  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.
