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8.1.4 F, R, and C Limitations
VCP2 has increased the traceback soft-decision memory and output FIFO compared to the memory in
VCP. The traceback soft-decision memory size has been increased from 1024 × 96 bits to 2048 × 64 bits.
The output FIFO memory has been increased from 32 × 64 bits to 64 × 64 bits. In addition, the
soft-decision resolution is increased internally to 13 bits and is clipped to 8 bits when output from VCP2.
These memory sizes have an effect on the F
max
parameter (i.e., the max. frame size for tailed processing),
and the (R+C)
max
parameter (i.e., the max. sliding window length for mixed/convergent processing).
The differences between VCP and VCP2 are related to F
max
, (R+C)
max
, and C. The impact on VCP2
performance is as follows:
• Allowed values for C are C = N*(K - 1), where N is a convergence multiplier. Table 26 and Table 27
show the possible values of N for each value of K for hard and soft decisions. The larger the
convergence length is, the better the BER. However, larger convergence lengths require more VCP
clock cycles.
• Soft decisions:
– R is constrained to be less than or equal to 248.
– F
max
and (R+C)
max
for VCP2 have been increased over VCP. This change reduces the number of
sliding windows and, therefore, decreases the number of VCP2 cycles needed for traceback.
Overall, there is no major improvement in the processing delay because a larger portion of cycles is
spent in state metric accumulation, which is largely unaffected by the choice of R and C.
• Hard decisions:
– (R+C)
max
for mixed/convergent processing for hard decisions has been increased from 605 to 635
for K = 6, and from 1020 to 2044 for K = 5. As with soft decisions, this change results in a small
decrease in VCP2 cycle counts during traceback.
The procedure to calculate the reliability length is as follows [the reliability length cannot be larger than
1920, C
≤ 1920 - (k - 1)]:
1. Determine the convergence length C = N*(K - 1).
2. Determine the number of sliding windows: N
sw
= ceil(f/[(r + c)
max
- c]).
3. Determine the reliability length: R = m × ceil[f/(Nsw × m)].
4. If hard decisions are being used and R > 1920 or if soft decisions are being used and R > 248, then
increment the number of sliding windows and go back to step 3.
5. For hard decision and soft-decision limits, see Table 26 and Table 27 .
Table 26. Traceback Soft Decision Sliding Window Limits
K F
max
(r + c)
max
Multiple (m) Convergent Multiplier (N) Special Constraint
9 56 60 4 3,6
8 105 105 7 3,6,9,12
7 186 186 6 3,6,9,12,15,18
6 315 315 5 3,6,9,12,15,18 r ≤ 248
5 1020 320 4 3,6,9,12,15,18 r ≤ 248
Table 27. Traceback Hard Decision Sliding Window Limits
K F
max
(r + c)
max
Multiple (m) Convergent Multiplier (N)
9 120 124 4 3,6,9,12,15
8 217 217 7 3,6,9,12,15,18
7 378 378 6 3,6,9,12,15,18
6 635 635 5 3,6,9,12,15,18
5 2044 2044 4 3,6,9,12,15,18
The maximum value for the FL input parameter is F
max
for tailed mode or (r+c)
max
for mixed or convergent
modes.
36 TMS320TCI648x/9x Viterbi-Decoder Coprocessor 2 SPRUE09E – May 2006 – Revised December 2009
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