3 October 2023

QPSK 2400Bd unid waveform (Chinese modem?)

QPSK 2400Bd waveform heard on 10221.0 KHz USB around 1400 UTC, probably a Chinese modem.

Fig. 1

Autocorrelation of the signal produces sharp 16.6 ms spikes tnat makes 80 bit or 40 dibit symbols (QPSK modulation) period at the rate of 2400 symbols/sec. Indeed, after demodulation the resulting bitstream has a framing of 40 symbols length consisting of 20 known symbols (probe)


followed by 20 unknown symbols (data): obviously, since QPSK, 1 symbol = 2 bit. 

Fig. 2 - autocorrelation and bitstream

After the removal of the 20 known symbols, the initial & ending data blocks show 64-symbols/128-bit patterns even if - actually - the ending blocks consist of a 32-symbols/64-bit pattern (as it was already visibile in Figure 2).

Fig. 3

Fig. 4

 As usually, comments are welcome.

[1] https://disk.yandex.com/d/5g-pEgBTLIxSmg

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