[Discuss-gnuradio] Re: gr-trellis: convenc and viterbi with own mapper/de-mapper

[colby . boyer]

On Mon, 16 Aug 2010, Jonas M. Börner wrote:

> Hi all,
>
> I am trying to use the convolutional encoder and Viterbi provided by the 
> gr-trellis class within another environment. I have my own mapper and de-mapper 
> blocks which I want to use. So I tried to use the feed the viterbi_combined 
> with this arguments:
>
> va_combined = 
> trellis.viterbi_combined_fb(fo,nsymbols,0,-1,1,[-1,1],trellis.TRELLIS_EUCLIDEAN)
>
> My de-mapper outputs soft bits between -1 and +1. Here is an example output of 
> my test script:
>
> data:           [0, 1, 1, 0, 0]
> encoded:        (0, 3, 2, 2, 0)
> unpacked:       (0, 0, 1, 1, 1, 0, 1, 0, 0, 0)
> modulated:      ((1+0j), (1+0j), (-1+0j), (-1+0j), (-1+0j), (1+0j), (-1+0j), 
> (1+0j), (1+0j), (1+0j))
> demodulated:    (-1.0, -1.0, 1.0, 1.0, 1.0, -1.0, 1.0, -1.0, -1.0, -1.0)
> decoded:        (0, 0, 1, 0, 0, 1, 0, 1, 0, 1)
>
> Another thing I don't understand is why the decoder outputs 10 values instead 
> of 5. I would be glad if someone told me what I am doing wrong.
>
> Regards,
>         Jonas

If I remember correctly, when you start pushing symbols through the 
trellis, it has some zero values before the input and some zero values 
after your last symbol pushed into the trellis. I think this is typically the 
length of your trellis. So at some point you will have to truncate the 
trellis output.

Any one else care to comment?

--Colby Boyer