LTE网络优化方案：上下行链路不均衡的优化分析

13GPPTSGRANWG2#58bisTdoc-R2-072721Orlando,U.S.A.,25th–29thJune2007Agendaitem:4.5.1Source:NTTDoCoMo,TelecomItalia,T-MobileTitle:UseofcellspecificoffsetsandreadingneighbourBCHDocumentfor:Discussion1.IntroductionInRAN2#58inKobe,RAN2hasdecidedthat,toallowforsufficientmobilitycontrolwithoutNCL,anoffsetvalueshallbeincludedinBCH,andthattheUEshallreadtheneighbourcellBCHtoobtainthisoffsetvaluebothinACTIVEandIDLEmodes[1].Theoffsetvaluebiasesthemeasuredquantityofthecorrespondingcellformobilitycontrol.ItwasexpressedbyoperatorsthatthisoffsetisnecessaryprimarilytocontrolthecellboudariesconsideringtheDLandULcoverageimbalance,causedbyDL/ULfeedercablelossdifference(duetoTMA)andeNBshavingdifferenttransmissionpowersadjoininginthenetwork[2].However,inRANPlenary#36inBusan,thedecisionwastakenbackaftersomevendorsexpressedconcernsonthehandover/cellreselectiondelaysandUEbatteryconsumption[3].Revisitingthisissue,thisdocumentexplainswhycellspecificoffsetsarethoughtnecessary,summarisesconcernsofreadingneighbourBCH,andpresentsourpositionontheissue.NotethatthesupportforoptionalNCLforintra-frequencycellshasalreadybeenagreedinRAN2,andthishasnotbeenreopened.TheoptionalNCLshouldservepurposessuchastosetserving-neighbourpairwisespecificoffsetsortoblacklistcertaincells.Itcanalsobeusedtospeedupcelldetection,althoughrelevanceofthisispendingRAN4response.Hence,theonlyopenquestionthatneedstobeaddressedis“whetherUEreadsneighbourBCHandobtainstheoffsetvalueincludedtherewith,”andthisistheexactfocusofthispaper.2.Useofcellspecificoffsets2.1DL/ULimbalanceproblemAsmentionedin[2],theneedforacellspecificoffsetismainlymotivatedbythefactthateNBsofdifferentpowerclassescanbeadjoininginmanyplacesthroughoutthenetwork,andthateachcellhasdifferentDLandULfeedercablelosses(i.e.,DL/ULfeederlossdifferenceduetoTMA).Bysettingappropriteoffsetvalues,theDL/ULimbalancecanbemitigated.Beforegoingintohowoffsettingworks,theDL/ULimbalanceproblemhastobeunderstood.Figure1showstheprincipleofDL/ULimbalancecausedbycablelossdifference.Assumingtwobasestations,havingthesameantennasandpropagationcoefficients,thecellboundarywillbeatthecentre(equidistant)basedonpathloss(ULoriented).However,ifthetwobasestationshavedifferentcablelosses(ordifferenttransmissionpowers),thecellboundarywilldeviatefromthecentrebasedonEc/N0(DLoriented),hencecausingDL/ULimbalance.2Fig.1DL/ULimbalanceprinciple.2.2MitigatingDL/ULimbalanceTheDL/ULimbalanceproblemcanbemitigated/toleratedinanumberofways:Alt.1:MobilitycontrolbasedonEc/N0(donothing)Alt.2:AdjustDLtotaltransmissionpowerbasedontheULcoverage(DL/ULbalancing)Alt.3:AdjustDLpilottransmissionpowerbasedontheULcoverageAlt.4:UsecellspecificoffsetsEachsolutionisdescribedindetailinthesequel.Notethatthesesolutionsarenotexclusive,andcanbecombinedifsodesired.Tosimplifythediscussion,thetrafficdistributionaspectisomittedforthequalitativeassessmentbelow.Howeverinpractice,cellplanninghastotakeintoaccountrealtrafficdistributions,whichcanbefarfromidealuniform.Itshouldbenotedthatthisaddsanotherdimensiontonetworkplanning,whichcanbequitecomplicated.2.2.1Alt.1:Ec/N0basedmobilitycontrol(donothing)Thefirstsolutionistodonothingspecial,andjustrelyonEc/N0tocontrolmobility(Fig.2).ThiscanbeoptimumfortheDL,however,theULwilldeteriorateespeciallyatcelledge.IftheUEhassufficienttransmissionpower(typ.smallcells),itwilltransmitatalargerpowertosatisfyitsQoS(i.e.,therequiredSIRforadesiredrate).IftheUEdidnothaveenoughpower(typ.largecells),thelikelyconsequenceinaschedulerbasedsystemasinLTEisthatittransmits(isscheduled)morefrequentlysothatthedesiredratecanbemet.Ineithercase,thiswillcreatelargerinterferenceattheneighbourcell,andhencedecreasessystemcapacityintheUL.Fig.2Ec/N0basedmobilitycontrol(donothing).2.2.2Alt.2:DLtotalTxpoweradjusting(balancing)3ThesecondsolutionistoadjustthetotalDLtransmissionpowerofthebasestationsuchthattheDLboundarymatchestheULboundary,whilethepowerratioofthepilotsymbolsusedformobilitymeasurementsismaintained.AnexamplecaseisshowninFig.3.Thetotalpowerofthebasestationhavingasmallercablelossisreduced,suchthattheemittedpowerfromtheantennaisequaltothatoftheneighbourhavingalargercableloss.ThiswillbalancetheDL/ULboundaries,henceresolvingimbalance,andwillbetheoptimumintermsoftheUL.ThishasabenefitinthatitlimitsDLinterferenceattheneighbour,andgenerallyimprovesIor/(Ioc+N0)atcellboundaries.However,ithassomedrawbacksasthecellwiththereducedpowernowhaslimitedcapacityduetoreducedpower.Itmayalsoreducechannelestimationandcelldetectionperformancesduetoreducedpilotpower.Thiswillbemoreevidentinnoiselimiteddeployements(e.g.,largecells).Fig.3DLtotalTxpoweradjusting(balancing).2.2.3Alt.3:DLpilotTxpoweradjustingInsteadofadjustingthetotaltransmissionpower,thetransmissionpowerofthepilotsymbolscanbereduced,whilemaintainingthetotalpower(Fig.4).ThiswouldalsoresolveDL/ULimbalance.Sincethetotalpowerismaintainedanddatatransmissionscanbeallocatedlargerpowers,thecellcanprovidelargercapacitycomparedtoAlt.2.However,thiswillcreatelargerinterferenceattheneighbourcell,andwillreducechannelestimationandcelldetectionper