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/
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