Efficient Beam Training for Fluid Antenna Systems

Fluid antenna systems (FASs) possess the capability to freely adjust their positions within a seamlessly connected spatial domain. The acquisition of channel state information (CSI) is crucial to maintaining optimal antenna placements for these flexible antenna systems. In large-scale antenna systems, beam training is a common method for user position estimation. By scanning different directions in space, users can determine their locations based on received power and provide feedback to the base station, enabling subsequent information transmission. However, as the number of antennas increases, the system can form finer beams, resulting in an expanded set of candidate directions and consequently leading to significant overhead in beam training. To realize effective beam training in FASs, we propose an efficient beam training framework in this paper. Specifically, the beam training framework comprises two phases. In the first phase, the antenna spacing is expanded to several times the half-wavelength, forming directional beams pointing to multiple directions. On the user side, the approximate candidate orientations are efficiently determined based on the received energy levels. In the second phase, building upon the roughly identified candidate directions from the previous phase, a detailed search for user positions is conducted. This process ultimately pinpoints the user locations, allowing secure data transmission by configuring directional beams for transmission. Simulation results reveal that the proposed scheme can realize effective beam training in FASs.