10 July 2016

"Serdolik" 40Bd mixed mode: OFDM 45 and 60 tones, MFSK-31, and a pich of FSK

One of the numerous variant of the nicknamed "Serdolik" system heard on 11469.0 KHz on USB. Such composite signals are not frequent in the air but sometimes, and for a short period of days, they spring up like magic. Who knows?  maybe the Russian Academy of Communication students take graduate work :).
Most of the transfer is performed with OFDM with short MFSK/FSK inserts at irregular cadences: it's interesting to note that all these modulations are characterized by the same 40 symbols/sec data rate. 

The used OFDM are the 60 (60 +1 pilot) and 45 (45 +1 pilot) tones waveforms, both with channel separation of 50Hz and - as said - 40 Baud speed (figs 1,2). The MFSK/FSK inserts occur in both the two modes. The 60+1 tones waveform was already meet here.

fig. 1 - OFDM 60-tones (+ 1 Pilot)
fig. 2 - OFDM 45-tones (+ 1 Pilot)
The switch bewteen the two modes just occured after an MFSK/FSK insert (fig-3)

fig. 3 - switch between the two OFDM waveforms
The parameters of the MFSK/FSK inserts are easy to measure: MFSK-31 40BD speed and 40Hz separation, FSK 40Bd 200Hz and 600 Hz shifts. I only show the MFSK detection in figure 4.

fig. 4 - MFSK parameters
Trasmission was closed by a series of MFSK/FSK bursts as shown in fig. 5

fig. 5
Apart from the affinity to the Serdolik "family" (clues are the MFSK-31 and the OFDM waveforms), it's improbable to say something more precise since the lack of official documentation.

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