12 October 2017

OFDM modem 12+12 tones 48Bd (Marconi 25-tones)


Probably a simplex transmission heard on 6246.5 KHz/USB. Although the SNR is poor, it's possible to see a central un-modulated tone a + 1500Hz  that splits the signal in two identical parts which I analyzed separately. The results show an OFDM modulation of 12 + 12 tones at a rate of 48 symbols/sec; the on-air constellation seems to be QPSK on each channel.

Fig. 1
Fig. 2
Fig. 3
 The initial part exhibits an upper un-modulated tone and a short FSK

Fig. 4

 Modem name and users are unknown to me.

Update:
thanks to hf_linkz who pointed me to the correct identification of this modem: it's the "Marconi 25-tones" as depicted here:
http://signals.radioscanner.ru/base/signal125/ 
https://www.sigidwiki.com/wiki/Marconi_Selenia_25-Tone_Modem