25 September 2016

MIL 188-110C App.D: BW6 KHz, SR4800 Bd, Walsh

yet another 188-110C App.D signal (WBHF,  Wide Band High Frequency) spotted around 1850 UTC on 5407.0 KHz/USB by my friend Karapuz: it worth noting the bandwidth of the signal, 6000Hz, and consequently the sampling frequency adopted for the recording, 24000 Hz, which allows a good signal resolution and accommodation. Since the presence of an annoying fading and the signal strength,  the block #2 is the most suitable for a good analysis.
The basic parameter of the waveform are shown in figs 2,3

fig. 2 - baudrate line
fig. 3 - PSK-8 constellation
The cited value of 5407.0 KHz is the tuning frequency used to mantain the signal at the center of the band and thus it isn't the real dial frequency: indeed, you may note that the carrier frequency is almost the double of the expected 3300Hz.

preamble section
From the 188-110C App.D documentation, the orthogonal Walsh modulation is used in the Synchronization Section of the preamble and the length of each super-frame is 18 channel-symbols, ie:
9 (fixed) + 4 (downcount symbols) + 5 (waveform identification symbols) 
Since in 6KHz bandwidth waveforms the preamble channel-symbol is 64 symbol length, the length of each repeated superframe is: 18 (channel-symbols) x 64 (length of one channel-symbol) = 3456 bit. 

fig. 4 - repeated superframes in the Sync Section of the preamble
The lenght of the Sync Section superframes generates the 3456 bit period which is apparent in the bitstream of the preamble after its demodulation (fig. 5).

fig. 5 - 3456 bit period in the preamble due to the superframes length
data section
The data section exhibits ~426ms ACF spikes (fig. 6) that make a 6144-bit length period(!), corresponding to 2048 tribit symbols. The period does not have the Known/Unknown data structure, so mini-probes are not sent but rather the data symbols are sent continuously after the initial preamble: this means that the block #2 is the wavfeorm Id 0 and Walsh Orthogonal Modulation is used.

fig. 6 - ACF value measured in the data section
from D.  (MIL 188-110C Appendix D):
Waveform ID 0 utilizes a different modulation technique, Walsh Orthogonal Modulation. For each pair of coded and interleaved data bits, the method produces a 32 symbol repeated Walsh sequence. The Walsh Orthogonal Modulation is accomplished by taking each pair of bits, or di-bit, and selecting a corresponding Walsh Sequence. The selected four element Walsh sequence is repeated 8 times to yield a 32 element Walsh sequence. For example, if the di-bit is 01, the sequence 0404 is repeated to generate the 32 symbol sequence:
0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4, 0, 4

Processing the bitstream of the data section, we get a value of 6144 bit (fig. 7) that matches the ACF value obtained in fig 6:

fig. 7 - 6144 bit period of the data section
Why this 6144 bit? 
For the Walsh Orthogonal Modes (waveform id 0) the data scrambling implementation generates 256 x 8 = 2048 values and the scrambling sequences are continuously wrapped around the 2048 symbol boundary: ie just 2048 x 3 = 6144 bit and then the ACF of the data section is due to the scrambler lenght.
Athough data are modulated using Walsh ortogonal modulation, they are scrambled to appear on-air as the PSK-8 constellation seen in fig. 3.