Spatial Degrees of Freedom in Small Cells: Measurements with Large Antenna Arrays

In this paper, we study the spatial structure of multiple-input multiple-output (MIMO) links in small urban macro-cells at 2.6 GHz. We resolve the double-directional structure of the radio channel. Also we compute common metrics used to characterize MIMO links, i.e. the structure of singular values and the resulting capacity. In a line-of-sight (LOS) link, we find that local scattering is not enough to create full-rank MIMO channels. Even behind a sector, we observe a low-rank channel. In a non-line-of-sight (NLOS) scenario, although we resolve several multi-paths with individual delay, direction of departure (DoD) and direction of arrival (DoA), the impact of local scattering is limited. Singular values indicate few more degrees of freedom for NLOS channels, but less than for a random matrix. We can model the spatial degrees of freedom in small cells better by assuming a random but small number of nearly specular paths feeding some local scattering widely spread in azimuth and elevation. Based on our results, we discuss modeling aspects and the value of large antenna arrays in mobile networks.