Resource-Efficient Near-Field Misfocus Mitigation in RIS-Assisted Wideband Multi-User Systems

In this paper, we investigate the challenge of beam focusing in near-field communications with reconfigurable intelligent surfaces (RIS), which becomes critical in wideband systems. We propose a low-complexity beam focusing strategy that leverages frequency-independent RIS phase adjustments to achieve a nearly flat channel response across a wide bandwidth. Unlike conventional methods that rely on power-hungry auxiliary components or iterative optimization, our approach exploits the passive nature of RIS to reduce hardware complexity while maintaining beam precision. To support multi-user scenarios, we extend single-user near-field beamforming by introducing a hybrid partitionsuperposition strategy, which enables user-specific phase profile modeling based on spatial configuration. The proposed design facilitates efficient bandwidth allocation and mitigates beam misfocus across all subcarriers. Simulation results demonstrate substantial spectral efficiency gains over baseline methods, confirming the practical effectiveness of our approach for RIS-assisted wideband near-field multi-user systems.