Exploring Frontiers of Polar-Domain Codebooks for Near-Field Channel Estimation and Beam Training: A comprehensive analysis, case studies, and implications for 6G

As sixth-generation (6G) wireless networks approach, leveraging the millimeter-wave (mmWave) and terahertz (THz) bands’ abundant spectrum becomes crucial, promising ultrahigh data rates and enabling immersive communication experiences. This transformation, characterized by the integration of ultramassive multi-in multi-out (UM-MIMO) systems, facilitates significant increases in throughput and capacity by utilizing densely packed antenna arrays. The resulting shift from traditional far-field communication, with its planar wavefronts, to near-field communication, where spherical wavefronts predominate, necessitates a reevaluation of conventional beamforming and channel estimation methods. Effective codebooks, based on the spherical wavefront phenomenon, are vital for both channel estimation and beam focusing, providing predefined sets of beam steering vectors or codewords needed to efficiently probe the channel and direct the beams toward desired directions. This article explores the development of polar-domain codebooks tailored for these near-field conditions. We reveal the exact boundaries of near-field communication, design polar codebooks based on our sparsity analysis findings, and demonstrate their efficacy in channel estimation and beam training without relying on user range information. These codebooks significantly reduce dimensionality, offering a practical solution to the challenges of minimal pilot overhead. Through two case studies, one on channel estimation and the other on beam training using the defined polar-domain codebooks, we illustrate the potential of these methodologies to enhance system performance and spectral efficiency in near-field wireless systems.