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PostedbyleopedriniFriday,June22,201211:12:00AMCategories:Course

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It'sveryimportanttousesolutionsthatimprovetheefficiencyoftheadoptedmodelinanydatacommunicationsystem.Ifthetransmissionis'Wireless',thisneedisevengreater.

In this scenario we have techniques that basically checks, or verify if the information sent by thetransmittercorrectlyarrivedinthereceiver.Inthefollowingexample,wehaveapacketbeingsentfromthetransmittertothereceiver.

Iftheinformationarrivedproperly(complete),thereceiverisreadytoreceive(andprocess)newdata.Iftheinformationarrivedwithsomeproblem,corrupted,thereceivermustrequestthatthetransmittersentthepacketagain(retransmission).

Let's understand a little more about these concepts increasingly used (and required) in the currentsystems?

Note:AlltelecomHallarticlesareoriginallywritteninPortuguese.FollowingwetranslatetoEnglishand

Spanish. As our time is short, maybe you find some typos (sometimes we just use the automatic

translator, with only a final and 'quick' review).We apologize andwe have an understanding of our

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Error Checking and Correction

What is Retransmission, ARQ and HARQ?

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Westarttalkingabouterrors.Errorsarepossible,andmainlyduetothetransmissionlink.Infact,wecaneven'expect'errorswhenitcomestoWirelessDataTransmission.

Ifwe have errors,we need to take some action. In our case,we can divide it into two steps: errorcheckinganderrorcorrection.

Errorcheckingisrequiredtoallowthereceivertoverifythattheinformationthatarrivediscorrectornot.

Oneof themostcommonmethodsoferrorchecking is theCRC,or 'CyclicRedundancyCheck',wherebits(CRC)areaddedtoagroupofinformationbits.TheCRCbitsaregeneratedbasedonthecontentsoftheinformationbits.Ifanerrorhappenswiththeinformationbits,theCRCbitsareusedtoverifyandhelprecoverthedegradedinformation.

The level of protection provided is determined by the ratio: number of CRC bits by the number ofinformationbits.Aboveacertainerrorlevel,theprocessiseliminated.CRCprotectionisusedpracticallyinallexistingVoiceandDataapplications.

ThefollowingdiagramshowsasimplifieddemonstrationofhowtheCRCisused.

AndtheCRCisdirectlyconnectedtotheErrorCorrectionmethods.TherearevariouswaysofFowardErrorCorrection(FEC),but themain idea is,givena levelofquality in the link, try toget the lowestnumberofrequiredretransmissions.

Minimizing the number of retransmissions we ended up having a more efficient data flow result,includingmainlythe'Throughput'.

Insimplifiedway:theCRCletsyouknowifapackagearrived'OK'or'NOTOK'.EverypacketthatissenthasaCRC,ora'Signature'.Asananalogy,it'slikewhenwesendalettertosomeone,andintheendwe sign: 'My Full Name'. When the other person receives this letter (information), he checks thesignature:'MyWrong'.Inthiscase,hetellstheMessenger:'Idon'tknow'MyWrong',thisinformationhassomeproblems.Pleaseasksendertosenditagain!'.

I.e.IdoCRCchecks.IftheCRCis'wrong',theinformationis'wrong'.IftheCRCis'correct',probablytheinformationis'correct'.

Retransmissions

Retransmissionsarethen:sendinformationagain(repeat)tothereceiver,afteritmakesucharequest.Thereceiverrequeststhatthe informationberetransmittedwhenever itcannotdecodethepacket,orthe result of decoding has been an error. That is, after checking that the information reached thereceiverisnot'OK',weshouldrequestittoberetransmitted.

Of course, when we have a good link (SNR), without interference or problems thatmay affect dataintegrity,wehavevirtuallynoneedforretransmissions.

Inpractice,inrealWorld,thisisverydifficulttohappen,becausethelinkscanfacethemostdifferentadversities.Thus,anefficientmechanismtoenableandmanagetheretransmissionisessential.

We consider such amechanism as efficientwhen it allow data communication in a linkmeet qualityrequirementsthattheservicedemands(QoS).

Voiceforexample,isaservicewhereretransmissiondoesnotapply.Ifapieceofinformationislost,andisretransmitted,theconversationbecomesintelligible.

On the other hand, data services practically rely on retransmission, sincemost have or allows acertaintolerancetodelayssomemore,someless.Withtheexceptiononlyfor'RealTime'services.

But it is also important to take into account that the greater the number of needed retransmissions,lowerthedatatransmissionratethatiseffectivelyreached:Iftheinformationhavetoberetransmittedseveraltimes,itwilltakelongforthereceivertoobtainthecompletefinalinformation.

