Part 4: Near-Field, XL-MIMO, and Hardware-Aware Design

Chapter 17: Near-Field Communications

Advanced~210 min

Learning Objectives

  • Compute the Fraunhofer distance dF=2D2/λd_F = 2D^2/\lambda and decide whether a link operates in the far field, radiating near field, or reactive near field
  • Derive the spherical-wavefront array response and identify the Taylor-expansion terms that the far-field plane-wave model drops
  • Distinguish beam steering (far-field, angle only) from beam focusing (near-field, angle plus range) and compute the depth of focus
  • Explain spatial non-stationarity and visibility regions for XL-MIMO, and why they force per-subarray processing
  • Argue why the number of spatial degrees of freedom in the near field can exceed min⁡(Nt,Nr)\min(N_t, N_r) through continuous-aperture (holographic) MIMO

Sections

Prerequisites & Notation
nextSpecial
From Far Field to Near Field
The Spherical Wavefront Model
Beam Focusing and Depth of Focus
Spatial Non-Stationarity and Visibility Regions
Near-Field Degrees of Freedom and Holographic MIMO
Chapter Summary
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Exercises
References & Further Reading
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Prerequisites

Array processing fundamentals: steering vectors, beam patterns (Telecom Ch. 7, MIMO Ch. 2)MIMO channel matrix, singular value decomposition, capacity (Telecom Ch. 15–18)Wave propagation: plane waves, spherical waves, Fresnel–Fraunhofer diffraction (Telecom Ch. 7)Large antenna arrays: massive MIMO capacity scaling (MIMO Ch. 1)Basic electromagnetics: wavelength, wavenumber, $\kappa = 2\pi/\lambda$ (Telecom Ch. 6)

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