  Correlated scattering occurs naturally in frequency-selective fading channels
and its impact on the performance needs to be understood. In particular, we
answer the question whether the uncorrelated scattering model leads to an
optimistic or pessimistic estimation of the actual average capacity. In the
paper, we use majorization for functions to show that the average rate with
perfectly informed receiver is largest for uncorrelated scattering if the
transmitter is uninformed. If the transmitter knows the channel statistics, it
can exploit this knowledge. We show that for small SNR, the behavior is
opposite, uncorrelated scattering leads to a lower bound on the average
capacity. Finally, we provide an example of the theoretical results for an
attenuated Ornstein-Uhlenbeck process including illustrations.
