16 November 2021

unid OFDM-121 system

Unid Russian OFDM-121 system recently heard on 11071.0 KHz (CF) until monday morning, frequency spacing between channels is 25 Hz, symbol rate in channels is 21 Baud, the used modulation seems to be a form of PSK4. As from figure 1, not all of the 121 available channels are used and it's possible to note a different arrangement of the channels in different days. The modulation used in the 2400Bd bursts preceeding the OFDM segments remains to understand, although there is a prevalence of PSK2 blocks.

Fig. 1

For what concerns the 2400Bd bursts which preceede the OFDM segments, I could not get a clean constellation; anyway looking at SA phase detector and 2nd order harmonics (figure 2), I think it is some kind of PSK2 modulation, or at least it's the prevalent one.
Fig. 2 - 2nd order harmonics, phase detector e constellation

4-ary constellation test

Moreover, the ACF has a 20ms value (figure 3) that makes 48 over-the-air symbols per frame, each frame consisting of 32 unknown data symbols followed by 16 known symbols. If the 20ms pattern in figure 3 represented bits then it would count a 32-bit length for the data block, therefore 1 symbol = 1 bit and thus we have a PSK2 modulation. As rightly suggested by my friend KarapuZ, in this case, the palette of the pattern consists of more than two or three colors: what does this mean? That we see more than just two or three levels of manipulation.
Fig. 3 - 2400Bd bursts framing

Apparently, this is one of the command and control systems and is associated with the recent military exercises in Belarus. Direction Finding indicate the Moscow area as Tx site location, as shown in figure 3.

Fig. 3


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