Experiments with compact antenna arrays for MIMO radio communications

Realizing antenna arrays that preserve channel capacity in multiple-input multiple-output (MIMO) radio communications is a challenge because of the detrimental effects of mutual coupling. The fact that mutual coupling becomes more pronounced as antenna spacing decreases is therefore a critical issue with compact arrays used in portable MIMO radios. The contributions made in this paper include the realization of compact MIMO arrays designed to preserve channel capacity and the characterization of these compact arrays' performance in field measurements using MIMO radios. A multiport metric is used to characterize the compact arrays in a way that more appropriately measures array performance under MIMO signalling conditions than the traditional scattering matrix characterization. Field measurements show the extent and proportion of the effect that correlation and radiation efficiency have on MIMO channel capacity. The compact arrays' strong performance in field measurements is explained by their ability to preserve the MIMO channel's eigenstructure. Results also show that mutual coupling prevents channel capacity from scaling linearly with the number of antennas. Guidelines for compact MIMO array design are given with supporting results from field measurements. Index Terms--Antenna arrays, channel capacity, mobile antennas, multiple-input multiple-output (MIMO) systems.