Extremely large-scale arrays (XL-arrays) have emerged as a promising technology to achieve super-high spectral efficiency and spatial resolution in future wireless systems. The large aperture of XL-arrays means that spherical rather than planar wavefronts must be considered, and a paradigm shift from far-field to near-field communications is necessary. Unlike existing works that have mainly considered far-field beam management, we study the new near-field beam management for XL-arrays. We first provide an overview of near-field communications and introduce various applications of XL-arrays in both outdoor and indoor scenarios. Then, three typical near-field beam management methods for XL-arrays are discussed: near-field beam training, beam tracking, and beam scheduling. We point out their main design issues and propose promising solutions to address them. Moreover, other important directions in near-field communications are also highlighted to motivate future research.

翻译：超大阵列（XL-array）已成为未来无线系统中实现超高频谱效率和空间分辨率的极具前景的技术。XL-array的大孔径意味着必须考虑球面波前而非平面波前，因此需要从远场通信范式向近场通信转变。与现有主要考虑远场波束管理的工作不同，我们研究了XL-array的新型近场波束管理。我们首先概述了近场通信，并介绍了XL-array在室外和室内场景中的各种应用。然后，讨论了三种典型的XL-array近场波束管理方法：近场波束训练、波束追踪和波束调度。我们指出了它们的主要设计问题，并提出了有前景的解决方案。此外，还强调了近场通信中的其他重要方向，以推动未来研究。