MIMO Evolution Beyond 5G Through Reconfigurable Intelligent Surfaces and Fluid Antenna Systems

TLDR

MIMO systems are advancing, yet growing demands push research beyond them, with reconfigurable intelligent surfaces (RIS) seen as a key technology that can act as an intelligent propagation medium to enhance radio efficiency. This article reviews conventional and advanced RIS architectures—including reflection-based, active/semiactive, and surface-wave-assisted designs—and explores fluid antenna systems, aiming to illustrate their combined potential with MIMO. The authors provide an updated overview of RIS technologies, discuss surface-wave-assisted metasurface designs, and examine fluid antenna technology, showing how these can be integrated with cell-free massive MIMO. The article concludes that integrating MIMO, RIS, and fluid antennas offers promising synergies for ultrahigh capacity, power efficiency, and scalability.

Abstract

With massive deployment, multiple-input–multiple-output (MIMO) systems continue to take mobile communications to new heights, but the ever-increasing demands mean that there is a need to look beyond MIMO and pursue the next disruptive wireless technologies. Reconfigurable intelligent surface (RIS) is widely considered a key candidate technology block to provide the next generational leap. The first part of this article provides an updated overview of the conventional reflection-based RIS technology, which complements the existing literature to include active and semiactive RIS, and the synergies with cell-free massive MIMO (CF mMIMO). Then, we widen the scope to discuss the surface-wave-assisted RIS that represents a different design dimension in utilizing metasurface technologies. This goes beyond being a passive reflector and can use the surface as an intelligent propagation medium for superb radio propagation efficiency. The third part of this article turns the attention to the fluid antenna, a novel antenna technology that enables a diverse form of reconfigurability that can combine with RIS for ultrahigh capacity, power efficiency, and scalability. This article concludes with a discussion of the potential synergies that can be exploited between MIMO, RIS, and fluid antennas.