From Single to Multi-Functional RIS: Architecture, Key Technologies, Challenges, and Applications

Although reconfigurable intelligent surfaces (RISs) have demonstrated the potential to boost network capacity, existing RIS architectures have certain limitations, such as double-fading attenuation and restricted half-space coverage. In this article, we delve into the progressive development from single to multi-functional RIS (MF-RIS) that enables simultaneous signal reflection, refraction, and amplification. We begin by detailing the hardware design and signal model that distinguish MF-RIS from traditional RISs. Subsequently, we introduce the operating strategies and key technologies underpinning MF-RIS-aided communication systems, along with the fundamental issues and challenges inherent to its deployment, such coupled coefficients and distributed networking. We then outline the promising applications of MF-RIS in communication, sensing, and computation systems. Lastly, we present simulation results to demonstrate the superiority of MF-RIS in enhancing network performance under different settings.