ARQ

Tillnowwetalkedinagenericwayaboutdataretransmissions,errorcheckingandcorrection.Let'snowseesomerealandpracticalschemes.

The simplest way (or more common) control using what we described above is known as ARQ, or'AutomaticRepeatRequest'.

InARQ,whenwehavea'bad'package,thesystemsimplydiscardsit,andasksforaretransmission(ofthesamepackage).Andforthis,itsendsafeedbackmessagetothetransmitter.

Thesefeedbackmessagesaremessagesthatthereceiverusestoinformwhetherthetransmissionwassuccessfulornot:'ACKnowledgement'(ACK)and'NonACKnowledgement'(NACK).Thesemessagesaretransmittedfromthereceivertothetransmitter,andrespectivelyinformsagood(ACK)orbad(NACK)receptionofthepreviouspackages.

If in the new retransmission the packet keep arriving with errors, the system requests a newretransmission(stillforthissamepackage).Thatis,sendsanother'NACK'message.

Thedatapacketsthatarenotproperlydecodedarediscarded.Thedatapacketsorretransmissionsareseparatelydecoded.Thatis,everytimeapacketthatarrivesisbad,itisdiscarded,anditisrequestedthatthissamepackageberetransmitted.

Butseethatiftherewerenoretransmissions,theperformanceofthedataflowwouldbemuchbetter.Intheexamplebelow,comparedwiththeprevious,wetransmitmoreinformation3timesinthesametimeinterval.

Unfortunatelywedon'thavemuchtodoaboutthelinkconditions.Orbetter,weareabletoimprovethe

linksperformance,forexamplewithconfigurationparametersoptimization,butwe'llalwaysbesubjecttofaceadverseconditions.Inthiscase,ouronlywayoutistotrytominimizeretransmissions.

Andthat'swhereariseothertechniquesormore'enhanced'schemesforretransmission.ThemainoneisHARQ.

Hybrid ARQ (HARQ)

The HARQ is the use of conventional ARQ along with an Error Correction technique called 'SoftCombining',whichnolongerdiscardsthereceivedbaddata(witherror).

Withthe'SoftCombining'datapacketsthatarenotproperlydecodedarenotdiscardedanymore.Thereceivedsignalisstoredina'buffer',andwillbecombinedwithnextretransmission.

Thatis,twoormorepacketsreceived,eachonewithinsufficientSNRtoallowindividualdecodingcanbecombinedinsuchawaythatthetotalsignalcanbedecoded!

The following image explains this procedure. The transmitter sends a package [1]. The package [1]arrives,andis'OK'.Ifthepackage[1]is'OK'thenthereceiversendsan'ACK'.

Thetransmissioncontinues,andissentapackage[2].Thepackage[2]arrives,butlet'sconsidernowthatitarriveswitherrors.Ifthepackage[2]arriveswitherrors,thereceiversendsa'NACK'.

Onlynowthispackage[2](bad)isnotthrownaway,asitisdoneinconventionalARQ.Nowitisstoredina'buffer'.

Continuing,thetransmittersendanotherpackage[2.1]thatalso(let'sconsider)arriveswitherrors.

Wehavetheninabuffer:badpackage[2],andanotherpackage[2.1]whichisalsobad.

Doesbyadding(combining)thesetwopackages([2]+[2.1])wehavethecompleteinformation?

Yes.Sowesendan'ACK'.

Butifthecombinationofthesetwopackagesstilldoesnotgiveusthecompleteinformation,theprocessmustcontinueandanother'NACK'issent.

Andtherewehaveanotherretransmission.Nowthetransmittersendsathirdpackage[2.2].

Let'sconsiderthatnowitis'OK',andthereceiversendsan'ACK'.

Herewecanseethefollowing:alongwiththereceivedpackage[2.2],thereceiveralsohaspackages[2]and[2.1],thathavenotbeendroppedandarestoredinthebuffer.

In our example, we see that the package arrived 2 times 'wrong'. And what is the limit of theseretransmissions?Upto4.IE,wecanhaveupto4retransmissionineachprocess.Thisisthemaximumnumbersupportedby'buffer'.

Different HARQ Schemes

Goingbacka little in thecaseofConventionalARQ,wheneverwesendapackageand itarriveswithproblems,itisdiscarded.

Takingtheaboveexample,whenwesendthepackage[2],anditarriveswitherrors,itisdiscarded.Andthissamepackage[2]issentagain.

