Channel coding questions

Hi all,

I’ve been exploring gr-trellis recently and have a few questions
relating to channel coding in gnuradio, and more specifically about
getting constellation objects to work with it.

Questions relating to channel coding:

  • Is gnuradio fast enough for channel coding for real applications?

  • At the moment Viterbi and BCJR (I think that’s what the siso blocks
    are) are implemented. What other algorithms would be useful? The
    docs suggest some suboptimal decoders (“eg, sphere decoding, M- and T-
    algorithms, sequential decoding, etc.”).

  • It might also be nice to have more python hierarchical blocks for
    example turbo coding and decoding blocks. What other hierarchical
    blocks would be most useful?

  • How much of a problem is buffering really? I can imagine that if
    the trellis.metrics_X is used with a big constellation then things
    could get a bit hairy, but otherwise I don’t see any problem. At the
    end of the gr-trellis docs there is a comment about turbo-decoding
    having issues with buffering between the blocks, but the siso blocks
    only output one float per bit, which doesn’t seem too outrageous
    considering that the complex symbols that are getting passed around
    all the time are two floats long.

Getting constellation objects to work with gr-trellis:

  • I’ve been adding constellation objects to gnuradio. It’s a minor
    change and will just mean that algorithms that should be optimised
    differently for different constellations are stored together with the
    constellation points in an object. The generation of metrics for the
    Viterbi and BCJR algorithms fall into this category.

  • gr-trellis is very flexible in that constellations are not
    restricted to sets of complex points but rather can be sets of
    n-dimensional real or complex points. Is it worth adding this
    flexiblity into the constellation objects?

  • My inclination is to add trellis.constellation_metrics_X,
    trellis.constellation_viterbi_combined_XX, and
    trellis.constellation_siso_combined_XX blocks that use the
    constellation objects while leaving the existing blocks alone. Any
    objections?

Cheers,
Ben

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