28 July 2016

HDL+ (a proposed enhanced protocol for STANAG-4538)

At a first glance this transmission resembles the STANAG-4538 forward+ACK structure, but the lengths of the bursts and their frame formats do not match the ones specified for the HDL and LDL protocols. Encouraged by my colleague KarapuZ, I tried the HDL+ approach in the analysis of these signals.

The HDL+ data link protocol combines high data rate waveforms similar to those of STANAG 4539 or MIL-STD-188-110C Appendix C with incremental redundancy (Type II Hybrid-ARQ) techniques similar to those of the STANAG 4538 LDL and HDL protocols.

fig. 1 - BW7 waveforms
As in STANAG-4538, the HDL+ transmisssion is composed of data forward bursts and ACK messages bursts. The forward transmission (datagram) in HDL+  consists of a header, which is transmitted using the Burst Waveform 6 (BW6) for robustness, and a data section sent with the Burst Waveform 7 (BW7) for higher speed: the BW6, which is also used for the ACK messages, is much more robust than BW7.

The structure of the HDL+ forward burst can be verified isolating a single burst and zooming the FFT as in figure 2, it's worth noting the presence of the 2 mini-probes initial sequence that follows the header and marks the beginning of the data section.

fig. 2 - BW7 header and data section
The constellation in figure 3, obtained from that same burst, reveals a PSK-8 modulation with constant symbol rate of 2400Bd: it's another clue in favor of HDL+ since it is based on STANAG-4539/188-110B waveforms.

fig. 3 - PSK-8 constellation for BW7 4800 bps
From documentation available in the web, the BW7 data section consists of an initial probe sequence of 64 symbols followed by a data sequence of 256 data symbols alternated with 32 symbols long mini-probes (fig. 4): so total frame symbols sums to 288, 256/32 unknown/known format (by the way, STANAG-4538 HDL provides PSK-8 32/16 unknown/known frame format for its forward transmissions).

fig. 4 - BW7 frame format
My results exactly match such frame format (fig. 5): the ACF value is 120ms, that makes 288 symbols @ 2400 Baud. Note that the two mini-probes initial sequence indicated in fig. 4 was seen above in the zoomed FFT in fig. 2.

fig. 5 - ACF and frame format or HDL+ BW7
As further confirmation, the analysis of the demodulated bitstream produces the same results: the frame period is 864 bit long and the mini-probes length is 96 bit (fig. 6).

fig. 6 - analysis of the demodulated BW7 bitstream
Since the results (header structure, signal constellation, and frame format) the signal can be identified as HDL+ BW7, in this case the 4800bps PSK-8 waveform.

One could confuse  "HDL+ BW7" with "188-110C App.D 3KHz WID7" and conversely since they just use the same 288 symbols 256/32 frame structure. In the absence of appropriate analysis tools, one  may discriminate between HDL+ and 188-110C by carefully analyzing, at physical layer, the preambles (the "synchronization preamble" in 188-110C and the  "header" in HDL+) and the data sections:

- "The 188-110C synchronization preamble consists of two main sections, a transmitter level control (TLC) settling time section, and a synchronization section containing a repeated preamble super-frame." [D.5.2.1 MS188-110C App.D]

- HDL+ does not have TLC, its header is shorter than 188-110C App. preamble and is  transmitted  in  386.67  ms

fig. 7 - 188-110C synchronization preamble
fig. 8 - HDL+ header
- the HDL+ data section begins with 2 mini probes
- the 188-110C data section begins with 1 mini-probe

fig. 9 - data sections

No initial synchronization preamble is required, since this role is filled by the BW6 burst waveform that is used to transmit the header immediately preceding each BW7 transmission (the payload section). Instead, an initial probe sequence containing two repetitions of a 32-symbol Frank-Heimiller sequence (a total of 64 known symbols) is transmitted.
The payload section is used to convey between one and fifteen (inclusive) packets of payload data, after their contents have been coded and interleaved. Each packet is conveyed by a sequence of unknown/known (“UK”) frames. Each UK frame contains a data block, a sequence of 256 unknown symbols modulated with payload data, followed by a 32-symbol mini-probe. The number of UK frames used to convey each payload data packet depends on the signal constellation, the code rate, and the packet payload size as shown in fig. 10 (quoted from STANAG 4538 Annex C Edition 1, Amendment 21, Draft 0.3).
fig. 10

For what concerns the Burst Waveform 6 (BW6), it is used to convey the BW7 header, ACK, and EOT (EOM) PDUs of the HDL+ data link protocol, and to convey PDUs of the FLSU and FTM protocols on a packet link established for delivery of data traffic using HDL+. BW6 burst has an on-air duration of 386.67 msec and 51 bits of payload that are Walsh modulated using a Walsh sequence and then PN-spread (Pseudo Noise) to produce 544 channel symbols.
The initial TLC/AGC guard sequence of 192 tribit symbols and the 544-length sequence of BW6 channel symbols is used to PSK-8 modulate an 1800 Hz carrier signal at 2400 channel symbols/sec. The 544 PSK-8 symbols, ie 1632 bit, are clearly visible once removed the TCL/AGC section (fig. 11 and 12).

fig. 11
fig. 12

The end of a HDL+ data transfer is reached when the sending PU has transmitted BW7 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 ACK 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 BW7 data PDU, to indicate to the receiving PU that the data transfer will be terminated (quoted from STANAG 4538 Annex C Edition 1, Amendment 21, Draft 0.3)
This link termination scenario is depicted in figure 13 where the two PUs are distinguishable by the strengths of the signals:

fig. 13
The link is terminated by the FLSU_TERM PDU which is transmitted using the burst waveform 6, according to STANAG 4538 Annex C Edition 1, Amendment 2, Draft 03 (in HDL+ links FLSU and FTM PDU shall be use BW6 waveforms). It's interesting to see that the receive peer transmits the optional FLSU_TERM confirm PDU (fig. 14)

fig. 14

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