6 Line coding 华科.docx
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6 Line coding 华科.docx
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6Linecoding华科
Linecode
1.Concept:
Intelecommunication,alinecode(alsocalleddigitalbasebandmodulation)isacodechosenforusewithinacommunicationssystemfortransmissionpurposes.
Linecodingisoftenusedfordigitaldatatransport.
Linecoding
Linecodingconsistsofrepresentingthedigitalsignaltobetransportedbyanamplitude-andtime-discretesignalthatisoptimallytunedforthespecificpropertiesofthephysicalchannel(andofthereceivingequipment).
Thewaveformpatternofvoltageorcurrentusedtorepresentthe1sand0sofadigitalsignalonatransmissionlinkiscalledlineencoding.
Thecommontypesoflineencodingareunipolar,polar,bipolarandManchesterencoding.
Forreliableclockrecoveryatthereceiver,oneusuallyimposesamaximumrunlengthconstraintonthegeneratedchannelsequence,i.e.themaximumnumberofconsecutiveonesorzerosisboundedtoareasonablenumber.Aclockperiodisrecoveredbyobservingtransitionsinthereceivedsequence,sothatamaximumrunlengthguaranteessuchclockrecovery,whilesequenceswithoutsuchaconstraintcouldseriouslyhamperthedetectionquality.
Afterlinecoding,thesignalisputthrougha"physicalchannel",eithera"transmissionmedium"or"datastoragemedium".Sometimesthecharacteristicsof2verydifferent-seemingchannelsaresimilarenoughthatthesamelinecodeisusedforthem.Themostcommonphysicalchannelsare:
⏹theline-codedsignalcandirectlybeputonatransmissionline,intheformofvariationsofthevoltageorcurrent(oftenusingdifferentialsignaling).
⏹theline-codedsignal(the"basebandsignal")undergoesfurtherpulseshaping(toreduceitsfrequencybandwidth)andthenmodulated(toshiftitsfrequencybandwidth)tocreatethe"RFsignal"thatcanbesentthroughfreespace.
⏹theline-codedsignalcanbeusedtoturnonandoffalightinFreeSpaceOptics,mostcommonlyinfraredremotecontrol.
⏹theline-codedsignalcanbeprintedonpapertocreateabarcode.
⏹theline-codedsignalcanbeconvertedtoamagnetizedspotsonaharddriveortapedrive.
⏹theline-codedsignalcanbeconvertedtoapitsonopticaldisc.
DCbalanced:
Mostlong-distancecommunicationchannelscannottransportaDCcomponent,unfortunately.TheDCcomponentisalsocalledthedisparity,thebias,ortheDCcoefficient.Thesimplestpossiblelinecode,calledunipolarbecauseithasanunboundedDCcomponent,givestoomanyerrorsonsuchsystems.MostlinecodeseliminatetheDCcomponent-suchcodesarecalledDCbalanced,zero-DC,zero-biasorDCequalizedetc.Thereare2waysofeliminatingtheDCcomponent:
ØUseaconstant-weightcode.Inotherwords,designeachtransmittedcodewordsuchthateverycodewordthatcontainssomepositiveornegativelevelsalsocontainsenoughoftheoppositelevels,suchthattheaveragelevelovereachcodewordiszero.Forexample,ManchestercodeandInterleaved2of5.
ØUseapaireddisparitycode.Inotherwords,designthereceiversuchthateverycodewordthataveragestoanegativelevelispairedwithanothercodewordthataveragestoapositivelevel.Designthereceiversothateithercodewordofthepairdecodestothesamedatabits.DesignthetransmittertokeeptrackoftherunningDCbuildup,andalwayspickthecodewordthatpushestheDClevelbacktowardszero.Forexample,AMI,8B10B,4B3T,etc.
Linecodingshouldmakeitpossibleforthereceivertosynchronizeitselftothephaseofthereceivedsignal.Ifthesynchronizationisnotideal,thenthesignaltobedecodedwillnothaveoptimaldifferences(inamplitude)betweenthevariousdigitsorsymbolsusedinthelinecode.Thiswillincreasetheerrorprobabilityinthereceiveddata.
Itisalsopreferredforthelinecodetohaveastructurethatwillenableerrordetection.
Notethattheline-codedsignalandasignalproducedataterminalmaydiffer,thusrequiringtranslation.
Alinecodewilltypicallyreflecttechnicalrequirementsofthetransmissionmedium,suchasopticalfiberorshieldedtwistedpair.Theserequirementsareuniqueforeachmedium,becauseeachonehasdifferentbehaviorrelatedtointerference,distortion,capacitanceandlossofamplitude.
2.Commonlinecodes
2.1Non-return-to-zero(NRZ)
non-return-to-zero(NRZ)linecodeisabinarycodeinwhich"1s"arerepresentedbyonesignificantcondition(usuallyapositivevoltage)and"0s"arerepresentedbysomeothersignificantcondition(usuallyanegativevoltage),withnootherneutralorrestcondition.ThepulseshavemoreenergythanaRZcode.UnlikeRZ,NRZdoesnothaveareststate.NRZisnotinherently(naturely)aself-synchronizingcode,sosomeadditionalsynchronizationtechnique(perhapsarunlengthlimitedconstraint,oraparallelsynchronizationsignal)mustbeusedtoavoidbitslip.
