The RAKE receiver is used in CDMA systems to implement diversity combining of multipath signals. It utilizes multiple correlators to separately detect the strongest multipath components which have arrived via different paths due to reflection, scattering, etc. The correlator outputs are weighted and combined to improve the overall signal quality compared to using a single multipath component. In IS-95 CDMA systems, the mobile receiver employs a searcher receiver and three digital data receivers as fingers of a RAKE receiver to track and process multipath components from individual or multiple base stations, implementing space/macro diversity on the downlink. On the uplink, the base station receiver uses antennas and digital receivers to track multiple multipath components from mobiles and combine them

1. Q.1). Present the use of RAKE Receiver in the CDMA system,
emphasizing that it implements a diversity combining scheme and
discussing how the requirements for diversity are satisfied?
Ans.
RAKE Receiver
RAKE receiver, used specially in CDMA cellular systems,can combine multipath components, which are
time-delayedversionsof the original signal transmission.Thiscombiningisdone inorderto improve the
signal to noise ratio (SNR) at the receiver. RAKE receiver attempts to collect the timeshifted versions of
the original signal byprovidingaseparate correlationreceiverforeachof the multipathsignals.Thiscan
be done due to multipath components are practically uncorrelated from another when their relative
propagation delay exceeds a chip period.
Due to reflections from obstacles a radio channel can consist of many copies of originally transmitted
signals having different amplitudes, phases, and delays. If the signal components arrive more than
duration of one chip apart from each other, a RAKE receiver can be used to resolve and combine them.
The RAKE receiver uses a multipath diversity principle. It is like a rake that rakes the energy from the
multipath propagated signal components.
Multipath Channel Model
Multipath can occur in radio channel in various ways such as, reflection and diffraction from buildings,
and scattering from trees presented in Figure 1.

2. An M-ray multipath model is shown in Figure 2, which is an extension to the multipath channel model
presentedinFigure 3.Eachof the Mpathshasan independentdelay,ţ(tau),andanindependentcomplex
time-variant gain, G.

3. M-finger RAKE Receiver
A RAKEreceiverutilizesmultiplecorrelatorstoseparatelydetectMstrongestmultipathcomponents.The
outputs of each correlator are weighted to provide better estimate of the transmitted signal than is
providedbyasingle component.Demodulationandbitdecisionsare thenbasedonthe weightedoutputs
of the Mcorrelators.
Each correlatordetectsa time-shiftedversionof the original CDMA transmission,andeach fingerof the
RAKE correlates to a portion of the signal, which is delayed by at least one chip in time from the other
fingers.
Assume M correlators are used in a CDMA receiver to capture M strongest multipath components. A
weighting network is used to provide a linear combination of the correlator output for bit decision.
Correlator1 is synchronized tothe strongestmultipathm1.Multipathcomponentm2 arrivedt1 laterthan
m1 but has low correlation with m1. The M decision statistics are weighted to form an overall decision
statisticas shownin Figure 3. The outputsof the M correlatorsare denotedas Z1, Z2,…,and ZM. They are
weightedbyα1,α2,…,andαM,respectively.The weightingcoefficientsare basedonthe powerorthe SNR
(Signal-to-Noise Ratio) from each correlator output. If the power or SNR is small out of a particular
correlator, it will be assigned a small weighting factor, α. If maximal-ratio combining is used, following
equation 1 can be written for Z’.
The weightingcoefficients, m, are normalizedtothe outputsignal power
of the correlator in such a way that the coefficients sum to unity, as shown in following equation 2.

