How Much Power Is Needed for RIS to Beat Relays? A Sustainability Framework

Future wireless networks are expected to utilise reconfigurable intelligent surfaces (RISs) to enhance coverage and communication quality. However, to ensure efficient RIS deployment and operation, it is crucial to understand the operational costs of RIS-assisted communication systems. This study aims to investigate the potential of RIS integration to improve the sustainability and energy efficiency of wireless communication networks, focusing on quantifying and minimising the operational cost of RIS-assisted communications. We present an in-depth analysis of power dissipation in RIS circuitry, emphasising both static and dynamic elements and offering a thorough breakdown of the operational costs associated with RIS-assisted communications. Our study examines control circuit components, control boards, and other factors contributing to dynamic power dissipation. We also develop a precise optimisation problem that addresses RIS power dissipation, aiming to minimise total power consumption in RIS-assisted communication systems while meeting essential communication constraints. In particular, we consider the rate requirements and the phase shift constraints, which enables our optimisation approach to be both theoretically valid and applicable to practical network scenarios. To address the optimisation problem effectively, we have developed a robust algorithm based on the particle swarm optimisation approach (PSO). This algorithm chooses the optimum number of RIS elements and adjusts the phase shifts matrix to reduce power dissipation while maintaining the channel gain required to achieve service requirements. Also, we present a comparison with active solutions in terms of power requirements, spectral efficiency, and environmental impact. Our analysis provides valuable insights into the advantages of RIS deployment, especially at higher data rates. It also highlights the importance of considering alternative relay techniques when tailoring wireless communication systems to specific rate requirements and operating conditions.