Unified Far-Field and Near-Field in Holographic MIMO: A Wavenumber-Domain Perspective

• Published in: IEEE communications magazine 2025-01, Vol.63 (1), p.30-36
• Main Authors: Chen, Yuanbin, Guo, Xufeng, Zhou, Gui, Jin, Shi, Ng, Derrick Wing Kwan, Wang, Zhaocheng
• Format: Magazinearticle
• Language: English
• Subjects: Discrete Fourier transforms; Harmonic analysis; MIMO; Near field communication; Philosophical considerations; Signal processing; System analysis and design
• ISSN: 0163-6804; 1558-1896
• DOI: 10.1109/MCOM.001.2300845

This article conceives a unified representation for near-field and far-field holographic multiple-input multiple-output (HMIMO) channels, addressing a practical design dilemma: "Why does the angular-domain representation no longer function effectively?" To answer this question, we pivot from the angular domain to the wavenumber domain and present a succinct overview of its underlying philosophy. In re-examining the Fourier plane-wave series expansion that recasts spherical propagation waves into a series of plane waves represented by Fourier harmonics, we characterize the HMIMO channel employing these Fourier harmonics having different wavenumbers. This approach, referred to as the wavenumebr-domain representation, facilitates a unified view across the far-field and the near-field. Furthermore, the limitations of the DFT basis are demonstrated when identifying the sparsity inherent to the HMIMO channel, motivating the development of a wave-number-domain basis as an alternative. We then present some preliminary applications of the proposed wavenumber-domain basis in signal processing across both the far-field and near-field, along with several prospects for future HMIMO system designs based on the wavenumber domain.