This book offers an introduction to the methods and concepts applied in modern communications, and suggests areas for future development and refinement. The term spread spectrum is used to describe a class of modulation techniques in which the data stream is coded in such a way that the total information rate is higher than the data rate. However, this book also describes a range of other electronic measures which are available to the engineer. It is therefore necessary to introduce the degrees of freedom given by the physical nature of electromagnetic waves and to give an overview of the corresponding possibilities in communication engineering. In order to design optimal communication systems, it is of vital importance that the essentials of the problems and possibilities of the various communication techniques are thoroughly understood.

This chapter discusses coding for bandwidth spreading. This chapter discusses some of the requirements for the spreading codes with respect to synchronization, multiple access systems and the spectral occupancy of the codes. The last is of special interest in a system to achieve concealment.

This chapter will derive expressions for the signal and interference components useful in SNR calculations under the condition of both nonselective and frequency or time selective channels. For nonselective channels the capacity of a multiple user system with code multiplexing is simulated for Rician fading statistics. As an example of the effect of time dispersion on spread spectrum modulation, an experiment with wideband signals on an HF skywave channel is analysed. Finally the problem of receiver synchronization and tracking for time dispersive channels is discussed.

In this book we have limited the discussion to spread spectrum systems with a burst signalling format, and in particular we have addressed the importance of spread spectrum to military communication systems. The majority of spread spectrum systems are still military, the main reason for this being, as mentioned, the growth in cost for systems with this modulation.

This appendix provides crosscorrelation parameters for maximal length sequences.

To provide some experimental data on the useful bandwidth for spread spectrum systems, test equipment consisting of a spread spectrum 2-PSK transmitter was constructed. The receiver consisted of a spread spectrum passive correlator for direct sequence modulation. The passive correlator consists of two lowpass inphase and quadrature filters implemented as two identical digital matched filters constructed to match the spreading code used. The receiver was designed to offer linear phase response to the variable signal bandwidths. The linear phase requirement emerged from the wish to measure the nonlinearities of the ionosphere undistorted by the test equipment.