21 October 2015

MFSK-4 150Bd 4000Hz (CIS FTM-4)

CIS FTM-4 unknown Russian "domestic" system: MFSK-4, speed 150Bd, Shift 4000Hz and CCF ~80 ms. Heard on 16343.0 and 16222.0 KHz cf (16335.0 and 16230.0 f0 USB). This signal is discussed here in radioscanner.ru: http://signals.radioscanner.ru/base/signal201/
At least in this sample, the start tone is the higher one while the ending tone is the lower.



the effective data rate
This signal (is logged as FSK 4-tone with baudrate 150Bd and shift 4000Hz, or else CIS FTM-4 (Pic. 1). Looking in depth, they use only 4 of 24 combinations of the four tones (Pic. 2), so the encoded characters which are transmitted are reduced to 4.  This mode is called Frequency-Time Matrix (FTM) or, in this case, FTM-4. The coding method is five-digit group of characters plus spaces. The four symbols are:
A) 1-2-3-4
B) 2-4-1-3
C) 4-3-2-1
D) 3-1-4-2
Looking at the segment in Pic. 2, we could find the occurences of each symbol, for example "A" occurs 4 times, "B" occurs 3 times, "C" occurs 3 times and "D" occurs 3 times.
Since the baudrate of the signal, i.e. 150Bd, the real data-rate is 150/4 = 37.5 Baud. 

Pic. 1
Pic. 2
The repetitions of 4 symbols is clearly visible analyzing the demodulated bistream, unless some few errors due to the demodulation:

Pic. 3
 as well as the structure of the message seen as 48 bits period: some parts are well distinct
although it is unclear what they mean (sinc?, address?, commands?, ...)

Pic. 4

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