Uncoupled Impedance Matching for Capacity Maximization of Compact MIMO Arrays

We extend the study of Fei (IEEE Transactions on Antennas and Propagation, vol. 56, no. 11, pp. 35663575, Nov. 2008) on single-port matching impedance for capacity maximization in compact 2 × 2 multiple-input-multiple-output (MIMO) systems to N × N MIMO systems. In addition, we increase the degrees of freedom of the uncoupled matching where each port is terminated by an arbitrary load impedance. By maximizing an upper bound of the ergodic capacity for a N × N MIMO system with coupling at the receivers in high signal-to-noise ratio (SNR) scheme, a set of equations is formulated to find the optimized load impedance matrix. Input impedance match is proved to be an optimum solution when the MIMO system operates in high SNR scenarios. For three-element arrays, we show numerically that a significant performance improvement can be achieved by introducing input impedance matching. Observations suggest that the achieved improvement varies with array geometry, antenna spacing, and SNR.