From Single to Multi-Functional RIS: Architecture, Key Technologies, Challenges, and Applications
Wanli Ni, Ailing Zheng, Wen Wang, Dusit Niyato, Naofal Al-Dhahir, Merouane Debbah
TL;DR
The paper tackles the limitations of conventional RISs, notably half-space coverage and double-fading, by introducing MF-RIS, a three-function metasurface capable of reflection, refraction, and amplification to create a full-space, enhanced radio environment. It provides a comprehensive hardware architecture and signal model, where outputs satisfy $\beta_m^r+\beta_m^t \le \beta_{\max}$ and signals are $y_m^r=(\sqrt{\beta_m^r}e^{j\theta_m^r})s_m$, $y_m^t=(\sqrt{\beta_m^t}e^{j\theta_m^t})s_m$, with STAR-RIS and SF-RIS as special cases. The paper then details enabling technologies (operating strategies, CSI methods, robust beamforming, and joint location/rotation optimization) and discusses core challenges (discrete phase shifts, coefficient coupling, static/dynamic trade-offs, two-timescale design, and distributed networking). It surveys seven MF-RIS applications across mmWave, PLS, NTN, cell-free networks, SWIPT, ISAC, and AirComp, and validates performance with a case study showing superior sum-rate performance of MF-RIS over traditional RIS architectures, especially as power budgets and element counts grow. Overall, MF-RIS demonstrates substantial potential to enhance spectral efficiency and network flexibility, offering new degrees of freedom for future 6G systems.
Abstract
Although reconfigurable intelligent surfaces (RISs) have demonstrated the potential to boost network capacity and expand coverage by adjusting their electromagnetic properties, 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 amplification, reflection, and refraction. We begin by detailing the hardware design and signal model that distinguish MF-RIS from traditional RISs. Subsequently, we introduce the key technologies underpinning MF-RIS-aided communications, along with the fundamental issues and challenges inherent to its deployment. We then outline the promising applications of MFRIS in the realm of communication, sensing, and computation systems, highlighting its transformative impact on these domains. Lastly, we present simulation results to demonstrate the superiority of MF-RIS in enhancing network performance in terms of spectral efficiency.