Umts Chapter 3

7/27/2019 Umts Chapter 3

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Chapter 3

Introduction to WCDMA


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3.2 Summary of the Main Parameters inWCDMA

3.3 Spreading and Despreading

3.4 Multipath Radio Channels and RakeReeption

3.! Po"er Control

3.# Softer and Soft $ando%ers


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&'( Multiple aess method

WCDMA is a "ide)and Diret*Se+uene CodeDi%ision Multiple Aess &DS*CDMA( system

user information )its are spread o%er a "ide)and"idth )y

multiplying user data "ith +uasi*random )its&alled hips( deri%ed from CDMA spreading

odes in order to support %ery high )it rates &up to

2M)ps(, the use of a %aria)le spreading fatorand multiode onnetions is supported


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&2( Duple-ing methodWCDMA supports )oth DD and /DD modes of

operation

re+ueny Di%ision Duple- &DD(

separate ! M$0 arrier fre+uenies are used foruplink and do"nlink, respeti%ely

/ime Di%ision Duple- &/DD(

only one ! M$0 is timeshared )et"een uplink anddo"nlink


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&3( 1asi station synhroni0ation

WCDMA supports the operation of asynhronous)ase stations

no need for a glo)al time referene suh as a PS

deployment of indoor and miro )ase stations is

easier "hen no PS signal needs to )e reei%ed


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&4( Chip rate

hip rate of 3.4 Mps leads to a arrier )and"idth&hannel )and"idth( of appro-imately ! M$0

hip the length of time to transmit either a 5or a ' in a )inary pulse ode

hip rate num)er of hips per seond

DS*CDMA systems "ith a )and"idth of a)out ' M$0

&narro")and CDMA systems("ide arrier )and"idth of WCDMA supports high

user data rates


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&!( rame length 6 slot length

frame length

'5ms &' frame length 7 3455 hips(

slot length

'! slots 8frame &' slot length 7 2!#5 hips(


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( Ser%ie multiple-ing

multiple ser%ies "ith di9erent +uality of ser%iere+uirements multiple-ed on one onnetion


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&:( Multirate onept

use a %aria)le spreading fator and multiode to

support %ery high )it rates &up to 2 M)ps(multiode

in multiode CDMA systems, eah user an )epro%ided "ith multiple spreading odes of ;-edlength, depending on users< rate re+uests

moti%ations for multiode CDMA

inrease the information rate o%er a gi%en spread)and"idth

allo" for the =e-i)ility of multiple data rates


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&( Detetion

WCDMA employs oherent detetion & ( on uplink

and do"nlink )ased on the use of pilot sym)ols & (or ommon pilot & (

oherent detetion &oherent demodulation(

a method of reo%ering the original signal thatre+uires an e-atly same arrier fre+ueny andphase &propagation delay auses arrier*phase

o9set( as those used in the transmitting endthe reei%ed signal is mi-ed, in some type of

nonlinear de%ie, "ith a signal from a loal osillator,to produe an intermediate fre+ueny, from "hihthe modulating signal is reo%ered &deteted(


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use of oherent detetion on uplink "ill result inan o%erall inrease of o%erage and apaity onthe uplink


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&>( Multiuser detetion and smart antennas supported )y the standard

deployed )y net"ork operator as a systemoption to inrease apaity and8or o%erage


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Smart antennas &also kno"n as adapti%e arrayantennas, multiple antennas and reently M?M@(

antenna arrays "ith smart signal proessingalgorithms used to identify spatial signal

signature suh as the diretion of arri%al &D@A( ofthe signal, and use it to alulate )eamforming%etors, to trak and loate the antenna )eamon the mo)ile8target

the antenna ould optionally )e any sensor

smart antenna tehni+ues are used nota)ly inaousti & ( signal proessing, trak and san

RADAR, radio astronomy & (and radio

telesopes & (, and mostly in ellular

systems like W*CDMA and M/S


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WCDMA supports highly %aria)le user datarates, in other "ords the onept of o)taining1and"idth on Demand &1oD( the user data rate is kept onstant during eah

'5 ms frame ho"e%er, the data apaity among users an

hange from frame to frame

this fast radio apaity alloation "ill typially )e

ontrolled )y the net"ork to ahie%e optimumthroughput for paket data ser%ies


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$ando%ersWCDMA is designed to )e deployed in

onBuntion "ith SM

hando%ers )et"een SM and WCDMA are

supported to le%erage SM o%erage


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Spread*spetrum transmission a tehni+ue in "hih the users original signal is

transformed into another form that oupies alarger )and"idth than the original signal "ouldnormally need

the original data se+uene is )inary multiplied"ith a spreading ode that typially has a muhlarger )and"idth than the original signal

the )its in the spreading ode are alled hips todi9erentiate them from the )its in the datase+uene, "hih are alled sym)ols


