17 November 2023

16Kbps/16KHz encrypted wideband FSK (WBFSK)

Unid 16Kbps/16000 wideband FSK (WBFSK) heard and sent me by my friend Martin on 31875 KHz, just a bit beyond the limit of the HF region (LVHF). Notice in Figure 1 the sharp filtering at the edge of FSK transitions.

Fig. 1

The signal has strong 196 ms ACF spykes which are due to the inserted bursts limiting the blocks in which the transfer is arranged (Figure 2).

Fig. 2

The bitstream resulting after demodulation of part of the signal has a 3136-bit length framing, as expected since the 196 ms ACF, consisting of a 191-bit known sequence (the above mentioned inserted bursts) followed by 2945-bit of unknown data; the inserted burst are probably used for (re)synchronization purposes. Despite several runs, I did not find a valid LFSR for that sequence. Also note - zooming the bitstream - further 33 bits that could not be part of the following data block: this way it could be a 192-bit sequence followed by a 32-bit CRC... but it's only a mine speculation!

Fig. 3

Voice or data? The 16 Kbps speed and the way it sounds (like a white noise, to make it difficult to intercept) make me think of a vocoder. Indeed, in many VHF radio stations the analog speech signal is converted into digital form using CVSD (Continuous Variable Slope Delta) [1] just running at 16 kbit/s and then encrypted probably using a VINSON family crypto device such as KY-57/KY-58 [2].
L3Harris RF-7800H-MP could be a possible candidate among other radios since it allows both VHF WBFSK and CVSD vocoder.

Likely, the 191-bit (or 224-bit) inserts are not a CVSD specific feature but are added by the crypto device.


[1] https://en.wikipedia.org/wiki/Continuously_variable_slope_delta_modulation
[2] https://en.wikipedia.org/wiki/VINSON

13 November 2023

a new family of wideband OFDM waveforms

A new family of wideband OFDM signals has recently appeared on-air, most likely tests of Russian origin, and which - at least according to the observations carried out so far - seems to be composed of six waveforms which for convenience I call OWF (OFDM Waveform):

OWF3: 3 KHz bandwidth, OFDM 64-tone
OWF6: 6 KHz bandwidth, OFDM 128-tone  *
OWF12: 12 KHz bandwidth, OFDM 256-tone
OWF24: 24 KHz bandwidth, OFDM 512-tone  
OWF48: 48 KHz bandwidth, OFDM 1024-tone    
OWF96: 96 KHz bandwidth, OFDM 2048-tone

 * the OWF6 waveform has not been observed "directly" but its characteristics and main parameters can be obtained from early-test transmissions [1]

Note that, unlike the western 188-110D App.D (single-tone waveforms, not OFDM), the bandwidths of 9, 15, 18, 21, 30, 36, 42 are missing.
The six waveforms share some common features:

1. initial preamble consists of an LFM segment followed by MFSK-64 37.5 Bd segment
2. MFSK segent is followed by up-chirped FMCW (Frequency Modulation Continuous Wave) sweeps
3. data transfer is arranged in 15 OFDM blocks, separated by chirps as in 2
4. OFDM blocks have all the same speed of 37.5 Baud and same channel separation of 46.8 Hz
5. in some waveforms (OWF1, OWF24, OWF48) the initial MFSK preamble is followed by the same up-chirped sweeps as in 2

It's interesing to note that OFDM parameters fit DRM-B ones.

OWF3 (3 KHz bandwidth)

Fig. 1 - 3 KHz bandwidth waveform

Fig. 2 - 3 KHz bandwidth OFDM

OWF6 (6 KHz bandwidth)

Fig. 3 - 6 KHz bandwidth waveform 

Fig. 4 - 6 KHz bandwidth OFDM

OWF12 (12 KHz bandwidth)

Fig. 5 - 12 KHz bandwidth waveform 

Fig. 6 - 12 KHz bandwidth OFDM

OWF24 (24 KHz bandwidth)

