Pattern-Reconfigurable Fluid RIS (FRIS)-Enhanced Wireless Networks

This paper proposes a novel pattern-reconfigurable fluid reconfigurable intelligent surface (FRIS) framework where each fluid element can dynamically modify its radiation pattern based on channel conditions. To explore the potential of the pattern-reconfigurable FRIS, we extend its application to a multi-user communication scenario. In this context, the spherical harmonics orthogonal decomposition (SHOD) method is employed to accurately model the radiation patterns of the fluid elements, enabling the pattern design process more tractable. Subsequently, an optimization problem with the aim of maximizing the weighted sum rate among users is formulated through jointly designing the active beamforming and spherical harmonics coefficients, subject to power budget and radiation pattern energy constraints. To address this non-convex optimization problem, an iterative based minimum mean-square error (MMSE) method and Riemannian conjugate gradient (RCG) method is proposed to alternatively solve the active beamforming and spherical harmonics coefficients. Simulation results reveal that the proposed pattern-reconfigurable FRIS significantly outperforms traditional RIS using 3GPP 38.901 and isotropic radiation patterns, achieving average performance enhancement of 161.5% and 176.2%, respectively.