9 August 2016

Chinese 4+4 π/4 DQPSK

Weak signal heard on 14021.3 KHz (cf) at 1450 UTC, just in the HAM 20mt band. The signal consists in two groups of four channels (4+4), the two groups are spaced by 450 Hz and channel separation is 300 Hz. The signal spreads ~2500 Hz bandwidth (fig. 1).

fig. 1 channels and groups separation
Despite the low signal quality, it is possible to measure the modulation rate in the channels in 75 symbols/sec. and fig. 2 shows the two methods used to confirm the result.

fig. 2 - measurement of the baud rate
The accomodation of the channels, pattern and manipulation speed, identify the signal as the Chinese 4+4 π/4 DQPSK: unfortunately the faint traces in the involution module does not allow to get a clear constellation. Talking about that signal, Karapuz sent me a clean record for study and comparison so it was possible to get the characteristic π/4 DQPSK of fig. 3

fig. 3

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