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eah user has its o"n spreading ode

the idential ode is used in )othtransformations on eah end of the radiohannel

spreading the original signal to produe a"ide)and signal

despreading the "ide)and signal )ak to theoriginal narro")and signal


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the ratio )et"een the transmission )and"idth andthe original )and"idth is alled the proessing gainalso kno"n as the spreading fator &S(this ratio simply means ho" many hips are used

to spread one data sym)ol in the /RA, the spreading*fator %alues an )e

)et"een 4 and !'2 in the /DD mode also S7' is allo"edthe lo"er the spreading fator, the more payload

data a signal an on%ey on the radio interfae


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1 symbol = 8 chips


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Spreading and despreading operation user data is assumed to )e a 1PSE*modulated &1inary Phase

Shift Eeying( )it se+uene of rate R

user data )its are assumed the %alues of ' or *' spreading operation

the multipliation of eah user data )it "ith a se+uene of ode )its, alled hips &the spreading fator is (

the resulting spread data is at a rate of F R

despreading operation

multiply the spread user data8hip se+uene, )it duration)y )it duration, "ith the %ery same ode hips as "eused during the spreading of these )its

as sho"n, the original user )it se+uene has )eenreo%ered perfetly


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the inrease of the signaling rate )y afator of orresponds to a "idening &)ya fator of ( of the oupied spetrum ofthe spread user data signal

despreading restores a )and"idthproportional to R for the signal


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Rake Receiver

Rake receiver

a radio receiver designed to counter the effects ofmultipath fadinguses several "sub-receivers" (called fingers) each delayed

slightly , that is, several correlators each assigned to adifferent multipath component

each component is decoded independently, but at a laterstage combined in order to make the most use of the differenttransmission characteristics of each transmission path


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the digital section of a CDM receiver !hich

permits the phone (or cell) to separate out the

relevant signal from all the other signals

can receive multiple signal sources and add them

together using multiple fingers

Rake receivers are common in a !ide variety of

radio devices including mobile phones and !ireless

# e$uipment


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RF: Radio Frequency


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Digiti%ed input samples received from R& (Radio &re$uency) front-end circuitry in the

form of ' and branches

Code generators and correlatorperform the despreading and integration to user data symbols

Channel estimator and phase rotator

channel estimator uses the pilot symbols ) *for estimating thechannel state !hich !ill then be removed by the phase rotatorfrom the received symbols

Phase rotator: a device used in radio astronomy [ ] to adjust the phase of an incomin sinal!


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Delay e$uliser

the delay is compensated for the

difference in the arrival times of the

symbols in each finger

Rake combiner

sums the channel compensated

symbols, thereby providing

multipath diversity against fading


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Matched filter

used for determining and updating the

current multipath delay profile of the

channel

this measured and possibly averaged

multipath delay profile is then used to

assign the Rake fingers to the largest

peaks


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ast po"er ontrol is in partiular on theuplink"ithout it, a single o%erpo"ered mo)ile ould

)lok a "hole ell


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Po"er ontrol in WCDMA

open*loop po"er ontrol

lose*loop po"er ontrol

inner*loop po"er ontrol

outer*loop po"er ontrol


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@pen loop po"er ontrol in WCDMAattempt to make a rough estimation of path

loss )y measuring do"nlink )eaon signal

pro)lem

far too inaurate

fast fading is essentially unorrelated)et"een uplink and do"nlink due to largefre+ueny separation of uplink and do"nlink

)and of WCDMA DD modeopen*loop po"er ontrol is used in WCDMA to

pro%ide a oarse initial po"er setting of MS atthe )eginning of a onnetion


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?nner*loop po"er ontrol in WCDMA uplink1S performs fre+uent estimates of the reei%ed

Signal*to*?nterferene Ratio &S?R( andompares it to a target S?R

if the measured S?R is higher than the targetS?R, 1S "ill ommand MS to lo"er the po"er

if S?R is too lo", it "ill ommand MS toinrease its po"er


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measureGommandGreat ylee-euted at a rate of '!55 times per seond

&'.! k$0( for eah MS faster than any signi;ant hange of path

loss ould possi)ly happen faster than the fast Rayleigh fading speed for

lo" to moderate mo)ile speeds inner*loop po"er ontrol

pre%ent any po"er im)alane among all the

uplink signals reei%ed at 1S


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?nner*loop po"er ontrol in WCDMA do"nlinkadopt the same tehni+ues as those used in

uplink

operate at a rate of '!55 times per seond

no nearGfar pro)lem due to Hone ell to manymo)ilesI senario


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do"nlink losed*loop po"er ontrol

pro%ide a marginal amount of additionalpo"er to MS at the ell edge as they su9erfrom inreased other*ell interferene

enhane "eak signals aused )y Rayleighfading "hen other error*orreting methodsdoesnt "ork e9eti%ely


