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Stacked Intelligent Metasurface-Based Transceiver Design for Near-Field Wideband Systems

Qingchao Li, Mohammed El-Hajjar, Chao Xu, Jiancheng An, Chau Yuen, Lajos Hanzo

TL;DR

The paper presents a SIM-based transceiver design for near-field wideband communications, formulating a joint optimization of multi-layer holographic beamforming and MMSE digital precoding under phase-noise. A layer-by-layer SIM optimization (SPGM-based) maximizes baseband eigen-channel gain, while MMSE precoding and iterative waterfilling manage inter-user interference and power allocation, accounting for phase-tuning statistics. Analytical results show a high-SNR spectral-efficiency ceiling caused by hardware-imperfection in the SIM, and performance is demonstrated to exceed state-of-the-art single-layer metasurfaces, with near-field channels enabling multi-user support in both angle and distance domains. The findings offer practical design insights into layer count, inter-layer spacing, and phase-noise mitigation for robust, high-rate near-field wideband SIM systems.

Abstract

Intelligent metasurfaces may be harnessed for realizing efficient holographic multiple-input and multiple-output (MIMO) systems, at a low hardware-cost and high energy-efficiency. As part of this family, we propose a hybrid beamforming design for stacked intelligent metasurfaces (SIM) aided wideband wireless systems relying on the near-field channel model. Specifically, the holographic beamformer is designed based on configuring the phase shifts in each layer of the SIM for maximizing the sum of the baseband eigen-channel gains of all users. To optimize the SIM phase shifts, we propose a layer-by-layer iterative algorithm for optimizing the phase shifts in each layer alternately. Then, the minimum mean square error (MMSE) transmit precoding method is employed for the digital beamformer to support multi-user access. Furthermore, the mitigation of the SIM phase tuning error is also taken into account in the digital beamformer by exploiting its statistics. The power sharing ratio of each user is designed based on the iterative waterfilling power allocation algorithm. Additionally, our analytical results indicate that the spectral efficiency attained saturates in the high signal-to-noise ratio (SNR) region due to the phase tuning error resulting from the imperfect SIM hardware quality. The simulation results show that the SIM-aided holographic MIMO outperforms the state-of-the-art (SoA) single-layer holographic MIMO in terms of its achievable rate. We further demonstrate that the near-field channel model allows the SIM-based transceiver design to support multiple users, since the spatial resources represented both by the angle domain and the distance domain can be exploited.

Stacked Intelligent Metasurface-Based Transceiver Design for Near-Field Wideband Systems

TL;DR

The paper presents a SIM-based transceiver design for near-field wideband communications, formulating a joint optimization of multi-layer holographic beamforming and MMSE digital precoding under phase-noise. A layer-by-layer SIM optimization (SPGM-based) maximizes baseband eigen-channel gain, while MMSE precoding and iterative waterfilling manage inter-user interference and power allocation, accounting for phase-tuning statistics. Analytical results show a high-SNR spectral-efficiency ceiling caused by hardware-imperfection in the SIM, and performance is demonstrated to exceed state-of-the-art single-layer metasurfaces, with near-field channels enabling multi-user support in both angle and distance domains. The findings offer practical design insights into layer count, inter-layer spacing, and phase-noise mitigation for robust, high-rate near-field wideband SIM systems.

Abstract

Intelligent metasurfaces may be harnessed for realizing efficient holographic multiple-input and multiple-output (MIMO) systems, at a low hardware-cost and high energy-efficiency. As part of this family, we propose a hybrid beamforming design for stacked intelligent metasurfaces (SIM) aided wideband wireless systems relying on the near-field channel model. Specifically, the holographic beamformer is designed based on configuring the phase shifts in each layer of the SIM for maximizing the sum of the baseband eigen-channel gains of all users. To optimize the SIM phase shifts, we propose a layer-by-layer iterative algorithm for optimizing the phase shifts in each layer alternately. Then, the minimum mean square error (MMSE) transmit precoding method is employed for the digital beamformer to support multi-user access. Furthermore, the mitigation of the SIM phase tuning error is also taken into account in the digital beamformer by exploiting its statistics. The power sharing ratio of each user is designed based on the iterative waterfilling power allocation algorithm. Additionally, our analytical results indicate that the spectral efficiency attained saturates in the high signal-to-noise ratio (SNR) region due to the phase tuning error resulting from the imperfect SIM hardware quality. The simulation results show that the SIM-aided holographic MIMO outperforms the state-of-the-art (SoA) single-layer holographic MIMO in terms of its achievable rate. We further demonstrate that the near-field channel model allows the SIM-based transceiver design to support multiple users, since the spatial resources represented both by the angle domain and the distance domain can be exploited.

Paper Structure

This paper contains 20 sections, 45 equations, 11 figures, 1 table, 1 algorithm.

Table of Contents

  1. Introduction
  2. Related Work
  3. Motivation
  4. Organization
  5. Notations
  6. System Model
  7. Signal Model
  8. Stacked Intelligent Metasurface Architecture
  9. Channel Model
  10. Hybrid Digital and Holographic Design
  11. SIM Phase Shift Optimization for Holographic Beamformer
  12. Precoding Vector Optimization for Digital Beamformers
  13. Performance Analysis
  14. Numerical and Simulation Results
  15. Performance Comparison of SIM-aided Transceiver Design and SoA Single-layer Metasurface
  16. ...and 5 more sections

Figures (11)

  • Figure 1: System model of the SIM-aided transceiver design.
  • Figure 2: SIM-based hybrid beamforming architecture.
  • Figure 3: System setup of the simulation scenario.
  • Figure 4: The average spectral efficiency $R$ versus the average receive SNR $\gamma$ in our proposed SIM-aided transceiver design and the SoA metasurface.
  • Figure 5: The average spectral efficiency $R$ versus the number of intermediate layers $L$ in the SIM, with different number of elements $N$ in each layer.
  • ...and 6 more figures