  The optimal spectral efficiency and number of independent channels for MIMO
antennas in isotropic multipath channels are investigated when bandwidth
requirements are placed on the antenna. By posing the problem as a convex
optimization problem restricted by the port Q-factor a semi-analytical
expression is formed for its solution. The antennas are simulated by method of
moments and the solution is formulated both for structures fed by discrete
ports, as well as for design regions characterized by an equivalent current. It
is shown that the solution is solely dependent on the eigenvalues of the
so-called energy modes of the antenna. The magnitude of these eigenvalues is
analyzed for a linear dipole array as well as a plate with embedded antenna
regions. The energy modes are also compared to the characteristic modes to
validate characteristic modes as a design strategy for MIMO antennas. The
antenna performance is illustrated through spectral efficiency over the
Q-factor, a quantity that is connected to the capacity. It is proposed that the
number of energy modes below a given Q-factor can be used to estimate the
degrees of freedom for that Q-factor.
