Near-Field MIMO Communications for 6G: Fundamentals, Challenges, Potentials, and Future Directions

TLDR

Extremely large‑scale antenna arrays in 6G technologies require near‑field spherical‑wave modeling, making near‑field MIMO communications essential. The article systematically investigates emerging near‑field communication techniques. The authors present near‑field fundamentals, a range metric, classify recent studies into challenge‑mitigation and potential‑exploitation categories, discuss their principles and progress, and outline open problems and future directions. The article aims to inspire further innovations in near‑field MIMO communications for 6G.

Abstract

Extremely large-scale antenna array (ELAA) is a common feature of several key candidate technologies for sixth generation (6G) mobile networks, such as ultra-massive multiple-input-multiple-output (UM-MIMO), cell-free massive MIMO, reconfigurable intelligent surface (RIS), and terahertz communications. Since the number of antennas is very large for ELAA, the electromagnetic radiation field needs to be modeled by near-field spherical waves, which differs from the conventional planar-wave-based radiation model of 5G massive MIMO. As a result, near-field MIMO communications will become essential in 6G wireless networks. In this article, we systematically investigate the emerging near-field communication techniques. First, we present the fundamentals of nearfield communications and the metric to determine the near-field ranges in typical communication scenarios. Then we investigate recent studies specific to near-field communications by classifying them into two categories: techniques addressing the challenges and those exploiting the potentials in near-field regions. Their principles, recent progress, pros, and cons are discussed. More importantly, several open problems and future research directions for near-field communications are pointed out. We believe that this article will inspire more innovations in this important research topic of near-field MIMO communications for 6G.