14 October 2017

BPSK & QPSK 2400Bd, QPSK 1500Bd, ARQ system (Maritime Band)

Robust ISB SDR ARQ system heard on 6231.5 KHz/USB (Maritime Mobile band) 2800Hz offset from carrier, around 0630z on 12 October and in which the two channels are used as follows:

- on USB upper channel: user data transfer using adaptive 2400 Baud PSK-2 and QPSK burst waveform, according to the channel conditions and the feasible data-rate. Initial bursts use PSK-2 modulation at 2400 Baud;  
- on LSB lower channel: link and traffic management (ACKs and mode used for data transfer) using 1500 Baud PSK-2/QPSK waveform; 

is not clear if the change of the mode used for data transfer is signaled in the management channel by the caller or it is requested by the called station.

Fig. 1 - user data transfer waveform (upper channel)
Fig. 2 - link and traffic management waveform (lower channel)

At least in this sample, data bursts last 2088 msec for PSK-2 and 3144 msec for QPSK, the initial bursts have a duration of 520msec; link and traffic management bursts last 310 msec. PSK-2 and QPSK data bursts exhibit strong ACF spikes every 523 msec which suggest a 1256 symbols frame structure (Fig. 3); initial data bursts have ACF = 0, probably Walsh modulation is used (Fig. 4).

Fig. 3
Fig. 4
Together with IK1YDE we studied the bitstreams after PSK-2 and QPSK demodulations and found a frame structure similar to those provided by the recent MS-110 Appendix C for the waveforms ID 5 and 6 (Fig.5). Indeed both PSK-2 and QPsK frames consist of 288 symbols (Fig. 6): 32 knwon symbols (mini probes) followed by 256 unknown (data user) symbols. It must be noted anyway that the data rate of the analyzed waveform (in its USB channel) is 2400 Baud and doesn't match the one provided by the waveforms 5 and 6 of MS-110 App.C.
Most likely the 1256-bit ACF is due to the length of the interleaver or the scrambler sequences.

Fig. 5
Fig. 6 - data bits and symbol numbers

A similar system was heard on 5 November 2016: in that case ACK burst were sent after each BPSK data burst (Figs. 7,8)

Talking with KarapuZ about this system, he suggested a SDR equipment rather than a ISB mode (I edited the text of the post accordingly) and proposed to have a look at the KLN Networks website since they are testing a proprietary 3G HF hybrid system (termed CNHF) to support the ship traffic in Artic regions [1]. In that high latitudes satcomm links can't be easily used since geostationary satellites do not cover these areas, moreover due to the long dark periods the low portion of HF shall be used (lack of F layers). Perhaps may catches are related to their tests... but it's only my guess (I emailed them to ask a confirm and maybe shed some ligth on this system).

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