11 February 2018

OFDM 20 out of 26 tones modem (possibly a Chinese waveform?)

The transmission has been spotted by ANgazu on 4102.0 KHz/USB by using a remote Kiwi SDR in Sweden. The signals consists of 4500 msec bursts, sent contiguously, and occupies a bandwidth of 2000 Hz (Figure 1).

Fig. 1
The waveform uses OFDM technology for 26 channels although 20 of these are used, ie the six tones 5,8,9,13,18,23 are off (at least in this sample). Modulation is PSK-2 in each channel with a symbol rate of 60Bd. Speed and modulation are confirmed by analyzing the whole signal and also a single channel (Figs 2,3).

Fig. 2 - analysis of the full OFDM spectrum
Fig. 3 - analysis of a single OFDM tone
KarapuZ suggets that it's likely a Chinese waveform, in this case the signal should be tuned on LSB. On my side I tried to synthetize a similar waveform although the results obtained (bandwidth and speed) are slightly different as shown in Figure 4.

Fig. 4 -synthetized OFDM 20-of-26 waveform
In another recording I also found bursts with 24 out of 34 tones up and PSK-2 modulation at 50 Bd (Fig. 5) , these short bursts are in between the groups of 20/26 bursts.

Fig. 5
A distinctive feature of the 20/26 waveform is its period that is formed by 14040 bits (Fig. 6): thanks to KarapuZ for the demodulated bitstream.

Fig. 6 - 14040 bits period


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