22 March 2016

Japanese Military, OFDM-30 +2 (mutichannel hybrid modem)

For several days, at a frequency of 12384.0 and 16553.0 kHz on USB (16553.0 is a constant for the Japanese MIL 8 freq signal), we heard unmodulated carriers only, and then finally they went to the data! 
At a first glance the signal looks like an OFDM 32 tones, ~70Hz spaced and BPSK modulation at 50 Baud (pic.1). A separated unmodulated tone, the lower in the spectrum, acts as a pilot-tone for Doppler correction and is transmitted at a higher level that the other tones.
pic.1 - OFDM analysis
studying more carefully the individual tones and especially the first two tones in the lower part of the spectrum, the signal is not properly constructed with OFDM technology but rather is a multichannel waveform with a DPSK or MSK modulation with 25 Hz shift and 50 Baud speed for what concerns the 30 upper channels.
Indeed, once isolated the higher tone, there is no evicence of carrier harmonics in the 2^ power and the phase detector shows a characteristic FSK-2 shape with 25Hz shift (pic.2).
pic.2 - absence of the carrier in the 2^ power harmonics
The 4-ary phase plane related to such channel reveals no diagonal transitions and two-state  transitions in Diff.1: signs of a DPSK or MSK modulation (pic.3)
pic. 3
The upper 30 tones are then MSK 50Bd 25Hz shift, spaced by 70 Hz.
The two lower tones just after the pilot tone (the lowest one) exhibit a BPSK modulation and a speed of 25 Baud for the first (pic. 4) and 50 Baud for the second (pic. 5). It is worth noting that:
- the sequence "0101010101" which is transmitted with these two channels is maybe used for sync purposes,
- these two tones are tranmitted at a lower level that the upper 30 tones. 
pic. 4 - the lower BPSK channel
pic. 5 - BPSK 50Bd in the second channel
Sumarizing the characteristics (pic. 6):
30 data-channels DPSK/MSK 50Bd/25Hz, 25Hz spaced (OFDM)
2 service-channels BPSK 25 and 50 Baud, transmitted at lower level than the 30 upper tones
1 pilot-tone, transmitted at higher level that the 30 upper tones
Both 16553.5 kHz USB and 12384.5 kHz USB was previously channels for the old Japanese 8-tone mode, probably a litlle mis-tuning.

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