Cell-Free XL-MIMO Meets Multi-Agent Reinforcement Learning: Architectures, Challenges, and Future Directions

Cell-free massive multiple-input multiple-output (mMIMO) and extremely large-scale MIMO (XL-MIMO) are regarded as promising innovations for the forthcoming generation of wireless communication systems. Their significant advantages in augmenting the number of degrees of freedom have garnered considerable interest. In this article, we first review the essential opportunities and challenges induced by XL-MIMO systems. We then propose the enhanced paradigm of cell-free XL-MIMO, which incorporates multi-agent reinforcement learning (MARL) to provide a distributed strategy for tackling the problems of high-dimensional signal processing and costly energy consumption. Based on the unique near-field characteristics in XL-MIMO systems, we propose two categories of the low-complexity algorithm design, that is, antenna selection and power control, to adapt to different cell-free XL-MIMO scenarios and meet the increasing data rate requirements. For inspiration, several critical future research directions pertaining to green cell-free XL-MIMO systems are presented.