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.

翻译：第五代移动通信（5G）中的大规模多输入多输出（MIMO）技术正向着超大规模天线阵列（ELAA）演进，以将第六代（6G）通信的频谱效率提升数个数量级。ELAA引入了基于球面波的近场通信，可在单用户和多用户场景中显著提升信道容量。然而，在广泛研究的均匀线性阵列（ULA）中，大入射/出射角度的近场区域会大幅缩减。因此，许多随机分布的用户可能无法受益于近场通信。本文利用均匀圆形阵列（UCA）的旋转对称性，在所有角度上提供均匀且扩大的近场区域，从而使更多用户受益于近场通信。具体而言，通过利用UCA与用户之间的几何关系，本文开发了适用于UCA的近场波束赋形技术。基于近场波束赋形的分析，我们发现，在有效瑞利距离方面，UCA能提供比ULA更大的近场区域。此外，本文设计了一种同心圆码本，以实现近场区域内基于码本的高效波束赋形。进一步地，我们发现，当近场波束的焦点恰好位于其他波束的零点时，UCA可沿同一方向生成正交近场波束，与ULA相比，这有望进一步提升多用户通信中的频谱效率。仿真结果验证了理论分析的有效性，并证实通过扩大近场区域，UCA能够使更多用户受益于近场通信。