- 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).
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| 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.
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.
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| Fig. 2 |
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| 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.
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