Cellular MIMO system performance versus base station element spacing

Outdoor MIMO propagation tests were carried out in an urban area in the Danish city of Aalborg in order to investigate some parameters that affect cellular MIMO base-station antennas. We experimentally examined the 2-by-2 MIMO channel responses of base-station arrays with differently-spaced antennas. From these trials, a clear dependence was found between antenna spacing and fading correlation, going from about 0.6 to about 0.1 when the spacing is increased from 1 to 6 wavelengths. However, due to the overall low correlation, the corresponding MIMO capacity gain over SISO capacity only increases from about 1.78 to about 1.87, and thus only a small extra capacity obtained.