Low-Dimensional Near-Field Codebook Design for Extremely Large-Scale MIMO Systems With Uniform Circular Arrays

The large array aperture of extremely large-scale multiple-input multiple-output (XL-MIMO) systems necessitates the consideration of near-field (NF) spherical wavefront, which significantly increases the beam training codebook size and hence training overhead. In this paper, we propose a new effective low-dimensional NF codebook for uniform circular arrays (UCAs) that is designed to focus solely on the azimuth angle of beams, regardless of distance and elevation angle. Specifically, we first reveal that the NF spherical wavefront channel can be approximated by the product of far-field planar wavefront and a phase residual term. Then, a novel codebook is designed for NF UCA beam training by compensating for the residual NF phase. Numerical simulations and experimental measurements demonstrate that the beam pattern is approximately unaffected by the user distance and elevation angle. Furthermore, these assessments show that the proposed codebook can achieve close beamforming performance with that of existing NF codebooks while with a low dimension of codebook size.