Introduction
Errorcontrolisatechnologythatusescodingmethodstocontrolerrorsgeneratedduringtransmissionindigitalcommunicationtoimprovetheaccuracyandeffectivenessoftransmission.Errorcontrolincludeserrordetection,forwarderrorcorrection(FEC)andautomaticretransmissionrequest(ARQ).
Accordingtothedifferentnatureoferrors,errorcontrolisdividedintoerrorcontrolforrandomerrorsanderrorcontrolforbursterrors.Randomerrorsmeanthatchannelerrorsaremoreevenlydistributedindifferenttimeintervals;whilebursterrorsmeanthatchannelerrorsareconcentratedinashortperiodoftime.Sometimesseveralerrorcontrolmethodsaremixedandused,andacertainerrorcontrolcapabilityisrequiredforrandomerrorsandbursterrors.
Amethodtoensurethatthereceiveddataiscompleteandaccurate.Becausetheactualtelephonelineisalwaysimperfect.Datamaybecomedisorderedorlostduringtransmission.Inordertocatchtheseerrors,thesendingmodemperformsamathematicaloperationonthedatatobesentandsendstheresultoftheoperationtogetherwiththedata.Thereceivingmodemperformsthesameoperationonthedataitreceivesandcomparesthetworesults..Ifthedataisdamagedduringtransmission,thetworesultsareinconsistent,andthemodemreceivingthedatarequeststhesendertoresendthedata.
Errorclassification
Errorsinthecommunicationprocesscanberoughlydividedintotwocategories:oneisrandomerrorscausedbythermalnoise;theotherisburstscausedbyconflictnoisemistake.Suddenerrorsaffectthelocalarea,whilerandomerrorsaffecttheoverallsituation.
Reasons
Thecausesoferrorsaremainlyduetotherandomnoise(thermalnoise)generatedbytheelectricalcharacteristicsofthelineitself,theattenuationordistortionofthesignalamplitude,frequencyandphase,andtheelectricalsignalThereflectionechoeffectonthetransmissionmedium,thecrosstalkofadjacentlines,externalelectromagneticinterferenceandequipmentfailurearecausedbyfactors.
Errorcontrolmethod
Generalmethod
1.Forwarderrorcorrection.Thereal-timeperformanceisgood,andthesimplexcommunicationisadopted.
2,automaticrepeatrequest(ARQ).Emphasizeserrordetectioncapability,doesnotrequireerrorcorrectioncapability,andadoptsbidirectionalchannels.
3.Hybriderrorcorrection.Theabovetwomethodsareintegrated,butthetransmissionequipmentisrelativelycomplicated.
Classificationmethod
Errordetectionisthebasisoferrorcontrol.Codesthatcancorrecterrorsshouldfirsthaveerrordetectioncapabilities,andonlywhenitcandeterminewhetherthereceivedsignalisinerrorcanitbepossibletoasktheotherpartytoresendtheerrormessage.Acodewitherrordetectioncapabilitydoesnotnecessarilyhaveerrorcorrectioncapability.Becauseerrordetectioncannotimprovechannelutilization,itismainlyusedonchannelswithbettertransmissionconditionsasameansoferrorstatisticsandqualitycontrol.
AutomaticrequestforretransmissionofARQandforwarderrorcorrectionFECaretwomethodsforerrorcontrol.
InARQmode,whenthereceivingenddetectsanerror,ittriestonotifythesendingendtoretransmituntilthecorrectcodewordisreceived.TheARQmethoduseserrordetectioncodes,buttheremustbeatwo-waychanneltofeedbackerrorinformationtothesender.Atthesametime,thesendershouldsetupadatabuffertostorethesentdatasothatthewrongdatacanbere-sent.
IntheFECmode,thereceivingendcannotonlyfinderrors,butalsodeterminethelocationofthebinarycodeelementwheretheerroroccurred,soastocorrectit.TheFECmethoduseserrorcorrectioncodes,anddoesnotrequireareversechanneltotransmittheinformationrequestedforretransmission,andthesenderdoesnotneedtostorethedatabufferforretransmission.However,thecodingefficiencyislowandtheerrorcorrectionequipmentisalsomorecomplicated.
Errorcontrolcodescanbedividedintoerrordetectioncodesanderrorcorrectioncodes.
Theerrordetectioncodecanonlydetecterrorsinthetransmission,andthesendercanonlycorrecttheerrorsbyretransmittingthedata;whiletheerrorcorrectioncodecannotonlydetecttheerrorsbutalsoautomaticallycorrecttheerrors,avoidingretransmissions.
Generalerrordetectioncodesare:paritycheckcode,cyclicredundancycode.
