cellular network planning and optimization part6
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Cellular NetworkPlanning andOptimization
Part VI:WCDMABasicsJyriHmlinen,
Communications andNetworking Department,TKK,24.1.2008
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Networkelements
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Networkelements
Networkelementsina3GWCDMAbasedPLMN
Ourmainfocusarea
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Networkelements
TypicallyPLMNisoperatedbyasingleoperator
ConnectedtootherPLMNs andnetworkslikeInternet
UserEquipment(UE)contains
Mobileequipment(ME):RadiocommunicationoverUuinterface
UMTSSubscriberIdentityModule(USIM):Subscriberidentityinformation,authenticationalgorithms,encryptionkeysetc
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Networkelements
UMTSTerrestrialRadioAccessNetwork(UTRAN)
NodeB(BaseStation):Handles/managesthetrafficbetweenUu andIub interfaces.Basictaskslikecoding,interleaving,rateadaptation,modulation,spreadingetc.
RadioNetworkController(RNC):Controlradioresourcesinitsoperationarea.ProvideservicesforCoreNetwork(CN).Loadandcongestioncontrol,
admissionscontrol,codeallocation,radioresourcemanagementtasks.
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Networkelements
MobileServicesSwitchingCentre(MSC)/VisitorLocationCentre(VLR) HandlesswitchinginCircuitSwitched(CS)connectionsand
holdvisitingusersserviceprofiles. ServingGPRSSupportNode(SGSN)
SimilarfunctionalityasinMSC/VLRbutusedforPacketSwitched(PS)services
OtherCNelements GatewayMSC(GMSC):HandlesincomingandoutgoingCS
connections GatewayGPRSSupportNode(GGSN):LikeGMSCbutin
PSdomain HomeLocationRegister(HLR):Mastercopyofusers
serviceprofiles
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Physicallayer
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Uplinktransmissionpath
Spreadingcodesareusedtoseparatedataandcontrolofauser.
Scramblingcodesareusedtoseparatedifferentusers.
DualchannelQPSKmodulation(dataandcontrolintodifferentI/Qbranches)
Modulation Spreading
Modulation Spreading
Data
ControlScrambling
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Downlinktransmissionpath
Userswithinacell(sector)areseparatedbyorthogonalspreadingcodes(sometimesalsocalledaschannelizationcodes)
Cells(sectors)areseparatedbyscramblingcodes QPSKmodulation
Modulation Spreading
Modulation Spreading
.
.
.
.
.
.
Stream1
StreamN
Scrambling
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Spreading
Spreadingisdoneusingorthogonalcodes
Codesremainorthogonalonlyifsynchronizationis
perfect Multi-pathfadingwillreducetheorthogonality
[1]
[1,-1]
[1,1]
[1,-1,1,-1]
[1,-1,-1,1]
[1,-1,-1,1,-1,1,1,-1]
[1,-1,-1,1,1,-1,-1,1]
[1,1,-1,-1]
[1,1,1,1]
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Spreading
Data
Spreadingcode
Signalafterspreading
frequency frequency
SpreadingFactor(SF)defineshowmanychipsareusedtorepresentonedatasymbol
Spreadingexpandsthesignaltowideband
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Spreading
Spreadingprovidesprocessinggain.Letusdenote
W=systemchiprate
R=userbitrateThenprocessinggainisdefinedby
Whileuserdatarateincreases,theprocessinggaindecreasesaswellasthespreadingfactor.Hence,itisharderforthereceivertodetectthesignalcorrectly.
Sometimeswealsousetermspreadinggain.Itreferstovalue
=
R
WPG 10log10
( )SF10log10gainSpreading =
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Spreading
SomemeasuresthatareusedinWCDMAreceiverinvestigations
CINR=Carriertointerferenceandnoiseratio,alsoSINRisused
CIR=Carriertointerferenceratio,alsoSIRisused
SNR=Signaltonoiseratio E=Energyperuserbitdividedbythenoisespectral
density=processinggain*powerthatisneededto
overcometheinterferencefromotherusers.
NotationiscommonlyusedforE0/NEb
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Spreading
InWCDMAchiprateis3.84Mcps. Temporaldurationofthechipis1/3.84*10^6=
260.4ns. Signaltravels78.125metersduringthechipduration
Thisdistancedefinesthemaximumaccuracybywhichreceivercanresolvedifferentsignalpaths.
Signalamplitude
delaytime
longechos,usuallyonlyinHillyterrainenvironment
directpath
nearbyscatterers
RAKEoperations
Chipduration
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RAKE
AbasicreceiverthatisusedinWCDMAiscalledasRAKE Themultipathchannelthroughwhicharadiowavepropagates
canbeviewedasasumofmanydelayedcopiesoftheoriginaltransmittedwave,eachwithadifferentmagnitudeandtime-of-arrivalatthereceiver.Eachmultipathcomponentcontainstheoriginalinformation=>ifthemagnitudeandtime-of-arrivalofeachmultipathcomponentisknown(throughchannelestimation),thenallthemultipathcomponentscanbeaddedcoherently
RAKEisdesignedtocountertheeffectsofmultipathfading.Itdoesthisbyusingseveralfingers,eachdelayed(byorderofsomechips)inordertocatchtheindividualmultipathcomponents.
