8.
21
(i1) EGCperforms worse than MRC by
only a factor 4
in termns of mean SNR.
8.5 RAKE RECEIVER
8.5.1 Introduction
Why Rake receiver needed?
One of the advantages of the CDMAsystem is that
it can be reduced by combining direct
multipath interference and
and reflected signals in the receiver.
The receivers used are called rake
receiyers.
There is an useful infornation in the
multipath components in CDMA, the
RAKE receiver mnay combine the time delayed
versions of the original signal
transmission in order to improve the signal - to - noise ratio at the
receiver.
Building
Transmitter Multipath
interface
Direct Signal
RAKE
Receiver
Fig.8.18. RAKE receiver
CDMA spreading codes are designed to provide very low
correlation between
successive chips.
Propagation delay spread in the radio channel provides multiple versions of
the transmitted signal at the receiver.
Wireless Communication
Correlator 1
T
r(t) m(t)
IF or baseband Correlator 2
CDMA signal
with multipath
Correlator M ZM
Fig. 8.19.An M-branch RAKE receiver
In RAKE receiver, each correlator detects a time shifted version of the
original CDMA transmission. It utilizes multiple correlatorsto separately
detect the 'M' strongest multipath components.
The outputs of each correlator are then weighted to provide a better estimate
of the transmitted signal then is provided by a single component.
Demodulation and bit decisions are then based on the weighted outputs of the
Mcorrelators.
3.5.2 Performance of a Rake Receiver
Assume Mcorrelators are used in a CDMA receiver to capture the M
strongest multipath components. A weighting network is used to provide a
linear combination of the correlator output for bit detection.
do Correlator 1 is synchronized to the strongest
multipath mË. Multipath
component m2 arrives Ti later than component m, where T2Ti is assumed to
be greater than chip duration. The second correlator is
synchronized tO m2. It
correlates strongly with m2 but has a lowcorrelation with mË.
In a RAKE receiver, if the output from one
correlator is
corrupted fading,
by
the others may not be and the corrupted signal may be
discounted through the
weighting process.
Dversin 8.23
I Decisions based on the combination of the M separate decision statistics
offered by the RAKE provide a form of diversity which can overcome fading
and thereby improves CDMA reception.
The outputs of the Mcorrelators are denoted as Z, Za,... and Zy. They are
weighted by a, az... and ay respectively.
The weighting coefficients are based on the power or the SNR from each
correlator output. In the case of a MRCdiversity scheme, the overall signal Z'
is given by,
M
Z'=am Zm
m=!
... (1)
The weighting coefficients Cat, are normalized to the output signal power of
the corelator in such a way that the coefficients sum to be unity.
M ...(2)
m=!
& Choosing the weighting coefficients based on the actual outputs of the
corelators yields better RAKEperformance.
When we insert one more radio receiver, we need more space and also
increase complexity. Consequently, we increase costs.
