Characteristic Mode-Inspired Advanced Multiple Antennas: Intuitive insight into element-, interelement-, and array levels of compact large arrays and metantennas

This article, as part of the special issue on characteristic modes (CMs), updates the recent progress in developing advanced multiple antenna systems based on CM analysis (CMA). The multiple antennas include antenna arrays of multiple radiating elements fed by one single signal port, arrays of multiple antennas fed by multiple signal ports simultaneously, and combinations thereof. The challenges, opportunities, and the way in which CMA assists in the design of antenna elements, multiple antennas, and antenna arrays are addressed, including physical insights and dedicated advanced CMA methods. Two types of advanced multiple antenna systems inspired by the CMs are discussed as examples. One is the multimode, multiport antenna (M³PA). The other is the CMA-enabled metasurface (MTS) antenna (metantenna). Both types can serve as unit cells of massive multiple-input, multiple-output (MIMO) antennas.