To: rsgb_lf_group@blacksheep.org
References: <CA+DS1VJxhLJuHViTbGkcS_pkpu2Jj7h7AyEA=pUVhh-UD8CyKw@mail.gmail.com>
From: Paul Nicholson <vlf0403@abelian.org>
Message-ID: <29b0445b-bcb6-fc3c-add0-cbba03ad693a@abelian.org>
Date: Fri, 29 Dec 2017 18:27:30 +0000
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Subject: Re: LF: 8269.9 kHz EbNaut 27/12/17
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Dex wrote:

 > New message tonight [27th]

 > 3 chars 16K21A CRC20
 > 30 second symbols
 > Start 22:30 UT

Decoding from 8269.9 Hz, 2017-12-XX_22:30,+27900
vtfilter -h bp,f=8270,w=3000
vtblank -a12 -d0 -t100
ebnaut -dp16K21A -r1 -S30 -k20 -N3 -PU -L500000 -v

Todmorden (6194 km)
-------------------

  27/28 Eb/N0 +9.2 dB, S/N -22.9 dB/1Hz, phase -140.9 *Decoded*
  28/29 Eb/N0 +5.4 dB, S/N -26.7 dB/1Hz, phase  108.7 *Decoded*

Bielefeld (6917.7 km)
---------------------

  27/28 Eb/N0 -2.6 dB, S/N -34.7 dB/1Hz, phase  71.2;
  28/29 Eb/N0 -2.3 dB, S/N -34.4 dB/1Hz, phase -45.2;

  Fails to decode with stacking, Eb/N0 -5.0 dB due to
  phase change.

Warsaw (7681.3 km)
------------------

  27/28 Eb/N0  -7.4 dB, S/N -39.6 dB/1Hz, phase 143.3;
  28/29 Eb/N0 -11.5 dB, S/N -43.7 dB/1Hz, phase  85.6;

Cumiana (7173.4 km)
-------------------

  27/28 Eb/N0 +0.6 dB, S/N -31.6 dB/1Hz, phase 164.1 *Decoded*
  28/29 Eb/N0 -2.1 dB, S/N -34.2 dB/1Hz, phase  63.2;

Hawley TX (1816.2 km)
---------------------

  27/28 Eb/N0 7.8 dB, S/N -24.3 dB/1Hz, phase  -86.3 *Decoded*
  28/29 Eb/N0 1.3 dB, S/N -30.8 dB/1Hz, phase -163.5 *Decoded*

Forest VA (254.2 km)
--------------------

  27/28 Eb/N0 21.2 dB, S/N -10.8 dB/1Hz, phase 115.9 *Decoded*
  28/29 Eb/N0 20.4 dB, S/N -11.8 dB/1Hz, phase -14.0 *Decoded*

All the rx sites show a similar phase change between the two
nights, even Warsaw with the very weak signal.

So, do ethics accept a decode claim if it is necessary to use
published information about the transmit phase to facilitate
stacking?

One might argue that knowledge of the tx phase is no different
to knowing the tx frequency and start time.  Such knowledge
does not bypass any propagation.

And what about using information published from other sites
about the strength and success of the transmission?

A problem with stacking is the number of permutations as the
number of repeats increases.  Eg with four repeats, there are:

  One run with all four;
  Four runs with one repeat dropped;
  Six runs with two repeats dropped;
  Four runs with single repeats;

15 runs altogether and the operator is obliged to accept the
strongest looking decode out of all of them.  Each of the
15 runs has the chance of throwing up a false decode which
will beat the correct decode.  Therefore, knowledge that a
particular repeat performed poorly at other sites allows you
to drop that one with no cost in terms of false decodes.

Pushing this further, once the message is published, the
repeats can be selected for stacking based on their cross-
correlation.  You might for example have 10 repeats and be
able to get a decode from the best 5,  Without that knowledge
you would need 252 full runs of the decoder to go through all
permutations of 5 out of 10 repeats, surely enough to suffer
a stronger false decode.

--
Paul Nicholson
--