4. As in the case of adaptive equalizers and diversity combining, there are many ways to generate the
weighting coefficients. However, due to Multiple Access Interference (MAI), RAKE fingers with strong
multipath amplitudes will not necessarilyprovide strong output after correlation. Choosing weighting
coefficients based on the actual outputs .of the correlator yields better RAKE performance.
RAKE Receiver Block Diagram
Whena signal isreceivedina matchedfilterovera multipathchannel,the multiple delaysappearatthe
receiver, as depicted in Figure 4. The RAKE receiver uses several baseband correlators to individually
processseveral signal multipathcomponents.The correlatoroutputsare combinedtoachieve improved
communications reliability and performance.
Bitdecisionsbasedonlyasinglecorrelationmayproducealarge biterrorrate asthe multipathcomponent
processed in that correlator can be corrupted by fading. In a RAKE receiver, if the output from one
correlator is corrupted by fading, the others may not be, and the corrupted signal may be discounted
through the weighting process.

5. Impulse response measurementsof the multipathchannel profile are executedthrougha matchedfilter
to make a successful de-spreading.It revealsmultipath channel peaks and gives timing and RAKE finger
allocationstodifferent receiverblocks.Laterittracksandmonitorsthese peakswith ameasurementrate
dependingonspeedsof mobile station andon propagationenvironment.The numberof available RAKE
fingers depends on the channel profile and the chip rate. The higher the chip rate, the more resolvable
pathsthere are,buthigherchiprate will cause widerbandwidth.Tocatch all the energyfromthe channel
more RAKE fingers are needed. A very large number of fingers lead to combining losses and practical
implementation problems.
RAKE RECEIVER IN IS-95 SYSTEM
In the implementationof the IS-95 system, the mobile receiveremploysa“searcher” receiverandthree
digital data receivers that act as fingers of a RAKE in that they may be assigned to track and isolate
particularmultipathcomponentsof asingle cell site,asinglebase stationinsofterhandoverandmultiple
base stationsinsofthandover.The PN chiprate of 1,2288 MHz allowsforresolutionof multipathattime
intervals of 1,2288 x 10-6 s = 0,814 µs, which means that multipath difference in meterscan be around
244 m.
A. Downlink
The searcher receiver scans the time domain about the desired signal’s expected time of arrival for
multipath pilot signals from the same cell site and pilot signalsand their multipath from other cell sites.
Searching the time domain on the downlink signals is simplified because the pilot channel permits the
coherent detection of signals. The search receiver indicates to the mobile phone’s control processor
where,intime,the strongestreplicasof the signal can be found,and theirrespective signal strengths.In
turn, the control processor provides timing and PN code information to the tree digital data receivers,
enablingeachof themtotrackanddemodulateadifferentsignal.If aanothercell site pilotsignalbecomes
significantly stronger than the current pilot signal, the control processor initiates handover procedures
during which the downlinksof both cell sites transmitat the same call data on all their traffic channels.
Whenboth siteshandle the call,additional space diversityormacro diversityisobtained.The datafrom
all three digital receivers are combined for improved resistance to fading. Different base stations or
sectors are distinguished by different short PN code offsets. The downlink performs coherent post-
detection combining after ensuring that the data streams are time-aligned; performance is not
compromised by using post-detection combining because the modulationtechnique is linear. Coherent
combiningispossible because thepilotsignalfrom eachbase stationprovidesacoherentphase reference
that can be tracked by the digital data receivers.
B. Uplink
On the uplink, the base station receiver uses two antennas for space diversityreception, and there are
four digital data receivers available for tracking up to four multipath components of a particular
subscriber’ssignal.The searcher receiveratthe base stationcan distinguishthe desiredmobile signalby
meansof itsunique scramblinglongPN code offset, acquiredbefore voiceordatatransmissionbeginson
the link, using s special preamble for that purpose. During soft handover from one base station site to

6. another, the voice datathat are selectedcouldresultfromcombining uptoeightmultipathcomponents,
fourat each site.The uplinktransmission,nothavingacoherentphase reference like the downlink’spilot
signal, must be demodulated and combined non-coherently; maximal-ratio combining can be done by
weightingeachpath’s symbol statisticsinproportion tothe path’s relative powerpriorto demodulation
and decoding decision.