The design of an HF band direct sequence point-to-point link for rural telecommunications.

Abstract

The following work documents the design of an HF band direct sequence point-to-point link as used in a rural environment. The dissertation begins with a description of the overall document layout. An introduction into the problems associated with providing rural access is then given, with special emphasis on wireless technologies. It is argued that the attributes of HF band radio make it a good candidate for providing wireless communications links for under serviced rural areas in South Africa. The pitfalls and disadvantages of using an ionospheric-based medium in which to propagate an electromagnetic wave are discussed and several solutions are put forward. One of these solutions is Direct Sequence Code Division Multiple Access (DS-CDMA). A thorough analysis of the principles of DS-CDMA is given with emphasis on its abilities to combat the negating effects of the HF channel. A CDMA HF system is then proposed, outlining the various practical and theoretical aspects. Next, an HF channel model is designed and simulated, first with no spreading or coding, then with just spreading and finally with spreading and coding. It is found that although the extra diversity of the spreading and coding aid in reducing the bit error probability, more detailed local measurements and refinement in the design of the channel model and simulated system are needed to increase performance. Various aspects of a practical system that was built are then discussed, highlighting issues such as hardware interfacing and the software design of a man-machine-interface. Field measurements are also given with estimates on an upper limit on expected performance of a practical system. Finally conclusions are given, detailing the achievements and shortcomings of the research.