Interesting analogy Mal ! may be we should start a
CFRD group , Campaign For Real Data ? you can the 'KEY'
person : )
G..
-----
Original Message -----
Sent:
Monday, August 27, 2012 10:22 PM
Subject:
LF: WD2XES - Opera detected and identified by correlation
Recently I have been pondering
wether it is possible to detect QRSS or Opera transmissions by signal
correlation against a known waveform. This should far be more sensitive
than the conventional incoherent decoding process. John's stable and
phase-coherent Opera transmission last night's provided a welcome
opportunity to test this scheme.
Using SndInput from DL4YHF, I
recorded a long IQ file at 2 samples / second, ie. 2 Hz wide centered
on 137561 Hz. The audio was taken straight from the Rubidium-locked
receiver, with no noise blanking inserted. The data was then
postprocessed using a MathCad spreadsheet. Some results can be viewed
in
http://df6nm.bplaced.net/opera/xes/
,
along with TA grabber screenshots showing both XGJ and a weak trace
from XES in 21 mHz FFT (testTA.jpg).
First a high resolution
spectrogram was generated, at 1.9 mHz per bin (xes_spectrogram.png).
The central carrier component of the transmission can clearly be seen.
Best SNR occured between 3:30 and 4:30 UT (as indicated by marker
ticks), when the peak was 9.0 dB above the noise (xes_spectrum.png).
Scaling noise bandwidth from 2.9 mHz to 2.5 kHz (-49.5 dB), and adding
6 dB for 50% duty cycle, we get a peak-power SNR of -44.5 dB. This
corresponds to -48.5 dB on the Opera SNR scale, about 9 dB below the
current decoding threshold for Op-32.
The peak appeared about 0.2 Hz
off-center because the 12 kHz samplerate had not been not calibrated.
Once the peak frequency is accurately identified, the received signal
can be correlated against a "prototype" waveform, which contains the
Opera sequence for WD2XES, 16-fold oversampled. The correlation is
efficiently implemented as a multiplication in Fourier space.
The result
(xes_correlation_wd2xes.png) shows four distinct peaks in time domain
at 2:15, 2:48, 3:21 and 3:54 UT, which should correspond to the
a-priori unknown start times of John's Opera sequences. The repetition
period was apparently 32.92 minutes. As the DC component in the
reference waveform had not been removed, the peaks are riding on a
pedestal caused by the self-correlation of the carrier component.
To check the ability to identify
an unknown station, the correlation to a different callsign was also
plotted (WD2XGJ just as an arbitrary example, see
xes_correlation_wrongcode). In spite of the weak cross-correlation
peaks, we find that a correct selection from a list of potential
candidates would certainly be feasible.
It would not be too difficult to
automate this process and create an "Opera deep search" software, which
should be able to detect and identify signals reliably down to about 12
dB below the threshold of the current Opera decoder. This means at same
sensitivity we could go 16 times faster!
To reap this benefit, the
following prerequisits need to be fulfilled:
- As we need a carrier component, transmit keying has to be phase
coherent. Thus simple keying schemes which interrupt the oscillator or
divider would not work.
- as we look at phase-sensitive integration over the whole sequence
rather than a single symbol duration, the frequency stability has to be
much tighter.
- As there is no bit-wise decoding involved, we will need to supply a
list of potential candidate callsigns (similar to deep-search in K1JT
EME modes).
Thanks again to John and Warren
for the signals!
Best 73,
Markus
(DF6NM in JN59NJ)
