27 February 2021

15-channel DBPSK 500Bd 30KHz, CIS (wideband?) Akula

Quite rare Akula ("Shark") [1] signal catched by my friend KarapuZ. The usual Akula 500Bd/1000 FSK2 waveform is preceeded by the transmission of 15 PSK tones (MPSK-15) lasting the same time of the FSK signal. The 15 channels are about 2200 Hz spaced and occupy a bandwidth of 30 KHz, each channel consisting of a Differential BPSK modulated tone at the symbol rate of 500Bd (Akula II), ie the same of the following FSK2 burst; the 15 subcarriers are not orthogonal (Figs 1,2).

Fig. 1

Fig. 2
Either the DBPSK channels and the FSK2 transport the same data and use the well-known distinctive sign "1771/"  as shown in Figure 3.
As per [1], FSK2 Akula may be descrambled using the LFSRs described by the polinomyals x^5+x^3+x+1 or x^4+x^3+1 after differential decoding: well, in this case none of the two modes (DBPSK and FSK2) is successfully descrambled using the above polynomials.
Fig. 3 - ending parts of the demodulated bitstreams

The FSK2 signal is exactly centered on the passband of the preceeding 8th DBPSK tone (likely the "call" frequency): it could be that the FSK2 signal is targeted to "legacy" receivers while the wideband part to "stared" SDR receivers (as said, the contents are the same)... but that's only a my guess, who knows? anyway, using such a width signal (about 30 kHz) should provide good noise immunity. It's to notice how the power of the FSK signal appears spreaded on the DBPSK signals (same transmitter). 
The use of BPSK modulation (Akula II) is not new, as a 2015 recording demonstrates (Figure 4). Also in this sample, the FSK signal is centered on the passband of the preceeding BPSK subcarrier, although the switch time  BPSK -> FSK2 is longer than the one 15xDBPSK -> FSK2. The two demodulated bitstreams - at least in this sample - are not the same (Figure 5).
Fig. 4 

Fig. 5

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