Am 06.05.2017 18:17, schrieb Paul Nicholson:
These seem to be typical, where the decode goes astray for
several characters, then comes back. It seems to involve
runs of about 3 to 5 constraint lengths (eg 4 * 19 = 76 bits =
~13 chars). Such a run contains 3 to 5 bits of redundancy in
the source encoding (0.3 bits per char) which the list decoder
makes use of. Putting in a small CRC allows the list decoder
to discard these 'diversions'. False decodes aren't much
of a problem because usually the real decode comes in with a
higher path metric and trumps the false ones but sometimes a
false decode wins. A small CRC will kill these off.
For short messages, a very small or zero CRC remains best,
because of the significant inner code overhead.
Interesting.
Yes that sounds good. For short messages, the symbol length is usually
smaller anyway, so a few bits more do not change the duration of the
transmission.
On the other side, many CRC bits in short messages (2 chars) make a very
significant part of the whole transmission.
I just saw that on my last announcement, decreased CRC from 32 to 2 and
increasing the symbol length from 40 to 60 still results in a shorter
duration!!
This also means I have to revise my policy for calculating an
optimum list length. Later I'll update the signal calculator
web page.
Will see if i can see a difference then...
73, Stefan
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Paul Nicholson
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