Multiple-input multiple-output (MIMO) system has been the defining mobile communications technology in recent generations. With the ever-increasing demands looming towards the sixth generation (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 of all, we propose a suboptimal scheme, referred to as QR MIMO-FAS, to maximize the rate at high signal-to-noise ratio (SNR) 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 antenna systems. Further, we introduce a new 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）系统在近几代移动通信技术中占据核心地位。随着第六代（6G）通信需求的持续增长，我们亟需在MIMO之外获得更多自由度以提升性能。为此，流体天线系统（FAS）作为一种利用可重构位置切换天线实现空间分集的新方案应运而生。针对收发两端在二维流体天线表面上激活多个端口的情形，本文采用信息论方法研究MIMO-FAS的可达性能极限。首先，我们提出一种次优方案（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高信噪比（SNR）下的可达速率。随后，我们推导了MIMO-FAS的最优分集与复用权衡（DMT），结果表明MIMO-FAS的性能优于传统MIMO天线系统。此外，我们引入了一种新指标——$q$中断容量，该指标可同时考虑传输速率与中断概率。基于该指标的分析表明，MIMO-FAS的性能显著超越传统MIMO。