Bandwidth-Constrained Capacity Bounds on MIMO Antennas

The optimal spectral efficiency of MIMO antennas in Rayleigh and ideal 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 this solution is solely dependent on the so-called energy modes of the antenna.
These modes are compared to the characteristic modes and how to effectively excite them is investigated for a linear dipole array as well as a plate with embedded, and raised, antenna regions.
The performance is illustrated through spectral efficiency over the Q-factor, a quantity that is connected to the true capacity. It is demonstrated that the Q-factor and the spectral efficiency form a Pareto trade-off bound, and that a certain Q-factor is Pareto optimal.