Chiński modem 4x4 (prawdopodobnie PLA Navy)

Chinese 4x4 waveform consisting of 2 groups of 4 PSK channels modulated at a rate of 75 Bd, the 2 groups are spaced by 450 Hz and channel separation is 300 Hz. The signal spreads about 2500 Hz bandwidth (Figure 1). The modem is most likely utilized by the The People's Liberation Army Navy, besides known as the People's Navy, PLA Navy or simply Chinese Navy.

Fig.1 - Chinese 4x4 modem

I isolated a single channel to identify velocity and what kind of PSK modulation is used, the spectrum of the 3rd order harmonics (x^3) shows the typical central line (subcarrier frequency) of PSK8 modulation; indeed, the phase plane exhibits a 8-ary constellation, but there is no transition paths through the center (as in case of PSK-8) and the comparative constellation (Diff-1) is simply a 90 degrees rotated QPSK: this propose the usage of π/4 DQPSK (Differential Quadrature phase Shift Keying) modulation.
The π/4 DQPSK modulation uses 2 QPSK constellations offset by 45 degrees (π/4 radians) and transitions happen from 1 constellation to the another making the illusion of a PSK-8 modulation; data bits are encoded by phase changes, alternatively of absolute value of the phase. By the way, the π/4 DQPSK modulation format is besides utilized in TETRA.

Fig. 2 - π/4 DQPSK modulation @ 75 Baud (single data channel)

The resulting bitstream after differential demodulation has a 22-bit (11 dibit symbols) dimension period, as shown in Figure 3.

Fig. 3 - demodulated bistream (single data channel)

The preamble preceding the data is besides modulated in π/4 DQPSK mode at a velocity of 75 Baud (Figure 4). The bitstream resulting from its demodulation (Figure 5) is formed by the repetition of a 22 bits dimension pattern, likely for AGC, fine-tuning, and synchronizing. Attempts to find the generating polynomial propose x^23+x^22+x+1. In addition to the same period dimension (22 bits), the "similarities" between the 2 bitstreams (data Vs preamble) is to be noted.

Fig. 4 - π/4 DQPSK modulation @ 75 Baud (single preamble)

Fig. 5 - demodulated bistream (single preamble)

Fig. 6 - messages' formats

mode-A examples
3BLK 3BLK 3BLK DE JQ02 JQ02 JQ02
3BLK 3BLK 3BLK DE JQ02 JQ02 JQ02
3BLK 3BLK 3BLK DE JQ02 JQ02 JQ02
JYJYJYJYJYJY HR MSG GA
41149 25 51 1001 1605
UXEE---Y9R
1213 0044 4433 7814 2404 2166 5873 4084 6463 2053
3462 8669 3268 6541 0511 3039 3930 2944 3388 6895
7921 4851 3871 2507 0062
MSG AGN
41149 25 51 1001 1605
UXEE---Y9R
1213 0044 4433 7814 2404 2166 5873 4084 6463 2053
3462 8669 3268 6541 0511 3039 3930 2944 3388 6895
7921 4851 3871 2507 0062

B81L B81L B81L DE JQ02 JQ02 JQ02
B81L B81L B81L DE JQ02 JQ02 JQ02
B81L B81L B81L DE JQ02 JQ02 JQ02
JYJYJYJYJY HR MSG GA
82230 23 51 1001 1025
UXEE---YXE
1243 0255 1667 1611 3469 2053 0063 5501 7301 1940
2587 7681 6966 7814 0584 6978 0091 2647 7217 7042
7179 5854 5844
MSG AGN
82230 23 51 1001 1025
UXEE---YXE
1243 0255 1667 1611 3469 2053 0063 5501 7301 1940
2587 7681 6966 7814 0584 6978 0091 2647 7217 7042
7179 5854 5844

3BLK 3BLK 3BLK DE JQ02 JQ02 JQ02
3BLK 3BLK 3BLK DE JQ02 JQ02 JQ02
3BLK 3BLK 3BLK DE JQ02 JQ02 JQ02
3BLK called station address
from
JQ02 caller station address

JYJYJYJYJYJY HR MSG GA
JYJYJYJYJYJY ?
HR MSG GA are telegraphic abbreviations:
HR = here or hear
MSG = message
GA = good afternoon

it is common to read another abbreviations specified as "message repetition":
MSG AGN
MSG = message
AGN = again

or even the "link termination"
AHR ZNN SK
AHR = ?
ZNN = All clear of traffic now
SK = End of contact

41149 25 51 1001 1605
41149 ?
25 number of the 4FGs groups that make up the message (seems to be always odd)
51 message group identifier?
1001 date (mmdd)
1605 local time (hhmm), possibly for drafting

UXEE---Y9R
probably these are military addresses which are expressed as "source---destination"; at least in my recordings, the origin address seems to be composed of 4 digits. Cross-referencing the callsigns of the first calls gives this (small) table:

JQ02 = UXEE
82VP = YXY
3BLK = Y9R
B81L = YXE
IJDW = YXX
THGM = 21II
WMBZ = 227
F9ED = 201
LTPE = 811
FMRK = 818

The messages consists of 4-digit codewords (here referred to as 4FGs groups or simply "groups") which are sent 10 per row in enumbered blocks, each block consisting of 100 groups. Given that the Chinese writing strategy is by nature nonalphabetic and thus noncipherable, Chinese cryptography was bound to the usage of codebooks (Chinese Telgraph Code, Chinese Standard character table or another unknown military codebook) containing a max of 10000 characters (0000-9999).

1213 0044 4433 7814 2404 2166 5873 4084 6463 2053
3462 8669 3268 6541 0511 3039 3930 2944 3388 6895
7921 4851 3871 2507 0062