Message-ID: <4634FBB90BDE4D73AC834A86697F0D6E@gnat>
From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
References: <54A92E4E.2030903@abelian.org>
Date: Sun, 4 Jan 2015 13:41:12 -0000
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Subject: LF: Re: Transatlantic messages at 8822Hz
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Hi Paul and Dex
Great work, well done!! It is nice to see that the frequency will carry 
information rather than just allow identifiction.

Best wishes
Alan
G3NYK

----- Original Message ----- 
From: "Paul Nicholson" <vlf0403@abelian.org>
To: <rsgb_lf_group@blacksheep.org>
Sent: Sunday, January 04, 2015 12:13 PM
Subject: VLF: Transatlantic messages at 8822Hz


>
> W4DEX achieved another 'first' recently by sending a series of
> messages across the Atlantic at 8822 Hz which were successfully
> copied at Todmorden UK, range 6194km.
>
> Transmissions used coherent BPSK signalling with ERP of around
> 150uW.   The modulation encoded the messages using a rate
> 1/16 terminated convolutional code with constraint length 25,
> cascaded with an outer error detection code.
>
> The first message was received at 2014-12-30 03:00, a 4
> character message 'EM95'.  Eb/N0 was -0.8dB using 9 second
> symbols.
>
> A second test the following night managed 12 characters 'PAUL
> HNY DEX' using 14 second symbols giving Eb/N0 of +1.0dB.
> Conditions were good and we could have used shorter symbols
> and a longer message.
>
> The third test and best result so far was a 25 character message
> '8822HZ 2015 JAN 1 TA TEST' sent from 2015-01-01 00:00 using
> 8 second symbols.  This was received with Eb/N0 = -0.1dB.
>
> In the 0.125 Hz bandwidth of a code symbol, the S/N was -13.2dB.
> That corresponds to -56dB S/N in a 2.5kHz audio bandwidth
> after sferic blanking.  Before the blanker the S/N would be
> around -76dB.
>
> The source encoding uses 6 bits per character to produce a
> payload of 150 bits.  An outer code adds a 16 bit CRC and the
> convolutional encoder expands the message to 3040 signal bits.
> The effective code rate is therefore 150/3040 = 1/20.27.
>
> Of the 3040 signal bits, 1153 were demodulated incorrectly
> but the FEC was able to fix them all to reveal the message.
>
> Received signal was around 0.12 fT and it was necessary to
> combine H-field and E-field receiver outputs to obtain a
> sufficient S/N to decode.
>
> The decoder is a soft Viterbi list decoder.  The signals are too
> weak to reveal a reference phase by the usual method of summing
> the squared complex symbol amplitudes.  Instead the decoder has
> to do a brute force trial and error search.
>
> The information rate in the 3rd test was 24.6 bits per hour
> which is 80% of the channel capacity.
>
> --
> Paul Nicholson
> http://abelian.org/
> --
>