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).
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| 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).
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| 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].
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| 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.
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| 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
