15 April 2016

MIL 188-110C App.D: BW3 KHz, SR2400 Bd, WALSH and PSK-8

Both the two signals A and B have the same duration and both have a long preamble-segment followed by the data-segment. The signals spread ~3KHz bandwidth and consist of a 1800Hz carrier with PSK-8 modulation at 2400 symbols/sec.

synchronization preamble segment
From MS188-110C App.D "The synchronization preamble is used for rapid initial synchronization and provides time and frequency alignment. The synchronization preamble shall consist of two main sections, a transmitter level control (TLC) settling time section, and a synchronization section containing a repeated preamble super-frame. The preamble super-frame consists of three distinct subsections, one with a fixed (known) modulation, one to convey a downcount, and one to convey waveform identification." The superframe shall be repeated M times. The Synchronization section shall be immediately followed by the modulated data (pic 1).

Pic. 1
Both the two sync preamble segments have the same lenght (~ 5 seconds) and the same ACF structure: 239.98 ms frame that makes 576 symbols or 1728 bits
From the 188-110C App.D documentation, the orthogonal Walsh modulation is used in the synchronization section of the preamble and the length of the super-frame is 18 channel-symbols, ie: 
9 (fixed) + 4 (downcount) + 5 (waveform identification)  
Since in 3KHz bandwidth waveforms the preamble channel-symbol is 32 symbol length (pic. 2), the length of each repeated superframe is: 18 (channel-symbols) x 32 (length of one channel-symbol) that makes the measured 576 symbols or 1728 bits (pic. 3). 

Pic. 2
Pic. 3
That's ok in pic.4, where the synchronization section of the two preambles exhibits a clear 1728 bit period length.

Pic. 4
data segment
The data segments have the same lenghts but different frame structures (pic. 5).
Pic. 5 - over-the-air bitstreams after removed the sync preamble

The frame structure for the signal-A waveform is the one shown in figure D-7 of Appendix D: the initial synchronization preamble is followed by frames of alternating data (unknown-data) and probe symbols (known-data):
After demodulating the signal the bistream analysis reveals a 288 symbols (or 864 bits) length frame, consisting of 256 unknown-data + 32 known-data (96 bits probe). This signal  meet the waveform ID-7 of the 3KHz bandwidth set (pic. 6)

Pic. 6a - WID-7 frame structure

Pic. 6b - WID-7 32 known-data (96 bits probe)

The signal-B waveform does not exhibit a data+probe structure but rather strong 853.4ms ACF spikes (pic. 7) that makes 2048 symbols/sec at 2400Bd speed or 6144 bits.  This signal meet the waveform ID-0, which uses a different structure after the synchronization preamble. Data “frames” are 32-symbol Walsh sequences (channel symbols), each corresponding to a single unknown (data) bit.

Pic. 7 - 2048 symbols ACF (~853.4ms) for the signal B

As shown in pic. 8 (after demodulating the signal-B) mini-probes are not sent in waveform 0, Walsh-coded data symbols are sent continuously after the initial synchronization preamble and the 2048 symbols (6144 bit) period is due to the scrambler lenght. For this waveform the data scrambling implementation just generates 256 x 8 or 2048 values and the scrambling sequences are continuously wrapped around the 2048 symbol boundary. Athough data are modulated using Walsh ortogonal modulation, they are scrambled to appear, on-air, as an 8PSK constellation.

Pic. 8 - WID-0 6144 bit period caused by the scrambler lenght