this transmission was spotted on 6550.0 KHz/USB (16 May) at 2024 UTC and consists of a 3-bursts train: each burts has a duration of ~1048ms and the interval between two consecutive bursts is ~385ms. The signal spreads a bandwidth of 2880 Hz (pic. 1).
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| pic. 1 |
The signal shares the common PSK-8 waveform features, 1800Hz single tone carrier, and 2400Bd symbol-rate, although the 8-ary constellation is a bit confused: four "clear" points and four "nuanced" points, as shown in pic. 2. Such feature has been observed in some Chinese waveforms but in this case, since the strength of the signal, it's difficult to confirm this origin.
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| pic. 2 |
About the structure of the waveform, it's possible to detect the presence of a preamble sequence (CCF of pic. 3) which has a duration of 200ms and preceedes the data transfer.
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| pic. 3 |
Data are structured in a 32 symbols frame, or 96 bits (pic. 4).The frame duration of 13.33ms (32 symbols @ 2400Bd) is the same than the STANAG-4538 BW-3 (LDL traffic data PDUs) but the burst duration does not match the BW3 characteristics (pic 4,5): for example, burst duration is computed to 1226.45ms in case of a packet of 64 frames while the burst of the signal being analyzed has a duration of 1048ms.
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| pic. 4 |
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| pic. 5 |
By the way, as seen in some previous posts, the measurement of the period of a certain signal could not coincide with its frame structure: it has been shown that in particular circumstances the period lenght, and hence the ACF value too,is produced by the lenght of the scrambler or the interleaver combined with the real frame lenght.
Similar transmissions are often spotted on 14870.0 KHz/USB kHz and "rumors" say it could be a modified 3G-ALE waveform. Comments are welcome, short recording is available on e-mail request.