Double RIS-based Hybrid Beamforming Design for MU-MISO mmWave Communication Systems

Abstract

Reconfigurable intelligent surface (RIS), with large-scale passive reflection elements, can be used to assist future wireless communications, which can reduce hardware cost and energy consumption. In this paper, we propose a double RIS-based hybrid beamforming architecture for mutiuser multi-input single-output (MU-MISO) millimeter wave (mmWave) communication system, where one RIS is used to achieve the analog beamforming at the transmitter, while the other is used to aid MU mmWave communication system near the user cluster. Firstly, in order to overcome the complexity of joint optimization of three variables, the RIS at the transmitter can be designed in advance based on the information of arrival of angle (AOA) and departure of angle (DOA). Then, we maximize the weighted sum-rate (WSR) by jointly designing the active and passive beam-forming. This non-convex optimization problem is solved via the alternating optimization framework, where the multidimensional complex quadratic transform (MCQT) is used to reformulate the problem into two quadratically constrained quadratic program (QCQP) sub-problems. Moreover, the semidefinite relaxation (SDR) technique is used to solve the unit-modulus constraints. Simulation results demonstrate that our proposed architecture can enhance the communication performance compared with the existing single-RIS mmWave communication systems.