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：在最近几代人中,界定了移动通信技术。随着对6G的要求日益增长,我们需要更多程度的自由,从而在MIMO之外取得更多收益。为此,流线天线系统(FAS)已成为利用可重新配置的定位可控天线获得空间多样性的新途径。考虑到在2D流天线表面两端启动一个以上港口的情况,我们采用信息理论方法研究MIMO-FAS的可实现性能极限。首先,我们提出一个称为QR MIMO-FAS的次级最佳计划,通过联合港口选择、传输和接收波形和电力分配来最大限度地提高费率。然后,我们从MIMO-FAS获得最佳多样性和多重交割(DMTT)。我们从DMT中强调,IMO-FAS超越了传统的MIMO系统。此外,我们提出了一个新的性能指标,即$q- outage能力,即Qq$- outage,可以共同考虑MFAS的高比率和超传统概率。通过这一指标,我们提出了MIMFAS。</s>