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Re: LF: Coherent BPSK on LF using EbNaut

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Subject: Re: LF: Coherent BPSK on LF using EbNaut
From: Paul Nicholson <[email protected]>
Date: Sat, 26 Sep 2015 13:17:05 +0000
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Very nice to see this working at LF with short symbols.
The phase was stable enough for the 20 or so minutes of each
transmission on this path of circa 1000km.

S/N of IZ7SLZ at DF6NM suggests that a contact would not have
been possible (at this symbol rate and signal strength) without
using coherent detection - it would be a few dB too weak.

The phase looks pretty steady.  The ionosphere and the local
oscillators are both looking very good here.  I wonder how
long the phase can keep stable for - or in other words, the
max message duration?

A fine achievement, well done to both.  It's great to see a
two-way contact using this mode and this demonstration that
it works at LF opens up some interesting possibilities.

Andy Talbot wrote:

> Unfortunately, I see it is not standalone, and requires
> Spectrum lab to act as the audio "front end" ...

Spectrum Lab in fact does most of the work:  Digitising
the audio, frequency shifting and the delicate business of
resampling with reference to GPS pulses so that each audio
sample is UT synchronous and at a constant sample rate.
Then there is filtering and if necessary, noise (sferic)
blanking.  SL even handles the decoding of GPS messages so
that it can accurately time the reception.  After all that,
ebnaut is just a small 'back-end' decoder.

This mode is not easy to use.  It requires some technical
skill to arrange atomic quality reference oscillators at
each end and to get the software set up correctly.

For those that can rise to the challenge, the potential gain
is several dB over any other mode on paths where the phase
is fairly stable for the duration of a transmission.

Coherent BPSK has been used for amateur satellites and packet
networks, for a long time.  These applications extract a
reference phase from the received signal (using a Costas loop
or equivalent).  EbNaut works even when the signal is too weak
to extract a reference phase.

More info at

 http://abelian.org/ebnaut/

--
Paul Nicholson
--

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