Summary form only given. The performance of wideband communication systems is heavily dependent on the characteristics of the channel. A set of second order statistics are defined for the characterization of the diversity of multipath fading channels. The frequency mean square correlation (FMSC) characterizes the frequency selectivity of a channel, while the time mean square correlation (TMSC) represents the time selectivity. Frequency-time mean square correlation is defined to characterize both the frequency and the time selectivities of a channel. For a wide-sense stationary uncorrelated scattering (WSSUS) channel, these parameters can be obtained from the scattering function. We show that these parameters are very closely related to the system performance. Because of this close relationship with the system performance, these parameters can be used in the system design. In this paper we give several examples of the usage of these parameters in the system design.