CIS FTM-4 transitions

CIS FTM-4 (FTM stands for Frequency-Time Matrix) is an unknown Russian "domestic" system, also known as CIS 4FSK 150 Bd, using MFSK4 150Bd/4000Hz modulation.
The recorded transmission consists of two alternating FTM-4 sets (L = lower set, H = higher set) with a total bandwidth occupation slightly over 25 KHz, the separation between the two sets (i.e. H1-L4) is 1 KHz. It's worth noting that when the L set is transmitted, the signal on the H1 frequency (i.e. the lowest of the H set) is continuous; vice versa, when the H set is transmitted the signal on the L1 frequency (i.e. the lowest of the L set) is continuous (Figure 1). Also note that the two sets are transmitted simultaneously forming a sort of MFSK-8 "construct" for a period of about 3 seconds.

Fig. 1 - the two FTM-4 "sets"

As for FMT-4 specifications, both the MFSK4 sets are modulated at the rate of 150 Baud with a spacing of 4000 Hz (Figure 2).

Fig. 2 - CIS FTM-4 main parameters

The L set has an ACF value of ~718 ms which corresponds to a repeated sequence of 216 bits length (assuming that MFSK4 uses 2 binary digits (dibit) per modulation symbol (0-3), the ACF value of 718 ms @150 symbols/sec corresponds to a period of 108 dibit symbols or 216 bits). I could not find a specific generator polynomial for that sequences.

Fig. 3

The study of the ACF value of the set H can be done on three different intervals: main, intermediate and unitary (Figs. 4, 4b). 

Fig. 4

 

Fig. 4b

 

The main ACF measures about 1920 ms which corresponds to a period of 288 symbols or 576 bits (Figure 5).

 

Fig. 5

The intermediate ACF measures about 480 ms which corresponds to a period of 72 symbols or 144 bits (Figure 6).

 

Fig. 6

The unitary ACF has a value of ~80 ms which corresponds to a period of 12 symbols or 24 bits (Figure 7).

Fig. 7

In this case (H set) it is possible to see that the demodulated bitstream consists of a repeated scheme formed of 4 patterns, as in Figure 8:

(MSB first)
P1: 0x21BE41
P2: 0xB1BE41
P3: 0xB8D727
P4: 0x28D727 

Fig. 8

 

N.B. all the the "designations" I used here are only mine; the first pattern P1 is chosen just for convenient reference, choosing a different initial pattern the "logic" does not change.

A possible interpretation (just a guess) is that the L set is transmitted as "idle" or alignment sequence for the receiving modem and the transition to the H set occurs when sending data, even if, as in this case, the data are repetitive sequences. In this regard, remaining in the realm of hypotheses, the P1-P4 patterns could be telemetry data or tele-commands.

The recording was made using a remote Airspy HF+ Located in Haapavesi, Finland (belonging to the Airspy server network) [1].

https://disk.yandex.com/d/Flr5-Ops8dRBJQ

[1] sdr://178.55.138.222:5000