SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The VCP implementation implies that the soft inputs need to be quantized so that the branch metrics satisfy the following bound B1 (branch metrics upper bound - absolute value):
K is the constraint length and C determines the truncation of state metrics that can be performed without loss of decoding performance.
The VCP is designed with C = 13. The branch metrics can have a maximum dynamic range of 7+1 sign bits do [-128; +127]. This gives another branch metrics, upper bound B2 ≤ 128.
So for a given constraint length, min(B1, B2) will give the final branch metrics, maximum bound B.
To satisfy B in the branch metrics calculation, the soft input values, delivered as 8-bit-signed equalized values, are linearly scaled with the following formula where 1/n is the rate.
Example
K=9, then B1 3 227.5 and the branch metrics range B2 is [-128; +127]. So the branch metrics need to be in [-128; +127] range.
If the rate is 1/3, then 128/3 ≈ 42, so the soft inputs need to be scaled by a factor 0.333333 and saturated within the range [-42; +42].
Table 30-13 summarizes the calculations for the different constraint length and rate:
1/Rate | K | Scaling Factor | Range |
---|---|---|---|
2 | 5, 6, 7, 8, 9 | 0.5 | [-64; +63] |
3 | 5, 6, 7, 8, 9 | 0.333333 | [-42; +42] |
4 | 5, 6, 7, 8, 9 | 0.25 | [-31; +31] |