[aprssig] Interesting Findings - 300 Baud AX.25 on VHF-FM APRS

Ross Whenmouth ross at topwire.co.nz
Sat Apr 2 15:22:47 CDT 2016


AX.25 does not use forward error correction - typically, the corruption 
of a single bit results in the complete loss of a packet (though some 
packet software will try toggling every bit in a bad packet to see if it 
can get the CRC to match). FX.25 has been proposed as a backwards 
compatible method of adding forward error correction to AX.25 - 
https://en.wikipedia.org/wiki/FX.25_Forward_Error_Correction (some of 
the links at the bottom of that page are broken but archive.org is your 

If you are prepared to break backwards compatibility, then you could get 
even better weak signal performance by wrapping a convolutional coder + 
interleaver around a Reed-Solomon coded packet eg 
http://www.ka9q.net/ao40/ (using a better modem than AFSK helps too).

Ross Whenmouth

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