RIS-aided Near-Field MIMO Communications: Codebook and Beam Training Design

TL;DR

This paper introduces novel codebook designs and hierarchical beam training schemes for RIS-assisted near-field and hybrid-far-near-field MIMO systems, significantly reducing training overhead and improving achievable rates by leveraging distance and angular information.

Contribution

It proposes new distance-based and combined angular-distance codebooks with corresponding beam training schemes for near-field and hybrid channels, enhancing efficiency and performance.

Findings

Beam training schemes achieve near-optimal performance with less overhead.

Incorporating distance information improves achievable rate in near-field MIMO.

Proposed algorithms effectively optimize RIS coefficients and beamforming matrices.

Abstract

Downlink reconfigurable intelligent surface (RIS)-assisted multi-input-multi-output (MIMO) systems are considered with far-field, near-field, and hybrid-far-near-field channels. According to the angular or distance information contained in the received signals, 1) a distance-based codebook is designed for near-field MIMO channels, based on which a hierarchical beam training scheme is proposed to reduce the training overhead; 2) a combined angular-distance codebook is designed for mixed-far-near-field MIMO channels, based on which a two-stage beam training scheme is proposed to achieve alignment in the angular and distance domains separately. For maximizing the achievable rate while reducing the complexity, an alternating optimization algorithm is proposed to carry out the joint optimization iteratively. Specifically, the RIS coefficient matrix is optimized through the beam training process, the optimal combining matrix is obtained from the closed-form solution for the mean square error (MSE) minimization problem, and the active beamforming matrix is optimized by exploiting the relationship between the achievable rate and MSE. Numerical results reveal that: 1) the proposed beam training schemes achieve near-optimal performance with a significantly decreased training overhead; 2) compared to the angular-only far-field channel model, taking the additional distance information into consideration will effectively improve the achievable rate when carrying out beam design for near-field communications.