Multi-Functional RIS Integrated Sensing and Communications for 6G Networks

• Published in: IEEE transactions on wireless communications Vol. 24; no. 2; pp. 1146 - 1161
• Main Authors: Han, Dongsheng, Wang, Peng, Ni, Wanli, Wang, Wen, Zheng, Ailing, Niyato, Dusit, Al-Dhahir, Naofal
• Format: Journal Article
• Language: English
• Published: New York IEEE 01.02.2025; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Array signal processing; Beamforming; beamforming design; Budgets; Communication; Hardware; Integrated sensing and communication; Interference; ISAC; Iterative algorithms; joint optimization; multi-functional RIS; Optimization; Phase shifters; Protocols; Radar; Reconfigurable intelligent surfaces; Reflection; Refraction; Signal reflection; Signal to noise ratio; Simulation; Wireless communication
• ISSN: 1536-1276; 1558-2248
• DOI: 10.1109/TWC.2024.3505918

In this paper, we propose a novel multi-functional reconfigurable intelligent surface (MF-RIS) that supports signal reflection, refraction, amplification, and target sensing simultaneously. Our MF-RIS aims to enhance integrated communication and sensing (ISAC) systems, particularly in multi-user and multi-target scenarios. Equipped with reflection and refraction components (i.e., amplifiers and phase shifters), MF-RIS is able to adjust the amplitude and phase shift of both communication and sensing signals on demand. Additionally, with the assistance of sensing elements, MF-RIS is capable of capturing the echo signals from multiple targets, thereby mitigating the signal attenuation typically associated with multi-hop links. We propose a MF-RIS-enabled multi-user and multi-target ISAC system, and formulate an optimization problem to maximize the signal-to-interference-plus-noise ratio (SINR) of sensing targets. This problem involves jointly optimizing the transmit beamforming and MF-RIS configurations, subject to constraints on the communication rate, total power budget, and MF-RIS coefficients. We decompose the formulated non-convex problem into three sub-problems, and then solve them via an efficient iterative algorithm. Simulation results demonstrate that: 1) The performance of MF-RIS varies under different operating protocols, and energy splitting (ES) exhibits the best performance in the considered MF-RIS-enabled multi-user multi-target ISAC system; 2) Under the same total power budget, the proposed MF-RIS with ES protocol attains <inline-formula> <tex-math notation="LaTeX">\rm {52.2}\% </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">\rm {73.5}\% </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">\rm {60.86}\% </tex-math></inline-formula> sensing SINR gains over active RIS, passive RIS, and simultaneously transmitting and reflecting RIS (STAR-RIS), respectively; 3) The number of sensing elements will no longer improve sensing performance after exceeding a certain number.