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Near-Field Communications: A Tutorial Review

Yuanwei Liu, Zhaolin Wang, Jiaqi Xu, Chongjun Ouyang, Xidong Mu, Robert Schober

TL;DR

This tutorial highlights the properties of near-field beamfocusing and discusses NFC antenna architectures for both SPD and CAP antennas, and provides a comprehensive performance analysis framework for NFC.

Abstract

Extremely large-scale antenna arrays, tremendously high frequencies, and new types of antennas are three clear trends in multi-antenna technology for supporting the sixth-generation (6G) networks. To properly account for the new characteristics introduced by these three trends in communication system design, the near-field spherical-wave propagation model needs to be used, which differs from the classical far-field planar-wave one. As such, near-field communication (NFC) will become essential in 6G networks. In this tutorial, we cover three key aspects of NFC. 1) Channel Modelling: We commence by reviewing near-field spherical-wave-based channel models for spatially-discrete (SPD) antennas. Then, uniform spherical wave (USW) and non-uniform spherical wave (NUSW) models are discussed. Subsequently, we introduce a general near-field channel model for SPD antennas and a Green's function-based channel model for continuous-aperture (CAP) antennas. 2) Beamfocusing and Antenna Architectures: We highlight the properties of near-field beamfocusing and discuss NFC antenna architectures for both SPD and CAP antennas. Moreover, the basic principles of near-field beam training are introduced. 3) Performance Analysis: Finally, we provide a comprehensive performance analysis framework for NFC. For near-field line-of-sight channels, the received signal-to-noise ratio and power-scaling law are derived. For statistical near-field multipath channels, a general analytical framework is proposed, based on which analytical expressions for the outage probability, ergodic channel capacity, and ergodic mutual information are obtained. Finally, for each aspect, topics for future research are discussed.

Near-Field Communications: A Tutorial Review

TL;DR

This tutorial highlights the properties of near-field beamfocusing and discusses NFC antenna architectures for both SPD and CAP antennas, and provides a comprehensive performance analysis framework for NFC.

Abstract

Extremely large-scale antenna arrays, tremendously high frequencies, and new types of antennas are three clear trends in multi-antenna technology for supporting the sixth-generation (6G) networks. To properly account for the new characteristics introduced by these three trends in communication system design, the near-field spherical-wave propagation model needs to be used, which differs from the classical far-field planar-wave one. As such, near-field communication (NFC) will become essential in 6G networks. In this tutorial, we cover three key aspects of NFC. 1) Channel Modelling: We commence by reviewing near-field spherical-wave-based channel models for spatially-discrete (SPD) antennas. Then, uniform spherical wave (USW) and non-uniform spherical wave (NUSW) models are discussed. Subsequently, we introduce a general near-field channel model for SPD antennas and a Green's function-based channel model for continuous-aperture (CAP) antennas. 2) Beamfocusing and Antenna Architectures: We highlight the properties of near-field beamfocusing and discuss NFC antenna architectures for both SPD and CAP antennas. Moreover, the basic principles of near-field beam training are introduced. 3) Performance Analysis: Finally, we provide a comprehensive performance analysis framework for NFC. For near-field line-of-sight channels, the received signal-to-noise ratio and power-scaling law are derived. For statistical near-field multipath channels, a general analytical framework is proposed, based on which analytical expressions for the outage probability, ergodic channel capacity, and ergodic mutual information are obtained. Finally, for each aspect, topics for future research are discussed.
Paper Structure (65 sections, 33 theorems, 234 equations, 37 figures, 7 tables)

This paper contains 65 sections, 33 theorems, 234 equations, 37 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Distinction between Near-Field and Far-Field Regions
  3. General Field Regions
  4. Boundary between Near-Field and Far-Field Regions
  5. Related Overview Articles
  6. Motivation and Contributions
  7. Organization
  8. Notations
  9. Near-Field Channel Modelling
  10. Spherical Wave Based Channel Model for SPD Antennas
  11. MISO Channel Model
  12. MIMO Channel Model
  13. Non-Uniform Channel Model for SPD Antennas
  14. USW Model starer1994passivebjornson2021primer
  15. NUSW Model zhou2015sphericalfriedlander2019localization
  16. ...and 50 more sections

Key Result

Lemma 2

(Depth of focus) The 3 dB depth of focus of beamformer $\mathbf{f} = \sqrt{\frac{P}{N}}\mathbf{a}^*(\theta, r)$ in direction $\theta$ is given by where $r_{\mathrm{DF}} \approx \frac{N^2 d^2 \sin^2 \theta}{2 \lambda \eta^2_{3 \mathrm{dB}}}$ and $\eta_{3 \mathrm{dB}} = 1.6$.

Figures (37)

  • Figure 1: Near-field communications versus far-field communications.
  • Figure 2: Condensed overview of this tutorial and outlook on NFC.
  • Figure 3: Field regions with respect to an antenna array.
  • Figure 4: Far-field channel model.
  • Figure 5: Near-field multipath MISO channel model.
  • ...and 32 more figures

Theorems & Definitions (83)

  • Remark 1
  • Remark 2
  • Lemma 2
  • proof
  • Lemma 3
  • proof
  • Theorem 1
  • proof
  • Remark 4
  • Corollary 1
  • ...and 73 more