21 September 2020

48 KBaud OQPSK unid wideband transmissions


This post originates from an email from my friend KE9NS Darrin who noticed  a strange transmission around 6.8 MHz with an occupied bandwidth of about 48 KHz. According to his reports, the signal seems to start somewhere between 0000 and 0100 UTC, likely ON until sun rise. Interestingly, the signal tends to move around the band slightly probably trying to find an open slot in the band. Indeed, some breaks were observed and Darrin just noted that when it shut down for a break there was a STANAG 4285 signal within its 48 KHz passband: it must have realized and moved to an open spot.
Darrin kindly sent me his IQ recordings for analysys since it's impossible for me to get such samples using remote KiwiSDRs.

The waveform has a speed of 48000 Baud and occupies a band of about 48 KHz: as shown in Figure 1, the spectrum width, equal to the manipulation speed and the presence of the third line in the 4th power, lead to think to the Offset QPSK (OQPSK) modulation.

Fig. 1
Although GMSK and OQPSK have a lot in common, some further clues in favor of OQPSK come from the phase plane (Figs. 2a, 2b): OQPSK looks like GMSK with BT < 0.25 (the lower the BT index, the more it's similar to OQPSK).

Fig. 2a - OQPSK phase plane

Fig. 2b - syntesized OQPSK signal
 
Similar results were obtained from the analysis of CIS-1280 waveform (Figure 3).
 
Fig. 3 - CIS-1280 OQPSK waveform

OQPSK is a constant-envelope modulation that has no 180-deg phase shifts and, therefore, has a much higher spectral containment than non-offset QPSK when transmitted over band-limited nonlinear channels. To further bandlimit an OQPSK signal, Shaped OQPSK (SOQPSK) was introduced and its initial version was referred to as MIL-STD SOQPSK after it was adopted as part of a military standard. 
 
Since OQPSK is like a GMSK with a small index, it is possible to do some demodulation attempts using the "FSK3 method" introduced by guys from radioscanner.ru [1]. In this regard, I also tried that FSK3 method by demodulating the 48 KBaud signal on three FSK levels (Figure 4) and then appropriately converting the ternary symbols through a small program written with Octave. It is difficult to establish the accuracy of the final bitstream, anyway the links to download the intermediate FSK3 file are below: everyone can try the demodulation by following the method described and post their comments and the obtained bitstreams.

Fig. 4 - FSK3 demodulation

Back to the 48 KBaud signal, it's always very strong, likely a very powerful transmitter. Quoting Darrin "Its a long shot but, the company that was supposedly trying to transmit stock trades via HF radio has a radio tower located in a town near to me. Supposedly they have a huge antenna array in Elburn, IL and transmit 20kw with an ERP of 808kw (very big stacked curtain log-periodic antennas) pointed 48deg and a special FCC license. It turns out, that antenna array is pointed directly towards my home in Bartlett, IL". We also thought of 188-110D App.D (per STANAG-5069) tests but such a waveform is not indicated.

https://yadi.sk/d/J76M1y-l-u42Sg (wav file) 
https://yadi.sk/d/_3o-UTVKEQpKNw (FSK3 demod)

[1] http://www.radioscanner.ru/forum/topic43183.html#msg865791

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