SkyOFDM, still on air

Since some days I'm following transmissions on 14693.0 KHz/USB consisting of exchanges of messages between two nodes, as the different fading patterns in Figure 1 suggest.

Fig. 1

Messages are sent using OFDM modulation occupyng a 2400 Hz bandwidth and consisting of 28 tones with a frequency spacing of ~86 Hz, each tone is modulated using PSK2 at the symbol rate of 62.6 Bd (Fig. 2). The same results are obtained/verified by analyzing a single channel as shown in Fig. 3 (lowest tone).

Fig. 2 - OFDM parameters

Fig. 3 - single tone (yhe lowest) analysis

The parameters resulting from the analysis are very similar to those of the Skysweep Technologies proprietary "SkyOFDM" waveforms family (Table I). Quoting the SkySweeper Reference Manual #82.2 General Description: "SkyOFDM is a state of art high speed modem based on the OFDM and turbo coding technologies. It offers several baud rates (300 -9600 bps) and two different interleaving options (short and long). Also there are two bandwidth options: 2.0 (OFDM-22) and 2.4 kHz (OFDM-28)".

Table I

Note the different number, position and duration of the header tones compared to the values ​​of the "original" waveform: this is probably an improved version of the previous SkyOFDM waveforms (Figure 4). 

Fig. 4

The signal has an ACF value of approximately 957.8 ms which identifies a super-frame composed of 11 frames, the latter with an ACF value of approximately 79.8 ms (Figure 5).

Fig. 5 - ACF values

Direction Finding tests using TDoA algorithm (Figure 6) indicate an area north of Helsinki as the site of the transmitter (or rather, the radiating antenna): this makes sense because, acccording to some DXers, SkyOFDM waveforms were/are used by Finnish MFA and SkySweep Technologies was a Finnish high tech company. By the way, although there are still many references in the web to SkySweep, their official website is no longer online since SkySweeper software was discontinued on June 1st 2009.

Fig. 6 - Direction Finding

 https://disk.yandex.com/d/A9UHdyMAXeOUlA

SkyOFDM 28-tone 86Hz 65.6Bd PSK2 (2)

Just for background it might be helpful to read the previous post.

Most likely the signal that is continuously transmitted on 4150 KHz/usb is a modded or a new waveform of the Skysweep Technologies proprietary "SkyOFDM" family. SkyOFDM is a high speed modem based on the OFDM and turbo coding technologies.  It offers several baud  rates (300-9600  bps) and two different interleaving options (short and long). Also there are two bandwidth options: 2000Hz (OFDM 22 tones) and 2400 Hz (OFDM 28 tones).

Fig. 1 - SkySweeper running the SkyOFDM modem

I tried the SkyOFDM modes available in SkySweeper 5.13 (Fig. 1), a Windows based product for radio data decoding and signal analysis developed by SkySweep.  As expected, I could not synthesize the exact waveform running on 4158 KHz since the different speeds, modulations, and ACF:

params	SkyOFDM	4150 Hz OFDM
bandwidth (Hz)	2000, 2400	2400	✓
preamble	7 tones	7 tones	✓
tones	22, 28	28	✓
shift (Hz)	86	86	✓
Baud rate	60.56, 64, 79	65.57	x
modulations	PSK2, PSK4, QAM	PSK2/SDPSK	x
ACF (ms)	78, 113.4	76.2	x

A peculiar difference lies in the type of the used modulations: for example, if you filter out and look at the modulation used in the second channel, you will see that is not PSK2 but SDPSK (Simmetric Differential PSK), thus it seems that channels are mixed artfully (Fig. 2 ).

Fig. 2

In SA Phase-Plane using n-Ary = 4 and absolute mode (diff=0) the transitions between states are similar to QPSK but without diagonal paths; in differential mode (diff=1) we see transitions between two states (Fig. 3) (1).

Fig. 3

According to some utility DXers, SkyOFDM waveforms were used by Finnish MFA and that's correct since SkySweep Technologies is a Finnish high tech company. Although there are still many references in the web to SkySweep, their official website is no longer online: this suggests a ceased activity or an incorporation into another company. Indeed, looking at waybackmachine.com, the site skysweep.com was crawled last time on 13 June 2017; by the way, SkySweeper software was discontinued on June 1st 2009.
That said, the permanence of this signal on 4150 KHz and its purpose are still unknown to me as well as other friends.


