Energy-Efficient RIS-Aided Cell-Free Massive MIMO Systems: Application, Opportunities, and Challenges

Reconfigurable intelligent surfaces (RIS)-assisted cell-free massive multiple-input multiple-output (CF mMIMO) systems have emerged as a promising technology for sixth-generation communication systems. These systems capitalize on RIS to minimize power consumption, thereby achieving consistent performance and enhancing communication quality through the establishment and shaping of auxiliary signal propagation pathways between access points (APs) and users. However, integrating RIS into existing CF mMIMO infrastructures presents several technical challenges. This study delves into the signal transmission scheme and deployment architecture of RIS-aided CF mMIMO systems, addressing inherent challenges such as interference induced by RIS and the increased complexity in beam alignment. Furthermore, we address the complexities arising from the joint optimization of the reflection phase of RIS and beamforming technology at the APs, intending to fully exploit the reflection capabilities of RISs and beamforming technology to maximize the energy efficiency (EE) of the system. To overcome these challenges, we propose cooperation communication to suppress RIS-induced interference, beam tracking, and joint optimization to improve system EE. We also present specific examples of cooperative communication under the constraint of electromagnetic interference and the beam tracking of a mobile system. Additionally, we emphasize important research directions for RIS-aided CF mMIMO systems, aiming to inspire future investigations.