30 May 2024

a STANAG-4285 autobaud waveform?

Interesting STANAG-4285 transmission heard on 14000 KHz/USB and sent me by my friend GrandBleu from radiofrecuencias.es (Figure 1)

Fig. 1 - STANAG-4285 segments

The 35 sec segments seem a modified S-4285 waveform since they begin with a block, that I here refer to as "header", and which is not referenced in the standard. The header has a duration of 116ms and is modulated using PSK2, as you may see in Figure 2.

Fig. 2 - PSK2 modulation detected in the initial "header"

I used the SA phase detector and its relative bitmap in order to "browse" the signal and to better indagate the header. Looking at Figure 3 you may see a 13.333ms repeated pattern: well, 13.333ms @ 2400 symbols/sec makes a duration of 32 symbols (31,999) or 32 bits, since the header is PSK2 modulated (ie 1 bit = 1 symbol).

Fig. 3 - 32-bits repeated pattern in the header of the heard S-4285 waveform

Consequently, I tried a PSK2 demodulation of the headers of some segments and after their differential decodings I obtained  bitstreams which exhibit a well-defined structure consisting of initial and final "01"s sequences and characterized by a 32 bits sequence which is six times repeated immediately before of the final "01"s sequence and that exactly matches the pattern seen in the bitmap of Figure 3.


Fig. 4 - differential PSK2 decoding of a header

The same 32-bit sequence was found in all the headers I demodulated (just 3 of them are shown in Figure 5), even if it didn't appear in the same order I wrote it: one must consider the characteristcs of the SA's generic (!) PSK-n demodulator .

Fig. 5

I don't think this so-called header is actually a “transmit level control” (TLC) block. Indeed, no information is carried by the TLC since it's a sequence of symbols intended solely for the purpose of establishing the radio TGC (transmit gain control), ALC (automatic level control) and AGC (automatic gain control) before the actual preamble is sent/received. In my opinion this S-4285 waveform feature an “autobaud” facility (1) which is coded in the initial header (perhaps a Walsh coded sequence?). As shown in Figure 5, the autobauding information would consist of 6 frames, each with a duration of 13.3 ms and a length of 32 bits (total length of 192 bits), and precedes the S-4285's usual synchronization preamble.

And let's get to the data blocks. To identify which sub-mode is used I chose from time to time the various options made available by a S-4285 decoder (k500) until I found the option that had 100% confidence and 0 errors: that is, 300bps and zero length interleaving.  As a test, I used a second S-4285 decoder and always got the same result even if the resulting bitstreams didn't seem structured. Although these decoders indicated 100% confidence and 0 errors (corrections), curiously they did not detect/show the 32-bit words used for signaling the Start Of Message (SOM = 0x03873C3C MSB first) and End Of Message (EOM = 0x4B65A5B2 MSB first): could it be sign of a "fake" decoding? Finally, I used a third, more sophisticated, decoder configuring it in "auto-detect" mode: this third test also confirmed the 300bps/N sub-mode but with the reporting of corrections and a resulting bitstream with a 40-bit/5-byte period that has - in my opinion - a bit more sense.
The 40-bit length period is due to the presence of a sequence that is four times repeated near the end of all the decoded segments (Figure 6). Note that the same considerations made above apply to the sequence in question.


At first glance it could be an EOM/EOT signal but the bitstream should come from a higher level protocol (datalink layer) i.e. after the removal of the S-4285 overhead and therefore should have a different function.

Fig. 6 - a data blocks bitstream

That datalink protocol (if any ) is at present unknown to me.

Back to the initial headers, I remembered having seen something similar a while back while I was analyzing Harris' serial PSK8 waveforms [1] and by demodulating their initial headers I found a correspondence between those headers and the one analyzed here: that is, a sequence of 32 bits of length which is repeated six times between sequences of initial and final "01"s (Figs. 7,8)



From the above it seems that L3Harris (and perhaps not only them) have added the "autobaud" function to some waveforms such as STANAG-4285, obviously it is only my hypothesis which has no direct or indirect confirmation: your comments and other submissions will be as usual welcome and may assist in resolving this matter.


(1) the “autobaud” facility enables the receiver modem to automatically adapt the transmitter’s data rate and interleaver configuration without operator intervention

[1] http://i56578-swl.blogspot.com/2021/11/harris-psk8-2400-bd-digital-voice.htm

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