1 June 2016

CIS-112 preambles


In this sample, recordered today on 16311.0 KHz/USB starting from 0654 UTC, the bursts are probably used as a sort of selcall or wake-up signaling before sending the data. The parameters of the two signals are the standard ones for the CIS-112 modem: BPSK modulation, as part of the basic π/4 DQPSK, symbols rate of  22.22 symbols/sec and 25.6 Hz as carriers step (fig. 1)

fig. 1
They use two different synchronization preamble structures.

The preamble of the burst mode consists of three parts (fig. 2):

fig. 2
1) all symbols are transmitted for one symbol period element;
2) the seven tones 1, 7, 33, 49, 65, 81, 97 (step 16, dF = 410.25 Hz) are transmitted for a duration of 19 symbol element periods;
3) only the 56 odd tones (dF = 51,28 Hz) are transmitted for a duration of 11 symbol element periods.
The data transfer part of the burst mode follows the preamble and  consists of 45 symbol element periods, therefore no special cahrs are transmitted as it happens in the stream mode each 72 and 144 symbols (as discussed here).
It's worth noting that in the burst mode the continuous pilot tone is transmitted at higher level during the seven-tones phase: +6dB than the odd-tones phase and  +12dB that the data phase (fig. 3).

fig. 3
The preamble of the stream mode consists of two parts (fig. 4):

fig. 4
1) initial part resembling a BPSK modulation 187Bd;
2) only the 56 odd tones (dF = 51,28 Hz) are transmitted for a duration of 36 symbol element periods.
During the odd-tones phase the pilot tone is transmitted at +6dB level higher than the data phase as it happens in the burst mode.

In both the two modes the 56 odd tones are transmitted with different but constant phases (fig. 5)

fig. 5

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