In this paper, the circular ring model is proposed to model the signal statistics of a wideband multiple-input multiple-output (MIMO) channel. Based on the proposed model, a new space-time frequency cross-correlation function is derived for the MIMO channel, in a compact mathematical form. The cross-correlation function includes various parameters of interest such as the distance between the base station and the user, spacing among antenna elements, user's speed and direction, as well as the angle spread and delay spread. In addition, the time of arrival probability density function and power delay profile of the circular ring model are derived and compared with measured data. The proposed model and the associated wideband space-time cross-correlation provide a convenient unified framework for the simulation of frequency selective mobile MIMO fading channels and the design of proper space-time coding and signal processing techniques over such channels.