The advent of multi-user multiple-input multiple-output (MIMO) resulted in space division multiple access (SDMA), enabling concurrent service to multiple users via spatially directed beams. The rise of massive MIMO in fourth generation (4G)/fifth generation (5G) has greatly enhanced SDMA by enabling narrower beams, allowing a larger number of users to be served simultaneously in the same time-frequency slot. As a natural evolution, massive MIMO transitions to extreme MIMO, and larger antenna apertures push typical urban macro cellular areas into the near-field region, invalidating planar wave models and necessitating spherical ones. This shifts multiple access in mobile communications from traditional beamforming to beam focusing. Unlike in the current massive MIMO-based SDMA system, where users are primarily separated by angular bins, in a near-field extreme MIMO system, users can be separated by distance and angular bins. This work advances multiple access strategies by providing the technical foundation for accurate signal focusing within three-dimensional (3D) spatial volumes, called volumetric beam focusing. Specialized near-field beam profiles, notably Bessel beams and the proposed Padé–Bessel beams, facilitate accurate signal focusing within 3D spatial volumes. This work also provides key techniques for implementing volumetric beam focusing with phased antenna arrays and evaluates the performance of Bessel and Padé–Bessel beams across multiple scenarios.
Banerjee et al. (Thu,) studied this question.