The classical multipath model of mobile communications is reviewed and the importance of phase derivatives with respect to space and frequency is emphasised. Using an analogy it is shown that the group delay follows a Student's t-distribution, delay spread as the determining parameter, and does not depend on the details of the power delay profile. The delay spread is the second central moment of the average power delay profile. The distribution is important for frequency selective effects of data communication over a narrowband channel.
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