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）系统是近几代移动通信的核心技术。随着第六代（6G）通信需求的日益增长，我们需要在MIMO之外提供额外自由度的新技术突破。为此，流体天线系统（FAS）作为一种利用可重构位置切换天线获取空间分集的新方法应运而生。考虑在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，我们提出一种次优方案——QR MIMO-FAS，通过联合端口选择、收发波束赋形与功率分配实现高信噪比（SNR）下的速率最大化。随后推导出MIMO-FAS的最优分集-复用折中（DMT），分析表明MIMO-FAS性能优于传统MIMO天线系统。此外，我们引入一个新的度量指标——q-中断容量，该指标可同时考虑速率与中断概率。基于该指标的评估结果显示，MIMO-FAS相比传统MIMO具有显著优势。