Next-Generation RIS: From Single to Multiple Functions

In this letter, we propose a next-generation reconfigurable intelligent surface (RIS) architecture, termed multi-functional RIS (MF-RIS), which enables signal reflection, refraction and amplification simultaneously to overcome the challenges, such as half-space coverage and double-fading attenuation. In order to investigate the performance of MF-RIS, we aim at maximizing the sum-rate in an MF-RIS-aided non-orthogonal multiple access network with discrete phase shifts, where three practical operating protocols, namely energy splitting (ES), mode switching (MS), and time switching (TS), are studied. Then, an efficient iterative algorithm is developed to tackle the formulated non-convex problem, where the transmit beamforming and MF-RIS coefficients are designed jointly by solving a series of semidefinite programming problems. Simulation results show that: 1) The proposed MF-RIS-assisted NOMA network provides 57% and 41% sum-rate gains than single- and dual-functional RISs, respectively; 2) The TS protocol outperforms the ES and MS counterparts.