Spatial Degrees of Freedom of Correlated Multipath

In this paper we consider the spatial degrees of freedom in the context of a multi-antenna wireless communication system. We investigate how the degrees of freedom depend on important system parameters such as the spatial extent of a region containing the antennas and, more importantly, the angular correlation of multipath. These results naturally augment known results which show how the degrees of freedom are affected by multipath from a restricted range of angles. We clarify the distinction between the spatial degrees of freedom with respect to an orthonormal basis and the concept of richness of multipath which is related to the Karhenen Loeve expansions for a random multipath field.