Near-Field User Localization and Channel Estimation for XL-MIMO Systems: Fundamentals, Recent Advances, and Outlooks
Published in arXiv, 2024
Abstract: Extremely large-scale multiple-input multiple- output (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.
Index Terms: Radiative near-field, XL-MIMO, channel estimation, user localization, 6G
Fig. 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.
Table 1: Features of the user localization schemes developed for the XL-MIMO systems.
Fig. 2: The several new challenges introduced by the three new features of radiative near-field propagation.
Table 2: Features of the channel estimation schemes developed for the XL-MIMO systems.
Fig. 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.
Fig. 4: Channel estimation in wideband XL-MIMO systems, where the BS is equipped with a uniform linear array (ULA) to serve multiple users.
Recommended citation: Hao Lei, Jiayi Zhang, Zhe Wang, Huahua Xiao, Bo Ai, and Emil Björnson, "Near-Field User Localization and Channel Estimation for XL-MIMO Systems: Fundamentals, Recent Advances, and Outlooks," arxiv: 2407.10147, 2024. https://arxiv.org/pdf/2407.10147