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

John Wiseman john.wiseman at cantab.net
Sat Apr 2 16:00:46 CDT 2016


True, but UZ7HO already has a Reed Solomon based 4PSK FEC mode. You can keep
the standard ax.25 packet format. You do, of course, need UZ7HO at each end.

73,
John G8BPQ

-----Original Message-----
From: aprssig [mailto:aprssig-bounces at tapr.org] On Behalf Of Ross Whenmouth
via aprssig
Sent: 02 April 2016 21:23
To: aprssig at tapr.org
Subject: Re: [aprssig] Interesting Findings - 300 Baud AX.25 on VHF-FM APRS

Hi,

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 
friend)

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).


73 ZL2WRW
Ross Whenmouth
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