Each finger will despread the CCH, extract the pilot bits, and perform some averaging across multiple
time slots. We have used the well-known Weighted Multi Slot Averaging (WMSA) approach
[28]. Now since we make use of not only the present and past estimates but also the future estimates,
the data channel must have some memory (e.g., FIFO) to prevent loss of information. This FIFO
essentially allows the channel compensation to go back into time to perform channel compensation.
p2(17) 0.12 # h1 0.84 # h2 0.28 # h3
L2(x)
1
200
(x2 16x 39)
L1(x)
1
100
(x2 26x 69)
L0(x)
1
200
(x2 36x 299)
3G WIDEBAND CDMA 397
FIGURE 7.68 Channel estimation using Lagrange Interpolation.
1 2 3 4 5 6 10 11 12 13 14 15 20 21 22 23 24 25 30
h1(3) h2(13) h3(23)
X L1
+
X L0 X L2
Interpolated Channel Estimate
Time Slot (n ??“ 1) Time Slot (n) Time Slot (n + 1)
If we look at the RAKE output signal, we can observe the following (assuming K distinct mutipaths
are being demodulated):
(7.86)
which is essentially performing the MRC function, similar to what was presented when we discussed
receive antenna diversity techniques. What Eq. (7.86) shows us is that performance improvements can
be achieved as more multipaths with significant energy are actively demodulated.
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