Dynamics of spatial degrees-of-freedom in MIMO mobile channels

This paper presents insights on spatial degrees-of-freedom (SDoF) of multiple-input multiple-output (MIMO) mobile propagation channels. The SDoF depends on multipath richness of the propagation channel and antenna aperture size, and indicates the number of effective antenna elements for the spatial multiplexing on the antenna aperture. We first define the SDoF using expression of propagation channels in plane and spherical wave domains. The SDoF is estimated for indoor MIMO mobile scenarios based on measurements, revealing that at least two SDoFs corresponding to two orthogonal polarizations are always available. Furthermore, the SDoF is mostly less than 10 when the antenna aperture size at the base and mobile sides is 3λ², and less than 4 if the aperture size is 0.25λ², revealing feasibility of spatial multiplexing with at most 4 eigenmodes using an electrically small antenna aperture at the mobile end. Significant fluctuation of the SDoF is observed during a mobile is in motion even though there is always a line-of-sight (LOS). The fluctuation is attributed to difference in LOS path dominance over other multipath components and varying angular distribution of the multipaths.