CommunicationTheory
AvarietyoftheoreticalproblemstorealizetheidealcommunicationsystemofClaudeElwoodShannon.Shannon'sinformationtheorysolvestheproblemoftheexistenceofidealencodersanddecodersonthebasisofstipulatingthecharacteristicsofthesourceandchannelprobability.However,thespecificanalysisoftheactualsourceandchannelandthespecificrealizationoftheencoderanddecoderalsoinvolvemanytechnicaltheoreticalissues.Thisisthemainresearchobjectofthecommunicationtheory.
TheformationofcommunicationtheorybeganwiththeconceptofinformationandtherelationshipbetweeninformationrateandfrequencybandproposedbyRVLHartleyandH.Nyquistin1928,N.Wienerin1942andВ.Аin1956.Kachnikovcitedstatisticalpointsofviewtoillustratethecharacteristicsofnoiseandsignal.In1948,Shannonsystematicallyproposedinformationtheory.Thesetheoriesareconsideredthebasisofmoderncommunicationtheory.Digitaltechnology,especiallythedevelopmentofcomputertechnology,hasanincreasinginfluenceoncommunication.Ontheonehand,theyenablemanyprinciplesofcommunicationtheorytoberealized,andontheotherhand,theyraisemanytheoreticalissuestobediscussed.
Thescopeofcommunicationtheoryisquitewide.ItinvolvesShannon’sinformationtheory,probabilitytheoryandrandomprocesstheoryinsourceandchannelanalysis,aswellastheelectromagneticwavetheorythatformschannels,suchaselectricwavepropagationtheory,electromagneticcompatibilityandinterferenceTheoryetc.Encoderanddecodertheoryisanimportantpartofcommunicationtheory.Encoderreferstoalltheconversionequipmentfromthesourcesymboltothesymbolsuitableforchanneltransmission,whichisdividedintoasourceencoderandachannelencoder.Decoderreferstoallinverseconversionequipmentfromthechanneltothesink(informationreceiver).Themainpurposeofdesigningthesedevicesistoimprovetheeffectivenessandreliabilityofcommunication,sothemainproblemofcommunicationtheoryresearchistoreducetheinformationrateofthesource,makefulluseofthechannelandimprovethequalityofcommunication.
Reducingtheinformationrateofthesource
Theinformationraterequiredtotransmitanalogsignalswithoutdistortionwilltendtobeinfinite.Therefore,itisusuallytosampletheanalogsignalandconverteachsamplevalueintoanumber,thatis,toconvertacontinuousquantityintoafinitequantity.Thisconversionprocessiscalledquantization.
Makefulluseofthechannel
Onecaseistheproblemofbaseband-type(frequencycomponentsstartingfromclosetozero)signalstransmittedbyband-passchannels.Inactualcommunication,commonlyusedmodulatorscompletethistransformation.Therearemanymodulationmethods(seemodulation),anddigitalmodulationisveryimportant.Tomakethefrequencybandnarrowandtheout-of-bandenergyassmallaspossible,themodulatedsignalshouldbeahigh-ordercontinuousfunction.Butthedigitalsignalisdiscontinuous,sothemodulatedsignalofonesymbolisrequiredtobeextendedtothetimeofothersymbols,whichwillcauseinter-symbolinterference.Fromthepointofviewofinformationtheory,thiskindofexpansioncanstillextractinformationabouttheprevioussymbolduringthenextsymbol,whichistheoreticallyadvantageous.Ofcourse,thedelaydecisionshouldbeusedfordemodulation,butitisinevitablymorecomplicated.Oneofthetheoriesrepresentingthisaspectiscontinuousphasemodulation,andthepartialresponseisusedtoextendthephasefunctionofonesymbol,sothatthephasechangesassmoothlyaspossibletolimitthefrequencyband,andthenthemaximumlikelihooddecoding(seeconvolutionalcode)isusedfordelaydecision.Toreducethebiterrorrate.
Improvecommunicationquality
Atthechanneloutputend,trytomakethehighestinformationtransmissionquality(foranalogsignals,theoutputsignal-to-noiseratioisthehighest,andfordigitalsignals,thebiterrorrateLowest),thisiscalledthebestreceptiontheory.Itsfoundationisstatisticaldetectiontheory,includingWienerfiltering,Kalmanfiltering,hypothesistesting,estimationtheory,etc.Thesetheoriesarewidelyused.Indigitalcommunication,therearestillerrorcorrectioncodes(seeerrorcorrectioncodes),whichcanalsoreducethebiterrorrate.Thetheoryofautomaticfeedbackretransmissionsystemisalsomoreimportant.Whenthesenderadoptserrordetectioncoding,ifthereceiverfindsthatthereisanerrorinthetransmission(errordetectioniseasiertoimplementthanerrorcorrection),itusesthefeedbackchanneltonotifythesendertoretransmituntilitisdeemederror-freetocompletetheinformationtransmission.Thiswaycanbasicallyachieveerror-freetransmission.However,duetotheneedtoencodeandretransmit,theeffectivetransmissionrateofinformationwilldecrease.Theautomaticanswertoquery(ARQ)systemintelegraphcommunicationandtheansweringmethodincomputercommunicationbelongtothiscategory.