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Re: [Discuss-gnuradio] gr-fec LDPC comments / questions.

From: Sylvain Munaut
Subject: Re: [Discuss-gnuradio] gr-fec LDPC comments / questions.
Date: Sat, 26 Jul 2014 14:42:38 +0200

Hi Again,

> But in the paper I'm looking at (
> http://www.ldpc-codes.com/papers/encoding.pdf ) the T & E matrices are
> on the right side, which would match much better of course and
> honestly makes more sense to me since the reulting codeword often has
> the systematic bits first and then the parity bits.

Digging through the decoder I also found this assumption that the
codewords are provided with the parity bits first, then the systematic
bits :

        for (index = 0; index < d_frame_size; index++) {
          unsigned int i = index + n - d_frame_size;  /* <<<< HERE  */
          int value = gsl_matrix_get(x, i, 0);
          out[index] = value;

All the codes I know (granted it's only GMR and DVB) use the opposite
convention, so I think he'd be wise to either flip the convention in
the GR code, or make it configurable.

Do you know of any LDPC codes in common application where the parity
bits are placed/sent first ?



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