Active Reconfigurable Intelligent Surface for Energy Efficiency in MU-MISO Systems.

Reconfigurable intelligent surfaces (RISs) are proposed as a promising solution for future green wireless communications, owing to their ability to manipulate signal propagation environments. However, the severe double path-loss in the RIS's cascaded channel limits its applications in realistic systems. To address this issue, a new RIS architecture, named active RIS, is proposed which can not only reflect but also amplify the incident signals using reflection-type amplifiers embedded in RIS elements. This however may lead to higher energy consumption in practice due to the signal amplification. Motivated by this, we investigate the energy efficiency (EE) in an active RIS-assisted multiuser multiple-input single-output (MU-MISO) communication system. Joint transmit beamforming and energy-included RIS reflection matrix design algorithm is proposed to maximize EE by using quadratic transform based fractional programming (FP) technique. Simulation results verify the effectiveness of the proposed active RIS scheme against all baseline schemes, which indicates that deploying active RIS to assist wireless systems is a promising approach towards green communications.