As a promising technique, extremely large-scale (XL)-arrays offer potential solutions for overcoming the severe path loss in millimeter-wave (mmWave) and TeraHertz (THz) channels, crucial for enabling 6G. Nevertheless, XL-arrays introduce deviations in electromagnetic propagation compared to traditional arrays, fundamentally challenging the assumption with the planar-wave model. Instead, it ushers in the spherical-wave (SW) model to accurately represent the near-field propagation characteristics, significantly increasing signal processing complexity. Fortunately, the SW model shows remarkable benefits on sensing and communications (S\&C), e.g., improving communication multiplexing capability, spatial resolution, and degrees of freedom. In this context, this article first overviews hardware/algorithm challenges, fundamental potentials, promising applications of near-field S\&C enabled by XL-arrays. To overcome the limitations of existing XL-arrays with dense uniform array layouts and improve S\&C applications, we introduce sparse arrays (SAs). Exploring their potential, we propose XL-SAs for mmWave/THz systems using multi-subarray designs. Finally, several applications, challenges and resarch directions are identified.

翻译：作为一种有前景的技术，超大规模阵列（XL-arrays）为克服毫米波（mmWave）和太赫兹（THz）信道中的严重路径损耗提供了潜在解决方案，这对实现6G至关重要。然而，与传统阵列相比，XL-arrays在电磁传播中引入了偏差，从根本上挑战了平面波模型的假设。取而代之的是，球面波（SW）模型被引入以准确表征近场传播特性，这显著增加了信号处理复杂度。幸运的是，SW模型在感知与通信（S&C）方面展现出显著优势，例如提升通信复用能力、空间分辨率和自由度。在此背景下，本文首先概述了由XL-arrays驱动的近场S&C所面临的硬件/算法挑战、基本潜力及典型应用。为突破现有密集均匀阵列布局的XL-arrays的局限性并改进S&C应用，我们引入了稀疏阵列（SAs）。通过探索其潜力，我们提出了基于多子阵设计的用于毫米波/太赫兹系统的超大规模稀疏阵列（XL-SAs）。最后，确定了若干应用、挑战及研究方向。