CIS 2400 Bd serial bursts + WideBand OFDM-241

[Update April 3, 2026]: Further analysis of the OFDM-241 waveform has revealed additional parameters.
I would like to thank my friend and colleague ANgazu, who shared a high-fidelity signal recording. He also analyzed the OFDM-241 waveform in radiofrecuencias.es forum, identifying 16-QAM modulation on the sub-carriers. Tones 1 and 241 are pilots. There are intermediate pilots and symbols used as such in what are known as "scattered pilots" .On my end, I replicated the OFDM analysis on the signal provided by my colleague and can fully confirm his findings. 

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The original post follows below:

Interesting composite signal featuring a format consisting of a burst pair with a 3 kHz bandwidth (~750 ms duratione), followed by a longer burst (~5700 ms duration) occupying a 6 kHz bandwidth. The transmission was recorded at a center frequency of 20859.0 kHz a few weeks ago (Figure 1).

Fig. 1

3 KHz bandwidth bursts
The 3 kHz bandwidth bursts consist of a preamble followed by 33 frames. The modulation employed is PSK at a symbol rate of 2400 Bd. Signal autocorrelation (ACF) exhibits peaks at 20 ms intervals, corresponding to a framing of 48 symbols: likely 16 symbols used for probes (known symbols) and 32 symbols for data (unknown symbols).

Fig. 2: 3 KHz bandwidth, 2400 Bd bursts 

Constellation diagrams and state transitions provide little assistance in identifying the modulation scheme (Figure 3); however, the preambles and reinserted probes most likely employ BPSK. In accordance with previous analysis, the waveform employed is identical to the one discussed in posts [1] and [2].

Fig. 3: 3 KHz bandiwdth, 2400 Bd constellations

6 KHz bandwidth bursts
The 6 kHz bandwidth bursts employ an OFDM waveform, consisting of a 241-channel multiplex with 25 Hz spacing and a symbol rate of approximately 21.33 Bd per carrier. This results in an aggregate raw symbol rate exceeding 5000 Bd.

Fig. 4: OFDM-241 6 KHz bandwidth burst

Determining the modulation scheme employed for each channel remains challenging; while QAM is a likely candidate, this remains an unconfirmed hypothesis. Similarly, it can be hypothesized that these are test or experimental transmissions conducted by CIS diplomatic or military entities, as observed in previous cases.

https://disk.yandex.com/d/67gLhhUILzry0Q

[1] https://i56578-swl.blogspot.com/search?q=ofdm+121
[2] https://i56578-swl.blogspot.com/2021/12/cis-2400bd-serial-bursts.html

CIS 2400Bd serial bursts

 Just to point out some differences bewteen these two types of CIS bursts (figure 1) :

* the ones sent in group or in front of the OFDM-121
* the ones sent individually or before Serdolik MFSK/OFDM 

Fig. 1

 

Both the bursts are 2400 Baud keyed and have the same 20ms ACF value as well as - likely - the used modulation, although their constellations and state transitions are not much help in identifying the type of modulation (figure 2): most likely PSK2 in preambles and reinserted probes. The number of the transmitted frames is different, 33 Vs 22, but since they have the same 48-symbol length framing (same ACF) their duration differs (about 760 and 476 ms).

Fig. 2

For what concerns the structure of the frames, the classical method can be misleading, showing two different structures consisting of 33 unknown symbols + 15 known symbols for a type of burst and 32 unknown symbols + 16 known symbols for the other (figure 3a); the plain raster method anyway shows the same 32 + 16 framing for both the burst (figure 3b).

 

Fig. 3a

Fig. 3b

Given the different systems in which these bursts are used, and also the modalities of their transmission (isolated, in group or in front of OFDM), I think they also have different purposes and functionalities.

https://disk.yandex.com/d/wLHbQn4iGgZeZA

CIS 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 PSK4 modulation.

 

Fig. 2 - 2nd order harmonics, phase detector e constellation

 

The ACF has a 20ms value (figure 3) that makes 48 over-the-air symbols per frame, each frame apparently consisting of 32 unknown data symbols followed by 16 known symbols.

 

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

https://disk.yandex.com/d/qQrCn43mL0UO1Q