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igure 3. depits losed loop transmissionpo"er ontrol in CDMAMS' and MS2 operate "ithin the same

fre+ueny, separa)le at the 1S only )y their

respeti%e spreading odes it may happen that MS' at the ell edge su9ers a

path loss, say :5 d1 a)o%e that of MS2 "hih isnear the 1S


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if there "ere no po"er ontrol mehanism forMS' and MS2 to the same le%el at 1S

MS2 ould easily o%ershout MS' and thus)lok a large part of the ell, gi%ing rise to

the nearGfar pro)lem of CDMA


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igure 3.> sho"s ho" uplink losed looppo"er ontrol "orks on a fading hannel atlo" speed


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@uter*loop po"er ontrol in WCDMA adBusts the target S?R setpoint in 1S aording to

the indi%idual radio link +uality re+uirement, usuallyde;ned as )it error rate &1JR( or )lok error rate

&1KJR( the re+uired S?R or 1KJR depends on the mo)ile

speed, multipath pro;le, and data rate

should the transmission +uality is dereasing, theRC "ill ommand ode 1 to inrease the targetS?R

outer*loop po"er ontrol is implemented in RC)eause there might )e soft hando%er om)ining


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Why should there )e a need for hanging thetarget S?R setpointL the re+uired S?R for, say, 1KJR 7 ' depends on

mo)ile speed and multipath pro;le

if one "ere to set the target S?R setpoint for highmo)ile speeds, one "ould "aste muh apaityfor those onnetions at lo" speeds

the )est strategy is to let the target S?R setpoint=oat around the minimum %alue that Bust ful;lsthe re+uired target +uality


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/he target S?R setpoint "ill hange o%er timeas the speed and propagation en%ironmenthanges &igure 3.'5(

@uter loop ontrol is typially implemented

)y ha%ing 1S tag eah uplink user data frame "ith a

frame relia)ility indiator, suh as a CRC &CyliRedundany Chek( result o)tained during

deoding of that partiular user data frame


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should the frame +uality indiator sho"s the

transmission +uality is dereasingRC "ill ommand 1S to inrease target S?R

setpoint


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Softer hando%er &igure 3.''(MS is in the o%erlapping ell o%erage area of

t"o adBaent setors of a 1S

ommuniations )et"een MS and 1S take plae

onurrently %ia t"o air interfae hannels, onefor eah setor separately


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use of t"o separate odes in the do"nlinkdiretion, so MS an distinguish the signals

the signals are reei%ed in the MS )y means ofRake proessing, and the ;ngers need togenerate the respeti%e ode for eah setorfor the appropriate despreading operation

due to multipath propagation, it is neessary

to use multiple orrelation reei%ers in orderto reo%er the energy from all paths and8orantennas

suh a olletion of orrelation reei%ers,termed N;ngers, is "hat omprises the

CDMA Rake reei%er


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only one po"er ontrol loop per onnetion isati%e

softer hando%er typially ours in a)out !O'!of onnetions


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?n the uplink diretion the ode hannel of MS is reei%ed in eah setor

use ma-imal ratio om)ining &MRC( Rakeproessing


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Ma-imal Ratio Com)iner &MRC(

the om)iner that ahie%es the )est performane isone in "hih eah output is multiplied )y theorresponding omple-*%alued &onBugateP Q(

hannel gain

the e9et of this multipliation is to ompensate for

the phase shift in the hannel and to "eight the signal)y a fator that is proportional to signal strength


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Soft hando%er &igure 3.'2(MS is in the o%erlapping ell o%erage area of

t"o setors )elonging to di9erent 1Ss

ommuniations )et"een MS and 1S take plae

onurrently %ia t"o air interfae hannels fromeah 1S separately

)oth hannels &signals( are reei%ed at the MS )yma-imal ratio om)ining Rake proessing


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t"o po"er ontrol loops per onnetion areati%e, one for eah 1S

soft hando%er ours in a)out 25O45 ofonnetions


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?n the uplink diretion the ode hannel of the MS is reei%ed from )oth

1Ss, )ut the reei%ed data is then routed to RCfor om)ining

the same frame relia)ility indiator is used toselet the )etter frame )et"een the t"o possi)leandidates "ithin RC

this seletion takes plae e%ery '5O5 ms


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Soft and softer hando%er an take plae inom)ination "ith eah other

@ther hando%er types of WCDMA inter*fre+ueny hard hando%ers

e.g., to hand a mo)ile o%er from one WCDMAfre+ueny arrier to another

one appliation for this is high apaity 1Ss"ith se%eral arriers

inter*system hard hando%ertakes plae )et"een WCDMA DD system

and another system, suh as WCDMA /DD orSM

Umts Chapter 3

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