On 20 May 2012 17:47, Rick Muething <[email protected]> wrote:
> I took some time this weekend to read about the FDMDV (14 carrier +
> pilot, 50 baud QPSK) mod/demod used. I am not sure I have all the latest
> documentation.
>
> This is a good test vehicle for the CODEC2 and should allow some very
> useful data to be gathered.
>
> Longer term I think a slight modification in this could offer a
> substantial (2-3 db Power equivalent ) gain. (aside: in the HF modem world
> we fight for every fraction of a dB!!!)
>
> It might work like this.
>
> 1) Baud rate, user data rate (to/from the CODEC2), carrier frequencies etc
> all stay exactly the same. The bandwidth would be the same as the current
> 4PSK scheme also.
>
> 2) The 4 PSK symbols are replaced by Trellis coded 8PSK symbols carrying
> one FEC bit per symbol (e.g. code rate of 2/3)
> 3) A fairly simple R=1/2 K=7 or perhaps lower K value encoder is used to
> create the 8PSK code. Basically called Pragmatic Trellis code moduation.
> 4) Some interleaving is helpful but the tradeoff here is latency. For
> example if you interleaved across all the carriers and used a Viterbi look
> back buffer of say 28 symbols you would have added about 20 ms (+ Viterbi
> decoding time) of latency. That is probably acceptable but slightly
> longer latency and look back buffers might give better performance on
> certain channel types. Probably would need to experiment with that.
>
> This would require more CPU time but the bit rates required (1400
> bits/sec) are certainly possible with even older PCs and easy for modern
> ones.
>
> In a WGN environment 8PSK Trellis should give 3 dB or so improvement over
> uncoded 4PSK....equivalent to doubling the transmit power...a very
> worthwhile improvement. For HF the number will be different but still
> effective. I saw a substantial improvement in WINMOR going from 4 PSK to
> Trellis 8PSK but that also had a second outer layer of RS FEC which would
> not be needed here.
>
> Just some thoughts and a relatively easy to implement experiment. Maybe
> using a HF Channel simulator would allow capturing enough consistent data
> to determine the actual improvement (in dB or BER) compared to standard non
> FEC 4PSK.
>
> Rick Muething, KN6KB
>
>
>
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Just a point about latency. Interleaving over 28 symbols shouldn't in
theory add any latency. The entire 56 bit frame would still be delivered in
the same time. So the 40ms latency there would be the same as always.
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