Inactualcommunicationnetworks,differenterrorcontroltechnologiesareoftenusedindifferentapplications.Forwarderrorcorrectionismainlyusedinwiredandwirelesstransmissionswithpoorchannelqualityandstricttransmissiondelayrequirements;errordetectionisoftenusedinthemonitoringandmanagementofchannelswithhightransmissionqualityorafterforwarderrorcorrection>Automaticretransmissionrequestsaremostlyusedinfiletransfersthatrequirelowlatencybutveryhighdatareliability,suchascomputercommunications.
Controlmethod
Errorcontrolmethod
BitErrorRateBitErrorRatePe=NumberofErrorCodesAccepted/TotalAcceptedTherearetwomeasurestakenforthenumberofsymbols:improvingtheelectricalcharacteristicsofthelineandadoptingerrorcontroltechnology
Thecommonerrorcontrolmethodforerrorcontrolistoadderrorcontrolcodestothedata.Beforetheinformationbitstobesent,acertainamountofredundantbitsisaddedaccordingtoacertainruletoformacodewordandthentransmitted.Thereareusuallyfeedbackretransmissiontechnologyandforwarderrorcorrectiontechnology:1)FeedbackretransmissiontechnologyThesenderaddsanerrordetectioncodetotheinformationbit,andthereceiverusestheerrordetectioncodetoperformerrordetectionontheinformationbitafterreceivingthecodeword,ifitiscorrectThenitsendsbackanerror-freesignalandstartstotransmitthenextinformationbit.Ifitisincorrect,itsendsbackanerrorsignal,andthesenderretransmitstheinformationuntilthereceiverconfirmsthatitiscorrect.2)ForwarderrorcorrectiontechnologyThesendingendsendsdatathatcanbecorrected.Afterreceivingthedata,thereceivingendcannotonlyautomaticallyfinderrors,butalsoautomaticallycorrecterrorsintransmission.Theadvantageisthatthereisnoneedforafeedbackchannel,buttheequipmentiscomplicated.
Systemcompositionandprincipleofaction
Systemcomposition
ThecompositionandprincipleoferrorcontrolsystemareshowninFigure1.ThepartwithinthedottedlineinFigure1istheerrorcontrolsystemindigitalcommunication.Whenthereisnoerrorcontrol,thedigital(alsocalledsymbolorsymbol)sequenceoutputbythesourcewillbedirectlysenttothechannel.Duetointerferenceinthechannel,errorswilloccurintheoutputofthechannel.Theprobabilityofdigitalerrorsintransmission(biterrorrate)isamajorindicatoroftransmissionaccuracy.Afterthechannelisgivenindigitalcommunication,ifthebiterrorratecannotmeettherequirements,errorcontrolmustbeadopted.Accordingtodifferentspecificimplementationmethods,errorcontrolcanbedividedintothreetypes:forwarderrorcorrectionmethod,feedbackretransmissionmethodandhybridmethod.Errorcontrol
ForwarderrorcorrectionmethodTheerrorcontrolsystemonlyincludesthechannelencoderanddecoder.Thedigitalsequenceoutputfromthesignalsourceiscodedinthechannelencoder(seechannelcoding),andthensenttothechannel.Sincethechannelencoderusesanerrorcorrectioncode,thedecodercancorrectmostoftheerrorscausedduringtransmissionsothatthesinkcangetamorecorrectsequence.
Aftertheforwarderrorcorrectiondetectsthereceivedsymbolerroratthereceivingend,itdeterminesthespecificlocationoftheerrorthroughacertaincalculationandautomaticallycorrectsit.Alsoknownasautomaticerrorcorrection,itisaneffectivemeanstoimprovechannelutilization.Thechannelcodingtheoryininformationtheoryistostudythelimitoftheforwarderrorcorrectioncapabilityofagivenchannel,andtheerrorcorrectioncodingtheoryistostudythespecificcodinganddecodingmethodoferrorcorrectioncodesforforwarderrorcorrection.Traditionalerrorcorrectioncodingtheorybelievesthatinorderforacodetohaveerrorcorrectioncapability,extrasymbolsmustbeaddedtotheoriginalcodewordtoexpandthedifferencebetweencodewords(calledcodedistance).Itisgenerallybelievedthattheextrasymbolsaddedduetoerrorcorrectionduringtransmissionwillincreasetheoccupiedbandwidthandreducethetransmissionrateperunitbandwidth.Thetheoryofcombinedcodingandmodulationistoconsidermodulationanderrorcorrectioncoding.Byexpandingthesetofmodulatedsignals,theerrorcorrectioncapabilityofthecodingandmodulationsystemcanbeimprovedwithoutincreasingtherequiredchannelbandwidth.Forwarderrorcorrectionhasbeenwidelyusedinsatellitecommunications,mobilecommunications,andfrequencybanddatatransmission.
