The WSSUS (Wide-Sense Stationary Uncorrelated Scattering) model provides a statistical description of the transmission behavior of wireless channels.  "Wide-sense stationarity" means the second-order moments of the channel are stationary, which means that they depends only on the time difference, while "uncorrelated scattering" refers to the delay  due to scatterers.
Modelling of mobile channels as WSSUS (wide sense stationary uncorrelated scattering) has become popular among specialists.  The model was introduced by Phillip A. Bello in 1963.
A commonly used description of time variant channel applies the set of Bello functions and the theory of stochastic processes.