Upper Bounds and Design Guidelines for Realizing Uncorrelated Ports on Multimode Antennas Based on Symmetry Analysis of Characteristic Modes

The design of multiport antennas for multiple-input multiple-output (MIMO) applications utilizing characteristic modes is investigated. For good MIMO performance, uncorrelated antenna ports are generally required. In order to analyze the port correlation, symmetry analysis based on group theory and matrix representations is applied to the theory of characteristic modes. The characteristic surface current densities act as basis functions of the irreducible representations of the symmetry group of the antenna. Current densities belonging to different representations or belonging to different rows of the same representation are orthogonal to each other and can, thus, be excited separately. Therefore, an upper bound for the number of uncorrelated antenna ports can be derived for a given antenna structure based on the symmetry analysis of the characteristic modes. Furthermore, design guidelines on which antenna geometry to choose in order to realize a given number of uncorrelated antenna ports and on how to implement these ports can be deduced. The concept is illustrated by means of examples.