HARRIS PSK8 2400 Bd Digital Voice, an autobaud waveform?

Discussing with my fiend ANgazu about the HARRIS Digital Voice waveform (see this post), it turns out that likely that waveform is designed to provide the autobaud feature which is coded - in our opinion - in its initial header just before the normal frames' structure. As shown in figure 1, the "presumed" autobaud header consists of 8 frames each with a duration of 13.3 ms and a length of 32 bit (given the PSK2 modulation at 2400 Bd) for a total length of 256 bit. The synchronization functions would then be performed by the preamble sequences which are transmitted every 106.6 ms.  The autobaud function it may be necessary  since, according to the RF-5800 data sheet, the narrowband digital voice mode may use MELP and LPC-10 algorithms at 2400 and 600 bps.

Fig. 1 - the presumed autobaud header

Moreover, looking at the bitstream of an entire session (figure 2) it can be argued that the shorter segments are used for the management of the voice-link (ARQ mode); selcall, link setup and link closure are performed by the HARRIS specific waveform.

Fig. 2

Harris RF-5800 Digital Voice PSK8 serial waveform (yet another 106.6 ms ACF)

A friend of mine sent me these signals consisting of a complete session utilizing the Harris 600/2400 bps Digital Voice (DV) mode:

1- selcall & link setup
2- 600/2400 bps vocoder PSK segments
3- link terminate

Fig. 1 - Harris Digital Voice mode session

Selcall is quite clear:  it's an MSK/OQPSK modulation at 2000 Baud speed, followed by short MFSK-8 125 Baud in non-standard MS-188-141A fomat, ACF is 50 ms (100 bit) and the resulting bitstream is characterized by the presence of the usual pattern (figures 2a, 2b).

Fig. 2a - Harris selcall MSK/OQPSK part

Fig. 2b - Harris selcall MFSK segment part

The VD mode PSK8 serial waveform is used only when a voice link is selected, although it also allows data to be sent over the same link; both data and voice are secured with Citadel encryption. Its ACF is the quite common 106.6 ms and that causes false-positive STANAG-4285 detections by decoders. As shown in figures 3a 3b, each frame consists of 256 tribit symbols, according to SA raster and bitstream each frame consisting of 66 sync sequence symbols (instead of 80 as in other similar 106.6 ACF waveforms), followed by a data block consisting of 190 symbols. The sync sequence is transmitted recurrently every 106.6 ms and uses PSK2 modulation.

Fig. 3a - Harris VD PSK8 serial

Fig. 3b - Harris VD framing

From Harris RF-5800 datasheet: "The digital voice mode utilizes the latest military MELP and LPC-10 algorithms for high-quality secure narrowband voice at 2400 bps. The Harris 600 bps vocoders extend the communication range beyond conventional 2400 bps systems." [1]

About the 106.6 ms ACF waveforms (figure 4), as said here, it seems that decoders such as Sorcerer and therefore also K500 try to identify a signal by measuring its ACF and comparing it against an internal table that allows the identification: likely, the length of the ACF and the initial PSK2 sync sequence mislead the decoders which consequently give a false positive STANAG-4285 id.

Fig. 4 - some common 106.6 ms ACF waveforms

https://disk.yandex.com/d/jMGcjN4lD8RH9Q 

[1] https://disk.yandex.com/i/sFOl3aX98-d6SQ