Open-Loop Power Control Performance in DS-CDMA Net

BaltzerJournalsDecember6,1996Open-LoopPowerControlPerformanceinDS-CDMANetworkswithFrequencySelectiveFadingandNon-StationaryBaseStationsA.ChockalingamandLaurenceB.MilsteinDepartmentofElectricalandComputerEngineeringUniversityofCalifornia,SanDiegoLaJolla,CA92093-0407Inthispaper,westudytheperformanceofasimpleandeasy-to-implementdis-tributedpowercontrolstrategyapplicabletodirectsequencecodedivisionmul-tipleaccess(DS-CDMA)networks.Theschememakesuseofthereceivedpowermeasurementsmadeontheforwardlinkatindividualmobileunitstocontrolthetransmitpowersontheirreverselinks.Thealgorithm,whicheectivelycom-pensatesfortheslowlyvaryingdistanceandshadowlosses(duetotheirhighcorrelationonbothforwardandreverselinks),attemptstominimizetheeectoffastmultipathfadingbyaveragingitout.Weadoptaquasi-analyticapproachtoestimatethereverselinkcapacityperformanceofanopen-looppowercontrolschemeinbothasinglecellandamulti-cellenvironment,andwedothisbothforaxedbasestationaswellasamovingbasestationscenario.Non-stationarybasestationsaretypicalintacticalandemergencycommunicationsscenarioswherethebasestationscouldbemountedonmovingplatforms(e.g.,tanks,jeeps,un-mannedairbornevehicles).Weestimatethecapacitydegradation,whenbasestationsmoverelativetoothercells,asafunctionoftheamountofcelloverlapandthestandarddeviationofthepowercontrolerror.Wealsoprovideacom-parisonoftheperformanceoftheopen-looppowercontrolstrategywiththatofaclosed-looppowercontrolstrategy.Keywords:open-looppowercontrol,DS-CDMA,frequencyselectivefading,non-stationarybasestations.1{IntroductionTransmitterpowercontrolremainstobeacrucialissueincellulardirectsequencecodedivisionmultipleaccess(DS-CDMA)networks,asthecapacitymaximiza-TherstauthorispresentlywithQualcomm,Inc.,6455LuskBoulevard,SanDiego,CA92121-2779.E-mail:[e-mail]ortedbyTRWMilitaryElectronics&AvionicsDivisionunderGrantNB8541VK2S,AirtouchCommunications,andtheMICROProgramoftheStateofCalifornia.A.ChockalingamandLaurenceB.Milstein/Open-LoopPowerControl...2tionandfairallocationofresourcesamongdierentuserslargelydependontheeectivenessofthepowercontrolschemeemployed[1],[2].Anadaptivepowercontrol(APC)schemeisdesirabletocombattheeectsoffading,shadowinganddistancelosses.Suchaschemeattemptstomaintainaconstantaverageper-formanceamongtheusers,minimizetherequiredtransmitpowerateachuserterminal,andreducethemultiple-accessinterferenceeect.TherearetwoformsofAPC,viz:;open-loopandclosed-loopAPC.Inclosed-loopimplementations,thereverselink(mobile-to-basestationlink)channelstateisestimatedbythebasestation,whichthentransmitsthisinformationbacktothemobileontheforwardlink(basestation-to-mobilelink)foruseincontrollingthemobile’stransmitpower[3],[4].Inaterrestrialcellularenvironment,suchaschemeisfeasibleundermostconditions;however,evenwithlowearthorbit(LEO)satellites,theroundtrippropagationdelayisabout10msfora400nauticalmilesatelliteandupto60msfor800nauticalmilesatellitesatlowerelevationangles[5].Becauseclosed-loopschemesaresensitivetothisroundtripdelay,theiruseistypicallynoteectiveinalandmobilesatellitesystem,sincesuchasystemsuersfromrapidmultipathfading.Thatis,duetothelargeroundtrippropagationdelay,thefadesoccurtoorapidlyfortheclosed-loopAPCtotrack.Analternate,andsimplerformofpowercontrolisanopen-loopsystem[6].Insuchascheme,thechannelstateontheforwardlinkisestimatedbythemobileuser,andthisestimateisusedasameasureofthechannelstateonthereverselink.Thistechniqueisidealiftheforwardandreverselinksareperfectlycorrelated.However,duetomultipathfading,thisisnot,ingeneral,true.Nevertheless,open-loopAPCcancompensateforlargescalevariationssuchasshadowing,anditprovidesafast,inexpensivemethodtoequalizeaveragereceivedpoweratthebasestation.Anopen-loopAPCalgorithmshouldthereforeattempttominimizetheeectofthemultipathfadingcomponentbyaveragingitout.Thisresultsinarandomlyvaryingpowercontrolerror(PCE)thatcausesperformancedegradation.Theresultingdistributionoftheopen-looppowercontrolerroronafrequencyselectiveRayleighfadingchan-nelisshowntohaveanapproximatelylog-normaldistributionwiththestandarddeviationtypicallyvaryingintherange1to4dBdependingonthemeasurementinterval,vehiclevelocity,andthenumberofRAKEreceivertaps[7].Inthispaper,usingaquasi-analyticapproach,weestimatethereverselinkcapacityoftheopen-looppowercontrol(OLPC)schemeinbothasinglecellaswellasamulti-cellenvironment.Ourkeyfocushereistopredictthedegradationofthereverselinkcapacityduetoboththeopen-looppowercontrolerrorandthepresenceofnon-stationarybasestationsinthesystem.Non-stationarybasestationsbecomenecessarytoprovideaneectivecommunicationsinfrastructureintacticalandemergencycommunicationsenvironments,wherebasestationscouldbemountedonmovingplatformslikejeeps,tanks,unmannedairbornevehicles(UAV),etc.,whichcanmovealongwiththeusersasacell.Thus,dynamicbaseA.ChockalingamandLaurenceB.Milstein/Open-LoopPowerControl...3stationmobility,inadditiontotheusermobility,becomesakeyissuewhichaectsperformance.Thereverselinkcapacityinamulti-cellenvironmenthasbeeninvestigatedbymanyauthors[1