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 based on full channel state information (CSI): 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 based on partial CSI: In this approach, a piecewise-near-field approximation of near-field channels is first proposed to facilitate the design of TTD-based analog beamformers based on the outcomes of near-field beam training. Subsequently, the low-dimensional digital beamformer is optimized using knowledge of 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：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其近场区域内的多个用户提供服务。为支持近场多用户波束成形并缓解空间宽带效应，基站采用了基于真实时延器（TTD）的混合波束成形架构。除传统的全连接TTD混合波束成形架构外，还提出了一种新的子连接架构以提高能效并降低硬件需求。针对两种架构，提出了两种最大化频谱效率的宽带波束成形优化方法：1）基于全信道状态信息的全数字近似方法：该方法通过块坐标下降法和惩罚函数法优化TTD混合波束形成器，使其逼近最优数字波束形成器，并确保收敛到频谱效率最大化问题的驻点。2）基于部分信道状态信息的启发式两阶段方法：该方法首先提出近场信道的分段近场近似，基于近场波束训练结果指导TTD模拟波束形成器设计；随后利用低维等效信道信息优化低维数字波束形成器，从而降低计算复杂度和信道估计复杂度。数值结果表明：1）所提方法有效消除空间宽带效应；2）与全连接架构相比，所提子连接架构具有更高能效，且对TTD和系统带宽的硬件约束更少。