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

TL;DR

The fundamentals of near-field communications and the metric to determine the near- field ranges in typical communication scenarios are presented and the techniques addressing the challenges and those exploiting the potentials inNear-field regions are investigated.

Abstract

Extremely large antenna array (ELAA) is a common feature of several key candidate technologies for sixth-generation mobile networks (6G), 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 is opposed to the conventional planar-wave-based radiation model of 5G massive MIMO. As a result, near-field communications will become essential in 6G wireless networks. In this article, we systematically investigate the emerging near-field communication techniques. Firstly, we present the fundamentals of near-field 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, i.e., 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 would inspire more innovations for this important research topic of near-field communications for 6G.

Figures (6)

• Figure 1: Far-field planar wavefront vs. near-field spherical wavefront. The plots at the bottom illustrate the normalized received signal energy in the physical space achieved by near-field beamfocusing (bottom left) and far-field beamsteering (bottom right).
• Figure 2: Near-field ranges for typical scenarios.
• Figure 3: Near-field codebook with non-uniform grids.
• Figure 4: This figure illustrates the far-field beam split effect (left) and the near-field beam split effect (right). Far-field beam split makes beams at different frequencies transmit towards different directions, while near-field beam split makes beams at different frequencies be focused on various locations.
• Figure 5: The spatial DoF increases in the near-field region.