HF radio communications offer latency advantages over fiber-optics and are more economical than satellite links in long-distance communication applications. As a result, there is renewed interest in the field of HF from the high-frequency trading industry, as a means of transmitting financial data. Ideally, decisions should be made on received symbols as soon as they arrive, without waiting for other samples to accumulate. A significant problem is that the ionospheric HF channel is a dynamic medium that is considered one of the most challenging communication channels. In this thesis, we investigate alternative techniques towards reliable communication with minimum delay. We study HF channel models and communication link design tools, which indicate that our minimum-delay problem is feasible but difficult to solve. Extant HF modems are observed to add prohibitive amounts of latency to ensure robustness, motivating us to design receivers that provide good performance given a delay constraint. We adapt a coupled MAP-RLS receiver studied in literature to the HF channel to yield minimal decision-making delay. We introduce a multitrellis adaptive Viterbi algorithm (MAVA) that solves the problem of equalization for sparse and time-varying ISI channels, producing a robust MLSE receiver with several milliseconds of decision-making delay. Finally, we cascade this MLSE receiver with MAP detection to obtain high-fidelity low-delay performance, finding a practical solution to minimum-delay HF communications.
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