Re: [Discuss-gnuradio] Using the viterbi decoder from gr-dtv dvbt in gr-iee802-11

[Tom Rondeau]

On Fri, Jan 29, 2016 at 12:38 AM, Gonzalo Arcos <address@hidden> wrote:

Hello Tom,

Thank you for your answer.

I dont see how using the viterbi decoder from gr-fec would fix my problem. I would still need the viterbi decoder to work on K=5 to be able to integrate it with gr-ieee802-11, it is the same situation i am now with the decoder i linked. My question is first if there is a highly optimized decoder at least for 1/2 K=5, or in case it does not exist, if it is possible for me to modify an existing implementation. I repeat, i am a novice in the field of convolutional decoding.

Thanks

Apologies, I misread your first email and swapped the constraint lengths.

But no, modifying the implementation for different values of K is /not/ trivial.

Tom

 

2016-01-28 20:05 GMT-03:00 Tom Rondeau <address@hidden>:

On Thu, Jan 28, 2016 at 11:13 PM, Gonzalo Arcos <address@hidden> wrote:

The dvbt project im talking about is:
https://github.com/gnuradio/gnuradio/tree/master/gr-dtv/lib/dvbt

And the gr-ieee-80211 project is:
https://github.com/bastibl/gr-ieee802-11

thanks in advance

We have a highly optimized Viterbi decoder for K=7, rate 1/2 in gr-fec, which is done mostly in a VOLK kernel. Those parameters (R=1/2, K=7) are fixed, but the polynomials can be set externally.

Tom

 

2016-01-28 19:11 GMT-03:00 Gonzalo Arcos <address@hidden>:

I am trying to optimize the current performance of gr-iee802-11. Ive done some profiling and came to the conclusion that there is a lot of work being put in decoding the convolutional code at the receiver.

I have done some research, and found out that there exists a viterbi decoder that was developed for the digital tv branch, that supposedly outperforms IT++ by a factor of 9x!, IT++ being the library that gr-iee802-11 uses to decode the convolution.

When trying to adapt this decoder to gr-iee802-11 ive found some hassles. First, the constraint length for the viterbi decoder in dvbt is K = 7, while in gr-ieee802-11 uses a constraint length of K=5.
In addition, the generator polynomials also differ.

I would appreciate if someone could help me here, i am a beginner in the matter of convolutional codes, i would like if someone could asses me if it is possible to adapt this viterbi decoder to gr-iee802-11 at all, and in case it is indeed possible, if it is very complex task for a non expert on the matter.

The source file d_viterbi.c has a comment that says:
That changing the polynomials would need me to "regenerate the BUTTERFLY macro calls in viterbi()" I am not certain about what should the new parameters be for the polinomials 0133 0171.
https://github.com/gnuradio/gnuradio/tree/master/gr-dtv/lib/dvbt

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