The beamforming performance of the uniform circular array (UCA) in near-field wideband communication systems is investigated. Compared to uniform linear array (ULA), UCA exhibits uniform effective array aperture in all directions, thus enabling more users to benefit from near-field communications. In this paper, the unique beam squint effect in near-field wideband UCA systems is comprehensively analyzed in both the distance and angular domains. It is rigorously demonstrated that the beam focal point only exists at a specific frequency in wideband UCA systems, resulting in significant beamforming loss. To alleviate this unique beam squint effect, the true-time delay (TTD)-based beamforming architecture is exploited. In particular, two wideband beamforming optimization approaches leveraging TTD units are proposed. 1) Analytical approach: In this approach, the phase shifters (PSs) and the time delay of TTD units are designed based on the analytical formula for beamforming gain. Following this design, the minimum number of TTD units required to achieve a predetermined beamforming gain is quantified. 2) Joint-optimization approach: In this method, the PSs and the TTD units are jointly optimized under practical maximum delay constraints to approximate the optimal unconstrained analog beamformer. Specifically, an efficient alternating optimization algorithm is proposed, where the PSs and the TTD units are alternately updated using either the closed-form solution or the low-complexity linear search approach. Extensive numerical results demonstrate that 1) the proposed beamforming schemes effectively mitigate the beam squint effect, and 2) the joint-optimization approach outperforms the analytical approach in terms of array gain and achievable spectral efficiency.

翻译：研究了均匀圆阵在近场宽带通信系统中的波束成形性能。相较于均匀线阵，均匀圆阵在所有方向上具有均匀的有效阵列孔径，因此能使更多用户受益于近场通信。本文从距离域和角度域两个维度，系统分析了近场宽带均匀圆阵系统中特有的波束斜视效应，严格论证了宽带均匀圆阵系统的波束聚焦点仅存在于特定频率，从而导致显著的波束成形损失。为缓解这一独特的波束斜视效应，本文采用基于真时延的波束成形架构，并提出了两种利用真时延单元的宽带波束成形优化方法：1）解析法：基于波束成形增益解析公式设计移相器和真时延单元时延量，并量化了达到预定波束成形增益所需的最小真时延单元数量；2）联合优化法：在实际最大时延约束下，通过联合优化移相器和真时延单元来逼近最优无约束模拟波束成形器。具体而言，提出了一种高效交替优化算法，采用闭式解或低复杂度线性搜索方法交替更新移相器和真时延单元参数。大量数值结果表明：1）所提波束成形方案能有效抑制波束斜视效应；2）联合优化法在阵列增益和可达频谱效率方面均优于解析法。