OFDM-30 (+1 pilot, +2) DPSK 50Bd

updated

The signal was recorded at 8403.5 KHz (CF) thanks to the use of some KiwiSDRs [1], it occupies a bandwidth of 2500 Hz  and seemigly consists of 35 tones (Fig. 1a). The lower tone (in case of USB) most likely is the "pilot" one, used for Doppler correction, and its level is 7 dB higher than the normal level of any one of the the remaining 34 tones (Fig. 1b). The pilot seems followed by four tones: actually two PSK2 channels modulated at 25 and 50 Bd.

The remainig 30 tones are used for data transfer, they are ~71 Hz spaced and are formed using the OFDM technology. Curiously, the transition from idle/data phases does not happen simultaneously for all the channels, the delay is approximately 3500ms starting from the lowest channel (Fig. 1c). The same signal was already meet here (thanks pir3 for his comment in twitter).

Fig. 1a

Fig. 1b

Fig. 1c

The two lower 25Bd and 50Bd PSK2 channels after the pilot tone send a continuous sequence of zeros and ones which is most likely used for sync purposes.

Fig. 2a

Fig. 2b

The analysis of the 30 data tones shows a 4-ary constellation in absolute mode and a 2-ary constellation in relative mode (Fig. 3), in my opinion these tones are keyed using DPSK ,or MSK, modulation with symbol-rate of 50 Baud and 25Hz shift (note that MSK is a particolar form of QPSK); the analysis of a single tone confirms my guess (Fig. 4). No particular patterns were detected during the data phase.

Fig.3

Fig.4

A raw demodulation (!) results in a 100-bit period stream (Fig. 5). As you see, 100-bit period makes a 2000ms interval that matches the intervals in Fig. 1c.

Fig.5

Signal localization is rather difficult, indeed several TDoA runs result in the middle of nowhere in Pacific Ocean (Fig. 6).

Fig.5

4 April update (replying to my friend Daniel's comment)
I simply used DPSK but I should have specified if it is a CDPSK or a SDPSK.
Indeed, "the difference between PSK and DPSK (Differential PSK) is in their encoding of the input data sequence. PSK encodes the input data sequences in-pahes (states), while DPSK encodes it in the phase difference (transitions) between successive bits or symbols.
This means that there would be a phase change in the modulation signal if the two successive bits in the input data sequence are different (0 to 1 or 1 to 0), and no phase changes if the successive bits are the same. DPSK is called conventional DPSK (or CDPSK) if the phase differences is in the set of [0,π] and symmetrical DPSK (SDPSK, also called π/2-DPSK) if the phase difference is in the set of [π/2,-π/2]". As you see in Figs. 3 and 4 the transitions are in the set of [π/2,-π/2] so most likely it's a SDPSK (π/2-DPSK). 
As a further proof, I synthesized a OFDM-30 SDPSK 50Bd using the OCG tool: the results are shown in Figs 6 and 7 below.

Fig. 6

Fig. 7

The "delay" in the transitions from idle to data and from data to idle (Fig. 1c) are clearly visible in the demodulated bitstream in Fig. 8 that my friend Daniel linked in his comments.
 

Fig. 8

https://yadi.sk/d/2Qs8X9e1sr1Pgg  (OCG synthesized OFDM-30 SDPSK 50Bd)
https://yadi.sk/d/cn6iqtWQhn-Sig 
https://yadi.sk/d/CUxayO9wyIeeAw

[1] the recordings were possible mainly thanks to the owner of KiwiSDRs at:
AI6VN/KH6 - Kahakuloa, Maui, Hawaii 

other useful KiwiSDRs used in monitoring: 
Marahau, Tasman District, New Zealand 
Northeast Asian Broadcasting Institute - Seoul, Republic of Korea
ZMH292 - Bay of Islands, New Zealand
Yokohama, Japan