Beam Alignment for MIMO Fluid Antenna Systems: An Online-Learning Approach

Beam alignment for multiple-input and multipleoutput fluid antenna systems (MIMO-FAS) is studied. Compared with the conventional position-fixed MIMO setups, MIMOFAS leverages flexible beamforming to achieve higher gains. However, realizing these gains typically requires high-complexity channel estimation methods, especially in MIMO scenarios. To overcome this challenge, a channel estimation-free active-sensing framework for beam alignment in MIMO-FAS is proposed. In particular, a new ping-pong protocol is first utilized for full-dimensional beam alignment. Then, a greedy-policy-based method is developed to design the port activation matrices and associated beamforming vectors based on the active-sensing results. Numerical results demonstrate that the proposed activesensing framework can effectively utilize the advantages of FAS over conventional MIMO systems without channel estimations.