28 February 2017

a possible variant of the CIS-48 OFDM modem

This signal was copied on 9905.0 KHz/USB at 0922 UTC 22 February, although the poor quality of the recording (it mostly depends on the signal weakness) some interesting characteristics can be obtained from its analysis.
Once the signal has been processed with a pass-band filter, the spectrum reveals the presence of 48 tones (Fig. 1) which are confirmed by the SA-OFDM module: in particular the 48 tones are 62.5 Hz spaced and are keyed at a symbol rate of 50 symbols/sec (Fig. 2); the "magic K" value is very good, ie 1/4, meaning a quite consistent result.

Fig. 1
Fig. 2
The poor quality of this sample does not allow to detect what kind of modulation is used in the channels (possibly 4-ary?) anyway it's possible to see that a special/service symbol, consisting of a certain combination of used/unused & modulated/unmodulated tones, is sent each 102 symbols (Fig. 3):

Fig. 3
The body of the signal is preceeded and followed by 6 tones, the initial ones are modulated with BPSK at 100 symbols/sec (Fig. 4)

Fig. 4
The CIS-48 OFDM modem is mentioned several times in radioscanner.ru forum and my friend KarapuZ, who helped in the identification, addressed me to one of his posts just related to CIS-48:
From these readings I may understand that some variants of this modem are on-air and most likely this is one of these: better and further recordings just of this signal, as well as your comments, would be needed.


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