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RE: Prototype FEC telemetry encoder/decoder released



A suggestion:

Leak some carrier and have a very narrow tracking loop
for the leaked carrier.  By doing this, we do not take
the nonlinearity loss from a squaring or costas & even
ML carrier/data estimator will benefit from the a priori
estimates you are able to then provide.  Your a posteriori
estimates will proceed much more quickly to the right
place.  As always,  the problem is locking these demods up
and the leaked carrier will be a huge boon.   Several decades
of NASA doing it can't be wrong since their needs are the
mother of the other inventions we are using.  The residual
carrier is what made it possible for Maureen Quirk and
I to do the successful demodulation of the VEGA Balloon
probe.

The narrow tracking loop for the carrier will be self
defeating when we go through deep fades with tons of
phase jumps (reversals even) since after each phase
change we'll find ourselves hunting and pecking for
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
that we can track carriers in gaussian noise and
decode to many dB better when there is no way to use
this power because the detector won't track.  The
noise/fading environment as a whole must be taking into
account.



Bob


-----Original Message-----
From: owner-AMSAT-BB@AMSAT.Org [mailto:owner-AMSAT-BB@AMSAT.Org]On
Behalf Of Phil Karn
Sent: Sunday, February 10, 2002 5:02 AM
To: Dquagliana@aol.com
Cc: amsat-bb@AMSAT.Org
Subject: Re: [amsat-bb] Prototype FEC telemetry encoder/decoder released


>I agree.  We will need ground station application software that 
>can receive the new format.  If no one can receive it, it will
>be a long time before it's implemented on AO-40.  

I'm also interested in user reports from anyone trying it out via the
transponder, over a terrestrial path or even simply running it in
software simulation mode.  VK2TFG has already been a big help in
finding and fixing a few little gotchas that cropped up when he built
my code on a different Linux distribution with a different version of
GCC.

The modulation, demodulation and FEC techniques I used have been
around literally for decades. Their properties are now very well
understood and I believe they are nearly optimal for this particular
job. So the real fun now is in finding and fixing all the little
portability, usability and integration issues just like the ones Geoff
helped me find.

If this format is accepted, implemented into the spacecraft and
integrated into AO40RCV and other telemetry reception programs, it
will be no harder to use than the present format. The only observable
difference is that performance will be significantly better (by ~6-7
dB) when the beacon signal is weak, and *dramatically* better (by
*TENS* of dB) when spin and bad squint angle induce deep fades and
rapid doppler shifts. It will also be immune to "false lock" and
highly resistant to radio retuning transients.

I plan to add a coherent BPSK demod to a future version of my
prototype.  This will be compatible with the same FEC transmission
format but increase performance on weak, non-fading channels by
another 3-4 dB, i.e., to 10-11 dB better than the present uncoded
format.

Phil


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