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Practical methods for highly reliable HF communications

Since 1993, TCI/BR investigators have been examining the potential of frequency and path diversity in solving many of the problems which have longed plagued HF communication systems and have hindered its effectiveness. While our results have application to voice communication, our primary motivation is directed toward the improvement in reliability for conveyance of digital data. Paths include polar cap, auroral zone, high-latitude trough, as well as mid-latitude channel environments. These paths have been selected to ascertain the relative correlation properties under both benign and pathological conditions, and the resultant data are being used to validate emerging real-time ionospheric and HF performance prediction models. These results have application for design and operation of highly reliable HFDL service for DoD applications in stressed environments and in various aeronautical-mobile and maritime applications. In this paper, we will outline the importance of path and frequency diversity toward the improvement in system availability. Emphasis will be given to results which have been obtained from an analysis of radio paths within the high latitude and arctic regions. These have been selected since the pathological behaviour exhibited by the relevant skywave channels provides us with an abundant opportunity to test our several design hypotheses based upon application of diversity. We have developed estimates of performance for an HF data link service based upon Chirpsounder records and HFDL modem specifications.