Low-Complexity Beamforming Design for RIS-Assisted Fluid Antenna Systems

Reconfigurable intelligent surface (RIS) has recently drawn substantial attention toward performance enhancement in wireless communication systems. One of the key challenges in RIS-assisted systems is to design efficient joint beamforming algorithms. However, most existing algorithms rely on instantaneous channel state information (CSI) and iterative optimization methods, which suffer from high computational complexity. Therefore, by exploiting the property of the recent popular fluid antennas, this paper proposes a low-complexity joint beamforming scheme for an RIS-assisted fluid antenna system (RIS-FAS) requiring only statistical CSI. Specifically, by carefully adjusting the FA array at the transmitter side, it is possible to respectively steer its main-lobe (ML) and grating-lobe (GL) towards the user and RIS, where the line-of-sight (LoS) channels from the transmitter to the user and RIS are guaranteed to be identical, allowing independent beamforming for the three cascade channels. Theoretical analysis and simulation results validate that the proposed scheme achieves both high-performance and low-complexity characteristics compared with its conventional counterparts.