Overviewunit1.docx
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Overviewunit1.docx
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Overviewunit1
Unit1:
OverviewofCDMARFDesign
Author
Department
PhoneNumber
ChuRuiChang
GlobalNetworkDesign
(Dept.2779)
(972)685-7067
ESN445-7067
1.1Introduction
TherearesignificantdifferencesindesigningaCDMAsystemvs.designinganarrow-bandsystem.WhilethegeneraldesignprocessesforanyRFnetworksaresimilar,manydesigncriteriaforCDMAaredifferent.Inthischapter,anoverviewwillbegiventosummarizethekeypointsofthedesignconsiderations,mostofthemarespecifictoCDMA.Eachfollowingchapterswillgiveindepthtreatmentofaspecifictopic.
1.2UniqueFeaturesofCDMANetworks
OneofthekeyfactorsleadingtoCDMA’shighoverallcapacityisthe100%frequencyreuse,whichalsomakesCDMAa"self-jamming"systeminthesensethatenergyfromeveryotheruser(reverselink),everyothercell(forwardlink)couldbeinterference.Theoverallsystem’sperformanceisthushighlydependentonhowwelltheout-of-cellinterferencecanbecontrolled.Ontheotherhand,thenarrowbandsystemsare“orthogonal”inthesensethattheyusedifferentfrequenciesforeveryuserinwithinareusecluster.Therefore,theRFcoverage-controlbecomesmuchmorecriticalfortheCDMAsystemthanforanarrowbandsystem.WhenanAMPSdesignerdesigningaCDMAsystem,itisimportanttorealizethefollowingdifferences:
(1)FrequencyIsolationvs.CodeDistinction
Inanarrow-bandsystemsuchasAMPS,downlinkchannelswithdifferentfrequenciesdonotpresentasinterferencetoeachotheratthemobile.InaCDMAnetwork,energiesfromeveryneighborsectorsareco-channelinterference.Withinasector,onlytheforwardchannelsfromthesamemultipathareorthogonalduetotheorthogonalityoftheWashCodes.However,thisorthogonalitycannotbemaintainedforRFsignalsfromdifferentmultipathswithdelaydifferences>1chip.Inthissense,thePNcodesprovidenoisolationexceptforaparticulargroupofchannels(i.e.,forwardchannelsfromthesamesectorandthesamemultipath).
TheRFcapacityofaCDMAnetworkisinverselyproportionaltothelevelofinterference.Thatis,doubletheinterferencelevelwillhalvethecapacity,andviceversa.Therefore,thekeypointtodesignahighcapacityCDMAnetworkistheinterferencecontrol.
(2)SignalLevel(RSSI)vs.Signal-to-Interference-Ratio(SIR)
MostCDMAsystemsareinterferencelimited.Foraninterferencelimitedsystem,theabsolutesignallevel(RSSI)doesnotuniquelydeterminetheperformance.Forexample,inoneplace,thecallmaydropwherethereceivedsignallevelishigherthan–70dBm.Inanotherplace,thecallqualitymaybegoodevenifthereceivedsignallevelislowerthan–95dBm.
Itisthesignal-to-interference-ratio(SIR)thatdeterminesservicequality(suchasFER).Therefore,anymethodthatincreasessignallevelbutatthesametimealsoincreasestheinterferencelevelwillproducenogainintheSIRandthusnogaininsystem’sperformance.
(3)HardHandoffvs.SoftHandoff
Inanarrow-bandsystem,thehandoffthresholdsaredefinedintermsofabsolutesignallevels.However,inaCDMAsystem,thesethresholdsaredefinedintermsofEc/Io,whichissimilartoSIR.InCDMA,almosteverythingisdeterminedbySIRandnotbyRSSI.
Hardhandoffinnarrowbandsystemsdoesnotaffectthenetworkcapacity,butsofthandoffinCDMAdoes.Tomakethingsmorecomplicated,softhandoffwillhavedifferentimpactsontheforwardlinkcapacityandreverselinkcapacity.Onthereverselink,softhandoffalwaysimprovessignalreceptionandincreasescapacity.However,ontheforwardlink,thesituationmaybequitedifferent.AmobileinaM-way(M>1)softhandoffwillturnonanadditionalM-1BTStransmitters.Thisusuallyincreasestheforwardlinkinterferencelevelsandreducestheforwardlinkcapacity.Inaninterference-limitedsystem,forwardlinkcapacityisthelimitingcapacity,andtherefore,excessivesofthandoffwilldegradetheoverallnetworkcapacity.Reducingthepercentageofsofthandoff(particularlythoseinM-way(M>2)softhandoffs)isaneffectivewaytoincreasecapacity.
