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 fewer 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 utilized 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 supply small assistance in identifying the modulation strategy (Figure 3); however, the preambles and reinserted probes most likely employment BPSK. In accordance with erstwhile analysis, the waveform employed is identical to the 1 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 employment an OFDM waveform, consisting of a 241-channel multiplex with 25 Hz spacing and a symbol rate of about 21.33 Bd per carrier. This results in an aggregate natural symbol rate exceeding 5000 Bd.
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| Fig. 4: OFDM-241 6 KHz bandwidth burst |
Determining the modulation strategy employed for each channel remains challenging; while QAM is simply 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 erstwhile cases.
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[1] https://i56578-swl.blogspot.com/search?q=ofdm+121
[2] https://i56578-swl.blogspot.com/2021/12/cis-2400bd-serial-bursts.html