unid datalink protocol(s) over a PSK8 ST and STANAG-4539 (Thales? L3Harris?)

A few days ago I came across some transmissions that caught my attention for at least three good reasons:
1. the frequency used, i.e. 7312.7 KHz/USB, within the 42 meter broadcast band;
2. the use of different traffic waveforms, ie PSK8 serial Tone and STANAG-4539, in ARQ and non-ARQ modes. The ARQ mode is easily recognizable by the difference between the frequencies of the subcarriers (around 50Hz) of data and ACK segments (1);  

Fig. 1 - different traffic waveforms

3. the use of a waveform composed of GMFSK-2000Bd + MFSK8 for the link setup procedure (Figure 2): as far as I know, this particular waveform is used by both Thales and Harris. Notice that the MFSK8 part is 188-141A compatible (125Bd & 250Hz separation between the 8 tones) but use a diferrent tone library.

Fig. 2 - GMFSK + MSK8 link setup waveform

I had already met such transmissions, noting how the system was able to simultaneously demodulate 2/3 waveforms (or more if we consider the ALE exchanges) even during the same logical link. In these recordings a single waveform (PSK8 ST or S-4539) is mostly used and I took advantage of this to study the characteristics of the used datalink protocol; a protocol which - in my opinion and according to my analysis - turns out to be proprietary and quite complex.

PSK8 Serial Tone
Figure 3 shows the 8-ary constellation (states and transitions) as well as the rasters of the two PSK8 modulated waveforms A and B. In the phase states, and especially looking at the transitions, one can easily notice the presence of a PSK2 modulation which is certainly used for the synchronization sequences visible in the bitmaps below. As usual, the resulting PSK2 symbols are then mapped and scrambled to appear, on-air, as a PSK8 costellation. Bot the the waveforms have an ACF of 106.6 ms that makes a 256 PSK8 symbols frame at the modulation rate of 2400Bd. However, although of the same length, two different framings are adopted, in particular the Type B waveform uses a framing similar in composition to that of STANAG-4285. 

 

Fig. 3 - constellation and bitmaps of the PSK8 Serial Tone waveform

It is important to note both in the bitmaps of Figure 3 and in the demodulated bistream of Figure 4 (related to the Type A waveform) the presence of "regular" and similar patterns, as well as the "invariance" of the symbols of the synchronization sequences. In my opinion such patterns and sequences could indicate the use of a uncoded mode and even no interleaving (or 1 frame length interleaver), furthermore the length of the scrambler should coincide with that of the frame (256 symbols) or at least it should be initialized at the beginning of each frame (2).

Fig. 4 - PSK8 ST type "A" waveform: demodulated bitstream

Examining the symbols of the synchronization sequences offers further food for thought. In Type A waveform, the use of PSK2 modulation is confirmed by the 2-state transitions in the sync sequence, the latter consisting of a pseudorandom sequence of 31 symbols that is repeated twice for a total of 62 symbols (Figure 5).

 

Fig. 5 - sync sequence symbols, PSK8 type "A" waveform

Two state-transitions are also visible in the sync sequence of Type B waveform (Figure 6). Given its similarity to the S-4285 framing, the synchronization sequence consists of 80 symbols and it too is a pseudorandom sequence of length 31, which is repeated periodically within the 80-symbol window (2 periods of length 31 plus the first 18 symbols of another period).

Fig. 6 - sync sequence symbols, PSK8 type "B" waveform

The most interesting thing, apart from the state values which may be due to both the scrambler and the the possible phase-offset errors of the SA PSK demodulator (3), is that both the two Types of waveforms use the same 31-symbol sync sequence: indeed, as can be seen in Figure 7, the 2-state transitions are the same. Perhaps the length of the sync sequence is used by the receiving modem to figure out which of the two waveforms is incoming, but it's just a my guess.

Fig. 7 - sync sequence symbols, PSK8 type "B" and "A" waveforms

Since the Type B waveform has the same framing as S-4285, an S-4285 decoder recognizes the Type B waveform samples (100% confidence) but since the synchronization sequences are different (see the 2-state transitions in Figure 8) it does not successfully engage any sub-modes.

 

Fig. 8 - comparison between sync sequences of PSK8 Type "B"  and STANAG-4285

STANAG-4539
As per STANAG-4539, both the QAM16 and PSK8 waveforms have the same 287-symbol framing (119.6ms ACF, 2400Bd) although the user data rate is different: 6400bps and 3200bps respectively for QAM16 and PSK8.

