7 November 2016

STANAG-4538 3G-HF, HDL complete session

3G-HF HDL transfer heard on 10627.0 KHz/USB at 1258 UTC
The High-throughput Data Link protocol (HDL), defined in STANAG-4538, is a selective repeat ARQ protocol which can only be run in a point-to-point data packet connection. HDL is most efficient when large volumes of data are to be transmitted and the channel conditions are moderately good, while LDL is best suited for small data volumes and in poor quality channel
"Data transfer by HDL begins after the stations have already established the data link connection in the traffic setup phase (using FLSU BW5 waveform). In an HDL data transfer, the sending station and the receiving station alternate transmissions in the manner depicted in figure 1; the sending station transmitting HDL_DATA PDUs containing payload data packets, and the receiving station transmitting HDL_ACK PDUs containing acknowledgments of the data packets received without errors in the preceding HDL_DATA PDU. The end of a data transfer is reached when the sending station has transmitted HDL_DATA PDUs containing all of the payload data in the delivered datagram, and the receiving station has received these data without errors and has acknowledged their successful delivery. When the sending station receives an HDL_ACK PDU indicating that the entire contents of the datagram have been delivered successfully, it sends an HDL_EOM PDU repeated as many times as possible within the duration of an HDL_DATA PDU, starting at the time at which it would have otherwise transmitted the next HDL_DATA PDU." [1]

fig. 1a
Fig. 1b
As in STANAG-4538 Table 13-1, HDL protocol use the burst waveforms BW2 for data forward and BW1 for ACK and EOM/Term signal, all the burst waveforms use the basic PSK-8 modulation at 2400 baud centered at 1800hz also used in the MIL-STD 188-110A serial tone modem waveform (fig. 2) and can be identified by measuring their duration (fig. 3)

fig. 2
fig. 3
BW2 consists of 100 msec TLC section and a short (26,67 msec) preamble followed by a number of fixed-size data packets (3, 6, 12, or 24): the number of packets is negotiated before the HDL protocol starts and remains unchanged until the end of the data transfer. Each packet consists of 20 frames, each of which contains 32 unknown symbols (data) followed by 16 known symbols acting as probe (fig. 5). 
HDL protocol is packet-oriented, in contrast with LDL protocol which is byte-oriented, and it can be designated by a number as HDL<n> where n - as said above - is the negotiated  number of packets which are transmitted in one forward frame: for example, in the recorderd transfer we see HDL3-type frames.

fig. 5
Burst waveform 1 (BW1) is a general-purpose waveform used to carry short messages for many of the 3G protocols: traffic management, link maintenance, and data acknowledgments for the HDL protocol. It consists of 576 PSK-8 symbols preamble followed by 2304 PSK-8 symbols of data which are coded using 16-ary Walsh seqences (fig. 6).

fig. 6
It's worth noting that all the six burst waveforms specified by STANAG-4538 begin with a TLC/AGC guard sequence, why? Existing HF radios were generally not designed with burst waveforms in mind. For example, MIL-STD-188-141 military radios are allowed 25 ms to reach full transmit power after keying. While the transmitter radio frequency stages are ramping up, the input audio signal level is adjusted by a transmit level control (TLC) loop so that it fully modulates the transmit power. At the receiver, an automatic gain control (AGC) loop must also adjust to a new receive signal. To accommodate these characteristics of existing radios, the 3G burst waveforms just begin with a TLC section of “throwaway” 8-ary PSK symbols that are passed through the system while the transmitter’s and receiver’s level control loops stabilize.

[1] from: "Third-Generation and Wideband HF Radio Communications" 

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