16 July 2015

CIS-45 v2 HDR modem: modulation switching

16230.0 --- Russian Mil, RUS:
   1412 USB OFDM 45-tone HDR Modem v1, 33.33Bd 62.5Hz DBPSK bursts
   1410 USB OFDM 45-tone HDR Modem v2, 40Bd 62.5Hz pi/4 DQPSK then BPSK stream

Today I heard the CIS-45 OFDM HDR modem running on 16230.0 KHz on USB around 1410z. More precisely, the heard signals were the two well known CIS-45 waveforms: the 33.33 Baud version (v1) in burst mode and the 40 Baud version (v2) in bistream. The v2 modem came with a discrete signal so I decided to record it for later analysis. The signal clearly exhibits the CIS-45 v2 features: baudrate = 40, Sh = 62.5 and BPSK modulation in the channels as in:

I was surprised by analyzing another segment of that signal: the modulation in the channels is no more the expected BPSK but pi/4 DQPSK and still running at 40 Baud. So, it seems that the system has the capability to change the modulation mode (from pi/4 DQPSK to BPSK) on-the-fly, mantaining the same baudrate:

I do not know the reasons of such behavior, possibly an adaptive feature? I searched the web to get some other informations and found that KarapuZ too recently noticed this beahavior: http://www.radioscanner.ru/forum/topic36750-153.html#msg1183165

As the others, this signal is available on request: just email me.

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