12 June 2018

TDMA waveforms (STANAG-4539 Annex D,...) and NILE/Link-22

Recently, some friends and me happened to run into QPSK and 8PSK burst waveforms that are among those described by STANAG-4539 for TDMA operations (Time Division Multiple Access) and that are used by NILE/Link-22. Even if I do not have a direct confirmation, my prudent guess is that it is probably about Link-22 transmissions.

In TDMA mode each user is allowed to transmit only within specified time intervals (Time Slots) so that different users transmit in differents time slots. When users transmit, they occupy the whole frequency bandwidth (separation among users is performed in the time domain). 
According to S-4539 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 (Fig. 1).

Fig. 1 - TDMA Slot Time
A Media Code Frame is composed of 270 symbols to be transmitted/ received at the modulation rate of 2400 baud and following a certain waveform structure (Traffic Waveform) with different waveforms and modulation. Each Traffic Waveform is composed of a sequence of different DATA blocks and Mini Probe (MP) blocks: the DATA block contains coded user information symbols and the MP block contains known training symbols to be used by the equaliser. There are two different types of PSK modulations: QPSK and 8PSK. The QPSK modulation is used for preamble transmission, QPSK or 8PSK are used for Media Code Frame transmission.
As far as I know, and from S-4539 D, the 270 symbols of a Media Code Frame are arranged according the used Traffic Waveforms (TWF):
TWF1: 4 packages with 48 data and 19/20 probes 
TWF2: 8 packages with 18 data and 15/16 probes
TWF3: 4 packages with 48 data and 19/20 probes
TWF4 to WF7: 1 package with 240 data and 30 probes 8PSK 
TWF?: 3 packages with 60 data and 30 probes 

That said, the  270 symbols (length of the Media Code Frame) sent at 2400 symbols/sec, regardless the used Traffic Waveform, should produce an ACF value of 112.5ms.
The analysis of the signals was therefore targeted at checking that ACF value, the structure of the Media Code Frames (Traffic Waveforms) and some other possible regularities among the signals. Unfortunately I have only the annex D of S-4539 which specifies only the first three Traffic Waveforms while from some advertising on the internet, Fig. 2, one can see that they can be up to 18 (Annex G).

Fig. 2 - STANAG-4539/Link-22 Traffic Waveforms
The lack of official documentation means that these signals can not be identified exactly as Link-22, but there is a good chance that they will be.

QPSK modulations
Below an interesting  4 Media Code Frame slots transmission (Fig. 4). Each Media Code Frame (270 symbols, 112.5ms) seems composed of 3 packages with 60 data and 30 probes (Fig. 5)

Fig. 4 - 4 Media Code Frame slots
Fig. 5 - Traffic Waveform composed of 3 packages
In Fig.6 is shown a Media Code Frame that uses a Traffic Waveform composed of only 1 package (TWF 1-7) followed by a 3 packages Traffic Waveform (37.5ms).

Fig. 6

8PSK modulations
In this sample the Media Code Frame uses a Traffic Waveform composed of two packages (Fig. 7)

Fig. 7 - 2 packages Traffic Waveform

As verified by KarapuZ, all the analyzed signals have the same preamble: here below, Figs. 8 and 9, an example of two signals (8PSK and QPSK)

Fig. 8
Fig. 9
(to be continued)

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