A near-field wideband communication system is studied, wherein a base station (BS) employs an extremely large-scale antenna array (ELAA) to serve multiple users situated within its near-field region. To facilitate the near-field beamfocusing and mitigate the wideband beam split, true-time delayer (TTD)-based hybrid beamforming architectures are employed at the BS. Apart from the fully-connected TTD-based architecture, a new sub-connected TTD-based architecture is proposed for enhancing energy efficiency. Three wideband beamfocusing optimization approaches are proposed to maximize spectral efficiency for both architectures. 1) Fully-digital approximation (FDA) approach: In this approach, the TTD-based hybrid beamformers are optimized to approximate the optimal fully-digital beamformers using block coordinate descent. 2) Penalty-based FDA approach: In this approach, the penalty method is leveraged in the FDA approach to guarantee the convergence to a stationary point of the spectral maximization problem. 3) Heuristic two-stage (HTS) approach: In this approach, the closed-form TTD-based analog beamformers are first designed based on the outcomes of near-field beam training and the piecewise-near-field approximation. 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 unveil that 1) the proposed approaches effectively eliminate the near-field beam split effect, and 2) compared to the fully-connected architecture, the proposed sub-connected architecture exhibits higher energy efficiency and imposes fewer hardware limitations on TTDs and system bandwidth.

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服务于位于其近场区域的多个用户。为实现近场波束聚焦并缓解宽带波束分裂，基站采用了基于真时延器的混合波束赋形架构。除全连接型TTD架构外，本文还提出了一种新型子连接型TTD架构以提升能效。针对两种架构，提出了三种宽带波束聚焦优化方法以最大化频谱效率：1) 全数字逼近方法：该方法通过块坐标下降法优化基于TTD的混合波束赋形器，使其逼近最优全数字波束赋形器；2) 基于惩罚的全数字逼近方法：在该方法中，利用惩罚技术保证频谱最大化问题收敛至驻点；3) 启发式两阶段方法：该方法首先基于近场波束训练结果与分段近场逼近设计闭式TTD模拟波束赋形器，随后利用低维等效信道信息优化低维数字波束赋形器，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近场波束分裂效应；2) 与全连接架构相比，所提子连接架构具有更高能效，并对TTD器件与系统带宽的硬件限制更宽松。