  Massive multiple-input multiple-output (MIMO) for 5G is evolving into the
extremely large-scale antenna array (ELAA) to increase the spectrum efficiency
by orders of magnitude for 6G communications. ELAA introduces
spherical-wave-based near-field communications, where channel capacity can be
significantly improved for single-user and multi-user scenarios. Unfortunately,
the near-field region at large incidence/emergence angles is greatly reduced
with the widely studied uniform linear array (ULA). Thus, many randomly
distributed users may fail to benefit from near-field communications. In this
paper, we leverage the rotational symmetry of uniform circular array (UCA) to
provide uniform and enlarged near-field regions at all angles, enabling more
users to benefit from near-field communications. Specifically, by exploiting
the geometrical relationship between UCA and users, the near-field beamforming
technique for UCA is developed. Based on the analysis of near-field
beamforming, we reveal that UCA is able to provide a larger near-field region
than ULA in terms of the effective Rayleigh distance. Moreover, a
concentric-ring codebook is designed to realize efficient codebook-based
beamforming in the near-field region. In addition, we find out that UCA could
generate orthogonal near-field beams along the same direction when the focal
point of the near-field beam is exactly the zeros of other beams, which has the
potential to further improve spectrum efficiency in multi-user communications
compared with ULA. Simulation results are provided to verify the effectiveness
of theoretical analysis and feasibility of UCA to enable more users to benefit
from near-field communications by broadening the near-field region.
