This article focuses on the near-field effect in terahertz (THz) communications and sensing systems. By equipping with extremely large-scale antenna arrays (ELAAs), the near-field region in THz systems can be possibly extended to hundreds of meters in proximity to THz transceivers, which necessitates the consideration of near-field effect in the THz band both for the communications and sensing. We first review the main characteristics of the near-field region in the THz bands. The signal propagation in the near-field region is characterized by spherical waves rather than planar waves in the far-field region. This distinction introduces a new distance dimension to the communication and sensing channels, which brings new opportunities and challenges for both THz communications and sensing. More particularly, 1) For THz communications, the near-field effect enables a new mechanism for beamforming, namely, beamfocusing, in the focusing region. Furthermore, in THz multiple-input and multiple-output (MIMO) systems, the near-field effect can be exploited to combat the multiplexing gain degradation caused by the sparse THz channels. To address the near-field beam split effect caused by the conventional frequency-independent hybrid beamforming architecture in THz wideband communications, we propose a pair of wideband beamforming optimization approaches by a new hybrid beamforming architecture based on true-time-delayers (TTDs). 2) For THz sensing, joint angle and distance sensing can be achieved in the near-field region. Additionally, the near-field beam split becomes a beneficial effect for enhancing the sensing performance by focusing on multiple possible target locations rather than a drawback encountered in communications. Finally, several topics for future research are discussed.

翻译：本文聚焦于太赫兹通信与感知系统中的近场效应。通过配备超大规模天线阵列，太赫兹系统中的近场区域可能扩展至收发机附近数百米范围，这使得在太赫兹频段进行通信与感知时必须考虑近场效应。我们首先回顾了太赫兹频段近场区域的主要特征。近场区域的信号传播以球面波而非远场区域的平面波为特征。这一区别为通信和感知信道引入了新的距离维度，为太赫兹通信与感知带来了新的机遇和挑战。具体而言：1）对于太赫兹通信，近场效应在聚焦区域实现了新的波束赋形机制，即波束聚焦。此外，在太赫兹多输入多输出系统中，可利用近场效应缓解稀疏太赫兹信道造成的复用增益退化问题。针对太赫兹宽带通信中传统频率无关混合波束赋形架构引发的近场波束分裂效应，我们提出了一种基于真延时器的混合波束赋形新架构，并开发了两类宽带波束赋形优化方案。2）对于太赫兹感知，近场区域可实现角度与距离的联合感知。此外，相较于通信场景中的缺点，近场波束分裂可通过聚焦多个潜在目标位置成为增强感知性能的有利效应。最后，讨论了若干未来研究方向。