# Physical bounds and radiation modes for MIMO antennas

## TL;DR

This paper investigates fundamental limits of MIMO antenna performance for both small and large structures, using convex optimization to analyze spectral efficiency bounds based on size, shape, and efficiency constraints.

## Contribution

It introduces a convex optimization framework to determine spectral efficiency bounds for electrically small and large MIMO antennas, considering physical and design restrictions.

## Key findings

- Spectral efficiency bounds depend on antenna size and shape.
- Efficient computational methods for optimization are developed.
- Performance limits vary significantly between small and large antenna regimes.

## Abstract

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.