Flexible MIMO for Future Wireless Communications: Which Flexibilities are Possible?

TL;DR

This paper introduces a taxonomy for flexible MIMO technologies that adapt physical configurations and operations to enhance future wireless networks, addressing limitations of static array designs in dynamic environments.

Contribution

It systematically categorizes twelve flexible MIMO technologies, discusses their characteristics, enablers, and presents case studies illustrating new design possibilities.

Findings

Flexible MIMO enables adaptable antenna configurations.

Enablers include AI-driven optimization and efficient CSI schemes.

Case studies demonstrate potential for next-gen wireless networks.

Abstract

In conventional multiple-input multiple-output (MIMO), static array configurations struggle in dynamic environments, and further antenna scaling is bounded by cost, energy, and footprint. Emerging approaches, which can enable next-generation wireless communication networks with modest spectrum availability by leveraging flexibility and adaptability rather than sheer array growth, are therefore needed. In this paper, we present a taxonomy framework, referred to as flexible MIMO technology, that systematically categorizes a wide range of evolving MIMO technologies. The focus is on MIMO technologies with flexible physical configurations and integrated applications. We categorize twelve representative flexible MIMO technologies into three major classifications: flexible deployment characteristics-based, flexible geometry characteristics-based, and flexible real-time modifications-based. We then comprehensively overview their fundamental characteristics, potential, and challenges. In addition, we highlight three vital enablers for flexible MIMO technology, including efficient channel state information acquisition schemes, low-complexity beamforming design, and explainable artificial intelligence (AI)-enabled optimization, and discuss eight representative sub-techniques. Finally, two brief case studies -- pre-optimized irregular array for high-speed railway network and cell-free movable antenna -- are presented, showing how flexible MIMO can open new design possibilities and inspire future research directions for next-generation wireless networks.