True-time delayers (TTDs) are popular components for hybrid beamforming architectures to combat the spatial-wideband effect in wideband near-field communications. A serial and a hybrid serial-parallel TTD configuration are investigated for hybrid beamforming architectures. Compared to the conventional parallel configuration, the serial configuration exhibits a cumulative time delay through multiple TTDs, which potentially alleviates the maximum delay requirements on the TTDs. However, independent control of individual TTDs becomes impossible in the serial configuration. In this context, a hybrid TTD configuration is proposed as a compromise solution. Furthermore, a power equalization approach is proposed to address the cumulative insertion loss of the serial and hybrid TTD configurations. Moreover, the wideband near-field beamforming design for different configurations is studied for maximizing the spectral efficiency in both single-user and multiple-user systems. 1) For single-user systems, a closed-form solution for the beamforming design is derived. The preferred user locations and the required maximum time delay of each TTD configuration are characterized. 2) For multi-user systems, a penalty-based iterative algorithm is developed to obtain a stationary point of the spectral efficiency maximization problem for each TTD configuration. In addition, a mixed-forward-and-backward (MFB) implementation is proposed to enhance the performance of the serial configuration. Our numerical results confirm the effectiveness of the proposed designs and unveil that i) compared to the conventional parallel configuration, both the serial and hybrid configurations can significantly reduce the maximum time delays required for the TTDs and ii) the hybrid configuration excels in single-user systems, while the serial configuration is preferred in multi-user systems.

翻译：真时延迟器（TTD）是混合波束赋形架构中用于抑制宽带近场通信空间宽带效应的常用组件。本文研究了用于混合波束赋形架构的串行及混合串并行TTD配置。与传统的并行配置相比，串行配置通过多个TTD产生累积时延，这有望降低对TTD最大时延的要求。然而，在串行配置中无法独立控制单个TTD。在此背景下，本文提出一种混合TTD配置作为折中方案。此外，为解决串行与混合TTD配置的累积插入损耗问题，提出了一种功率均衡方法。进一步地，研究了不同配置下最大化单用户与多用户系统频谱效率的宽带近场波束赋形设计：1) 针对单用户系统，推导了波束赋形设计的闭式解，并刻画了各TTD配置的优选用户位置及所需最大时延；2) 针对多用户系统，开发了一种基于惩罚的迭代算法，以获取各TTD配置下频谱效率最大化问题的驻点。同时，提出一种前向-后向混合（MFB）实现方案以增强串行配置的性能。数值结果验证了所提方案的有效性，并揭示：i) 与传统并行配置相比，串行与混合配置均能显著降低TTD所需的最大时延；ii) 混合配置在单用户系统中表现优异，而串行配置更适用于多用户系统。