Componentsignalsfromfingersarecombinedcoherentlyforthesumsignalthatisusedindecoding.
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Scrambling
+1
-1
+1
-1
+1
-1
Signalafterspreading
Scramblingcode
Signalafterscrambling
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Scrambling
Scramblingcodesareusedtoseparateusersinuplinkandcellsindownlink
Scramblingisusedontopofspreading
Scramblingisnotchangingthesignalbandwidth
Indownlinkscramblingcodesareallocatedtothecells(sectors)innetworkplanningphase
Numberofscramblingcodesishigh=>codeplanningisatrivialtaskandcanbeautomated
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Spreadingandscramblingsummary
Noimpacttotransmissionbandwidth
IncreasestransmissionbandwidthBandwidth
UL:38400chips=10ms=framelength
DL:38400chips=10ms=framelength
UL:4-256chipsDL:4-512chips
Codelengthdefinessymbolrate
Length
UL:Separationofusers
DL:Separationofcells
UL:Separationofcontrolanddatafromthesameuser
DL:Separationofconnectionswithinacell
Usage
ScramblingcodesSpreadingcodes
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Importantchannels/uplink
Uplinkdedicatedchannel
PhysicallayercontrolinformationinDedicatedPhysical
ControlChannel(DPCCH),spreadingfactor=256 DataiscarriedinDedicatedPhysicalDataChannels
(DPDCH).Variablespreadingfactor
TherecanbemultipleDPDCHs butonlyoneDPCCH.
Note:Thereis
usuallyapowershiftbetweendataandcontrol
channels
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ControlinformationinDPCCH
Pilotbitsforchannelestimation Alwayspresent
TransmitPowerControl(TPC)bitsfordownlinkpowercontrol Alwayspresent
TransportFormatCombinationIndicator(TFCI) Informreceiveraboutactivetransportchannels
FeedbackBitInformation(FBI)
Presentonlywhendownlinktwo-antennaclosedlooptransmitdiversityisapplied
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UplinkDPDCHdatarates
Dataratesinthetableachievedwith rate
coding Parallelcodesnotused
inpracticeduetoreducedpoweramplifier
efficiency
Maximumratebelow500kbps.
Note:Inuplinkeachuserhaveallspreadingcodesinitsuse
2.8Mbps4,6parallelcodes
480kbps4
240kbps8
120kbps16
60kbps32
30kbps64
15kbps128
7.5kbps256
UserdatarateSpreadingfactor
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Importantchannels/downlink
Downlinkdedicatedchannel
DownlinkcontrolinformationiscarriedinDedicated
PhysicalControlChannel(DPCCH) DownlinkdataiscarriedinDedicatedPhysicalData
Channel(DPDCH)
Spreadingfactordependsontheservice
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Importantchannels/downlink
CommonPilotChannel(CPICH)
CPICHaidchannelestimationattheterminal
Spreadingfactor=256 Terminalmakeshandoverandcellselectionmeasurements
fromCPICH=>CPICH shouldbeheardeverywhereinthecell
CellcoverageandloadcanbeadjustedthroughCPICH IfCPICHpowerisreducedpartoftheterminalswillhand
overtoadjacentcells
Synchronizationchannel(SCH)
Synchronizationchannelisneededforcellsearch
Spreadingfactor=256
(*)Importantpropertyfromnetworkplanningperspective
(*)
(*)
(*)
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Importantchannels/downlink
PrimaryCommonControlPhysicalChannel(PrimaryCCPCH)
Carrybroadcastchannelandallterminalsinthesystemshouldbeabletoreceiveit.
IfCCPCHdecodingfailsthenterminalcannotaccesstothesystem=>CCPCHtransmissionpowerhigh.
Nopilotbits,channelestimationdonefromCPICH
whichistransmittedwithsameantennaradiationpattern
Spreadingfactor=256, ratecoding
(*)Importantpropertyfromnetworkplanningperspective
(*)
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DownlinkDPDCHdatarates
Dataratesinthetableachievedwith rate
coding Indownlinkallusers
sharethespreadingcodes=>numberof
orthogonalcodesdefinesahardlimitforcellcapacity
Partofthespreadingcodesarereservedforcontrolchannels
1-3kbps512
2.8Mbps4,3parallelcodes
936kbps4
456kbps8
215kbps16
105kbps32
45kbps64
20-24kbps128
6-12kbps256
UserdatarateSpreadingfactor
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Downlinkcontrol
Fromnetworkplanningperspectiveitisimportanttokeepinmindthatcontrolchannels
takepartoftheDLpower
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Radioresourcemanagement
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General
RadioResourceManagement(RRM)iselementarypartofWCDMA.
