RACH.docx
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RACH.docx
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RACH
RACH
Whatisthemosttrickypartindevicetroubleshooting?
Myexperiencesays"Ifaproblemhappensinthemiddleofdoingsomething,itisrelativelyeasytofindtherootcauseandtroubleshootit(probablyImighthaveover-simplifiedthesituation-:
),butifsomethinghappenedbeforeanythingstarted,itwouldbeanightmare."Forexample,yousetthealltheparametersatthenetworkemulatorforaUEyouwanttotestandthenturnedontheUE.InaseveralsecondUEstartbootingandtheninacoupleofsecondyouseeacoupleofantennabarssomewhereatthetopofUEscreen..andtheninseveralsecondsyousee'SOS'or'ServiceNotAvailable'insteadofyournetworkoperatornamedisplayedonyourscreenandnormalAntennabars.ThisiswhatImeanby"probleminthemiddleofdoingsomething".Inthiscase,ifyoucollectUElogandequipmentlog,atleastyoucaneasilypinpointoutthelocationtheproblemhappensandstartfromthereforfurtherdetails.Butwhatifyouareinthissituation?
yousetthealltheparametersatthenetworkemulatorsideandturnontheUE..UEstartbootingup..showingthemessagesaying"SearchingNetwork...."andgotstuckthere..withnoAntennabars..noteven'SOS'..justsaying"Noservice".AndIcollectedUEsidelogandNetworkEmulatorsidelog,butnosignallingmessage.Thisiswhereourheadachestarts.
Asexamples,
i)Whatifyoudon'tsee'RRCConnectionRequest'whenyourturnedontheWCDMAUE?
ii)Whatifyoudon'tsee'ChannelRequest'whenyourturnedontheGSMUE?
iii)Whatifyoudon'tsee'RACHPreamble'whenyourturnedontheLTEUE?
Firstthingyouhavetodoistounderstandtheeverydetailsofthisprocedurenotonlyinthehighersignalinglayer,butalsoallthewaydowntothephysicallayersrelatedtothesefirststep.Andalsoyouhavetouseproperequipmentwhichcanshowthesedetailedprocess.Ifyouhaveanequipmentthatdoesnotprovidetheloggingoritprovideslogbutonlyhigherlayersingnalinglog,itwillbeextremlydifficulttotroubleshoot.Giventhatyouhavethepropertools,thenextthingyouhavetobereadyistounderstandthedetailedknowledgeoftheseprocess.Withouttheknowledge,howevergoodtoolsIhaveitdoesn'tmeananythingtome.So?
IwanttoteachmyselfhereaboutthefirststepofLTEsignalingwhichisRACHprocess.(SomebodywouldsaytherearemanyofotherstepsevenbeforetheRACH,likefrequencySync,TimeSync,MIB/SIBdecoding..butitputtheseasidefornow..sinceitismorelikebasebandprocessing).
∙WhenRACHProcessoccurs?
∙TwotypesofRACHprocess:
Contention-basedandContention-free
∙ExactlywhenandWhereaUEtransmitRACH?
∙Whatispreambleformat?
∙HowdoesNetworkknowsexactlywhenUEwilltransmittheRACH?
∙PRACHPreambleSignalStructure
∙HowtogenerateRACHSignal?
∙ExactlywhenandwhereNetworktransmitRACHResponse
∙PRACHParametersandit'sPhysicalMeaning
∙RACHProcedureduringInitialRegistration-RACHProcedureSummary
∙HowcanwegetRARNTI?
∙AnExampleofFullRACHProcess
∙PRACHRetransmission
∙RACHProcessOverviewInDiagrams
oRACHProcedureonInitialRegistration
oRACHProcedureonHandover-ContentionBased
oRACHProcedureonHandover-NonContentionBased
oRACHProcedureonDLDataArrivalwhenOut-of-Sync-NonContentionBased
oRACHProcedureonDLDataArrivalwhenOut-of-Sync-ContentionBased
oRACHProcedureonULDataArrivalwhenOut-of-Sync
oRACHProcedureonRRCConnectionRe-establishmentwhenOut-of-Sync
∙PRACHRFSnapshot
∙3GPPStandardforRACHProcess
WhenRACHProcessoccurs?
Itwouldbehelpfultounderstandifyouthinkaboutwhen'RRCConnection'happens(orwhenPRACHprocesshappensifyouareinterestedinlowerlayerstuffs)inWCDMA.Itwouldalsobehelpfulifyouthinkaboutwhen'ChannelRequest'happensinGSMUE.
MyimpressionofLTERACHprocessislikethecombinationofPRACHprocess(WCDMA)andChannelRequest(GSM).Itmaynotbe100%correctanalogy..butanywayIgotthiskindofimpression.InLTE,RACHprocesshappensinfollowingsituation (3GPPspecification,10.1.5RandomAccessProcedureof36.300 )
i)InitialaccessfromRRC_IDLE
ii)RRCConnectionRe-establishmentprocedure
iii)Handover
iv)DLdataarrivalduringRRC_CONNECTEDrequiringrandomaccessprocedure
E.g.whenULsynchronisationstatusis搉on-synchronised�
v)ULdataarrivalduringRRC_CONNECTEDrequiringrandomaccessprocedure
E.g.whenULsynchronisationstatusis"non-synchronised"ortherearenoPUCCHresourcesforSRavailable.
vi)ForpositioningpurposeduringRRC_CONNECTEDrequiringrandomaccessprocedure;
E.g.whentimingadvanceisneededforUEpositioning
TwotypesofRACHprocess:
Contention-basedandContention-free
WhenaUEtransmitaPRACHPreamble,ittransmitswithaspecificpatternandthisspecificpatterniscalleda"Signature".IneachLTEcell,total64preamblesignaturesareavailableandUEselectrandomlyoneofthesesignatures.
