Physical Bounds and Radiation Modes for MIMO Antennas

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 article, a computationally efficient method for calculating the fundamental performance bound on spectral efficiency is presented. The performance bound is calculated for multiple-input-multiple-output (MIMO) antennas with a radiation efficiency requirement. A convex optimization problem is formulated in the current density of the antenna, this problem is solved by constructing its dual problem. The solution to this problem is based on a set of modes known as the radiation modes. This article shows how to perform modal analysis using these modes to evaluate design strategies for maximal spectral efficiency. This is illustrated both for electrically small and large structures. For electrically small structures, sub-regions for antenna design are evaluated and analyzed. These are compared to $N$ -degree MIMO systems. For electrically large structures, the degrees of freedom per unit area of different shapes are analyzed using radiation modes.