Experimental Evaluation of the Effect of BS Antenna Inter-Element Spacing on MU-MIMO Separation

In this paper, multiple-input multiple-output (MIMO) channel measurements in an outdoor micro-cell environment are used to study the effect of base-station (BS) inter-element spacing on multi-user MIMO signal separation. Users with two dual-polarized patch antennas at the mobile station (MS), and four equally-spaced dual-polarized patch antennas at the BS are considered. At the BS, the inter-element spacing (i.e., the spacing between the four dual-polarized patch antennas), is varied from half a wavelength to 8 m. For each BS inter-element spacing, the 4×8 MIMO channels are used to evaluate the system's capability separating closely located users. This evaluation is done by means of the correlation matrix distance metric (between each pair of closely located users) and the sum-rate capacity of the system (when 8 equally-spaced closely located users are served simultaneously). It is found that the system is capable of separating closely located users as long as the distance between them and the BS is less than the Fraunhofer distance associated with the BS antenna array. In the measured environment, for both LOS and NLOS propagation conditions, users located as close as 0.5 m from each other, having the same orientation, are separated successfully in the multi-user MIMO sense.