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Re: Prototype FEC telemetry encoder/decoder released



>Leak some carrier and have a very narrow tracking loop
>for the leaked carrier.  By doing this, we do not take

I would very much have liked to do this for all the reasons you cited.
But I'm constrained by the existing AO-40 beacon hardware. It requires
400 bps, differential manchester encoding, and no residual carrier.

I'd do things differently for a mode specifically designed for
user-to-user operation through the transponder.

I point out that any transmitter power you put into a carrier is power
that can't carry data. So you don't want to overdo it. Also, given the
very low data rates and enormous computing power now available to us
on the ground, the brute force maximum likelihood approach starts
looking increasingly practical. Don't want the squaring loss
associated with a Costas or squaring loop? Just try BPSK demodulation
and FEC decoding for all possible frame offsets, carrier frequencies
and phases until one succeeds! To help the searching, you can still
form noisy estimates of the signal parameters, sort them, and try the
most likely candidates first.

>the carrier.  I suggest that in any package that
>attempts to include demods based on the code Phil
>has done for general QSO use, we include both
>coherent and differential detectors.  It won't matter

This is exactly what I plan to do. The differential detector is
already in place and works extremely well on the spin-induced deep
fades that were my original motivation for this project. The coherent
detector is a secondary priority, but it will still be useful.

I do wonder why so many existing Phase 3 and Microsat telemetry
demodulators are coherent given the difficulties many have in tracking
even mild doppler and fading. The penalty for noncoherent DBPSK
demodulation at the ~10 dB Eb/No required by the current uncoded
format is less than 1 dB, a small price to pay for its simplicity and
robustness. It's only at the much lower Eb/Nos made possible by FEC
that coherent demodulation can be significantly better -- but only in
the nonfading case.

Phil



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