A Tutorial on Near-Field XL-MIMO Communications Towards 6G

TL;DR

This paper provides a comprehensive tutorial on near-field XL-MIMO for 6G, arguing that XL-scale arrays transition wireless channels from far-field UPW to near-field NUSW with spatial non-stationarity, necessitating new modelling, analysis, and design paradigms. It develops a thorough near-field array response framework, covers LoS and multipath modelling, and discusses spatial correlation and VR-based extensions, supported by measurement insights. The performance analysis reveals nonlinear SNR scaling with antenna count via angular span, enhanced beam focusing and distance resolution, and opportunities in DoF and sensing. The design sections propose practical near-field beam codebooks, training strategies, channel estimation tailored to polar-domain sparsity and CKM, and an ISI-free DAM approach, alongside cost-efficient hardware and distributed processing techniques. Together, these insights establish a roadmap for robust near-field XL-MIMO system design, and point to future research in multi-cell, multi-path, hybrid active/passive setups, and ISAC integration.

Abstract

Extremely large-scale multiple-input multiple-output (XL-MIMO) is a promising technology for the sixth-generation (6G) mobile communication networks. By significantly boosting the antenna number or size to at least an order of magnitude beyond current massive MIMO systems, XL-MIMO is expected to unprecedentedly enhance the spectral efficiency and spatial resolution for wireless communication. The evolution from massive MIMO to XL-MIMO is not simply an increase in the array size, but faces new design challenges, in terms of near-field channel modelling, performance analysis, channel estimation, and practical implementation. In this article, we give a comprehensive tutorial overview on near-field XL-MIMO communications, aiming to provide useful guidance for tackling the above challenges. First, the basic near-field modelling for XL-MIMO is established, by considering the new characteristics of non-uniform spherical wave (NUSW) and spatial non-stationarity. Next, based on the near-field modelling, the performance analysis of XL-MIMO is presented, including the near-field signal-to-noise ratio (SNR) scaling laws, beam focusing pattern, achievable rate, and degrees-of-freedom (DoF). Furthermore, various XL-MIMO design issues such as near-field beam codebook, beam training, channel estimation, and delay alignment modulation (DAM) transmission are elaborated. Finally, we point out promising directions to inspire future research on near-field XL-MIMO communications.

Figures (26)

• Figure 1: An example of the Rayleigh distance. For fixed $D$, Rayleigh distance increases with the increase of frequency, whereas for fixed $M$, it increases as the frequency gets lower.
• Figure 2: Illustration of far-field UPW versus near-field NUSW.
• Figure 3: Different architectures of XL-MIMO.
• Figure 4: Illustration of XL-MIMO application scenarios in future wireless networks.
• Figure 5: Organization of this paper.