Achievable Rate Optimization of the RIS-Aided Near-Field Wideband Uplink

In this work, we investigate the performance of reconfigurable intelligent surface (RIS) assisted near-field wideband system. By considering the large-scale effect of a high-dimensional RIS and frequency-selective channels, we derive an accurate array manifold of the RIS in the near-field from the scattering point of view. Subsequently, we conceive a near-optimal RIS phase design for a single-user scenario to alleviate the beam-squint effect of the wideband system. As for the multi-user case, we provide a virtual-subarray-based phase shift design, which mitigates the beam-squint effect as well as the mitigates deleterious effects of beam concentration. Numerical results show that the achievable data rate can be significantly improved by the proposed schemes compared to the benchmarks both in the single-user and multi-user cases. Explicitly, in the multi-user system, by leveraging the virtual-subarray-based phase design, the achievable sum-rate can be doubled compared to the conventional benchmarks.