Fundamental Bounds on MIMO Antennas

Antenna current optimization is often used to analyze the optimal performance of antennas. Antenna performance can be quantified in, e.g., minimum Q-factor and radiation efficiency. The performance of multiple-input-multiple-output antennas is more involved and, in general, a single parameter is not sufficient to quantify it. Here, the capacity of an idealized channel is used as the main performance quantity. An optimization problem in the current distribution for optimal capacity, measured in spectral efficiency, given a fixed Q-factor and radiation efficiency is formulated as a semidefinite optimization problem. A model order reduction based on characteristic and energy modes is employed to improve the computational efficiency. The performance bound is illustrated by solving the optimization problem numerically for rectangular plates and spherical shells.