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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