By flexibly manipulating the radio propagation environment, reconfigurable intelligent surface (RIS) is a promising technique for future wireless communications. However, the single-side coverage and double-fading attenuation faced by conventional RISs largely restrict their applications. To address this issue, we propose a novel concept of multi-functional RIS (MF-RIS), which provides reflection, transmission, and amplification simultaneously for the incident signal. With the aim of enhancing the performance of a non-orthogonal multiple-access (NOMA) downlink multiuser network, we deploy an MF-RIS to maximize the sum rate by jointly optimizing the active beamforming and MF-RIS coefficients. Then, an alternating optimization algorithm is proposed to solve the formulated non-convex problem by exploiting successive convex approximation and penalty-based method. Numerical results show that the proposed MF-RIS outperforms conventional RISs under different settings.

翻译：通过灵活调控无线电传播环境，可重构智能表面（RIS）是未来无线通信中的一项有前景技术。然而，传统RIS面临的单侧覆盖和双衰落衰减问题极大限制了其应用。为解决此问题，我们提出了一种多功能RIS（MF-RIS）的新概念，可同时实现入射信号的反射、传输和放大。为提升非正交多址接入（NOMA）下行链路多用户网络的性能，我们部署MF-RIS，通过联合优化主动波束成形和MF-RIS系数来最大化总速率。随后，提出一种交替优化算法，利用逐次凸近似和基于惩罚项的方法求解所构建的非凸问题。数值结果表明，在不同设置下，所提出的MF-RIS性能优于传统RIS。