Thefeedbackretransmissionmethodonlyusestheerrordetectioncodetofindtheerrorscausedbythetransmission.Atthesametime,aftertheerrorsarefound,thesenderisnotifiedtoretransmitthecorrespondingsetofnumbersthroughthereversechanneltoimproveAccuracyoftransmission.Accordingtodifferentretransmissioncontrolmethods,thefeedbackretransmissionmethodcanalsobedividedintoseveralimplementationmethods.Thesimplestoneiscalledwaitingforretransmission.Inthisway,thesenderstopsandwaitsfortheanswerfromthereceivereverytimeitsendsasetofnumbers.Atthistime,thechanneldecodersendsacorrectanswertothesenderthroughthereceiverretransmissioncontrollerandthereversechannelifnoerrorisfound.Afterreceivingit,thesenderusesthesenderretransmissioncontrollertocontrolthesourcetotransmitthenextsetofnumbers,otherwisethesourcewillretransmittheoriginalsetofnumbers.
FeedbackretransmissionAfterthereceiverdetectsanerrorinthetransmission,itwillautomaticallynotifythesendertoretransmittheerrormessage.Differentfromforwarderrorcorrection,theautomaticretransmissionrequestcandynamicallyadjustthetransmissionrateaccordingtothechangeofchanneltransmissionqualityundertherequirementofafixederrorrate.Itisanadaptiveerrorcontrolmethod,buttheremustbeAfeedbackchannel.Automaticrequestretransmissionisthemostwidelyusedinend-to-enderrorcontrolthatrequiresstricterrorcodes.
Themaindifferencebetweentheabovetwomethodsis:①Forwarderrorcorrectiondoesnotrequireareversechannel,whilefeedbackretransmissionmusthaveareversechannel.②Forwarderrorcorrectionuseserrorcorrectioncodes,andfeedbackretransmissionuseserrordetectioncodes.Generallyspeaking,theimplementationoferror-correctingcodesismorecomplicated,withfewererrorsthatcanbecorrected,whiletheimplementationoferror-detectingcodesiseasierandmoreerrorscanbefound.③Themessagedelaycausedbyforwarderrorcorrectionisfixed,andtherateofmessagetransmissionisalsofixed.Themessagedelayandmessagetransmissionrateinfeedbackretransmissionwillchangewiththechangeofretransmissionfrequency.④Forwarderrorcorrectiondoesnotrequirecontrolofthesource,whilefeedbackretransmissionrequirescontrolofthesource.⑤Theaccuracyofthetransmittedmessageafterforwarderrorcorrectionwillstillchangegreatlywiththechangeofchannelinterference,whiletheaccuracyofthetransmittedmessageafterfeedbackandretransmissionisrelativelystable,andgenerallydoesnotchangewiththechangeofinterference.Therefore,theapplicationoccasionsofthetwoareverydifferent.
Inthehybridmethod,whenthechannelinterferenceislarge,thetransmissionrateofthemessagewilldroptoomuchduetothecontinuousretransmissionduetothefeedbackretransmissionalone,andtheforwarderrorcorrectionalonecannotguaranteesufficientaccuracyAtthistime,itismoreadvantageoustouseboth,thisisthehybridmethod.Thechannelcodingusedinthismethodisacodethatcannotonlycorrectsomeerrorsbutalsofindmosterrors.Thechanneldecoderfirstcorrectsthoseerrorsthatcanbecorrected,andonlyrequiresretransmissionsforthosethatcannotbecorrectedbutcanbefound.Thiswillgreatlyreducethenumberofretransmissions.Atthesametime,duetothestrongerrordetectioncapabilityofthecode,theaccuracyofthefinaldigitalmessageisrelativelyhigh.
Role
Errorcontrolhasbeensuccessfullyappliedtosatellitecommunicationsanddatacommunications.Insatellitecommunication,convolutionalcodesorconcatenatedcodesaregenerallyusedforforwarderrorcorrection,whileindatacommunication,blockcodesaregenerallyusedforfeedbackandretransmission.Inaddition,errorcontroltechnologyisalsowidelyusedincomputers.Thereareroughlytwospecificimplementationmethods:①Usingerrorcorrectioncodestoautomaticallycorrecterrorsgeneratedbyhardware;②UsingerrordetectioncodesthroughrepeatedexecutionofinstructionsorprogramsaftererrorsarefoundPartialreturntoeliminateerrors.