The flexible-position multiple-input multiple-output (MIMO), such as fluid antennas and movable antennas, is a promising technology for future wireless communications. This is due to the fact that the positions of antennas at the transceiver and reflector can be dynamically optimized to achieve better channel conditions and, as such, can provide high spectral efficiency (SE) and energy efficiency (EE) gains with fewer antennas. In this article, we introduce the fundamentals of flexibleposition MIMO systems, including hardware design, structure design, and potential applications. We shall demonstrate that flexible-position MIMO, using fewer flexible antennas, can match the channel hardening achieved by a large number of fixed antennas. We will then analyze the SE-EE relationship for flexible-position MIMO and fixed-position MIMO. Furthermore, we will design the optimal trajectory of flexible antennas to maximize system sum SE or total EE at a fixed travel distance of each antenna. Finally, several important research directions regarding flexible-position MIMO communications are presented to facilitate further investigation.

翻译：柔性位置多输入多输出（MIMO）技术（如流体天线和可移动天线）是未来无线通信领域的一项重要技术。这是因为收发器和反射器上天线的位置可以动态优化，以获得更优的信道条件，从而在更少天线数量下实现高频谱效率和能量效率增益。本文介绍了柔性位置MIMO系统的基础知识，包括硬件设计、结构设计和潜在应用。我们将证明，采用较少柔性天线的柔性位置MIMO系统能够达到大量固定天线所实现的信道硬化效果。随后，本文分析了柔性位置MIMO与固定位置MIMO的谱效-能效关系。此外，我们将设计柔性天线的最优运动轨迹，以在每根天线固定移动距离下最大化系统总谱效或总能效。最后，本文提出了柔性位置MIMO通信的几个重要研究方向，以促进进一步探索。