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
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| fig. 2 - baudrate line |
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| 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.
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| 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).
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| 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.
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| fig. 6 - ACF value measured in the data section |
from D.5.1.2.3 (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:
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| 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.
