Hybrid Beamforming with Discrete Phase Shifts for RIS-assisted Multiuser SWIPT System

—Reconﬁgurable Intelligent Surfaces (RIS) provides an emerging affordable solution by leveraging massive low-cost reconﬁgurable reﬂective array elements. In this letter, by utilizing low-cost passive reﬂecting elements, a joint hybrid beamforming with discrete phase shifts design is studied in RIS-assisted multiuser simultaneous wireless information and power transfer (SWIPT) systems. However, the formulated sum rate maximization problem is nonconvex and very challenging to solve. To overcome this difﬁculty, we decouple the original maximization problem into the digital-, phase shifts and analog subproblem. Speciﬁcally, for the nonconvexity of the digital sub- problem, we perform the ﬁrst order Taylor series expansion on the objective function, and then an iterative strategy is developed to tackle the difﬁculty of the maximum transmit power constraint imposed by discrete optimization variables. Subsequently, an effective convex optimization technique was employed to optimize reﬂecting elements of RIS. Finally, the greedy selection method based on minimum mean squared error (MMSE) scheme is proposed to solve the analog subproblem. Numerical simulations are conducted to demonstrate the performance advantages of the proposed scheme.