Near-Field Channel Modeling for Holographic MIMO Communications

TL;DR

This work surveys near-field channel modeling for holographic MIMO (H-MIMO) systems, motivated by electrically large metasurface apertures that enable spherical-wave propagation and EM-domain wave control in the Fresnel region. It categorizes models by communication distance, scatterer presence, parameter randomness, modeling principles, and methodologies, and it reviews state-of-the-art EM-domain approaches such as Tensor Green's Functions and Fourier plane-wave expansions. The paper identifies distinctive near-field features (vectorial fields, distance-angle dependence, enhanced DoF, mutual coupling, spatial non-stationarity) and outlines challenges and evaluation criteria (accuracy, complexity, measurability, and tractability) to guide model development. To address practical limits, it presents computationally and measurement-efficient EM-domain models (CDCM/CICM and PSCM/FSCM) and demonstrates their performance via numerical results, while highlighting future research directions in EM-domain NLoS modeling, dimensionality reduction, and validation through channel sounding.

Abstract

Empowered by the latest progress on innovative metamaterials/metasurfaces and advanced antenna technologies, holographic multiple-input multiple-output (H-MIMO) emerges as a promising technology to fulfill the extreme goals of the sixth-generation (6G) wireless networks. The antenna arrays utilized in H-MIMO comprise massive (possibly to extreme extent) numbers of antenna elements, densely spaced less than half-a-wavelength and integrated into a compact space, realizing an almost continuous aperture. Thanks to the expected low cost, size, weight, and power consumption, such apertures are expected to be largely fabricated for near-field communications. In addition, the physical features of H-MIMO enable manipulations directly on the electromagnetic (EM) wave domain and spatial multiplexing. To fully leverage this potential, near-field H-MIMO channel modeling, especially from the EM perspective, is of paramount significance. In this article, we overview near-field H-MIMO channel models elaborating on the various modeling categories and respective features, as well as their challenges and evaluation criteria. We also present EM-domain channel models that address the inherit computational and measurement complexities. Finally, the article is concluded with a set of future research directions on the topic.

Figures (3)

• Figure 1: Categories of H-MIMO channel modeling.
• Figure 2: Distinctive features of near-field H-MIMO channels.
• Figure 3: Performance evaluation of the near-field HMIMO LoS channel models in Gong2023HMIMOGong2023Transmit versus the TX-RX distance: (a) NMSE and (b) achievable spectral efficiency.