UEselect"Randomly"oneofthesesignatures?
DoesthismeanthatthereissomepossibilitythatmultipleUEssendPRACHwithidenticalsignatures?
Yes.
Thereissuchapossibility.ItmeansthesamePRACHpreamblefrommultipeUEreachestheNWatthesametime..thiskindofPRACHcollisioniscalled"Contention"andtheRACHprocessthatallowsthistypeof"Contention"iscalled"Contentionbased"RACHProcess.InthiskindofcontentionbasedRACHprocess,Networkwouldgothroughadditionalprocessatlatersteptoresolvethesecontentionandthisprocessiscalled"ContentionResolution"step.
Butthereissomecasesthatthesekindofcontentionisnotacceptableduetosomereason(e.g,timingrestriction)andthesecontentioncanbeprevented.Usuallyinthiscase,theNetworkinformseachoftheUEofexactlywhenandwhichpreamblesignatureithastouse.Ofcourse,inthiscaseNetworkwillallocatethesepreamblesignaturesothatitwouldnotcollide.ThiskindofRACHprocessiscalled"ContentionFree"RACHprocedure.Toinitiatethe"ContentionFree"RACHprocess,UEshouldbeinConnectedModebeforetheRACHprocessasinHandovercase.
Typical'ContentionBased'RACHProcedureisasfollows:
i)UE-->NW:
RACHPreamble(RA-RNTI,indicationforL2/L3messagesize)
ii)UE<--NW:
RandomAccessResponse(TimingAdvance,T_C-RNTI,ULgrantforL2/L3message)
iii)UE-->NW:
L2/L3messageiv)Messageforearlycontentionresolution
Nowlet'sassumethatacontentionhappenedatstepi).Forexample,twoUEssentPRACH.Inthiscase,bothoftheUEwillrecievethesameT_C-RNTIandresourceallocationatstepii).Andasaresult,bothUEwouldsendL2/L3messagethroughthesameresourceallocation(meaningwiththesametime/frequencylocation)toNWatstepiii).WhatwouldhappenwhenbothUEtransmittheexactsameinformationontheexactsametime/frequencylocation?
OnepossibilityisthatthesetwosignalactasinterferencetoeachotherandNWdecodeneitherofthem.Inthiscase,noneoftheUEwouldhaveanyresponse(HARQACK)fromNWandtheyallthinkthatRACHprocesshasfailedandgobacktostepi).TheotherpossibilitywouldbethatNWcouldsuccessfullydecodethemessagefromonlyoneUEandfailedtodecodeitfromtheotherUE.Inthiscase,theUEwiththesuccessfulL2/L3decodingonNWsidewillgettheHARQACKfromNetwork.ThisHARQACKprocessforstepiii)messageiscalled"contentionresolution"process.
Typical'ContentionFree'RACHProcedureisasfollows:
i)UE<--NW:
RACHPreambleAssignment
ii)UE-->NW:
RACHPreamble(RA-RNTI,indicationforL2/L3messagesize)
iii)UE<--NW:
RandomAccessResponse(TimingAdvance,C-RNTI,ULgrantforL2/L3message)
ExactlywhenandWhereaUEtransmitRACH?
Toanswertothisquestion,youneedtoreferto3GPPspecificationTS36.211-Table5.7.1-2.
Didyouopenthespecificationnow?
ItshowsexactlywhenaUEissupposedtosendRACHdependingonaparametercalled"PRACHConfigurationIndex".
Forexample,iftheUEisusing"PRACHConfigurationIdex0",itshouldtransmittheRACHonlyinEVENnumberSFN(SystemFrameNumber).Isthisgoodenoughanswer?
DoesthismeanthatthisUEcantransmittheRACHinanytimewithinthespecifiedtheSFN?
Theanswertothisquestionisin"SubFrameNumber"colulmnofthetable.Itsays"1"for"PRACHConfigurationIdex0".ItmeanstheUEisallowedtotransmitRACHonlyatsubframenumber1ofeveryevenSFN.
Checkingyourunderstandingofthetable,Iwillgiveyouonequestion.Withwhich"PRACHConfigurationIdex",itwouldbetheeasiestfortheNetworktodetecttheRACHfromUE?
andWhy?
Theanswerwouldbe14,becauseUEcansendtheRACHinanySFNandanyslotswithintheframe.
Inabigpicture,youshouldknowallthedimmensionsinthefollowingdiagram.(TheRedrectangleisPRACHsignal).
TheR_SlotisdeterminedbyPRACHConfigurationIndexandR_lengthisdeterminedbyPremableformat.F_offsetisderminedbythefollowingequationwhenthepreambleformatis0~3.n_RA_PRBoffsetinthisequationisspecifiedbyprach-FreqOffsetinSIB2.
Whatispreambleformat?
Ifyouseethetable5.7.1-1showabove,youseethecolumntitledas"PreambleFormat".Whatisthepreambleformat?
Itisdefinedasfollowingdiagram.
YouwouldseethatthelengthofPRACHpreamblevariesdependingonthepreambleformat.Forexample,thelengthofPRACHwithpreambleformatis(3186+24567)Samples.(Asyouknow,onesample(Ts)is1/30.072us).
Youmayask"Whyweneedthiskindofmultiplepreambleformat?
",especiall
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