Let
us still assume we need to store 2 time slots where each chip is oversampled 4 times. We will assume the
I/Q samples require 5 bits of finite precision representation. Then the total number of bytes is given as
[bytes] (7.112)
If we assume the number of fingers is a design parameter, then we can plot the memory requirements
comparing the chip-level buffer to the symbol-level buffer.
In Fig. 7.80, we plot the symbol-level buffering requirements for voice (SF 64) and 384-kbps
data service (SF 4). Notice a crossover region occurs when the number of fingers is greater than 10
(assuming the above finger architecture parameters).
CL
2 # 2560 chips # 4 times # 5 bits # 2I/Q
8
FIGURE 7.80 Chip- and symbol-level comparison.
Chip- & Symbol-Level Buffering Comparison
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
0 5 10 15
Number of Fingers
Memory [Bytes] Symbol Level (Voice)
Chip Level
Symbol Level (384 kbps)
Typically, a RAKE receiver is implemented with the worse-case scenario in mind, namely, the
maximum number of fingers supported are all used. If the modem was solely designed for voice applications,
then using the chip-level buffer would significantly increase the memory requirements over
the symbol-level buffer.
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