STANAG-4538, HDL+ transmissions on a bidirectional link

According to the different signals strength, two radio stations (also termed Partecipating Units, PUs, in STANAG-4538) alternatively forward their data on the same channel with a fixed interval of about 7.5 sec, while ARQ acknowledgements flow in the reverse direction of data.  These half-duplex transmissions have been copied on 6973.5 KHz at 1300 UTC, since the schema used by the stations it could be probably a test.

Unless the two initial BW5 exchanges which convey fast link setup PDUs (Fig. 1), all the protocol data units (PDUs) are sent using the BW6 and BW7 waveforms.

Fig. 1

Actually the transmission of the PDUs (either data, control and management packets)  is performed using the sequence BW6, BW6, BW7, BW6, BW6 which - as seen - is repeated each ~7.5 seconds (Fig. 2)

Fig. 2

The role of the two BW6 bursts that initiate each sequence could be misleading but it's important to note that "[...] if a link has been established for delivery of packet traffic using the HDL+ data link protocol, all FTM and FLSU PDUs transmitted for the remaining duration of the packet link shall be transmitted using the BW6 burst waveform, up to and including the FLSU_TERM PDU transmitted to terminate the link, and any optional response to the FLSU_TERM" (STANAG 4538 Annex C Edition 1, Amendment 2, Draft 03). 

This means that BW6, other than the DHDR (BW7 header), ACK, and EOT (EOM) PDUs of the HDL+ protocol, is also used to convey PDUs of the fast link setup (FLSU) and fast traffic management (FTM) protocols in HDL+ links! Since FLSU and FTM PDUs generally use a 50-bit structure (BW5), when in conjunction with HDL+ they will use the 51st bit of BW6 as an additional CRC bit.

That said, the HDL+ sessions can be read as in Fig. 3. Each change of data flow is preceeded by two TM handshake that synchronizes the PUs

Fig. 3

The two ending BW6 bursts of each sequence, as usual, signal the end of the data transfer. When the sending PU receives an HACK PDU indicating that the entire contents of the datagram have been delivered successfully, it sends one or more (up to four) EOT PDUs, starting at the time at which it would have otherwise transmitted the next data PDU, to indicate to the receiving PU that the data transfer will be terminated.

Since in HDL+ the ACK/EOT PDUs and the FLSU/FTM PDUs use the same 51-bit burst wavefom 6, they will have the same on-air duration, 386.67 msec, and the same number of on-air symbols, ie 544 PSK-8 symbols or 1632 bit after removed the initial TLC/AGC guard sequence of 192 symbols (Fig. 4).

Fig. 4 - BW6 544 on-air symbols

For what concerns the data exchanged between the two PUs, the HDL+ protocol sends its datagrams using a forward transmission composed of two PDUs: an HDL+ data header PDU, which is transmitted using the BW6 burst waveform, and an HDL+ data PDU, transmitted using the BW7 burst waveform. These two PDUs are transmitted contiguously with no dead time separating them. No initial synchronization preamble is required, since this role is filled by the BW6 burst waveform.
The payload section is used to convey between one and fifteen (inclusive) packets of payload data Each packet is conveyed by a sequence ofunknown/known (“UK”) frames. The number of UK frames (Figs. 5a, 5b) used to convey each payload data packet depends on the signal constellation, the code rate, and the packet payload size.

Fig. 5a - four UK frames

Fig. 5b - two UK frames

According to the table of Fig. 6, the modulation used in BW7 bursts is QAM-64 since there are 4 or 2 UK frames in the HDL+ payloads (if it was PSK-8 we would have seen the heigth harmonic in Fig. 7).

Fig. 6

Fig. 7

Link is disconnected by the FLSU "link termination" PDU followed by the (optional) "link termination confirm"; both the FLSU PDUs, the FLSU_TERM, are sent using bust BW6 waveform as specified in STANAG-4538 for HDL+ packets links.

I want to thanks my friends KarapuZ who recordered the signal and Marco for sending me the STANAG-4538 Annex C Amendment.