With Contributions of the following partners (in order of ap(5)

Dimitriouetal.[132]triedtosolvetheSIRexpressionsinordertodeterminethetargetreceivedpowersatthebasestation.Thepresentedapproachisaniterativeonethatcalculatesthedeterminantsofthesystemwithnactiveusersbasedonthedeterminantswhenn 1activeusersareinthecell.Thereceivedpowersarethendeducedandthenewcallisblockedifthecorrespondingtransmittedpowersexceedthemaximaltransmissionone.AtunableguardfactorFisproposedtocontrolthedroppingratesinthesystem.Fisincreasedwhentheobserveddroppingprobabilityduetotheusers’mobilityistoohigh.However,thisanalyticalstudyhasbeenconductedforasingle-cellcase,anditwasfollowedwithsimulations[133]thatshowthattheother-cellinterferencereducesthecapacity.Alimitationofthisalgorithmisthecomplexity

A state of the art

2.2CDMA-orientedCACalgorithms

ofthecalculationsitrequireswhichmakesverydif culttheintroductionofenhancementslikeprioritizationbetweenhandoffandnewcalls,ortheintroductionofmultimediaservices.

2.2.2.2ICACorPower-CAC?Thetwoapproachesdescribedabove(ICACandPower-CAC)arecom-paredbysimulationsin[213],anditisshownthattheappropriatethresholdsforICACmayleadtoaperfor-mancecomparabletothatofPower-CAC.However,itisshownin[82]thatthesystemmaybeinterference-limitedifthepowerthresholdneverpreventsthepowercontroltoreachanequilibriumpoint,anditispower-limitedotherwise.ItisalsoshownthattheoptimalthresholdvalueofICACdependsonthepropagationenvironment,andthatwecannotde neasimplegeneralcriteriatotunetheinterferencethresholdPthr.TheproblemoftuningPthrwasalsoinvestigatedbysimulationsin[314]todeterminethetradeoffbetweentheblockinganddroppingprobabilitiesinthechoiceofthecapacitybound.Power-basedCACremainsthenthemostaccuratemethodtocontrolthesystem.

2.2.3SIR-basedCAC

AnotherapproachtoCDMA-orientedCACistobasethedecisionofacceptingornotanewarrivalonitseffectontheSIRofexistentusers.ThischoiceisbasedonthefactthatSIRisanimportantmeasureoftheQoSinCDMAradiointerface:itcanbemappedintocorrespondingbitrates,biterrorrates,orbitenergytointerferenceratio(Eb/N0)(Chahedetal.[90]).Thisapproach,combinedwithanadequatepowercontrolmechanism,makesitpossibletoconsiderthemaximaltransmissionpowerofmobilestationsthatisanimportantissueintheuplinkasweshowedabove.

The rstSIR-CACalgorithmwaspresentedbyLiuetal.in[290].ItisbasedontheideathatSIRisinverselyproportionaltothecapacity.Theresidualcapacityforeachcellkisthencalculatedas:

Rk=11 ,fortargetcellTH111

( TH ),foradjacentcells

where X denotesthegreatestintegerlessthanorequaltoX,andαisanempiricalconstantrepresentingtheestimateoftheinterferencecouplingbetweenadjacentcells.

Theoverallresidualcapacityisthentheminimalresidualcapacitiesbetweenalltheadjacentcells,andthenewcallisacceptedifthisquantityispositive.

Onecanseethatthisother-cellinterferenceestimationisnotveryaccurateanddoesnottakeintoaccountthevariousintercellinterferenceconditions.Kimetal.[248]proposedanextensionof[290]withmorerealisticassumptionsontheinterference,astheyconsidernotonlytheinterferencebetweenadjacentcells,butalsobetweencellsthatarenotadjacent,butclosetoeachother.Again,thealgorithmisdesignedforasingleclassoftraf c,andnocoverageconstraintsweretakenintoaccount.

2.2.3.1Handlingmultipleclassesoftraf cTheSIR-CACalgorithmpresentedbyJeonetal.[223]aimstoinsureprioritiesbetweenhandoffandnewcalls,andtosupportmultimediatraf cinbothuplinkanddownlink.Infact,inresourcesharingbetweencallrequests,sinceprematureterminationofconnectedcallsisusuallymoreannoyingthanrejectionofanewcallrequest,itiswidelyrecommendedthatasystemgiveshigherprioritytohandoffcallrequestsascomparedtonewones.Inadditiontothisnew/handoffcleavage,realtimecalls(voice,video,etc)maybegivenhigherpriorityoverelasticones(data).

A state of the art

2.2CDMA-orientedCACalgorithms

Whenaclassicallarrives,thealgorithmin[223]measurestheaverage(Eb/N0)foreachclassofusersandusesittopredictthe(Eb/N0)valueifthenewcallisaccepted.Athresholdisthen xedforeachclassofusersandthenewarrivalisacceptedifthepredicted(Eb/N0)doesnotgobelowthisthreshold.ThisalgorithmcanbeconsideredasaSIR-CAConeas(Eb/N0)canbedirectlyrelatedtotheSIR[253][90].

dMoreexplicitly,letΦuk,iandΦk,irespectivelydenotetheuplinkanddownlinkthreshold(Eb/N0)sofaclass

dkcallforcontrollingadmissionofaclassinewcallrequestandlet uk,iand k,irespectivelybetheuplink

anddownlinkthreshold(Eb/N0)sofaclasskcallforcontrollingahandoffcallrequestfromclassi.Toguaranteethepriorityofhandoffcallsovernewones,wemusthave:

u uk,i<Φk,i,d dk,i<Φk,ifor0≤k≤L 1

Fortheprioritiesbetweencallsofdifferentclasses,CACthresholdvaluesshouldbedeterminedsoastoguaranteetheprioritybetweencallclasses,consideringthedataratesandtherequired(Eb/N0)sofallcallclasses.

ThesystemismodeledasamultidimensionalcontinuoustimeMarkovchainwhose owbalanceequationsaredetermined.Asperformancemeasures,theauthorscalculatedtheblockingandoutageprobabilitiesaswellasthecellthroughput.Thethresholdsarethencalculatedtomaximizethethroughputwhileminimizingthefailureprobabilities.

Thismathematicalstudyisquitecompleteandoffersmanyanalyticalinsights.Itconsidersdifferentclassesofmultimediatraf cwithdistinctionbetweenhandoffandnewarrivals,forbothuplinkanddownlink.However,itisverycomplexandtheCACdecisionisonlyobtainedaftercumbersomecalculations.Italsofailstoconsiderpowerandcoverageconstraintsandtheanalyticalmodelforthe(Eb/N0)isnotaccurateasitisbasedonaveragecalculations.

2.2.3.2Theeffectivebandwidth:apossiblesolutioninWCDMATheeffectivebandwidth,anotionthatoriginatedinCACalgorithmsforATM-basedB-ISDNusingthelargedeviationstheory[362],ismakingitswaystothecontextof3Gsystems.Evansetal.[154],appliedATM’seffectivebandwidthtechniquesinmulti-cellCDMA.Intheirwork,thepresentedCACisaSIR-basedoneastheQoSofmobileiisexpressedintermsoftherequiredbitrateRiandthebitenergytointerferencedensityratio(Eb/I0)i,encapsulatedintheminimumSignaltoInterferenceDensityRatio(SIDR)values

Γi=Ri(Eb/I0)i,i=1...N

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