1.3CommentsaboutCDMALinkBudgets
AtypicalCDMAlinkbudgetmaylooklikeTable1:
Table1.1:
ATypicalCDMALinkBudget
ForwardLink
ForwardLink
ReverseLink
ReverseLink
Max.TxPowerforwardtrafficchannel
32[dBm]
HandsetTxPowerLevel
23[dBm]
BTSTxAntennaGain
17[dBi]forSuburban
BTSReceiveAntennaGain
17[dBi]forSuburban
AntennaCableLoss
-3[dB]
AntennaCableLoss
-3[dB]
FadeMargin
-7.6[dB]forSuburban
FadeMargin
-7.6[dB]forSuburban
SoftHandoffGain
0[dB](SHOgainandaddedinterf.washout)
SoftHandoffGain
4[dB]
HandsetReceiverLoading
-3[dB]
BTSReceiverLoading
-3[dB]
BuildingPenetrationLossforSuburban
-10[dB]forSuburban
BuildingPenetrationLossforSuburban
-10[dB]forSuburban
HandsetReceiverSensitivity
-117[dBm]
BTSReceiverSensitivity
-122[dBm]
DownLinkPathLoss
142.4[dB]forSuburban
UpLinkPathLoss
142.4[dB]forSuburban
Itisimportanttorealizethatlinkbudgetisonlyasimplifiedmodelforaroughestimationofthecellcoverage.Sobecarefulwhentryingtousethisoversimplifiedmodeltoscalethenetworks.Thedangeristhatbysimplylookingattheabovetableitmaynotbeobviousaswhichwaysomevariablesmaychangewiththechangeofthenetwork’sdimension.An“uneducatedguess”couldleadtocompletelywronganswer.
Forexample,byjustlookingattheabovetable,onemayconcludethatthemoreextramarginisgiventothebuildingpenetrationloss,thebettertheRFdesignsincethemoreextramarginisgiventothelinkbudget.Thisisnottrueingeneral.Whatismissingfromthat“scalinglaw”isthattheloading(markedredinthetable)foroutdoorhandsetsisalsoincreasingwiththeincreaseofthe“extramargininthelinkbudget”!
IfonegivesanextraXdBofmarginbydesigningBTSsmuchclosertogetherthannormal,boththeforwardandreverseloadingwillalsoincreaseddrasticallyduetotheincreasedout-of-cellinterference.Consequentlythereceiverwillbedesensitizedaccordinglyandtheremaynotbeanygaininthelinkbudget.
Rigorouslyspeaking,theassumed3dBforwardlinkloadingvalueinthetableisalmostnevercorrect.Toestimatethecorrectforwardlinkloading,onemusttakeintoaccountthetotalinterferenceeffectfromallneighboringBTSinthenetwork.Soitisaffectedbymanyfactors,andthereisnowayasimple“guess”willbecorrect.OneneedsaMonteCarlosimulationthattakesintoaccounttheterrainprofiles,BTSsandsubscriberlocations…,sothatbothsignallevelandtotalinterferencelevel(=loading),canallbeaccuratelyestimated.
Thekeypointisthatwhenscalinganetwork,onemusttakeintoaccountalleffects,andthatisbeyondthecapabilityofanoverlysimplifiedmodel.
1.4SomeDesignPracticestoAvoidCommonProblems
Problem#1:
PilotPollution
OnecommonmisconceptionaboutCDMAforanAMPSdesigneristhattheybelievethatthehigherthesignallevelonthecelledge,thebetterthecelledgereliability.Consequently,oneshouldeitherdesignaveryhigh-powerHPA,ordesigntheBTSantennasveryclosetoeachothersothatthecompositesignallevelatthecelledgeis–70dBm,-60dBm,orhigher.
Foraninterference-limitedCDMAsystem,thisintuitionisnottrue.ItisimportanttorealizethattheRFsignalwillnotsimplystopatthecelledge,butwillovershoottotheneighborcellsandcauseinterference.Thehigherthesignallevelatthecelledge,thefartheritwillovershoot.TheproblemofexcessiveRFenergyspillovertoneighboringcellsistheso-called“pilotpollution”problem.Thisisusuallythe#1problemforurbancells,andareverycommonforsuburbancellsaswell.
Howtoreducethepilotpollutionproblem?
Agooddesignpracticeistryingto
(A)Maximizethesignallevelinsidethecelland
(B)Minimizetheenergylevelovershoottotheneighborcells.
LargenumberofdrivetestdatashowthattheRFpropagationcantypicallybecharacteristicsbyatwo-slopemodelwitha“breakpoint”,asshowinFig.1.Itiscanbeseenthatbeforethebreakpoint,thesignallevelreducesslowlywiththedistance,butafterthebreakpoint,itreducesmuchfaster.NotealsothatdistanceofthebreakpointisafunctionofBTSantennaheight.
Fig.1Atwo-slopepropagationmodel.H=BTSantennaheight,h=mobileantennaheight,λ=wavelength.Typicallys1=2to2.5,s2=3to4.
Oneshouldtakeadvantageofthisfeaturebydesigningtheantennaheightandcellradiussothatthebreakpointstaysinsideofthecelledge.Thisway,signalofthein-cellregionfollowsaslowerpathlossslope,buttheout-of-cellinterferencefollowsafasterpathlossslope.ThustheSIRwillbeenhanced.Ifthisprincipleisviolated,thatis,iftheantennasaretootall,ortheBTSsaretooclosetoeachother,theout-of-cellinterferencewillincreasemuchfasterthansignal,andtheSIRreducessignificantly,asshowinFig.2.
(A)(B)
Fig.2TheantennaheightandBTSseparationshouldbedesignedsothatthebreakpointstaysinsidethecellradius.(A)Properlydesigned;(B)EithertheBTSsaretoocloseorantennaheightsaretoohigh.
NotethatbydesigningtheBTSstooclosetoeachother,anotherfactorwillmakeinterferenceincreasemuchfasterthansignalincrease.Fig.3showsthesituationofonecellinaclusterofcells.Ifinter-celldistancesandantennaheightsareadequate,thenonlythefirsttierofneighborcells(6ofthem)aresignificantinterferencesources,theinterferencefromthesecondtierandbeyondarenegligible.Iftheseparationistoosmall,theninterferencefromthesecondtie
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