  Modern antenna design for communication systems revolves around two extremes:
devices, where only a small region is dedicated to antenna design, and base
stations, where design space is not shared with other components. Both imply
different restrictions on what performance is realizable. In this paper
properties of both ends of the spectrum in terms of MIMO performance is
investigated. For electrically small antennas the size restriction dominates
the performance parameters. The regions dedicated to antenna design induce
currents on the rest of the device. Here a method for studying fundamental
bound on spectral efficiency of such configurations is presented. This bound is
also studied for $N$-degree MIMO systems. For electrically large structures the
number of degrees of freedom available per unit area is investigated for
different shapes. Both of these are achieved by formulating a convex
optimization problem for maximum spectral efficiency in the current density on
the antenna. A computationally efficient solution for this problem is
formulated and investigated in relation to constraining parameters, such as
size and efficiency.
