SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
The branch metrics (BM) are calculated by the DSP and stored in the DSP memory subsystem as 8-bit signed values. Per symbol interval T, for a rate R = k/n and a constraint length K, there are a total of 2(K- 1+k) branches in the trellis. For rate 1/n codes, only 2(n-1) branch metrics need to be computed per symbol period and passed to the VCP module. Moreover, n symbols are required to calculate 1 branch metric.
Assuming BSPK modulated bits (0 → 1, 1→ -1), the branch metrics are calculated as follows:
where r(t) is the received codeword at time t (2 symbols, r0(t) is the symbol corresponding to the encoder upper branch, see Figure 30-4).
where r(t) is the received codeword (3 symbols, r0(t) is the symbol corresponding to the encoder upper branch; see Figure 30-4).
where r(t) is the received codeword (4 symbols, r0(t) is the symbol corresponding to the encoder upper branch, see Figure 30-4).
The data must be sent to the VCP verision 2 as described in Table 30-10, Table 30-11, and Table 30-12 for rates 1/2, 1/3, and 1/4, respectively (the base address must be double-word aligned).
The branch metrics can be saved in the DSP memory subsystem in either its native format or packed in words (user implementation). When working in big-endian mode, the VCP endian mode register (VCP_VCPEND) indicates if the data is 32-bit word packed or Native 8-bit format.
Address (hex) | Data | |||
---|---|---|---|---|
MSB | LSB | |||
Base | BM1(t=T) | BM0(t=T) | BM1(t=0) | BM0(t=0) |
Base + 4h | BM1(t=3T) | BM0(t=3T) | BM1(t=2T) | BM0(t=2T) |
Base + 8h | ... |
Address (hex) | Data | |||
---|---|---|---|---|
MSB | LSB | |||
Base | BM3(t=0) | BM2(t=0) | BM1(t=0) | BM0(t=0) |
Base + 4h | BM3(t=T) | BM2(t=T) | BM1(t=T) | BM0(t=T) |
Base + 8h | ... |
Address (hex) | Data | |||
---|---|---|---|---|
MSB | LSB | |||
Base | BM3(t=0) | BM2(t=0) | BM1(t=0) | BM0(t=0) |
Base + 4h | BM7(t=0) | BM6(t=0) | BM5(t=0) | BM4(t=0) |
Base + 8h | BM3(t=T) | BM2(t=T) | BM1(t=T) | BM0(t=T) |
Base + Ch | BM7(t=T) | BM6(t=T) | BM5(t=T) | BM4(t=T) |
Base + 10h | ... |
The state metric accumulation resolution is 13 bits on the VCP modules. Consequently, full 8-bit dynamic range is available for branch metrics on the VCP modules, for all constraint lengths and all code rates.