The issue of spreading in multipath time-varying channels

The capacity of a multiple-access, time-varying but known multipath channel is considered in the presence of white Gaussian noise, a bandwidth constraint, and power constraints. It is assumed that the product of the time spread and the Doppler spread is small. The general case is specialized to particular cases such as flat fading and specular paths with fixed Doppler shifts. The results provide insight into questions of CDMA and orthogonal systems.