A near-field wideband communication system is investigated in which a base station (BS) employs an extra-large scale antenna array (ELAA) to serve multiple users in its near-field region. To facilitate near-field multi-user beamforming and mitigate the spatial wideband effect, the BS employs a hybrid beamforming architecture based on true-time delayers (TTDs). In addition to the conventional fully-connected TTD-based hybrid beamforming architecture, a new sub-connected architecture is proposed to improve energy efficiency and reduce hardware requirements. Two wideband beamforming optimization approaches are proposed to maximize spectral efficiency for both architectures. 1) Fully-digital approximation (FDA) approach: In this method, the TTD-based hybrid beamformer is optimized by the block-coordinate descent and penalty method to approximate the optimal digital beamformer. This approach ensures convergence to the stationary point of the spectral efficiency maximization problem. 2) Heuristic two-stage (HTS) approach: In this approach, the analog and digital beamformers are designed in two stages. In particular, two low-complexity methods are proposed to design the high-dimensional analog beamformers based on approximate and exact line-of-sight channels, respectively. Subsequently, the low-dimensional digital beamformer is optimized based on the low-dimensional equivalent channels, resulting in reduced computational complexity and channel estimation complexity. Our numerical results show that 1) the proposed approach effectively eliminates the spatial wideband effect, and 2) the proposed sub-connected architecture is more energy efficient and has fewer hardware constraints on the TTD and system bandwidth compared to the fully-connected architecture.

翻译：本文研究了一种近场宽带通信系统，其中基站采用超大规模天线阵列为近场区域内的多个用户提供服务。为促进近场多用户波束成形并缓解空间宽带效应，基站采用基于真实时延器的混合波束成形架构。除传统的全连接式基于真实时延器的混合波束成形架构外，本文提出一种新型子连接架构以提高能效并降低硬件需求。针对两种架构，提出了两种宽带波束成形优化方法以最大化频谱效率：1）全数字逼近法：该方法通过块坐标下降法和惩罚函数法优化基于真实时延器的混合波束成形器，以逼近最优数字波束成形器，确保收敛至频谱效率最大化问题的稳定点；2）启发式两阶段法：该方法分两阶段设计模拟与数字波束成形器，特别提出了两种低复杂度方法，分别基于近似与精确视距信道设计高维模拟波束成形器，随后基于低维等效信道优化低维数字波束成形器，从而降低计算复杂度与信道估计复杂度。数值结果表明：1）所提方法能有效消除空间宽带效应；2）相较于全连接架构，所提子连接架构具有更高能效，且对真实时延器与系统带宽的硬件约束更少。