Message-ID: <54AB0110.3030709@abelian.org>
Date: Mon, 05 Jan 2015 21:24:32 +0000
From: Paul Nicholson <vlf0403@abelian.org>
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Subject: Re: VLF: Transatlantic messages at 8822Hz
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Status: O

By reversing the modulation of Dex's Jan 1st transmission,
the carrier is reconstructed.  S/N and phase in in 1 hour
(278uHz) steps:

  00:00 to 01:00  11.6 dB  -122.1
  01:00 to 02:00  13.1 dB  -119.4
  02:00 to 03:00  12.6 dB  -131.9
  03:00 to 04:00  14.3 dB  -135.0
  04:00 to 05:00  12.7 dB  -111.0
  05:00 to 06:00  15.2 dB  -118.9
  06:00 to 07:00  13.4 dB  -111.7

  Average of above: 13.27 dB

This agrees well with the S/N of 13.33 dB in 278uHz calculated
from the symbol error rate.

Note the steady phase which is what allows coherent BPSK to be
used when the signal is too weak to extract a reference phase.
The decoder merely has to try phases (in say 30 degree steps)
until it gets a decode.

I wrote:
 > information rate ...  80% of the channel capacity.

I made a mistake and forgot to allow for the 16 CRC bits
(they belong to the coding, not the information payload).
Actually the rate was 73% of capacity.  I suppose we'll just
have to do better.

In the same series of tests, I received a 9 character message
from Markus DF6NM with symbol error rate 0.395, Es/N0 = -14.4dB,
Eb/N0 = 0.0 dB at an effective code rate of 1/27.85.   This
achieved 71% of channel capacity.   In fact it was quite an
easy decode of the 10 second symbols (it did not require list
decoding) and with hindsight we could have used 8.5 symbols
to obtain 83% of capacity.  This illustrates the challenge
of selecting a symbol rate for any given test.  It is essential
to have some reliable carrier S/N measurements beforehand.

Another result: On 30th December a 4 character message
from W4DEX was decoded from the natural radio receiver at
Sebring FL, distance 869 km. Eb/N0 was +2.8 dB so it was
nowhere near the channel capacity and we could have used a
much longer, faster message.

I am very grateful to Markus and Dex for making these tests
happen and turning what would have been just a computing
exercise, into reality.

It seems there are three essentials for success at VLF:

Teamwork;   A GPS;  And the following advice -

  "Get started and see what happens...
   Turn your DREAM into REALITY!!"
         ...Mal Hamilton G3KEV, Feb 2010.

--
Paul Nicholson
http://abelian.org/
--