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

TL;DR

This paper reviews the fundamentals, recent advances, and future outlooks of near-field user localization and channel estimation in XL-MIMO systems, emphasizing the unique electromagnetic phenomena and challenges in 6G wireless networks.

Contribution

It provides a comprehensive overview of near-field effects in XL-MIMO, highlighting new algorithms and challenges for localization and channel estimation in 6G systems.

Findings

Near-field effects enable depth perception and precise localization.

New algorithms address near-field challenges in XL-MIMO.

Future research directions are identified for 6G applications.

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.