24 July 2018

the unid Kongsfjord OFDM (1)

If you open the Norwegian Kongsfjord kiwiReceiver and tune to 468 kHz/usb you will find a continous signal 12 kHz wide with a carrrier at +1,5 kHz from the lower edge and two smaller carriers at the signal's midband. 
I suspect this signal (and 3 other signals found a bit lower in the same band) to be Russian maritime broadcasts. My assumption is based on the fact that it was announced a couple of years ago that Russia intended to build a chain of MF broadband stations along the North Eastern Sea route [1], and that Russia had shown interest in the NAVDAT MF broadband system developed by Kenta, France, and adopted by ITU [2]. Anyway, it could also belong to a Narrowband (Under 500 kHz) Power Line Communications (NB-PLC) system used not only for smart metering but also for many other Smart Grid applications.

After waveform analysis by me and AngazU (1), the strong 12 KHz wide signal (cf at 474 KHz) turns out to be a OFDM 95-tone, 125 Hz spaced, using a mix of QAM-16, QAM-32, and QAM-64 modulations at the same time, ie in different subcarriers, with a symbol-rate of 116.8 baud (Fig. 1)

Fig. 1 - signal spectrum
The signal seem structured as follows (Figs. 2,3):
- 2 groups of 46 (data) subcarriers,
- 2 unmodulated pilot subcarriers, 
- 1 empty place (exactly at center band), although - according to the SA OFDM module - it seems actually a modulated tone:

Fig. 2 - main parameters of the OFDM waveform

Fig. 3 - different constellations: QAM-16, QAM-32, and QAM-64

It's worth noting that the QAM-64 constellation is ~9.75 degrees rotated (Figs.4,5). A smilar feature is offered by DVB-T2 specification (tilt angle is 16,8 degr in this case) to establish a form of diversity. Also noteworthy is that DVB-T2 uses concatenated BCH and LDPC coding for FEC as does the Russian system for the Northern Sea Route. It seems that this system has borrowed features from DVB-T2. 

Fig. 4 - QAM-64 rotation in consecutive subcarriers
Fig. 5 - using GIMP software to measure the tilt angle after six rotations
This system seems operate in a “permanent mode”, stations are transmitting continously, rather than a "sequential mode" (based on time slots). As you see in the title figure, the spectrogram shows other three similar signals with lower SNR at 378, 414 KHz and 438 KHz (central frequencies), notice in Fig. 6 that the four signals are separated by intervals which are related to the 12 KHz bandwidth (3x12 and 2x12 KHz).

Fig. 6 - up to four transmitters ?
Kongsfjord SDR seems to be the only SDR which receives these signals, probably it's due to the propagation limits in this band and the power levels used since the radiated power from the regional coast station transmitter should be what is sufficient to cover the intended service area of that coast station. If you look at the map at page 3 in the cited document about a Russian Arctic MF communication system [1], below in Figure 7, you will notice the chain of stations are plotted and that one (service zone 2) covers the position in Norway where the Kongsfjord SDR is located. Also a transmitter in zone 6 (there is no service circle on the map), likely Murmansk or Severomorsk port, probably could be received at the same location in Norway. Signal's levels in Figure 6 could be a confirm of the Tx sites. 

Fig. 7 - Approximate position of the local service zones. Zone radius is 200 km.

Me and friends who collaborate to this analysis emailed Mr. Bjarne, Kongsfjiord SDR admin and arcticdx.blogspot.com owner, asking his opinion: he kindly replied by also sending  two jpgs which show the regional and local layout of the power grid. He tends to exclude the PLC option for political/strategic reasons and also for tech reasons ("the signal is also strong 14 km away from the KongSDR location", Bjarne says).

It's very interesting to notice in Fig. 8 that only 2 signals were visible on September 2017 (vs. the 4 which are visible today) during ALA1530LF Loop and Longwire comparative tests at KongSDR site: maybe other systems/sources were set in operation after September?

Fig. 8 - LF spectrum at Kongsfjord on September 2017

It will be interesting to follow these signals after the summer and look if they will be received in some other close SDRs as the one located in Haparanda (Sweden), the reason is the high latitude and the propagation mechanisms in the low portion of MF: as you know, in this month (July) in that Arctic region the sun comes up for more than 20 hours.
(next post here)

(1) Since the 11968 KHz bandwidth available for IQ recording, limited by the KiwiSDR itself, the analysis could return some inaccurate estimates, anyway we resampled the recordings to 48 KHz. 

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