchange.
②Improvetheeffectivenessofsignaltransmission.Thatis,undertheconditionofensuringcertaintransmissionquality,useasfewdigitalpulsesaspossibletorepresenttheinformationgeneratedbythesource.Sourcecodingisalsocalledbandcompressioncodingordatacompressioncoding.
3.Kódování kanálu:
①Thepurposeofchannelcoding:Channelcodingmainlysolvesthereliabilityproblemofdigitalcommunication.
②Theprincipleofchannelcoding:addsomeredundantcodes(supervisioncodes)tothetransmittedinformationsymbolsaccordingtocertainrulestoformnewcodewords,andthereceivingendperformserrordetectionorevenerrordetectionaccordingtotheagreedrulesErrorcorrection.
③Channelcodingisalsocallederrorcontrolcoding,anti-interferencecoding,anderrorcorrectioncoding.
4.Digitální modulace
①Theconceptofdigitalmodulationtechnology:movethefrequencyspectrumofthedigitalbasebandsignaltoahighfrequencytoformafrequencybandsignalsuitablefortransmissioninthechannel.
②Themainfunctionofdigitalmodulation:improvetheefficiencyofsignaltransmissiononthechannel,andachievethepurposeoflong-distancesignaltransmission.
③Basicdigitalmodulationmethods:amplitudekeyingASK,frequencyshiftkeyingFSK,phaseshiftkeyingPSK.
5.Synchronizace
①Theconceptofsynchronization:referstothatthereceivingandsendingpartiesofthecommunicationsystemhaveaunifiedtimestandard,sothattheirworkis"instep."
②Theroleofsynchronization:itisessentialfordigitalcommunication.Ifthereisanerrororlossofsynchronization,alargenumberoferrorswilloccurduringthecommunicationprocess,whichwillcausetheentirecommunicationsystemtofail.
6. Kanál:
Channelisthegeneraltermforsignaltransmissionmedia.Therearetwotypesoftransmissionchannels:wiredchannel(suchascable,opticalfiber)andwirelesschannel(suchasfreespace).
7. Zdroj hluku:
Inherentinvariousequipmentandchannelsinthecommunicationsystem,fortheconvenienceofanalysis,thenoisesourceisregardedastheconcentratedexpressionofnoiseeverywhereandabstractlyaddedTothechannel.
Historie vývoje
Theearlyhistoryofdigitalcommunicationisconnectedwiththedevelopmentoftelegraph.
V roce 1937 Britský A.H. Reeves navrhl pulzní kódovou modulaci (PCM), která podpořila digitalizaci analogových signálů.
V roce 1946 vynalezl Francouz E.M. Deloraine deltamodulaci.
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.
"Digitální komunikace" byla založena v roce 1974. Po vstupu do nového století, aby se přizpůsobila vývoji tržní ekonomiky, byla upravena, zabalena a zdůrazněna její "digitální" základna; "mobilní" jako jádro;
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Výhody digitálního komunikačního systému
1.Silná odolnost proti rušení
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.Ovladatelné chyby
Theerrors(errors)thatoccurduringthetransmissionofdigitalsignalscanbecontrolledbyerrorcorrectioncodingtechnologytoimprovethereliabilityoftransmission.
3. Snadné šifrování
Comparedwithanalogsignals,digitalsignalsareeasiertoencryptanddecrypt.Therefore,theconfidentialityofdigitalcommunicationisgood.
4.Snadná integrace s moderní technologií
Duetotherapiddevelopmentofmoderntechnologiessuchascomputertechnology,digitalstoragetechnology,digitalexchangetechnology,anddigitalprocessingtechnology,manydevicesandterminalinterfacesareItisadigitalsignal,soitisveryeasytoconnectwithadigitalcommunicationsystem.
Nevýhody digitálního komunikačního systému
1. Míra využití frekvenčního pásma je nízká
Thefrequencybandutilizationrateofthesystem,theavailablesystemallowsthemaximumtransmissionbandwidth(bandwidthofthechannel)Comparedwiththeeffectivebandwidthofeachsignal,thefrequencybandwidthoccupiedbydigitalsignalsindigitalcommunication.Taketelephoneasanexample.Ananalogphoneusuallyonlyoccupies4kHzbandwidth,butadigitalphonewiththesamevoicequalitymayoccupy20-60kHz.Bandwidth.Therefore,ifthesystemtransmissionbandwidthisfixed,thefrequencybandutilizationrateofanalogphonesshouldbe5to15timeshigherthanthatofdigitalphones.
2.Systémové vybavení je složitější
Indigitalcommunication,toaccuratelyrestorethesignal,thereceivingendneedsastrictsynchronizationsystemtokeepthestrictrhythmandgroupingofthereceivingendandthesendingend.Unanimous.Therefore,digitalcommunicationsystemsandequipmentaregenerallymorecomplexandbulky.
However,withtheadoptionofnewbroadbandtransmissionchannels(suchasopticalfiber),narrowbandmodulationtechnologyandthedevelopmentofverylarge-scaleintegratedcircuits,theseshortcomingsofdigitalcommunicationhavebeenweakened.Withtherapiddevelopmentandwideapplicationofmicroelectronictechnologyandcomputertechnology,digitalcommunicationwillgraduallyreplaceanalogcommunicationandoccupyadominantpositioninfuturecommunicationmethods.
Klasifikace digitálních komunikačních systémů
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.
Digitální přenosový komunikační systém v základním pásmu
Correspondingtothefrequencybandtransmissionsystem,wecallthedigitalcommunicationsystemwithoutmodulator/demodulatoradigitalbasebandtransmissioncommunicationsystem,asshowninFigure3.Shown.
ThebasebandsignalgeneratorinFigure3mayincludeanencoder,anencryptor,andawaveformtransform,andthereceivingfiltermayalsoincludeadecoder,adecryptor,etc.
Analogový digitální přenosový komunikační systém
Inthedigitalcommunicationsystemdiscussedabove,thesignalsoutputbythesourcearealldigitalbasebandsignals.Infact,mostofthesignalsindailylife(Suchasvoicesignal)isacontinuouslychanginganalogsignal.Then,torealizethetransmissionofanalogsignalsindigitalsystems,theanalogsignalsmustbedigitizedatthesendingend,thatis,A/Dconversion;atthereceivingend,theoppositeconversion,thatis,D/Aconversionmustbeperformed.ThesystemthatrealizesthedigitaltransmissionofanalogsignalsisshowninFigure4.