Two NILE/Link-22 close channels recorded this morning (26th October) on 14.656 KHz and 14.659 KHz utilizing a distant AirSpy server located in Romania [1], traffic in the 2 channels flowed in alternating mode.
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| Fig. 1 - NILE/Link-22 transmissions |
Link-22 usage the TDMA (Time Division Multiple Access) waveforms, as for STANAG-4539 Annex D: modulation method consists of phase shifting of a 00 Hz sub-carrier and velocity of 2400 baud. In TDMA mode each user is allowed to transmit only within specified time intervals named as "Time Slots" so that different users transmit in differents time slots (1). According to S-4539 Annex D, a TDMA slot is the advanced level structure in which information will be transmitted/received and it is composed of a Preamble, a certain number of Media Code Frames and a defender Time.
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A Media Code Frame is composed of 270 symbols to be transmitted at the modulation rate of 2400 baud and utilizing different Traffic Waveforms and modulations. Each Traffic Waveform is composed of a series of different Data blocks and Mini Probe (MP) blocks: the Data block contains coded information symbols and the MP block contains known training symbols to be utilized by the equaliser. The Mini Probes symbols, prior to scrambling, are all symbols number 0. Since the 270 symbols of the Media Code Frame, regardless the utilized Traffic Waveform, it's ACF has a value of 112.5 ms.
Figures 2 & 3 show that the 2 Link-22 channels usage 2 different Traffic Waveforms to arrange the 112.5-ms/270-symbol dimension of the media code frame; besides announcement that they usage the same time slot duration since both send 15 media code frames per time slot.
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| Fig. 2 |
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| Fig. 3 |
Given the mediocre SNR of the signals I was incapable to find clear constellations and so identify the kind of the Traffic Waveforms that were used; attempts propose the usage of QAM modulations (Figure 4).
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| Fig. 4 |
Unfortunately I don't have the 2019 edition of Stanag-4539 but only the first edition dated 2005 which specified only 3 waveforms (QPSK & PSK8): as you may see, the framings resulting in figures 2,3 do not correspond.
https://disk.yandex.com/d/xPhBz07F5Oh5tg
(1) separation among users is performed in the time domain https://en.wikipedia.org/wiki/Time-division_multiple_access
