RIS-aided MIMO Channel Capacity with Near-Field Channel Model

As wireless networks evolve toward 6G, MIMO and RIS have emerged as key enablers for achieving ultra-high data rates and enhanced spectral efficiency. While existing studies on MIMO-RIS systems often rely on far-field assumptions, nearfield propagation effects become significant at high frequencies and large array sizes, introducing additional spatial degrees of freedom (DoF). In this paper, we investigate the capacity scaling behavior of MIMO-RIS systems under a near-field channel model. Furthermore, we uncover structured phase shift patterns in the optimized RIS configuration, and we analyze the impact of phase quantization and show that while coarse quantization is sufficient in the far-field, it fails to approximate continuous phase shifts in the near-field. The findings of this paper provide critical insights into MIMO-RIS design, offering practical guidelines for optimizing DoF, phase shift computation, and hardware constraints in next-generation wireless systems.