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-4539/188-110B (MIL 188-110B Appemdix 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)

It's worth noting that 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).

https://yadi.sk/d/A_kkMKYl3RnEZP 

STANAG-4538 HDL+, BW7 QAM-16 waveform

- Burst Waveform 6 (BW6) is used to convey the HDLP_DHDR, HDLP_ACK, and HDLP_EOT 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+ (note the Link terminate PDU that is conveyed by a BW6 burst). BW6 PDUs bursts have 51 bits of payload, an on-air duration of 386.67 ms, and are transmitted using a PSK-8 modulation.

- Burst Waveform 7 (BW7) is used for transfers of traffic data by the HDL+ protocol. The HDL+ protocol combines high data rate waveforms similar to those of MIL-STD-188-110C Appendix D with incremental redundancy techniques. 

Given the variable lengths and modulation formats of HDL+ data, it's necessary to include a header at the beginning of each BW7 PDU (which was unnecessary in LDL and HDL) that announces the number of packets and modulation format of the following payload section of the transmission (HDLP_Data PDU). For this header, the HDL+ uses a BW6 PDU (HDLP_DHDR PDU). 

Since BW6 symbols are modulated using a PSK-8 constellation, the structure composed of BW6-BW7 PDUs

will be clearly visible in those cases where BW7 use a different constellation for its symbols, such as QAM-16 or QAM-64 (BPSK and QPSK are scrambled to appear on-air as a PSK-8 constellation). 

Just yesterday, I copied a such HDL+ data transfer on 11132.0 KHz/USB. As displayed in Figure 1, the 8th power harmonics are present for all the duration of the BW5 and BW6bursts, but only in the initial segments of the BW7 bursts, ie in the BW6 PDUs that work as headers. The HDLP_DATA PDUs are instead modulated using a QAM-16 constellation (12 points in the outer ring, 4 in the inner ring).

Fig. 1

For what concerns the analysis of the BW7 waveform, no initial synchronization preamble is required since this role is filled by the BW6 HDLP_DHDR PDU. Instead, an initial probe sequence containing two repetitions of a 32-symbol Frank-Heimiller sequence (a total of 64 known symbols) is transmitted.
The following section is used to convey between one and fifteen (inclusive) packets. Each packet is composed of 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 data packet depends on the signal constellation, the code rate, and the payload size.

Fig. 2

Fig. 3

 

Other than the recording of the transmission, a short video (from my YouTube channel) that illutrates the analysis with SA  is also available.

https://youtu.be/rRI-kgf_b9Y

https://yadi.sk/d/4H_XVAL43HfXKh 

STANAG-4538, HDL+ and LDL protocols swapping in a bidirectional link

This is a very interesting 3G-HF on-air scenario where as many as 5 burst waveforms and 2 datalink protocols are used, moreover, thanks to the different strength of the signals, it's also possible to notice the swap of the data-flow directions.
In the first part, after the 2-way Fast link setup PDUs exchange, data flows from PU1 to PU2 using HDL+ protocol. Immediately after the HDL+ transfer is complete, ie after the three HDL+ ACK PDUs (BW6) sent by PU1, both stations remain linked and initiate the Fast Traffic Management (FTM) protocol to negotiate further traffic. This gives PU2, which was the called station, the opportunity to send reverse traffic to PU1 (which was the caller) so data now flow from PU2 to Pu1.

After the LDL transfer is complete, ie after the ACK PDU (BW4) sent by PU2, stations wait for possible FTM PDUs and after the link timeout has occurred, the last station to receive an xDL transfer (PU1 in this case) terminates the link with an FLSU_Term PDU request.

Fig. 2

It's important to notice the use of the BW6 waveform for FLSU and FTM protocols PDUs (marked with an "*" in Figure 2) which are usually conveyed using the BW5 waveform. Indeed, as stated in STANAG 4538 Annex-C Edition1 Amendment2, 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.
This means that BW6, other than 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. It's worth noting that altough the link is subsequently used for LDL, it was initialized for HDL+ protocol.

Transmission was copied on 9175.0 KHz/USB at 1454 UTC (March, 22): MIL 188-110B traffic from Algerian Army was logged on this QRG, are we facing the same user? 
For what concerns the nature of the exchanged data, we can only state that LDL is used by PU2 to send an HARRIS Citadel encryped message to PU1 (Fig.3): probably the end-to-end acknowledgment mechanism which is part of P-MUL protocol; this is and end-to-end ACK at application layer and it's related to the delivery of the message (not the ACKs issued in xDL link protocols). The use of Citadel encryption is a clue in favor of HARRIS sw/hw equipments, e.g. Falcon II RF-5800H radios.

Difficult to say what sits on top of S-4538: it could be an email messaging system (eg HARRIS WMT RF-6710) as well as a  STANAG-4406 application (Thales XOmail): the presence of the reverse message from PU2 to PU1 leads to think to the latter, since WMT does not provide delivery confirmations. I will discuss this topic in further posts.

Fig. 3

https://yadi.sk/d/0ju0lxSE3GFc37

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.

STANAG-4538, HDL+ BW7 waveforms

Bearing in mind the results and considerations about STANAG-4538 HDL+ protocol, as reported in the previous post, one can easily face the analysis of 3G-HF burst signals like this (fig. 1)

fig. 1

Once checked the headers and the length of the data sections, and found to be 288 symbols, I used the "harmonics" tool in order to get the modulations: I also used the zoom tool to better clarify the differences in each forward burst. A partial result at 8^ power is shown in figure 2: the PSK-8 harmonics due to BW6 headers are well visible, while the data sections (BW7) use different modulations according to the table of figure 3.

fig.2

Fig. 3

 Analyzing all the forward bursts, it's possible identify the presence of the QAM-16 and QAM-64 BW7 waveforms, other than the PSK-8 BW6 waveform. 

fig. 4 - 4800 bps PSK-8 BW7 waveform

fig. 5 - 6400 bps BW7 QAM-16 waveform

fig. 6 - 9600/12800 bps BW7 QAM-64 waveform

It is worth noting in figs. 5,6 that the used constellations for QAM modulations are in 'circular rings' and then they are modified with respect to the standard ones.

fig.7

fig.8

 

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