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Date: Sun, 8 Jan 2017 19:40:08 -0500
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Subject: LF: RE: Wideband EbNaut with prefix?
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Very cool.

Mine safety folks might like that.

Jim AA5BW
 

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Paul Nicholson
Sent: Sunday, January 8, 2017 6:18 PM
To: rsgb_lf_group@blacksheep.org
Subject: LF: Wideband EbNaut with prefix?


A decode from a near field test at VLF:

  Rank 6938 Eb/N0 -0.4 dB 8K19A 60 chars

The interesting thing is, the symbol period is 6.25 milliseconds
(3152 symbols for 19.7 second total duration) and the message was sent by magnetic coupling between two undisciplined soundcards.  Received against the natural VLF background.
The message was also 'unannounced'.

EbNaut here is sending a coded prefix.  The decoder monitors a band and searches for prefixes, then measures the frequency and timing accurately enough to decode the following message coherently.

It relies on the undisciplined soundcard clock or RX LO being stable over the message duration, say up to a minute or so.

The overhead of the prefix is at the moment 1.25dB, hoping to bring that down a bit.  A delicate matter of balancing the prefix energy and message energy.  No point in a detectable prefix if the message wont decode, or vice versa.

In principle then, with a small Eb/N0 overhead, the decoder can take a live untimed I/Q stream from SpecLab or your favourite SDR and monitor a band a few Hz wide. The only thing you have to know in advance is the symbol rate.  Coding in the prefix supplies the rest.

No good of course for slow narrow band, but there might be applications for short duration wide band messages coherently detected and decoded with signals a dB or so from the limit.

--
Paul Nicholson
--