26 October 2023

NILE/Link-22, likely QAM traffic waveforms

Two NILE/Link-22 close channels recorded this morning (26th October) on 14.656 KHz and 14.659 KHz using a remote AirSpy server located in Romania [1], traffic in the two channels flowed in alternating mode.

Fig. 1 - NILE/Link-22 transmissions

Link-22 use the TDMA (Time Division Multiple Access) waveforms, as for STANAG-4539 Annex D: modulation technique consists of phase shifting of a 00 Hz sub-carrier and speed 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 high 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 Guard Time.


A Media Code Frame is composed of 270 symbols to be transmitted at the modulation rate of 2400 baud and using different Traffic Waveforms and modulations. Each Traffic Waveform is composed of a sequence 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 used 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 used Traffic Waveform, it's ACF has a value of 112.5 ms.
Figures 2 & 3 show that the two Link-22 channels use two different Traffic Waveforms to arrange the 112.5-ms/270-symbol length of the media code frame; also notice that they use the same time slot duration since both send 15 media code frames per time slot.

Fig. 2 

Fig. 3

Given the poor SNR of the signals I was unable to find clear constellations and therefore identify the type of the Traffic Waveforms that were used; attempts suggest the use of QAM modulations (Figure 4).

Fig. 4

Unfortunately I don't have the 2019 edition of Stanag-4539 but only the first edition dated 2005 which specified only three waveforms (QPSK & PSK8): as you may see, the framings resulting in figures 2,3 do not correspond.


(1) separation among users is performed in the time domain https://en.wikipedia.org/wiki/Time-division_multiple_access

[1] sdr:// 

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