Numerical communication

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.

"DigitalCommunication"wasfoundedin1974.Afterenteringthenewcentury,inordertoadapttothedevelopmentofthemarketeconomy,werepositionedit,packagedit,andemphasizedits"digital"basis;"mobile"asthecore;Thefeatureof"mobilephone"hasbecomethefirstpracticalmobilephonemagazineforthegeneralpublicinChina.

"DigitalCommunications"hasfourmajorsections:"Product","Market","Application"and"Technology":includingmobilenews,mobilephonefrontline,newproducts,productzone,DCshoppingguide,mobilephonesecrets,industryThereareseveraltopicsinmultiplecolumnssuchasobservationandmobilephonepark.

"DigitalCommunication"integratespracticality,knowledge,andinterest,withahumorouslanguageandclosetolife."DigitalCommunication"hasacirculationofmorethan250,000everyhalfmonth,andaneffectivereaderforeachissueofthemagazineAbout750,000people.AsthenumberonemobilemagazineincirculationinChina,"DigitalCommunication"hasreadersalloverthecountryandisthemobilemagazinewiththewidestdistributionareaandthehighestpenetrationrate.

"DigitalCommunication"isthenation'slargestcirculationandthemostauthoritativeprofessionalmobilephoneandwirelesscommunicationjournalinthefieldofmobilephones.Itiswellreceivedbyhundredsofthousandsofreadersforitsprofessionalevaluationarticles,playingguidescoveringallpopularmodels,andtimelymobilephoneinformationandmarketinformation.

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.

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