30 September 2018

PSK-8 bursts at inconsistent 12800bps data rate (2)

Some other observations on  the weird PSK-8 12800bps STANAG-4539 bursts, which was discussed here. Assumptions rely on the reliability of SA and K500.

Preamble & data constellations
Look at the on-air symbols of S4285 and S4539 shown in Fig. 1, both the constellations are obtained using SA.  S4285 preamble and data symbols have exactly the same phase (Fig. 1a) but they have a phase shift,  prob. pi/8, in case of S4539 (Fig. 1b). This also clarify the "flips" which sometimes are visible in the 8-ary constellations shown by SA.

Fig. 1
The phase shift means that if we only synchonize with the standard sync sequence we'll end up to get a shifted 8-ary constellation as in Fig. 2a (note that the decoder of Fig. 2a plots also the preamble states). The same shifted constellation is shown in Fig. 2b using a second decoder which does not plot the preamble but only the symbols adopted for the modulation of data (i.e., not the symbols on-air). This second decoder shows that PSK-8 is the used modulation and it matches the on-air 8-ary constellation shown by SA, i.e. QAM does not seem to be the used modulation. As said, this assumption relies on the reliability of SA and K500.

Fig. 2

Sync sequence and D0,D1,D2 sets
Maybe it's an implementation of STANAG-4539 Annex H "Technical specifications to ensure interoperability of application communication systems using multiple discrete HF channels serial waveform" (V0.4, 2016), also mentioned in 110C Appendix F. Noticeably, "The waveforms in this appendix have been designed to be compatible with the Appendix C waveforms, and use identical preamble processing with the exception that these waveforms employ settings for specifying data rate and interleaver that are reserved in Appendix C.", i.e.:

0,0,0  0,0,2  0,0,6  0,2,0  0,2,2  0,6,0 
0,6,6  2,0,0  2,0,2  2,2,0  2,2,2  2,2,4 
2,4,2  2,4,6  4,2,2  4,2,4  4,4,0  4,4,2 
4,4,4  4,4,6  4,6,0  4,6,2  4,6,4  4,6,6 
6,0,0  6,0,6  6,4,0  6,4,2  6,4,4  6,4,6 
6,6,0  6,6,4  6,6,6

They might use a different sync sequence and/or a different D0,D1,D2 set: in both cases a standard S4539 decoder is not aware of and so the 12800bps/PSK-8 ambiguity.
I mean that the data rate and interleaver setting that we get is function of the sync sequence: i.e. the 6,6,2 setting is obtained through the standard sync sequence; probably with an ad-hoc sequence you will get any of the "n,n,m" sets among the reserved ones. By the way, notice in Figure 3 that a shifted sync sequence originates the 0,0,4 setting.
 
Fig. 3

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