RIS-assisted beamforming for energy efficiency in multiuser downlink transmissions

Reconfigurable intelligent surface (RIS) based reflections is a promising approach to increase spectral efficiency (SE) and reduce power consumption of wireless communications systems. This paper investigates the trade-off between these two metrics by considering the energy efficiency (EE) maximization of an RIS-assisted multiuser downlink transmission from a multiantenna base station (BS) to multiple single-antenna users while satisfying constraints on quality-of-service (QoS), RIS phase shifts, and BS maximum transmit power. We consider a coordinated beamforming scheme and propose a joint optimization procedure based on the Dinkelbach, fractional programming, and semi-definite relaxation (SDR) methods. Simulation results show that the RIS-assisted system is more energy-efficient than its counterpart without RIS and that the RIS, particularly when it is equipped with a large number of antenna elements, can simultaneously improve the SE and power consumption of the transmission. Furthermore, the presented algorithm achieves good-quality solutions that are competitive to the obtained via exhaustive search with branch-reduce-and-bound (BRnB) methods and requires fewer iterations to converge.