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
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