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TCP-group 1995


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Re: FEC paper, implementation



According to Phil Karn:
> >I think a nice platform to do these experiments would be the DSP4.  It
>
> I'm not convinced that a DSP has any special advantage over a general
> purpose CPU in executing FEC algorithms.  A general purpose CPU (such
> as the one already running NOS) is fast enough for ham use, especially
> with sequential decoding. My Fano decoder in C runs at roughly 280-320
> kilobranches/sec on a 66 MHz 486DX2 in protected (32 bit) mode. That
> translates directly to 280-320 kilobits/sec on "clean" packets with
> little backward decoder motion.
>
> Even my Viterbi (K=7, r=1/2) decoder program runs over 40 kb/s on the
> same machine.

Ok, I realize that I forgot to mention it, but my main reason for suggesting
the DSP4 was that you could run the code tightly coupled with the modem,
and therefore have the advantage you outline below.
Another thing is that you have a separate processor per modem, so that
you can run multiple ports without overloading the central processor.
However, it looks like this won't be a problem on fast systems like yours.

Unfortunately, to get the best performance out of a 56001 you need to code
in assembly language...

> What I *would* like from a DSP is a modem that gives me "soft
> decision" samples. This results in an additional coding gain of 2dB on
> AWGN channels with linear modulation (PSK), bringing the Eb/N0
> requirement for the r=1/2 K=32 sequentially decoded code down to 3dB.
> Hardware modems could be modified to do this by replacing the slicer
> with an A/D converter having at least 3 bits of precision, but on a
> DSP it would just be a matter of software.

Right.  And the ideas like moving the sampling interval to get best eye
pattern are easier to realize when there is a tight coupling between the
A/D and the FEC processing.

> I'm currently packaging up my Fano decoder and will be releasing it
> shortly for people to experiment. I'll follow up with my Viterbi
> decoder.

I just received your shar file.

Rob

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| Rob Janssen         rob@knoware.nl | AMPRnet:   rob@pe1chl.ampr.org       |
| e-mail: pe1chl@wab-tis.rabobank.nl | AX.25 BBS: PE1CHL@PI8UTR.#UTR.NLD.EU |
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