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[Discuss-gnuradio] Question about reverse-engineering a new mode

From: Mark Haun
Subject: [Discuss-gnuradio] Question about reverse-engineering a new mode
Date: Tue, 19 May 2015 15:04:16 -0700
User-agent: Mutt/1.5.21 (2010-09-15)

This is a bit of an idle question, but I'm hoping some knowledgable folks on
here can offer advice.  Mostly I'm trying to understand better what I
don't know, and the size of the challenge, before jumping in to a project:

I'd like to try decoding some AVL traffic in the 700-MHz band (GPS locations
broadcast by transit vehicles to a central collector, where predictors are
used to generate the ETAs displayed on electronic bus-stop signs).  The
modulation is 4-FSK, similar to P25 except wider with a higher symbol rate,
emission designator 20K0F1D.  The particular frequency(s) should be easy
enough to discover.  Transmissions are short packets on shared channels with
some kind of slotted aloha or CSMA MAC.  A rate-3/4 convolutional code is
used.  The preceding is public information gleaned from the web.  I haven't
captured any signals yet.

The known unknowns:  preambles and framing stuff, symbol mapping,
the particular rate-3/4 code used (only a couple of candidates though), and,
the scrambler (whitener) and its initialization.  AFAIK there is no
encryption per se.  The payload is supposed to be TCP/IP, so there could be
some sort of header compression.

My question, then, is given this information, are there reasonable odds of
success?  I have some digital comms background from grad school but little
to no practical experience.  Wondering if this might be an excuse to pick up
a HackRF etc. and learn GNU Radio, or if it's likely to be a dead end.



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