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Date: Sun, 11 May 2014 12:06:55 +0200
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Subject: Re: VLF: Coherent BPSK at 8270 from DF6NM
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Hello Paul, Markus,

Well done! Interesting stuff. I see you never stop to find some new 
things to try :-) Well well, i should think about...

What was the contest of the message? A complete callsign, DF6NM?

73, Stefan

Am 11.05.2014 08:36, schrieb Paul Nicholson:
> I am pleased to report reception of two test transmissions from
> DF6NM on 8270.000 Hz which took place on Saturday 2014-05-10
> morning.
>
> Markus was sending coherent BPSK with UT synchronous symbols
> using a symbol period of 30 seconds.  The FEC is a terminated rate
> 1/4 convolutional code with constraint length 21, cascaded
> with an outer error detecting code using a 16 bit CRC.
>
> Two transmissions of 46 bits were made, each lasting 132
> minutes with a 20 minute carrier test in between.  ERP was
> probably around 5 or 10 uW and the range is 1028 km.
>
> Eb/N0 was about -0.5dB in the first test and about -1.5 dB
> in the second, which is below -7dB in the symbol bandwidth
> of 33.3 mHz.
>
> Both messages were decoded with some margin to spare.
>
> The decoder is a list Viterbi decoder using the tree trellis
> algorithm with a list length of 2000 and stack size 20000.
> Both messages decoded at the top of the list so the list
> decoding wasn't actually necessary for this strength of signal.
>
> The transmitter uses a rubidium source and the receiver is
> GPS timed.   A reference phase at the receiver is obtained by
> averaging the phase of the squared signal but at such low signal
> strengths the resulting reference is unreliable and the decoder
> makes a search for the correct phase and phase drift rate.
>
> The signal is completely invisible in any spectrogram at the
> receiver.   In a spectrogram running at the symbol bandwidth
> the signal is too far below noise and the bandwidth of the
> transmission is such that, in a resolution capable of seeing
> the signal above noise, the signal is too wide.
>
> For example, this spectrogram uses the symbol bandwidth,
>
>  http://abelian.org/vlf/tmp/df6nm_140510a.png
>
> Maybe someone can see some very faint line at 8270.000 ?
>
> The first test ran 07:02 to 09:12 and the second
> from 09:32 to 11:44.   The 20 minute carrier in between is
> also below noise at this resolution.
>
> Following the second test, an hour of carrier is visible in
> the 278 uHz spectrogram at
>
>  http://abelian.org/vlf/fbins.shtml#p=1399780800&b=110&s=sp
>
> More information on the FEC codes, trials, and search for
> good polynomials can be found at
>
>  http://abelian.org/fec
>
> Stronger codes with constraint length up to 25 are available
> and hopefully further tests will be made soon.
> -- 
> Paul Nicholson
> -- 
>