(1) PSK encodes the input data sequences in pahes (states), while Differential PSK (DPSK) encodes the input data in the phase difference (transitions) between successive bits or symbols. This means that there would be a phase change in the modulation signal if the two successive bits in the input data sequence are different (0 to 1 or 1 to 0), and no phase changes if the successive bits are the same. DPSK is called conventional DPSK (or CDPSK) if the phase differences is in the set of [0,π] and symmetrical DPSK (SDPSK, also called π/2-DPSK) if the phase difference is in the set of [π/2,-π/2]. As you see in Fig. 3 the transitions in differential mode (diff=1) are in the set of [π/2,-π/2] so most likely it's a SDPSK (π/2-DPSK). 

unid SkyOFDM 28-tone 86Hz 65.6Bd PSK2

Continuous ofdm bursts transmission picked up on 4158 KHz/USB thanks to the "ArcticSDR" in Kongsfjord Arctic Norway: a KiwiSDR managed by my friend Bjarne Mjelde
http://arcticsdr.ddns.net:8073/ 
https://www.facebook.com/groups/1628656197277661/

Timings of the transmission and its spectrum are shown in Fig. 1 

Fig. 1 - timing and spectrum

The analysis of the OFDM signal clearly shows 28 channels and a frequency spacing of ~86 Hz, each channel is modulated using PSK2 at the symbol rate of 65.57 Bd (Fig. 2). The same results are obtained/verified by analyzing a single channel as shown in Fig. 3 (higher channel).
 

Fig. 2 - OFDM analysis

Fig. 3 - anlysis of the higher channel (#28)

As you see in Fig. 2, I did a further analysis after resampling the signal at 10109 Hz. Indeed, I used the tool OCG [1] in order to calculate and sythesize an OFDM waveform having the same parameters (channels, Br, Shift, modulation, width,...) and got 10109 Hz as one of the possible "native" sampling rate. The analysis of the synthesized OFDM is visible in fig. 4: notice the similarity between the PSK2 constellation of the synthesized signal and the one of the real signal (although resampled).

Fig. 4 - analysis of the synthesized OFdM-28 signal

The seven initial tones last 30 symbol periods and are derived from the OFDM generator as shown in Fig. 5; more precisely the used tones are: 2, 5, 6, 9, 13, 16, and 19.

Fig. 5 - initial seven unmodulated tones

The autocorrelation has a value of 76.2 ms (Fig. 6) that makes a 140 symbols length frame if considering an aggregate speed of ~1836 Bd (65.57 x 28).

Fig. 6 - autocorrelation

A similar OFDM waveform but with shorter and different bursts (Fig. 7) was reported on 2016.02.05 by my friend Cryptomaster [2] just on the same frequency of 4158 KHz/USB. In that case the modulation used was a form of PSK4, anyway number of tones, shift, Br, and ACF are the same; thus, that signal is on-air since several years.

Fig. 7

As regards the signal source, several TDoA tries always indicated an area north to Helsinki as probable Tx site (Fig. 8) although qrg.globaltuners.com reports exactly the same waveform/spectrum (and frequency too) indicating it as a signal sourced by the Spanish Navy [3].  In my opinion that's quite odd since the signal is fairly well received in the northern European countries such as Sweden, Norway, and Finland, while it is rather weak or inaudible at all in south Europe... I don't think of such a long skip.

 

Fig. 8 - TDoA reults

In my opinion it's an evolution of the original Skysweep Technologies proprietary waveform named "SkyOFDM", probably used by Finnish MFA (thanks to Roland Proesch for the hint). Indeed, the mentioned recording by my friend Cryptomaster just matches the features of the "original" SkyOFDM waveform (Fig. 9).

Fig. 9 - Skysweep Technologies OFDM-28

It's worth noting that SkySweeper Pro 5.13 software does not recognize the "new" OFDM-28 PSK2 that is analyzed  in this post.

(to be continued)

arcticsdr.ddns.net_2020-01-15T04_36_08Z_4159.70_iq.wav
synthetized_ofdm28_r10109Hz.wav

[1] OCG is a program for calculating and synthesizing OFDM signals, it can be downloaded from here
[2] http://www.radioscanner.ru/files/unknown/file19060/
[3] http://qrg.globaltuners.com/details.php?id=17420

SkyOFDM 28-tone, 65.6Bd BPSK, 2.6KHz bandwidth

...ooooops, the post you are looking for has been moved here:
http://i56578-swl.blogspot.com/2020/01/unid-ofdm-28-tone-86hz-656bd-psk2.html 
sorry for drawback 😀