Multiple RIS-Assisted Multisensor Wireless Communication Network: Downlink Spectral Efficiency Maximization Utilizing Secondary Reflections

Wireless sensor networks and reconfigurable intelligent surfaces (RISs) have attracted extensive exploration. In this article, a multiple RIS-assisted multisensor wireless communication network is investigated to maximize the downlink spectral efficiency (SE) of the system, which considers the full range of secondary reflections between RISs. The SE maximization problem is presented and optimized by cooperatively designing transmit precoding of the base station (BS) and reflecting matrix (or passive beamforming) of each RIS, where the Lagrangian dual transform and the quadratic transform are utilized to solve this nonconvex problem. The problem is reformulated and then divided into several subproblems corresponding to decision variables. The channel state information (CSI) model is reconstructed to decouple the phase shifts of each element on RISs. Then, an alternating optimization (AO) algorithm with derived closed-form expression of decision variables is proposed to reduce the complexity of the algorithm. Moreover, simulation results verify that the SE of the proposed multisensor communication system considering sufficient secondary reflections is roughly $1.4\times $ higher than that of the existing systems when the transmit power is 60 dBm and the numbers of the sensors and the elements on RISs are 4 and 64, respectively.