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：本文研究了一种近场宽带通信系统，其中基站（BS）采用超大规模天线阵列（ELAA）为其近场区域内的多个用户提供服务。为了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（TTD）的混合波束成形架构。除了全连接的TTD架构外，本文还提出了一种新的子连接TTD架构以提高能量效率。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）全数字近似（FDA）方法：该方法利用块坐标下降法优化基于TTD的混合波束成形器，使其逼近最优全数字波束成形器。2）基于惩罚项的FDA方法：该方法在FDA中引入惩罚项，确保频谱最大化问题收敛至驻点。3）启发式两阶段（HTS）方法：该方法首先基于近场波束训练结果和分段近场近似，设计闭式TTD模拟波束成形器；随后利用低维等效信道的知识优化低维数字波束成形器，从而降低计算复杂度和信道估计复杂度。数值结果表明：1）所提方法有效消除了近场波束分裂效果；2）与全连接架构相比，所提子连接架构具有更高的能量效率，并对TTD和系统带宽的硬件限制更少。