9 September 2015

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