Near-Field User Localization and Channel Estimation for XL-MIMO Systems: Fundamentals, Recent Advances, and Outlooks

TL;DR

XL-MIMO enables depth-aware localization and enhanced channel estimation by exploiting radiative near-field effects, including spherical-wave propagation, finite-depth beamfocusing, and spatial non-stationarity. The paper surveys fundamental near-field EM properties and reviews state-of-the-art localization and channel-estimation algorithms such as MUSIC, MLE, polar-domain CS, GTBC, Turbo-OAMP, and DL-based methods, supported by illustrative case studies. It highlights key challenges—rank-deficient channels, non-sparse representations, and VR-induced non-stationarity—and outlines three forward-looking directions: refined EM models, hardware distortion-aware designs, and an electromagnetic information-theoretic framework to guide capacity-focused XL-MIMO design. The work emphasizes the potential of ISAC and advanced signal processing to enhance localization accuracy and spectrum efficiency in 6G deployments, with fundamental DoFs for planar XL-MIMO characterized by $\pi \cdot \textrm{Area} / \lambda^2$ guiding capacity-oriented design at upper mid-band.

Abstract

Extremely large-scale multiple-input multipleoutput (XL-MIMO) is believed to be a cornerstone of sixth-generation (6G) wireless networks. XL-MIMO uses more antennas to both achieve unprecedented spatial degrees of freedom (DoFs) and exploit new electromagnetic (EM) phenomena occurring in the radiative near-field. The near-field effects provide the XL-MIMO array with depth perception, enabling precise localization and spatially multiplexing jointly in the angle and distance domains. This article delineates the distinctions between near-field and far-field propagation, highlighting the unique EM characteristics introduced by having large antenna arrays. It thoroughly examines the challenges these new near-field characteristics pose for user localization and channel estimation and provides a comprehensive review of new algorithms developed to address them. The article concludes by identifying critical future research directions.

Figures (4)

• Figure 1: The EM characteristics comparison of near-field and far-field, where the widely adopted boundary is the Fraunhofer/Rayleigh distance. The central carrier frequency is $100$ GHz. We set the number of BS antennas for near-field and far-field scenarios to $512$ and $128$, respectively.
• Figure 2: The several new challenges introduced by the three new features of radiative near-field propagation.
• Figure 3: User localization in wideband XL-MIMO systems with analog beamforming transceivers, where the BS is equipped with a uniform linear array (ULA) to serve four users.
• Figure 4: Channel estimation in wideband XL-MIMO systems, where the BS is equipped with a uniform linear array (ULA) to serve multiple users.