The sixth-generation (6G) wireless systems are expected to adopt extremely large aperture arrays (ELAAs), novel antenna architectures, and operate in extremely high-frequency bands to meet growing data demands. ELAAs significantly increase the number of antennas, enabling finer spatial resolution and improved beamforming. At high frequencies, ELAAs shift communication from the conventional far-field to near-field regime, where spherical wavefronts dominate and the channel response depends on both angle and distance, increasing channel dimensionality. Conventional far-field channel estimation methods, which rely on angular information, struggle in near-field scenarios due to increased pilot overhead and computational complexity. This paper presents a comprehensive survey of recent advances in near-field channel estimation. It first defines the near- and far-field boundary from an electromagnetic perspective and discusses key propagation differences, alongside a brief review of ELAA developments. Then, it introduces mainstream near-field channel models and compares them with far-field models. Major estimation techniques are reviewed under different configurations (single/multi-user, single/multi-carrier), including both direct estimation and RIS-assisted cascaded estimation. These techniques reveal trade-offs among estimation accuracy, complexity, and overhead. This survey aims to provide insights and foundations for efficient and scalable near-field channel estimation in 6G systems, while identifying key challenges and future research directions.

翻译：第六代（6G）无线系统预计将采用极大孔径阵列（ELAA）、新型天线架构，并在极高频段运行，以满足日益增长的数据需求。ELAA显著增加了天线数量，可实现更精细的空间分辨率和改进的波束赋形。在高频段，ELAA使通信从传统的远场转向近场区域，其中球面波前占主导地位，信道响应同时依赖于角度和距离，从而增加了信道维度。传统的依赖于角度信息的远场信道估计方法，由于导频开销和计算复杂度的增加，在近场场景中面临挑战。本文全面综述了近场信道估计的最新进展。首先从电磁学角度定义了近场和远场的边界，并讨论了关键的传播差异，同时简要回顾了ELAA的发展。然后，介绍了主流的近场信道模型，并与远场模型进行了对比。在不同的配置（单用户/多用户、单载波/多载波）下，回顾了主要的估计技术，包括直接估计和RIS辅助的级联估计。这些技术揭示了估计精度、复杂度和开销之间的权衡。本综述旨在为6G系统中高效且可扩展的近场信道估计提供见解和基础，同时指出关键挑战和未来研究方向。