Fig. 7 - 24 KHz bandwidth waveform
Fig. 8 - 24 KHz bandwidth OFDM

OWF48 (48 KHz bandwidth)

Fig. 9 - 48 KHz bandwidth waveform 

Fig. 10 - 48 KHz bandwidth OFDM

OWF96 (96 KHz bandwidth)

Fig. 11 - 96 KHz bandwidth waveform

Fig. 12 - 96 KHz bandwidth OFDM

While the vast majority of Western countries use single tone waveforms (see the aforementioned MS-110D) Russian friends favor the development of multi-tone waveforms, as OFDM in this case: I am not able to make a comparison of performances, there is certainly a valid reason behind preferring one approach or the other.

Thanks to my friends ANgazu, Joni, Linkz and KarapuZ for sending me their recordings.

(to be continued)


[1] http://i56578-swl.blogspot.com/2016/05/cis-ofdm-64-tone-qam-16-40bd.html

11 November 2023

unid MFSK-17 125Bd

Unid (probably Russian) MFSK-17/125 125Bd burst transmission heard on 21292 kHz USB (15mt HAM band) USB at 0830 UTC. 

Fig. 1

Fig. 2

The same waveform was already noted six years ago(!) and reported in this blog:


9 November 2023

Akula 250Bd/500 FSK version

A friend of mine, whom I'm grateful, sent me this interesting and quite rare example of the Akula 250Bd/500 FSK waveform. Transmission was recorded on 9202 KHz around 0800 UTC using a Japanese SDR: as you see, the values of the FSK parameters are the half of the usual ones (500Bd/1000) 

Fig. 1 - Akula 250Bd/500

The demodulated bitstream shows the normal Akula "stuff" (Figure 2):
- reversals
- sync group (6 code words followed by 6-bit "0"s separator)
- preamble group (7 code words with two different, but varying values arranged as 4 x 1st code word + 3 x 2nd code word)
- data block
- End-Of-Message group + EOT group (which never varies and consists of the five code words 101111 100010 100010 101111 011110)

Also notice in Figure 2 the slight difference between the preamble of this sample:
3 x 100101 + 110101 + 2 x 110001
and the characteristic one obtained from the demodulation of the 500Bd/1000 waveform:
4 x 100101 + 2 x 110001
further registrations are needed before we can say that this is the characteristic preamble of the 250Bd/500 waveform.

Fig. 2 - Akula 250Bd/500 demodulated bitstream

It's worth noting in Figure 1,3 the continuous carrier in absence of messages, as already seen in other recordings [1]: probably this is due to the adopted ship-shore "paradigma". While in many of the western navies the shore stations are used to broadcast a list of available listening frequency (FABs/CARBs) (1), it could be that the Russian shore stations transmit a carrier on their known listening frequencies at scheduled times on behalf of subs which have something to comunicate to the shore station itself.  That's obviously my and my frield's guess.

Fig. 3

It's very interesting to note that a day after, and on the same frequency, a short speech was noted: "GREYDER ya DALNIE", more over the Ministry of Defense of the Russian Federation web site reports about an anti-submarine exercise in Peter the Great Bay by the Pacific Fleet just on 8th November (Figure 4) [3]. Note that "Peter the Great Bay" is located in the Sea of Japan, northwestern Pacific Ocean, in the Maritime (Primorye) region of far eastern Russia and that the Akula sample was heard using a remote KiwiSDR in Nagano, Japan. Just a coincidence?

Fig. 4 - https://function.mil.ru/news_page/country/more.htm?id=12484855@egNews


(1) CARB is the acronym for Channel Availability and Receipt Broadcast, these transmissions radiate information on the frequencies available for ship-shore traffic and to pass control and receipt messages; sometimes also indicated as FAB or Frequency Availability Broadcast. These procedures are used to automatically perform a channel-link before a message could be sent [2]. 

[1] http://i56578-swl.blogspot.com/2022/04/akula-quite-unusual-session.html
[2] http://i56578-swl.blogspot.com/search/label/FAB
[3] https://function.mil.ru/news_page/country/more.htm?id=12484855@egNews