RRMisresponsibleforefficientutilizationoftheairinterfaceresourcesitisneededto
GuaranteeQualityofService(QoS)
Maintaintheplannedcoveragearea Optimizethecellcapacity
TheimportanceofRRMismostlyduetothe
featuresoftheUMTSsystem;interferencelimitednatureandadaptiveservices
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RRMalgorithms
FamilyofRRMalgorithms:
Powercontrol
Fastpowercontrol(NodeB,UE) Outerlooppowercontrol(RNC)
Handovercontrol(RNC)
Admissioncontrol(RNC) Loadcontrol(RNC)
Fastloadcontrol(NodeB)
Packetscheduling(RNC)
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Powercontrol
Objectives Maintainthelinkqualityinuplinkandindownlinkbycontrolling
thetransmissionpowers
Preventsnear-fareffect Minimiseeffectsoffastandslowfading Minimisesinterferenceinnetwork
Accuracyofthepowercontrolisimportant Notime-frequencyseparationofusers,allusethesame
bandwidth Inaccuracyinpowercontrolimmediatelyliftsthenetworks
interferencelevel,whichcorrespondinglylowersthecapacity Duetousersmobilitythespeedofpowercontrolisalsoa
criticalissue
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Near-farprobleminuplink
TherecanlargepathlossdifferencebetweenUE1(cell
centre)andUE2(celledge)
IfbothUEsaretransmittingwiththesamepowerthenUE1willblockUE2(andothercelledgeuserstoo)
PowercontrolwilldrivetransmissionpowersofUE1andUE2totheminimumlevelthatisrequiredtomeetQoS
InNodeBreceivedpowersfromUE1andUE2willbethesameforsameservices
UE1
UE2
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Powercontrol
PowerControlonthecommonchannels ensuresthattheircoverageissufficient bothtosetupUE-originatingandUE-terminatingcalls.
PowerControlonthededicatedchannels ensuresanagreedqualityof connectionintermsofBlockErrorRate(BLER),whileminimizingtheimpact onotherUEs.
UplinkPowerControl increasesthemaximumnumberofconnectionsthatcan beservedwiththerequiredQualityofService
(QoS),whilereducingboththe interferenceandthetotalamountofradiatedpowerinthenetwork.
DownlinkPowerControl minimizesthetransmissionpoweroftheNodeB and compensatesforchannelfading.Minimizingtransmittedpowermaximizesthedownlinkcapacity.
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Powercontrol
Mainpowercontrolapproaches
Fastpowercontrol:
Aimistocompensatetheeffectoffastfading Gainfromfastpowercontrolislargestforslowly
movingUEs andwhenfadingisflat,i.e.thereismulti-pathdiversity
Fastpowercontroldrivesthereceivedpowertoa
targetSIR.Thisvalueisdiscussedmorecloselyinconnectionwithdimensioning.
Outerlooppowercontrol AdjustthetargetSIRaccordingtoserviceQoS.
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PCmechanism
OuterloopPC:RNCadjustthetargetSIRinorder
tomeettargetBLER
FastPC:NodeBcommandterminaltochangetransmitpowerinordertomeettargetSIR ReceivedSIR
Outerlooppowercontrol
Fastpowercontrol
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UplinkouterloopPC
ThegoalistocontrolthetargetSIR inordertoremainthewantedQoS withminimumtransmitpower
ThetargetBLERisdefinedwiththeadmissioncontrolalgorithm
TheuplinkalgorithmiscontrolledinRNCanddownlinkalgorithminUE
Updatefrequencyfrom10Hzupto100Hz Outerlooppowercontrolwillraiseorlowerthetarget
SIRaccordingtostepsize,whichisdefinedbyradionetworkplanning.
Theequipments performancedefinestheminimumvaluefortargetSIR
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Fastpowercontrol
Idealfastpowercontrolinvertthechannel
Inpracticepowercontrolaccuracyisreducedby
feedbackerrors, Betterfigure,PCheadroometc
Fastfadingchannel
Transmittedpower
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UplinkfastPC
Update rate 1.5kHz=>fastenough totrack andcompensate fastfading up toxkm/hmobilespeed
If receivedSIR>targetSIRinNodeB=>UEiscommanded todecrease its transmitpower.Similarly UEiscommanded toincrease its transmissionpowerifreceivedSIR
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DownlinkfastPC
Similar asDLfastPC:
UEmeasures SIRonDLDPCCHduring thepilot
period UEmaintainstheQoS bysendingfastpowercontrol
commands(TPCbits)requestingpower adjustment
Poweroffsets can be used inDLinordertoimprovecontrolreliability.Offsets are network parameters thatcan be setinplanning phase