65 A RAKE finger-block diagram using CPICH-based CE.
Next, an MA filter is inserted to reduce the effects of noise. The output of the despreader for the
DCH channel is mathematically represented as follows:
(7.76)
Once again forcing the DCH and CCH codes to be orthogonal, the output is given as
(7.77)
If we neglect the noise terms for the moment and use the previously defined, depiloted output signal,
we can then perform channel compensation in the following manner:
(7.78)
(7.79)
(7.80)
Here k is a constant over the observed time interval. Since the estimated channel will slightly differ
from the actual channel, the compensated symbols z(k) will be complex valued. Hence the
imaginary part of the complex signal is used for DCH processing. The projection onto the real axis
is due to the estimation error of the channel estimate (see Fig. 7.66).
The uplink modulation scheme is BPSK with HPSK spreading. The CCH contains not only the
pilot bits but also other Layer 1 control information, such as TPC and TFCI bits. Hence the pilot bits
must be extracted from the received bit stream through the pilot-gating function. In this case, using
pilot bits from adjacent time slots can improve system performance. We will quantify this improvement
when the BER performance results are presented later.
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