Near-Field Multipath MIMO Channels: Modeling Reflectors and Exploiting NLOS Paths

TL;DR

This paper introduces a generalized statistical near-field MIMO channel model that accounts for large reflector reflections, demonstrating the importance of NLOS paths for multiplexing gains in near-field communications.

Contribution

It extends existing NF MIMO channel models to include imperfect reflections from large surfaces and analyzes their impact on system performance.

Findings

NLOS components are significant in near-field MIMO channels.

Relying only on LOS links may be insufficient for multiplexing gains.

The proposed model accurately predicts the influence of large reflectors.

Abstract

Near-field (NF) communications is receiving renewed interest in the context of multiple-input multiple-output (MIMO) systems involving large physical apertures with respect to the signal wavelength. While line-of-sight (LOS) links are typically expected to dominate in NF scenarios, the impact of non-LOS (NLOS) components at both in centimeter- and millimeter-wave frequencies may be in general non-negligible. Moreover, although weaker than the LOS path, NLOS links may be essential for achieving multiplexing gains in MIMO systems. The commonly used NF channel models for NLOS links in the literature are based on the point scattering assumption, which is not valid for large reflectors such as walls, ceilings, and the ground. In this paper, we develop a generalized statistical NF MIMO channel model that extends the widely adopted point scattering framework to account for imperfect reflections from large surfaces. This model is then leveraged to investigate how the physical characteristics of these reflectors influence the resulting NF MIMO channel. In addition, using the proposed channel model, we analytically demonstrate for a multi-user scenario that, even when users are located within the NF regime, relying solely on LOS NF links may be insufficient to achieve multiplexing gains, thus exploiting NLOS links becomes essential. Our simulation results validate the accuracy of the proposed model and show that, in many practical settings, the contribution of NLOS components is non-negligible and must be carefully accounted for in the system design.