16 July 2016

Unid PSK-8 4800/2400Bd & OFDM 15-tones QPSK Hybrid Modem

This is an example of a "teamgroup" analysis: the signals were originally spotted by KarapuZ, then analyzed together by me, cryptomaster and IK1YDE: the discussion can be read here ("ansanto" is my nickname in radioscanner.ru forum)

The signal show up in FEC burst and ARQ modes with symbol rates of 4800 Baud (6000Hz bandwidth) and 2400 Baud (3000Hz bandwidth). 
The first weirdness we saw is just related to the baudrate lines of the signal in both the two speeds. As shown in figure 1, each burst consists of the preamble segment followed by the data segment: a strong 4800Bd line is visible only in the firts segments of the bursts, preambles, while just weak nuances are visible in the second segments.

fig. 1 - strong 4800Bd lines for preamble segments
In order to understand the used modulation we have isolated a single burst and then tried its harmonics: results are another weirdness. As shown in figure 2, the signal exhibits a clear PSK-8 modulation in preamble segment "A" (as expected, since its baudrate line) followed by short PSK-2 insert but no signs of modulation in the second segment "B" (altough the weak baudrate lines in fig.1).

fig. 2 - modulation harmonics in a single burst
Other than the 4800 Baud 8-ary constellation (shown in fig. 3) the segments A are characterized by a 53.33ms ACF that matches a 256 bits frame structure (fig. 4).

fig. 3 - PSK-8 single tone modulation at 4800 Baud in segments A
fig. 4 - 53.33ms ACF in segments A
For what concerns the segments B I suggested to isolate each of them from the ARQ signal and merge them together to form one unique file and then use the OFDM approach to study just that file. Cryptomaster spotted OFDM 15-tones structure and also isolated a single channel, detecting the presence of QPSK modulation: figures 5 and 6 show my tests that confirm his OFDM-15 238Bd QPSK observations.

fig. 5 - OFDM 15-tones in segments B
fig. 6 - inspecting a single OFDM channel
What remains still unclear, other than the source/user of the transmissions, is the reason of the baudrate line in the segments B since OFDM normally does not cause such pattern in the modulation harmonics.
To be thorough, figs 7,8 show the same behaviors in the 2400Bd waveform.

fig. 7 - strong 2400Bd lines in segments A
fig.8 - PSK-8 modulation only in 2400Bd waveform segments A

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