The flexible-position multiple-input multiple-output (FLP-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 FLP-MIMO systems, including hardware design, structure design, and potential applications. We shall demonstrate that FLP-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 FLP-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 FLP-MIMO communications are presented to facilitate further investigation.

翻译：柔性位置多输入多输出（FLP-MIMO），如流体天线和可移动天线，是未来无线通信的一项有前景技术。这是因为收发端和反射器处的天线位置可以被动态优化以获得更好的信道条件，从而能在使用更少天线的情况下实现高频谱效率（SE）和能量效率（EE）增益。本文介绍FLP-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，使用较少柔性天线的FLP-MIMO能够匹配大量固定天线所实现的信道硬化效果。随后分析FLP-MIMO与固定位置MIMO的SE-EE关系。此外，我们将设计柔性天线的最优轨迹，以在每根天线固定行程距离下最大化系统总SE或总EE。最后，提出FLP-MIMO通信的几个重要研究方向以促进进一步探索。