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Branch metrics calculation in C6455 VCP2 Viterbi Co-Processor

Francisco Aguilar
Intellectual 380 points


Hello,

Could someone please help us in the following issue? We're working with VCP2 in C6455. We don't understand how we can calculate branch metrics as input data to VCP2. We're using QPSK, rate 1/2 and constraint length K = 7.

In the section 4.1 of the VCP2 reference guide (http://focus.ti.com/lit/ug/spru972c/spru972c.pdf), it is explained that we have to use the following calculation for branch metrics when we are using BPSK:

- BM0(t) = r0(t) + r1(t)

- BM1(t) = r0(t) - r1(t)

Could we use the same calculation of branch metrics in our case of QPSK? We don't understand fine how we can obtain these formula to calculate branch metrics.

Many Thanks in advance!

Kind Regards

  • 0
    Prodigy 30 points

    Branch metric calculation has nothing to do with you modulation type.

    After constellation de-mapping performa the following mapping and calculate the branch metric as per the equaiton you have said above.

    map 0 to 1 and map 1 to -1.

    Branch metrics BM0 and BM1 are computed for each bits of the non-encoded bits(i.e bits input to encoder).

    Hope you understood.