Direct sequence spread spectrum techniques for land mobile radio applications

This thesis describes an investigation into the application and performance of direct sequence spread spectrum techniques for land mobile radio systems. There is a brief description of the basic principles of operation of direct sequence systems. The multiple user facility is analysed and values obtained for the maximum number of simultaneous system users in terms of system parameters. This clearly illustrates the need for power control. A possible method of providing power control is described. Comparison of user density is made against conventional narrowband modulation methods. There is some discussion of the effect of sequence cross-correlations on the number of system users. The system organisation is mentioned, showing possible application of a calling channel. Consideration is given to the possibilities of bandsharing with narrowband modulation systems. Figures are derived for the resulting interference to existing systems which would be caused by such an arrangement. A brief resume of the pertinent features of the land mobile radio channel is given. The effects of shadowing on the output quality and spectral efficiency of direct sequence systems is discussed. There is an analysis of the effects of shadowing on the user density in small cell schemes. An analysis shows the effects of multipath propagation on direct sequence performance by reference to a simple two path channel. Details are given of a simple experimental direct sequence spread spectrum transmitter and receiver constructed. The measured results for the performance of the system against various forms of interference and channel degredation are compared with their theoretical values. Finally ideas for future work are discussed.