20 August 2019

OFDM-80 30Bd/37.5Hz

Unid (to me) OFDM 80-tones 30Bd/37.5Hz K=1/4, spotted with good SNR on 9400 KHz (cf) and resampled to 9600 Hz.

Fig. 1
Some channels show a clear PSK2 modulation while in other channels I could not successfully detect the used modulation (looks like a differential PSK but I could be wrong).

Fig. 2 - observed modulations
Data seem to be sent in blocks while the ACF of the transfer is 133.4 ms with a bit of instability which is probably due to the observed modulations in the channels (see Fig. 2). Since the aggregate speed of 2400 symbols/sec (30x80), the ACF value reveals a 320-symbols length frames. 

Fig. 3
The signal was recorded using the KiwiSDR http://sdr.ok2kyj.cz:8073/ (Pohorany near Olomouc, Czech Republic).

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