Foragivendatasignalingrate,i.e.,bitrate,theNRZcoderequiresonlyhalfthebandwidthrequiredbytheManchestercode.
Whenusedtorepresentdatainanasynchronouscommunicationscheme,theabsenceofaneutralstaterequiresothermechanismsfordatarecovery,toreplacemethodsusedforerrordetectionwhenusingsynchronizationinformationwhenaseparateclocksignalisavailable.
NRZ-Levelitselfisnotasynchronoussystembutratheranencodingthatcanbeusedineitherasynchronousorasynchronoustransmissionenvironment,thatis,withorwithoutanexplicitclocksignalinvolved.Becauseofthis,itisnotstrictlynecessarytodiscusshowtheNRZ-Levelencodingacts"onaclockedge"or"duringaclockcycle"sincealltransitionshappeninthegivenamountoftimerepresentingtheactualorimpliedintegralclockcycle.Therealquestionisthatofsampling--thehighorlowstatewillbereceivedcorrectlyprovidedthetransmissionlinehasstabilizedforthatbitwhenthephysicallinelevelissampledatthereceivingend.
However,itishandytoseeNRZtransitionsashappeningonthetrailing(falling)clockedgeinordertocompareNRZ-Leveltootherencodingmethods,suchasthementionedManchestercode,whichrequiresclockedgeinformation(istheXORoftheclockandNRZ,actually)andtoseethedifferencebetweenNRZ-MarkandNRZ-Inverted.
2.1.1UnipolarNon-Return-to-ZeroLevel
"One"isrepresentedbyonephysicallevel(suchasaDCbiasonthetransmissionline).
"Zero"isrepresentedbyanotherlevel(usuallyapositivevoltage).
Inclocklanguage,"one"transitionsorremainshighonthetrailingclockedgeofthepreviousbitand"zero"transitionsorremainslowonthetrailingclockedgeofthepreviousbit,orjusttheopposite.Thisallowsforlongserieswithoutchange,whichmakessynchronizationdifficult.Onesolutionistonotsendbyteswithouttransitions.
2.1.2BipolarNon-Return-to-ZeroLevel
"One"isrepresentedbyonephysicallevel(usuallyanegativevoltage).
"Zero"isrepresentedbyanotherlevel(usuallyapositivevoltage).
Inclocklanguage,inbipolarNRZ-Levelthevoltage"swings"frompositivetonegativeonthetrailingedgeofthepreviousbitclockcycle.
AnexampleofthisisRS-232,where"one"is−5Vto−12Vand"zero"is+5to+12V.
2.1.3Non-Return-to-ZeroSpace
"One"isrepresentedbynochangeinphysicallevel.
"Zero"isrepresentedbyachangeinphysicallevel.
Inclocklanguage,theleveltransitionsonthetrailingclockedgeofthepreviousbittorepresenta"zero."
This"change-on-zero"isusedbyHigh-LevelDataLinkControlandUSB.Theybothavoidlongperiodsofnotransitions(evenwhenthedatacontainslongsequencesof1bits)byusingzero-bitinsertion.HDLCtransmittersinserta0bitafterfivecontiguous1bits(exceptwhentransmittingtheframedelimiter'01111110').USBtransmittersinserta0bitaftersixconsecutive1bits.Thereceiveratthefarenduseseverytransition--bothfrom0bitsinthedataandtheseextranon-data0bits--tomaintainclocksynchronization.Thereceiverotherwiseignoresthesenon-data0bits.
2.1.4Non-Return-to-ZeroInverted(NRZI)
Nonreturntozero,inverted(NRZI)isamethodofmappingabinarysignaltoaphysicalsignalfortransmissionoversometransmissionmedia.ThetwolevelNRZIsignalhasatransitionataclockboundaryifthebitbeingtransmittedisalogicalone,anddoesnothaveatransitionifthebitbeingtransmittedisalogicalzero.
"One"isrepresentedbyatransitionofthephysicallevel.
"Zero"hasnotransition.
Also,NRZImighttaketheoppositeconvention,asinUniversalSerialBus(USB)signalling,wheninMode1(transitionwhensignallingzeroandsteadylevelwhensignallingone).Thetransitionoccursontheleadingedgeoftheclockforthegivenbit.ThisdistinguishesNRZIfromNRZ-Mark.
However,evenNRZIcanhavelongseriesofzeros(oronesiftransitioningon"zero"),soclockrecoverycanbedifficultunlesssomeformofrunlengthlimitedcodingisusedontop.Magneticdiskandtapegenerallyusesfixed-rateRLLcodes,whileUSBusesbitstuffing,whichisefficient,butresultsinavariabledatarate:
ittakesslightlylongertosendalongstringof1bitsoverUSBthanitdoestosendalongstringof0bits.(USBinsertsanadditional0bitafter6consecutive1bits.)
2.2Return-to-zero(RZ)
Return-to-zero(RZ)describesalinecodeusedintelecommunicationssignalsinwhichthesignaldrops(returns)tozerobetweeneachpulse.Thistakesplaceevenifanumberofconsecutive0'sor1'soccurinthesignal.Thesignalisself-clocking.Thismeansthataseparateclockdoesnotneedtobesentalongsidethesignal,butsuffersfromusingtwicethebandwidthtoachievethesamedata-rateascomparedtonon-return-to-zeroformat.
The"zero"betweeneachbitisaneutralorrestcondition,suchasazeroampli
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