3 March 2016

QPSK 1500Bd-19200Bd (Maritime Band)

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.
The most of these signals have wide-band high-speed performances (although they do not belong to 188-110C App. D) as the following 19200Bd DQPSK whauch occupies a band of  ~24KHz bandwidth (pic. 1).

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 1500Bd BPSK modulation in the preamble and trailer segments and 1500Bd QPSK modulation in the data block; the follwing three segments have 19200Bd QPSK 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) [1]

pic. 8 - frame structure

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.

[1] 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.

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