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The data link protocol HDL+, which has been proposed for incorporation into NATO STANAG 4538, employs an innovative combination of Type II Hybrid-ARQ techniques with real-time adaptation of signal constellation and code rate to achieve high throughput performance under a wide variety of channel conditions. The ionospheric HF channels for which HDL+ was designed exhibit important variations in channel quality (SNR, fading and multipath chcracteristics) at a variety of time scales from seconds to minutes. It appears plausible that the design characteristics of the HDL+ protocol couldprove especially valuable in coping with time-varying channel quality over these time scales. In this paper, we Â¿ Present and discuss a model of channel quality variation and a way of incorporating it into an ionospheric channel simulator suitable for performance characterization of HF communications waveforms andprotocols Â¿ Provide overviews of HDL+ and of the STANAG 5066 data link protocol, a widely-used protocol based on conventional (not hybrid) ARQ techniques Â¿ Present and discuss comparative performance data for the HDL+ and STANAG 5066 protocols obtained under test conditions including channel quality variation.