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 problem of high-dimension signal processing and costly energy consumption. Based on the unique near-field characteristics, we propose two categories of the low-complexity design, i.e., antenna selection and power control, to adapt to different cell-free XL-MIMO scenarios and achieve the maximum data rate. For inspiration, several critical future research directions pertaining to green cell-free XL-MIMO systems are presented.

翻译：无蜂窝式大规模多输入多输出（mMIMO）与极大规模MIMO（XL-MIMO）被视为下一代无线通信系统的创新技术。它们在增加自由度数量方面的显著优势引起了广泛关注。本文首先回顾了XL-MIMO系统带来的关键机遇与挑战。随后，我们提出了增强型无蜂窝式XL-MIMO范式，该范式融合多智能体强化学习（MARL），提供了一种分布式策略以应对高维信号处理和高能耗问题。基于独特的近场特性，我们提出了两类低复杂度设计方案（即天线选择与功率控制），以适应不同的无蜂窝式XL-MIMO场景并实现最大数据速率。最后，本文为绿色无蜂窝式XL-MIMO系统指出了若干关键未来研究方向。