User's Manual Part 1

Link Budget Analysis
InterReach Fusion Wideband Installation, Operation, and Reference Manual Page 93
D-620616-0-20 Rev H • TECP-77-044 Issue 7 • May 2013 © 2013 TE Connectivity Ltd.
ElementsofaLinkBudgetforCDMAStandards
ACDMAlinkbudgetisslightlymorecomplicatedbecauseyoumustconsiderthespreadspectrum
natureofCDMA.UnlikenarrowbandstandardssuchasTDMAandGSM,CDMAsignalsarespread
overarelativelywidefrequencyband.Uponreception,theCDMAsignalisde‐spread.Inthe
de‐spreadingprocessthepowerinthereceivedsignalbecomesconcentratedintoanarrowband,
whereasthenoiselevelremainsunchanged.Hence,thesignal‐to‐noiseratioofthede‐spread
signalishigherthanthatoftheCDMAsignalbeforede‐spreading.Thisincreaseiscalled
processinggain.ForIS‐95andJ‐STD‐008,theprocessinggainis21dBor19dBdependingonthe
userdatarate(9.6Kbpsforrateset1and14.4Kbpsforrateset2,respectively).Becauseofthe
processinggain,aCDMAsignal(comprisingoneWalshcodechannelwithinthecompositeCDMA
signal)canbereceivedatalowerlevelthanthatrequiredfornarrowbandsignals.Areasonable
levelis–95dBm,whichresultsinabout–85dBmcompositeasshownbelow.
Animportantissuetokeepinmindisthatthedownlin
kCDMAsignaliscomposedofmany
orthogonalchannels:pilot,paging,sync,andtraffic.Thecompositepowerlevelisthesumofthe
powersfromtheindividualchannels.Table79showsanexample.
Thistableassumesthatthereare
15activetrafficchannelsoperatingwith50%voiceactivity(so
thatthetotalpoweraddsupto100%).Noticethatthepilotandsyncchannelstogethercontribute
about25%ofthepower.WhenmeasuringthepowerinaCDMAsignalyoumustbeawarethatif
onlythepilotandsyncchannelsareactive,thepowerlevelwillbeabout6to7dBlowerthanthe
maximumpowerlevelyoucanexpectwhenallvoicechannelsareactive.Theimplicationisthat
ifonlythepilotandsyncchannelsareactive,andthemaximumpowerpercarriertablesaysthat
youshouldnotexceed10dBmforaCDMAsignal,forexample,thenyoushouldsettheattenuation
betweenthebasestationandtheMainHubsothattheMainHubreceives3dBm(assuming0dB
systemgain).
AnadditionalconsiderationforCDMA
systemsisthattheuplinkanddownlinkpathsshouldbe
gainandnoisebalanced.Thisisrequiredforproperoperationofsoft‐handofftotheoutdoor
networkaswellaspreventingexcessinterferencethatiscausedbymobilesontheindoorsystem
transmittingatpowerlevelsthatarenotcoordinatedwiththeoutdoormobiles.Thisbalanceis
achievedifthepowerleveltransmittedbythemobilesunderclose‐looppowercontrolissimilar
tothepowerleveltransmittedunderopen‐looppowercontrol.
Theopen‐looppowercontrolequationisas
follows:
whereP
TX
isthemobile’stransmittedpowerandPRXisthepowerreceivedbythemobile.
Table79.
DistributionofPowerwithinaCDMASignal
Channel Walsh Code Number Relative Power Level
Pilot 0 20% –7.0 dB
Sync 32 5% –13.3 dB
Primary Paging 1 19% –7.3 dB
Traffic 8–31, 33–63 9% (per traffic channel) –10.3 dB
forCellular,IS‐95: P
TX
+P
RX
=–73dBm
forPCS,J‐STD‐008: P
TX
+P
RX
=–76dBm