EMF-Aware Energy Efficient MU-SIMO Systems With Multiple RISs

In this article, we consider an uplink transmission of a multi-user single-input multiple-output (SIMO) system assisted with multiple reconfigurable intelligent surfaces (RISs). We investigate the energy efficiency (EE) maximization problem with an electromagnetic field (EMF) exposure constraint. In order to solve the problem, we present a lower bound for the EE and adopt an alternate optimization problem. Then, we propose the Energy Efficient Multi-RIS (EEMR) algorithm to obtain the optimal transmit power of the users and phase shifts of the RISs. Moreover, we study this problem for a system with a central RIS and compare the results. The simulation results show that for a sufficient total number of RIS elements, the system with distributed RISs is more energy efficient compared to the system with a central RIS. In addition, for both the systems the EMF exposure constraints enforce a trade-off between the EE and EMF-awareness of the system.