3 March 2016

Unid DQPSK-19200Bd (new waveforms tests from Inmarsat-GW ?)

Thanks to my friend KarapuZ, I recently had the opportunity to play with some signals heard in HF maritime segments, fixed and mobile services, mainly recordered on 8400 and 12300 KHz/USB. Although there are no definitive conclusions or official informations, some observers suggest to be the GW-OFDM system replacement after the Globe Wireless acquisition by Inmarsat:
All these signals exhibit a PSK modulation with spread spectrum and, following the trend of the waveforms described in the recent MIL-STD standards (WBHF waveforms family), the most of these signals have wide-band and high-speed performances, however not compatible with such standards. 
One of these signals is the DQPSK 19200 Baud, ~21KHz bandwidth, (pic. 1) described below.

pic. 1
As in almost all these signals, a 1500Bd starting block seems used to announce or precede the session: as shown in pic. 2, this block has a BPSK preamble and trailer and DQPSK data while the follwing three segments have a symbol rate of 19200 Baud and DQPSK modulation (pic. 3).

pic. 2 - 1500Bd starting segment
pic. 3 - 19200Bd segments
By selecting and analyzing the second 19200Bd segment, the longer one, we can get some clues about its frame structure. Looking at pic. 4 we can see frames of alternating data and miniprobe symbols. Each data frame consists of a data block followed by a BPSK mini-probe consisting of symbols of known data.  After 4 data blocks, the initial BPSK preamble (or an its symbol subset) is reinserted most likely to facilitate late acquisition of an ongoing transmission.

pic. 4 - frame structure
Frame structure and times are confirmed by running both CCF and ACF functions (pic. 5): note that 120ms frame makes 2304 QPSK symbols, ie 4608 bits, at 19200 Baud speed.
pic. 5 - CCF and ACF results
In order to find the data block and known data (miniprobe) lengths we need to investigate the 120ms frame by using a bitstream analyzer as shown in pic. 6.
pic. 6 - 19200Bd segments
As expected, the period legth is 4608 bits that matches the 120ms or 2304 QPSK symbols. Since the mini-probes consist of well known data, their pattern is easily recognizable into the bitstream and we can get a pretty acurate measurement of the length: 512 bits, ie 256 QPSK symbols (pic. 7)

pic. 7 - known data lenght
Unless my mistakes, each 2304 symbols frame consists of a data block consisting of 2048 data symbols followed by a mini-probe consisting of 256 symbols of known data. After 8192 data symbols, ie each four data blocks, a 584 known symbols set (preamble?) is reinserted (pic. 8)

pic. 8 - frame structure

Just about preamble (or an its subset) re-insertions, it's worth nothing that MIL-STD 188-110C W/ CHANGE NOTICE-1 (03-JAN-2012) removed the Paragraph D.5.4 sentence "The reinserted preamble facilitates acquisition (or re-acquisition) of an ongoing broadcast transmission." since it refers to a feature that is obsolete.

Little or nothing can be said about the secondary protocol: we can work on just the over-the-air symbols, unless to find the scrambler ploynomial, interelaver lenght and CRC algorithm... but that's another story.

My friend Alipio pointed me to an interesting question: the superframe length and the way to measure it.
We know that the superframe consists of 9544 symbols:

4 data-blocks: 8192 symbols
3 mini-probes: 768 symbols
reinserted-preamble: 584

while Alipio says that the reinserted-preamble is made up by the fourth miniprobe + the preamble itself (as in MS188-110 style):
4 data blocks: 8192 symbols
4 miniprobes: 1024 symbols
1 preamble: 328 symbols

Perhaps it's only a question of interpretation of the fourth mini-probe.

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