Fig. 9 - constellations and bitmaps of STANAG-4539 QAM16 and PSK8 waveforms

If in the case of PSK8 ST it was only possible to analyze the symbols after demodulation, in the case of STANAG-4539 it is possible to decode the signals and then analyze the composition of the upper layer datalink protocol(s).
Figure 10 shows a detail of a bitstream obtained after removing the S-4539 QAM16 overhead and consists of 96-byte (768 bits) Protocol Data Units (PDUs), each PDU consisting of 3 bytes header followed by 93 bytes of data:
1st byte: a ID/value field, in this sample: 0x09 (LSB first)
2nd byte: down-counter field (LSB first)
3rd byte: up-counter field (LSB first)
As one can see looking at the values of the two counters in Figure 10, the sample consists of 55 PDUs, numbered from 0 (00000000) to 54 (00110110).

Fig. 10 - headers and part fo bitstream after S-4539 QAM16 decoding

 

The same fields' structure can be found in the PDUs extracted from a sample of STANAG-4539 PSK8 (Figure 11). Since the half of the user data rate (3200bps Vs 6400bps), each PDU consists of 48 bytes: 3 bytes for the header fields followed by 45 bytes of data. It's interesting to see that a change in the first field (from 01001000 (36) to 00001000 (8)) occurs when the down-counter field restarts its value after reaching the 0: curiously, the up-counter does not "reset" but continues its counting.
It's worth noting that that patterns highlighted in the bitstream of Figure 4 (and the bitmaps of Figure 3) are most likely the two counter fields of Figure 11: if so, both tPSK8 ST and S-4539 traffic waveform transport the same datalink PDUs.

 

Fig. 11 - headers and part fo bitstream after S-4539 PSK8 decoding

 

Even more interesting. After removing the 3 bytes of the headers, I reshaped the stream into a 128 bit scheme (16 bytes), i.e. to the most probable value of its period, and I noticed the repetition of the string 0x3CF04F; so I synced the stream on this value (Figure 12), fixing a minimum length of 128 bits. The result highlights the presence of 45 PDUs of a "secondary" datalink protocol where each PDU has an header consisting of 4 bytes and a minimum length of 16 bytes (128 bits), the maximum is over 600 bytes (I was not able to establish it accurately):
bytes 1-3: a ID/value field, [001111001111000001001111] 0x3CF04 (LSB first)
4th byte: up-counter field (LSB first)

 

Fig. 12 - the emerging "secondary" datalink protocol PDUs

This may be a hasty statement, but it seems that the "secondary" datalink protocol PDUs > 16 bytes length are fragmented into small segments and then incapsulated into the 45/95 bytes payload of the "primary" datalink protocol PDUs. By the way, at least in these samples, the "secondary" PDUs have been found only in the primary PDUs which have the first byte of the hedaer equal to 0x48, maybe just a mere coincidence (Figure 13,14).

Fig. 13

Fig. 14

comments
Since the lack of clear-text callsigns it's impossible to id the user, we may speculate just some guess about the manufacturer of the used devices:

 

- as far as I know both Thales and L3Harris make use of the GMSK-MFSK8 waveform to manage HF links: unfortunately the GMFSK signals portions are too short to allow the analysis of the bitmaps (L3Harris GMFSK has a well recognizable pattern [1]);

L3Harris typical pattern in GMFSK-MFSK8 signals

- the patterns highlighted in Figures 3,4 are very similar to the ones visible in the demodulated bitmaps of PSK8 Voice Digital waveform (L3Harris VD mode) [2];

L3Harris VD mode bitstream

- from Harris RF-5800 datasheet "L3Harris VD mode also allows data to be sent...both data and voice are secured with Citadel encryption" [2]: well, I did not find the Citadel characteristic pattern within the decoded bitstream, even if they could be plain-text transmissions.

 

So: Thales? L3Harris? either of them? ...hints and comments are welcome.

(to be continued)

 

https://disk.yandex.com/d/tMk8PwVw1gu8dg 

 

(1)50Hz difference between 1800Hz sub-carriers

 

(2) FEC encoding and interleaving should provide time separation between contiguous values.

(3) SA is a signal analyzer and not a decoder, therefore its phase-plane demodulator does not sync  any particular protocol, as it happens for example in STANAG-4285 "suited" decoders. Working with phase keyed signals, the SA phane-plane demodulator produces right interpretations and views (number of phases, angles, modulation speed, carrier frequency,...) but it may return wrong demodulated streams due to the possible phase-offset errors.

 

[1] http://i56578-swl.blogspot.com/2015/06/harris-selective-call-msk-2000bd1000.html
[2] http://i56578-swl.blogspot.com/2021/11/harris-rf-5800-digital-voice-psk8.html