Multiple-input multiple-output (MIMO) system has been the defining mobile communications technology in recent generations. With the ever-increasing demands looming towards 6G, we are in need of additional degrees of freedom that deliver further gains beyond MIMO. To this goal, fluid antenna system (FAS) has emerged as a new way to obtain spatial diversity using reconfigurable position-switchable antennas. Considering the case with more than one ports activated on a 2D fluid antenna surface at both ends, we take the information-theoretic approach to study the achievable performance limits of the MIMO-FAS. First, we propose a suboptimal scheme, referred to as QR MIMO-FAS, to maximize the rate via joint port selection, transmit and receive beamforming and power allocation. We then derive the optimal diversity and multiplexing tradeoff (DMT) of MIMO-FAS. From the DMT, we highlight that MIMO-FAS outperforms traditional MIMO systems. Furthermore, we introduce a new performance metric, namely $q$-outage capacity, which can jointly consider rate and outage probability. Through this metric, our results indicate that MIMO-FAS surpasses traditional MIMO greatly.

翻译：多输入多输出（MIMO）系统已成为近几代移动通信的标志性技术。随着6G时代日益增长的需求，我们需要超越MIMO的额外自由度以获得更大增益。为此，流体天线系统（FAS）作为一种利用可重构位置可切换天线获取空间分集的新兴技术应运而生。针对二维流体天线表面两端激活多个端口的情况，我们采用信息论方法研究MIMO-FAS的可达性能极限。首先，提出一种次优方案（称为QR MIMO-FAS），通过联合端口选择、收发波束赋形与功率分配来最大化速率。随后推导出MIMO-FAS的最优分集-复用权衡（DMT）。DMT表明，MIMO-FAS性能优于传统MIMO系统。此外，我们引入一种新的性能指标——$q$-中断容量，该指标可联合考虑速率与中断概率。通过该指标，我们的结果表明MIMO-FAS显著超越传统MIMO系统。