Whathappensisthatwenolongerhavetheconceptof'packageversion'[2.1],[2.2],etc.Wedonothavethe'redundancy'version,orthegainwegetinHARQprocessing.

Tounderstandthis,weneedtoknowthatinformationisdividedasfollows:

[Information+Redundancy+Redundancy]

Whenwetransmitthepacket[2]wearetransmittingthis:

[Information+Redundancy+Redundancy]

Whenretransmitthesamepackage[2]weareretransmitingitagain:

[Information+Redundancy+Redundancy]

ButwhenweuseHARQ,andretransmitpacket[2.1]or[2.2],wehavethepossibilityof:

OrretransmitthatsameinformationagainOrretransmitonlytheredundancy.

Andthen,ifweretransmitlessinformation(onlyredundancy),wespendlessenergy,andthatwillrunmuchfaster.Withthiswehaveagain!

That is, we work with different 'versions of redundancy', that allows us to have a gain in theretransmission.Thisiscalled'RedundancyVersion',orwhatversionofredundancy.

The redundancy version, or HARQ scheme with 'Soft Combining' can be 'Chase Combination' or'IncrementalRedundancy'.

HARQ Chase Combination

ChaseCombination:whenwecombinethesameinformation(theretransmissionisanidenticalcopyoftheoriginalpacket).

Wetransmitaninformation,whicharrivedwrong,andweneedtodoaretransmission.Weretransmitthesameinformationandtherewedon'thavemuchgain.

HARQ Incremental Redundancy

IncrementalRedundancy:whereweretransmitonly theportion thatwedidn't transmittedbefore.Thusweretransmit less information.Less informationmeansfewerbits, lessenergy.Andthisgivesagain!

Redundancybitsareretransmittedgraduallytothereceiver,untilanACKisreceived.

With this,weadapt tochanges in theconditionof the link.The first retransmissioncan, forexample,containornotbitsofredundancy.Ifnecessary,asmallnumberofthesebitsisretransmitted.Andsoon.

Finishing for today: what are the 2 steps of HARQ? Why it gives me a Gain?

First because fromwrong packets 1 and 2 we can get a correct one, since we do not discard erroneous packetsanymore.Secondbecausewecanalsoinretransmissionsendlessinformation,andstreamlinetheprocess.

The use of HARQ with 'Soft Combining' increases the received Eb/Io effective value for eachretransmission, and therefore also increases the likelihood of correct retransmissions decoding, incomparisontoconventionalARQ.

Wesendapackage,and itarriveswitherrors:wekeepthispackage.Receivetheretransmissionandthenweaddorcombineboth.

HARQ Processes (Case Study)

Whatwehave seen so far clarifies the concepts involved. In practice, in retransmission, this type ofProtocoliscalled'StopAndWait'(thereareotherkindsofsimilarprotocols).

Whatwouldbe:sendtheinformationandstop.Waitfortheresponsetosendotherinformation.Send,waitforresponse.Send,waitforresponse...

No!Notsoinpractice.Inpractice,weworkwithanumberof'processes',whichmayvaryforexamplefrom4,6or8.Thefollowingimageillustratesthismoreclearly.

Other types of HARQ

Newschemesareconstantlybeingdevelopedandused,asthetypeIIIHARQ,whichusesselfdecodablepackages.

Butenterthesevariations,terminologyandconsiderations, isnotthescopeofourtutorial,whichwassimplytointroducetheconceptofRetransmission,ARQandHARQ.

Based on the key concepts illustrated here today, you can extend your studies the way you want,howeverwebelievethatthemostimportantthingwasachievedunderstandhowitworksandwhatareallthecitedconcepts.

JAVA Applet

Below,youcanseehowsomeretransmissionschemeswork.ThereareseveralAppletsavailable,forthemanypossibilities(ARQ,HARQ,WithSlidingWindows,Selective,etc).

ThenextisalinkforaJAVAAppletthatsimulatesa'SelectiveRepeatProtocoltransmission'.

http://media.pearsoncmg.com/aw/aw_kurose_network_4/applets/SR/index.html

Conclusion

This was another tutorial on important issues for those who work with IT and Telecom: dataTransmissionandRetransmissiontechniques,ARQandHARQ.

ARQisusedforapplicationsthatallowacertaindelay,asWebBrowsingandStreamingAudio/video.Itis used widely in Wimax and WiFi communication systems. However, it cannot be used in Voicetransmission,asforexampleinGSM.

HARQforexampleisusedinHSPAandLTE,andthereforemustbeawellunderstoodconceptforthosewhoworkorwanttoworkwiththesetechnologies.

Wehopeyouenjoyedit.Anduntilournexttutorial.

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