a new Russian OFDM modem: 4-ary start-tones

this is a modded version of the CIS-OFDM3100, a (new) Russian modem/waveform reported here.This is the most recent recording of this signal that we have in our hands and it dates back to September 10, 2014 (just one year ago). The updates made to the signal are related to pre-tones, and more precisely their modulation: in the original version (recording of April 2014) the 5 pre-tones had a 2-ary modulation (DBPSK) and 50 Baud speed while this version exhibits a 4-ary modulation. Although the signal name, for what we know, be "Preliminary DQPSK", it's not easy to say which of 4-ary modulations we are facing (DQPSK vs OQPSK) because there are some unclear nuances about the manipulation speed in these tones .

The baudrate auto-measurement method of SA returns the plain standard value of 50Bd (Pic. 1) but we know that this feature does not always determines the correct baudrate so we looked at the absolute phase changes in order to receive a confirm or a new value for Br.

In Pic. 2 all changes are periodic and the 4 mSec changes (associated with 250 Baud speed) take place near the standard 20 mSec changes (associated with 50 Baud speed). Forcing these two values in the phase-module of SA I get two different 4-ary constellations:
- OQPSK @ 50 Baud (no diagonal transitions), as in Pic. 3
- DQPSK @ 250 Baud, as in PIc. 4

Assuming that "Preliminary DQPSK" be something like an official item, we should conclude for a DQPSK 250 Baud but, as said, not all shadows are gone.

Pic. 1 

Pic. 2

Pic. 3

Pic. 4

The above pictures are related to the pre-tone #4 but the findings also apply to the other pre-tones. Below (Pic. 5) another SA method to receive the baudrate of a signal by the full-wave detector: this method also confirms the 250 Baud.

Pic. 5

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

 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.