Channel Estimation for 6G Near-Field Wireless Communications: A Comprehensive Survey

Sixth-generation (6G) wireless communication systems are expected to embrace extremely large aperture arrays (ELAAs), novel antenna architectures, and operation in high-frequency bands to meet the rapidly growing demand for data transmission. By increasing the number of antenna elements, ELAAs enable finer spatial resolution and enhanced beamforming capabilities. At these high operating frequencies, an ELAA aperture may span tens or even hundreds of wavelengths, causing the propagation conditions to gradually depart from the conventional far-field assumption, and making spherical-wave effects increasingly prominent. More generally, near-field behavior is jointly determined by the array size relative to the wavelength and the link distance, and it may also arise in short-range deployments. Consequently, near-field propagation is expected to play a key role in 6G systems. In the near-field region, the electromagnetic field impinges on the array with a non-negligible wavefront curvature. Consequently, the channel is parameterized by both angular information and the propagation distance (range) between transmitter and receiver. This additional distance-dependent degree of freedom increases the dimensionality of the channel parameters and alters the structural properties exploited by far-field channel estimators. As a result, straightforward extensions of conventional far-field channel estimation techniques, typically designed to exploit only angular information, to near-field scenarios may lead to significant high computational complexity. These challenges motivate the development of estimation methods tailored to the distinctive characteristics of near-field propagation. This paper provides a comprehensive overview of recent advances in near-field channel estimation. From an electromagnetic-wave perspective, we first delineate the boundary between near- and far-field regions and highlight the fundamental differences in their propagation mechanisms. We then summarize representative ELAA architectures, spherical-wavefront control techniques, emerging near-field applications, and ongoing standardization efforts related to near-field communications. Next, we introduce widely used near-field channel models and contrast them with their far-field counterparts. Finally, we systematically review major estimation techniques under various system configurations, including single- and multiuser, as well as single- and multi-carrier settings, covering both direct channel estimation between the base station and user equipments, and cascaded channel estimation assisted by a reconfigurable intelligent surface. The surveyed techniques reflect different trade-offs among estimation accuracy, complexity, and robustness. Overall, this survey aims to provide technical insights and theoretical foundations for efficient and scalable near-field channel estimation in 6G systems, while also highlighting key challenges and promising future research directions.