2 September 2015

a new Russian OFDM modem (say CIS-62/CIS-OFDM3100)

The wave file was sent to me by Karapuz and was recorded during the tests of this (new-entry ?) modem. Although the waveform does not fit the models that are built into SA software, I tried to determine the number of channels and their baudrate and the results are the same as those obtained by other friends. 
The official name for this waveform/modem, as well as its nickname, is unknown and CIS-62, or better "CIS-OFDM3100" since the no-CP assumpion (see later), are only my names just chosen in the wake of other similar identifications used by community.
As said, the recording has been sent by Karapuz and I never met this signal in the air neither logs or comments in the web about it. According to his experience this signal is quite rare in the air, he heard it this year too, but with a lower level and with modified preamble and trailer. Apparently, the tests are more distant from Karapuz QTH: perhaps performed in the far east part of Russia or beamed to North and/or Pacific area, but these are only speculative guess.

Assuming that the signal is an OFDM without CP, where channel separation is equal to the frequency manipulation (Br), the number of channels is given by the relationship: N = (BW/Br)-1. Then as first step, I began to calculate the baudrate (pictures 1 and 2) using two methods in order to receive a confirm:

Pic. 1
Pic. 2
Then the spread bandwidth (Pic. 3):

Pic. 3
Substituting the results, the number of channels is given by (3100/50) -1 = 62. 

Karapuz has got the same results in his analysis, but unfortunately he was not able to determine the used modulation. However the edge channels, 1 and 62, exhibit PSK-2 sequentially with the transfer of ones and zeroes, possibly for synchronization (Pic. 4):
Pic. 4

In his analysis Angazu used the tool CCF (Pic. 5), noting that at least in this recording it looks like an ARQ system with a 5.8 Sec ACK
(record starting is ACK) and, given the test purposes of the transmissions, it's very likely.

Pic. 5

All first pre-tones are BPSK modulated at 50Bd, speed manipulation has been calculated non only by the SA automatic mode (Pic. 6a and 6b), apparently to last for 78 symbols, as you can see by using SA (Pic. 7), they should carry some information useful for the tuning of the demodulator.

Pic. 6a

 Pic. 6b
Pic. 7
Taking the absolute phase changes of the pre-tones modulation, the result is very interesting, since they have the same group structure. Probably informations are different inside the chunks, but the absolute phase changes are constant in every tone. We do not know the reason.

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