On 27 August 2012 23:22, Markus Vester <[email protected]> wrote:

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)