9 October 2015

CIS-45 π/4-DQPSK "special" transitions

During these days there are a large variety of signals on the air, mostly on 14MHz and 17MHz bands; among these signals there is the OFDM 45-tone HDR modem (aka CIS-45), running in both the two waveforms: 40Bd π/4-DQPSK and 33.33Bd BPSK.
And just one of them, tuned on 17434.0 KHz USB, caught my attention because of a certain shapes in its spectrum (pic. 1). Apparently it looks like a version 2 modem (pic. 2), ie 40Bd and 62.5Hz step, but the relative constellations in the channels do not exhibit the expected π/4-DQPSK but rather an odd 3-ary constellation in some channels (in blue colour) and an unplanned BPSK in others (red color), as arranged in picture 3 related to the segments "A".

About the segments "A", by isolating the 44th (apparently 2-ary) and the 45th channel (app. 3-ary) you can get the used modulation and then the real constellations: as it is clearly visible in pictures 4 and 5, all the channels have π/4-DQPSK modulation (see Diff.0 constellation), as it should be, BUT with the special transitions 3-of-4 and 2-of-4 of the possible states.

The same results are obtained by analyzing the segments "B" (Pic. 7) and "C" (Pic. 8), with the presence of two 3-ary transitions. It's interesting to notice that the BPSK-like transitions occupy always the same channels along all (at least) this sample: 2,5,8,11,14,17,20,23,26,29,32,35,38,41 and 44.

Given the position of these segments along the signal, they could act as service or sync tones for the receiver modems, anyway in my little experience I never seen this arrangement before in CIS-45 signals.
Pic. 1
Pic. 2 - OFDM parameters
Pic. 3 - carriers arrangement
Pic. 4 - 44th channel, π/4-DQPSK with special transitions
Pic. 5 - 45th channel, π/4-DQPSK with special transitions
Pic. 6 - demod
Pic. 7 - transitions and their arrangement in segments "B"

Pic. 8 - transitions and their arrangement in segments "C"

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