Techniques for improving the reliability of mobile HF communications over high-absorption paths

System design aspects of the mobile HF communications problem in high-latitude regions are discussed. An attempt is made to identify the various techniques available for enhanced reliability of reception under high-absorption conditions and to indicate how these can be coalesced into an improved HF system design. The basic problem associated with excess absorption is that received signal-to-noise ratios (SNR's) are lower than those which might be expected for say mid-latitude paths. Specific methods by which the value of the SNR required at the receiver can be reduced, either directly or effectively are discussed. Techniques discussed include: (1) the use of various forms of diversity processing, emphasising the value of path/geographical diversity combining; (2) relaying out of the disturbed region with a view to avoiding the perturbed ionosphere and operating the transmitting antenna system more efficiently at a high frequency; (3) the use of meteor-burst and ionospheric scatter communication to complement normal HF communication; (4) the use of modulation and coding schemes requiring a relatively low SNR at the receiver; and (5) system control procedures.