Beamforming Design for RIS-Aided MIMO Communication: A Piece-Wise Near-Field Model

This work proposes a joint active and passive beam-forming design for reconfigurable intelligent surface (RIS)-aided wireless communication systems. We adopt a piece-wise near-field channel model that leverages the benefits of the traditional near-field approach while enhancing its robustness against channel estimation errors (CEEs). We analyze the impact of different channel models, including the existing near-field, the proposed piece-wise near-field, and far-field channel models, on the interference distribution caused by CEEs and model mismatches. Then, by considering the interference as noise, we formulate a joint active and passive beamforming design problem to maximize the spectral efficiency (SE) and address it exploiting a block coordinate descent (BCD) framework. Simulation results demonstrate that adopting the piece-wise near-field channel model leads to an improved SE compared to both the traditional near-field and far-field models in the presence of CEEs. Additionally, the proposed piece-wise channel model strikes an effective balance between modeling accuracy and system degrees of freedom (DoF).