25 January 2018

STANAG-4538, HDL+ BW7 frame structure

"The HDL+ protocol is used to provide acknowledged point-to-point delivery of datagrams from a transmitting PU to a receiving PU across an already-established HF link, with selective retransmission (ARQ) of data received in error. The datagram passed to the HDL+ entity for delivery is a finite-length ordered sequence of 8-bit data bytes (octets). Because it can adaptively employ a variety of signal constellations and degrees of coding, the HDL+ protocol can deliver datagrams of all sizes with high efficiency, under fair to very good channel conditions [...]"
"In an HDL+ data transfer, the sending PU and the receiving PU alternate transmissions in the manner depicted in Figure 1. Each transmission by the sending PU contains an HDL+ data header PDU (using BW6 waveform), followed immediately by an HDL+ data PDU (using BW7 waveform) containing payload data packets. After receiving each HDL+ header+data combination, the receiving PU transmits an HDL+ ack PDU (using BW6 waveform) containing acknowledgements of the data packets received without errors in the preceding HDL+ data PDU [...]"
Fig. 1

The end of a data transfer is reached when the sending PU has transmitted HDL+ data PDUs containing all of the payload data in the delivered datagram, and the receiving PU has received these data without errors and has acknowledged their successful delivery. When the sending PU receives an HDL+ ack PDU indicating that the entire contents of the datagram have been delivered successfully, it sends one or more (up to four) HDL+ EOT PDUs (using BW6 waveform), starting at the time at which it would have otherwise transmitted the next HDL+ data header PDU, to indicate to the receiving PU that the data transfer will be terminated. This scenario is depicted in Figure 2." (quoted from NATO STANAG-4538)

Fig. 2
Continuing to read S-4538 "Because it can use the high-order signal constellations (up to 64-QAM) of the STANAG 4539 High Data Rate waveforms, the HDL+ protocol can achieve very high data throughputs [...]" it may seem that HDL+ uses the waveforms described in S-4539 (or MIL 188-110C App. C) but actually uses a waveform with a frame structure of 288 symbols, as can be seen in Figures 3a and 3b (in this sample a PSK-8 2400 Bd modulation).
Fig. 3a - HDL+ frame structure (view)
Fig. 3b
Fig. 3b - HDL+ frame structure (analysis)
The structure of the frame is confirmed by the analysis of the demodulated bitstream in Figure 4: indeed the period consists of 864 bits that just make 288 tribit (PSK-8) symbols.

Fig. 4 - HDL+ demodulated bitstream (864-bit frame)
A frame structure of 288 symbols does not belong to NATO STANAG-4539, or the equivalent MIL 188-110C Appendix C, since all the S-4539 waveforms  - unless the preamble (obsoleted) - have a frame structure of 287 symbols! (Figure 5)

Fig. 5 - STANAG-4539 frame structure (287 symbols)
In terms of frame structure it is more correct to say that HDL+ BW7 PDUs, unless the sync preamble which is formed using BW6,  use the same structure of MIL 188-110C App.D 3 KHz waveforms, ie 288 symbols (Figure 6).

Fig. 6 - MIL 188-110C App.D frame structure
By the way, the analyzed transmission was recorded on 11430.0 KHz/USB at 1045z (Jan, 10).


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