Fluid Antenna Systems Enabled by Reconfigurable Holographic Surfaces: Beamforming Design and Experimental Validation

Fluid antenna systems (FASs) have emerged as a promising antenna technology for 6G networks by tapping into new degrees of freedom (DoF) in antenna positions to harness multiplexing gains. In this paper, we propose an implementation of FASs enabled by reconfigurable holographic surfaces (RHSs) and construct a 384-element prototype. Benefiting from its amplitude-modulation capabilities, RHSs can adjust the antenna positions by activating different subsets of elements. However, due to unknown user locations, multiple antenna ports, and varying channel environments, beamforming schemes relying on acquiring accurate channel state information (CSI) suffer high complexity. To avoid CSI acquisition, we design a low-overhead fluid beam training scheme for RHS-enabled FAS. Unlike fixed-position antennas, fluid beam training employs different antenna positions via element activation, namely sliding windows, for each codeword to improve the channel quality, thereby enhancing the received signal strength (RSS). Such an element activation method is also applied to reprogram the effective array aperture, hence the fluid beam training adopts a hierarchical structure where the beamwidth of codewords narrows across layers. Experimental and simulation results verify the variation of RSS with sliding windows. Compared to traditional schemes, the proposed fluid beam training utilizing sliding windows achieves higher training accuracy and data rates.