Information theory aspects of spread-spectrum communications

A novel definition of a spread-spectrum system as a communications system in which the Fourier bandwidth is much greater than the Shannon bandwith (the number of dimensions of signal space used per second) is proposed. Six different communication systems are analyzed in terms of this definition. It is shown that there is a fundamental difference between the bandwidth expansion due to coding and due to “spectrum spreading”. It is further shown that spectrum spreading plays no role in increasing the capacity of the system, but can perform other useful roles such as providing low probability of interception of the signal, good electromagnetic compatibility, and a multiple-access capability