Many statistical multiple-input multiple-output (MIMO) channel models have been extensively developed over the past few years. However, to better understand and exploit such MIMO channels, there is a need for a MIMO channel model that has a basis in the physical propagation mechanisms. In this paper, a wideband semideterministic MIMO model is derived to quantify the mean capacity in an urban line-of-sight (LOS) microcell. Specifically, the model considers scattering from large-scale discontinuities on building faces randomly distributed along a street canyon. Using geometrical optics and the uniform theory of diffraction, the spatial correlations between any two receiving signals are evaluated and are used to compute the upper bound of the average MIMO capacity.