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

TL;DR

This tutorial comprehensively reviews near-field XL-MIMO technology for 6G, covering modeling, performance analysis, design challenges, and future research directions, highlighting its potential to significantly enhance wireless communication capabilities.

Contribution

It provides a detailed overview of near-field XL-MIMO, including new modeling approaches, performance metrics, and design solutions, addressing key challenges for 6G implementation.

Findings

Near-field XL-MIMO exhibits non-uniform spherical wave characteristics.

Performance analysis reveals SNR scaling laws and beam focusing benefits.

Design issues like beam training and channel estimation are critical for practical deployment.

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.