Beamforming design for RIS-aided MIMO communication: a piece-wise near-field model

This work proposes a joint active and passive beamforming 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 nearfield 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 piecewise 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).