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.