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Stacked Intelligent Metasurface Enabled Near-Field Multiuser Beamfocusing in the Wave Domain

Xing Jia, Jiancheng An, Hao Liu, Lu Gan, Marco Di Renzo, Mérouane Debbah, Chau Yuen

TL;DR

The paper introduces stacked intelligent metasurfaces (SIM) as a wave-domain computing layer at the BS to enable near-field multiuser MISO communications with reduced RF chains and low-resolution DACs. It provides a detailed SIM model, a near-field channel model with spherical wavefronts, and a signal model that enables end-to-end interference-free beamfocusing via the SIM, coupled with a gradient-descent algorithm to optimize diffraction coefficients. The results show that near-field NFC with SIM achieves higher spatial DoF and sum-rate than far-field models and approaches digital ZF performance, while significantly reducing hardware complexity. The work demonstrates a practical wave-based processing paradigm for NFC that can deliver fast, interference-free multiuser transmission in THz and ultra-m large-array scenarios.

Abstract

Intelligent surfaces represent a breakthrough technology capable of customizing the wireless channel cost-effectively. However, the existing works generally focus on planar wavefront, neglecting near-field spherical wavefront characteristics caused by large array aperture and high operation frequencies in the terahertz (THz). Additionally, the single-layer reconfigurable intelligent surface (RIS) lacks the signal processing ability to mitigate the computational complexity at the base station (BS). To address this issue, we introduce a novel stacked intelligent metasurfaces (SIM) comprised of an array of programmable metasurface layers. The SIM aims to substitute conventional digital baseband architecture to execute computing tasks with ultra-low processing delay, albeit with a reduced number of radio-frequency (RF) chains and low-resolution digital-to-analog converters. In this paper, we present a SIM-aided multiuser multiple-input single-output (MU-MISO) near-field system, where the SIM is integrated into the BS to perform beamfocusing in the wave domain and customize an end-to-end channel with minimized inter-user interference. Finally, the numerical results demonstrate that near-field communication achieves superior spatial gain over the far-field, and the SIM effectively suppresses inter-user interference as the wireless signals propagate through it.

Stacked Intelligent Metasurface Enabled Near-Field Multiuser Beamfocusing in the Wave Domain

TL;DR

The paper introduces stacked intelligent metasurfaces (SIM) as a wave-domain computing layer at the BS to enable near-field multiuser MISO communications with reduced RF chains and low-resolution DACs. It provides a detailed SIM model, a near-field channel model with spherical wavefronts, and a signal model that enables end-to-end interference-free beamfocusing via the SIM, coupled with a gradient-descent algorithm to optimize diffraction coefficients. The results show that near-field NFC with SIM achieves higher spatial DoF and sum-rate than far-field models and approaches digital ZF performance, while significantly reducing hardware complexity. The work demonstrates a practical wave-based processing paradigm for NFC that can deliver fast, interference-free multiuser transmission in THz and ultra-m large-array scenarios.

Abstract

Intelligent surfaces represent a breakthrough technology capable of customizing the wireless channel cost-effectively. However, the existing works generally focus on planar wavefront, neglecting near-field spherical wavefront characteristics caused by large array aperture and high operation frequencies in the terahertz (THz). Additionally, the single-layer reconfigurable intelligent surface (RIS) lacks the signal processing ability to mitigate the computational complexity at the base station (BS). To address this issue, we introduce a novel stacked intelligent metasurfaces (SIM) comprised of an array of programmable metasurface layers. The SIM aims to substitute conventional digital baseband architecture to execute computing tasks with ultra-low processing delay, albeit with a reduced number of radio-frequency (RF) chains and low-resolution digital-to-analog converters. In this paper, we present a SIM-aided multiuser multiple-input single-output (MU-MISO) near-field system, where the SIM is integrated into the BS to perform beamfocusing in the wave domain and customize an end-to-end channel with minimized inter-user interference. Finally, the numerical results demonstrate that near-field communication achieves superior spatial gain over the far-field, and the SIM effectively suppresses inter-user interference as the wireless signals propagate through it.

Paper Structure

This paper contains 11 sections, 19 equations, 5 figures.

Table of Contents

  1. Introduction
  2. Proposed SIM-Aided Near-Field System Model
  3. The Proposed SIM Model
  4. Near-Field Channel Model
  5. Signal Model for SIM-Aided NFC
  6. Wave-based Beamfocusing Scheme
  7. Problem Formulation
  8. Proposed Diffraction Coefficient Optimization Algorithm
  9. The Algorithm Complexity Analysis
  10. Simulation Results
  11. Conclusion

Figures (5)

  • Figure 1: A SIM-aided MU-MISO NFC system.
  • Figure 2: The system sum rate versus $L$ under near-field and far-field channel. ($S=K=4$ and $P_\text{T}= -40$ dBm).
  • Figure 3: (a) The system sum rate versus the number of UEs, $K$. (b) The fitting NMSE ${\varpi }$ versus $K$. ($P_\text{T}= 5$ dBm, $M=225$ and $S=K$).
  • Figure 4: (a) The system sum rate versus the number of metasurface layers, $L$. (b) The fitting NMSE ${\varpi }$ versus $L$. ($P_\text{T}= 5$ dBm and $S=K=4$).
  • Figure 5: The visualization of the near-field beamfocusing for UEs in the same direction. ($S=K=4$).