Hybrid beamforming and phase shift optimization for energy efficiency with on-off control of RF chain and reflecting elements in 6G RIS-aided communications

Abstract

Reconfigurable Intelligent Surface (RIS) has emerged as a promising low-cost and energy-efficient solution for 6G wireless networks by enabling intelligent control of the wireless propagation environment. However, the energy consumption of RF chains and the large-scale deployment of reflecting elements remain critical challenges for practical RIS-assisted systems. In this paper, we propose a novel Beamforming and Phase-shift Optimization Framework (BPOF) that jointly optimizes hybrid beamforming, RIS phase shift design, and the on-off control of both RF chains and reflecting elements to enhance energy efficiency (EE) in downlink communications. The proposed BPOF scheme is applicable to both passive and active RIS architectures, where active RIS elements can amplify reflected signals to mitigate multiplicative fading effects. To address the non-convexity of the energy efficiency maximization problem, we adopt Dinkelbach’s method and quadratic transformation, combined with an alternating optimization strategy, to efficiently approximate the optimal solution. Simulation results demonstrate that the proposed BPOF scheme achieves a 1.04% improvement in energy efficiency compared to existing baseline approaches, highlighting its potential for future energy-efficient 6G RIS-aided communication systems.