These individual symbols are combined in order to produce an aggregate
symbol. The actual combining method shown in Fig. 7.16 follows the Maximal Ratio Combining
(MRC) approach presented in Chap. 5. For sake of completeness, we should mention that any of
the previously mentioned techniques used for receiver diversity can now be applied in the RAKE
finger combining operation.
As shown in Fig. 7.16, a RAKE receiver consists of multiple parallel demodulators, who individually
track each of the arriving replicas (or delay spread echos). This is a form of time diversity. One
common technique is to perform combining of the individual demodulated signals. In Fig. 7.17, we
show a simple high-level block diagram of the RAKE receiver for illustrative purposes.
In Fig. 7.17, we have shared a single, spectral down-conversion receiver, called the RF section.
Also we have placed the ADC directly before the RAKE to emphasize the RAKE operations are typically
performed in the digital domain. A finger can only despread multipaths that it is aware of, which
is accomplished by the use of an Echo Profile Manager (EPM) or a searcher. The EPM finds valid
multipaths by maintaining an up-to-date power delay profile P(). The EPM selects a particular arriving
multipath and assigns a finger to demodulate the information present on that arriving multipath.
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