  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 efficiency. The performance of MIMO 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 efficiency is formulated as
a semi-definite 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.
