Optimal Single-Port Impedance Matching for Compact MIMO Arrays

Recent theoretical and simulation studies reveal that closely coupled antennas with appropriately chosen impedance matching loads can yield the desired characteristic of high MIMO capacity. Because of the practical infeasibility of multiport- conjugate matching, the single-port matching impedance (Z_opt) which maximizes the capacity of a 2 times 2 MIMO system with coupled half-wavelength dipoles is derived in this paper. We derive Z_opt for an ergodic capacity upper bound including the mutual coupling and matching networks effect. We show that the closed-form of Z_opt in the high SNR regime is identical to the input impedance. The analytical results are validated by numerical simulations.