A Primer on Near-Field Communications for Next-Generation Multiple Access

TL;DR

The paper addresses NFC-enabled next-generation MA by integrating spherical-wave NFC channel models, information-theoretic limits, and near-field beamforming. It develops SPD and CAP channel models, including a novel Fourier plane-wave (wavenumber) representation that sparsifies the channel, and derives closed-form macroscopic capacity expressions for two-user MAC and BC in the near-field, showing energy-conserving, finite asymptotic limits in contrast to far-field growth. It then presents low-complexity near-field beamforming approaches, including RDMA in the spatial domain and WDMA in the wavenumber domain, with CAP-extension and robustness considerations, demonstrating that near-field RDMA can effectively manage interference even for co-directional users. Collectively, these results establish near-field NFC as a flexible and scalable framework for NGMA, enabling high spectral efficiency and robust interference management with ELAA-like apertures at mmWave/THz frequencies.

Abstract

Multiple-antenna technologies are advancing toward the development of extremely large aperture arrays and the utilization of extremely high frequencies, driving the progress of next-generation multiple access (NGMA). This evolution is accompanied by the emergence of near-field communications (NFC), characterized by spherical-wave propagation, which introduces additional range dimensions to the channel and enhances system throughput. In this context, a tutorial-based primer on NFC is presented, emphasizing its applications in multiuser communications and multiple access (MA). The following areas are investigated: \romannumeral1) the commonly used near-field channel models are reviewed along with their simplifications under various near-field conditions. \romannumeral2) Building upon these models, the information-theoretic capacity limits of NFC-MA are analyzed, including the derivation of sum-rate capacity and capacity region, and their upper limits for both downlink and uplink scenarios. \romannumeral3) A detailed investigation of near-field multiuser beamforming design is presented, offering low-complexity and effective NFC-MA design methodologies in both the spatial and wavenumber (angular) domains. Throughout these investigations, near-field MA is compared with its far-field counterpart to highlight its superiority and flexibility in terms of interference management, thereby laying the groundwork for achieving NGMA.

Figures (26)

• Figure 1: Illustration of the flattening of EM waves with range.
• Figure 2: Illustration of EM field regions and changes of antenna amplitude pattern shape from the reactive near field toward the far field, where $D$/$D_{{a}}$ and $\lambda$ denote the physical dimension of the antenna element/array and signal wavelength, respectively.
• Figure 3: Illustration of the array geometry.
• Figure 4: Illustration of the impact of the projected aperture. $d=0.0628$ m, $d=\lambda/2$, $A=\frac{\lambda^2}{4\pi}$, $r=10$ m, $M_x=1$, and $M_z=M$.
• Figure 5: Illustration of a near-field MIMO channel.

Theorems & Definitions (10)

• Remark 1
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• Remark 10