Problems of mobile HF communication and techniques for performance improvement

Abstract

The paper first outlines the inherent strengths and weaknesses of mobile and fixed terminals in HF communication systems. Conventional HF system design and operating procedures are then described. The improvement in performance achievable by the use of various forms of diversity combining is quantified, and it is shown how a number of fixed HF terminals can be combined into a geographical diversity network, which can be accessed by mobile terminals. Specific problems of interconnection, data transmission and system control are considered in greater depth. IntroductionThe HF (2-32 MHz) band provides a communication medium which can support information transfer over distances from optical line-of-sight to world-wide. Surfacewave or sky-wave (ionospheric) propagation modes can be used, the former being restricted to ranges of (at most) a few hundreds of kilometres. Sky-wave propagation is by far the most important physical mechanism operationally, since it provides the means for very long range communication [1]. Radio signals refracted by ionospheric layers suffer from multipath and time dispersion effects, together with variable amounts of frequency dispersion and high levels of non-Gaussian atmospheric noise; also, cochannel interference generated by other users of the HF spectrum is a primary source of performance degradation. This paper examines the fundamental problems of communicating with mobile terminals in such an environment, and identifies techniques which offer potential for more reliable operation.Any mobile terminal will generally be subject to performance limitations arising from the following factors:(a) Dependence on a propagation path which, in addition to the perturbations outlined above, also suffers further variability, owing to the constantly changing position of the mobile {b) Relatively low transmitter powers, limited by the independent power sources installed on the mobile platform (c) Simple, narrowband and largely nondirectional antenna systems, having limited transmission efficiency, particularly at the lower end of the HF range (d) High levels of ambient noise/interference due to emissions from other co-sited electromagnetic systems, such as radars and communications transmitters, operating in the HF and other frequency bands.In contrast, fixed (land-based) stations can employ the following:(i) High transmitter power levels (ii) Large, efficient and directive antenna installations with narrowband or broadband characteristics (iii) Electrically 'quiet' receiving sites removed from other electromagnetic equipments.It is also relatively simple to interconnect fixed terminals via communication links to provide networking capabilities.