15 September 2023

Chinese Navy (PLA Ny) MFSK-8 125Bd & PSK2 2400Bd mixed mode

Complete and good quality data transfer session recorded on 10346.0 KHz/USB at 2153 UTC thanks to a KiwiSDR located in Oita, Japan [1].

Fig. 1 - Complete data transfer session

In the first part of the recording, the one related to the link setup, we can see standard MS 188-141 exchanges preceeded by bursts which use short MFSK-8 125Bd 250Hz segments (just the same tones of MS-141 but w/out the 6th tone). Perhaps they do not use this MFSK waveform as an ALE resource since they just use the standard 188-141. The involved ALE callsigns are AN1 & BN2, according to UDXF logs these IDs belong to the China's Navy (PLA Navy, People's Liberation Army Navy).

Fig. 2

The most interesting part is the one related to the data transfer. In my opinion, although constellation and state's transitions indicate a PSK4 modulation, the trajectories and the phase detector indicate that the main transmission mode is PSK2 (Figs. 3,4).

Fig. 3

Fig. 4

Probably, the four-state constellation is due to the inserts you may see below in Figure 5 (Figure 6).

Fig. 5

After PSK4 demdoulation, the resulting bitstream shows a well-defined 8-bit format.

Fig. 6

A Chinese PSK2 2400Bd serial waveform was already commented here: unfortunately, the bitstreams have different structure and patterns.


[1] https://flydog.web-sdr.net/?f=10346.00usbz8