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NLOS-Aided Joint OTA Synchronization and Off-Grid Imaging for Distributed MIMO Systems

Xin Tong, Lechen Zhang, Yu Ge, Dario Tagliaferri, Henk Wymeersch

Abstract

Distributed multiple-input multiple-output (MIMO) architectures enable large-scale integrated sensing and communication (ISAC) by providing high spatial resolution and robustness through spatial diversity. However, practical phase-coherent sensing is challenged by phase synchronization errors and modeling mismatch caused by grid discretization. Existing over-the-air (OTA) synchronization methods typically treat synchronization and sensing tasks separately, which may lead to inaccurate phase alignment when multipath components are used for imaging. In this paper, we propose a non-line-of-sight (NLOS)-aided joint OTA synchronization and off-grid imaging framework for distributed MIMO ISAC systems. First, a line-of-sight (LOS)-assisted coarse synchronization is performed to establish initial phase coherence across distributed links. Subsequently, an iterative refinement stage exploits reconstructed NLOS components obtained from imaging results. By modeling off-grid effects via a first-order Taylor expansion, we transform measurements with nonlinear off-grid offset into an augmented linear model with jointly sparse reflectivity and off-set variables. The imaging problem is reformulated as a structured sparse recovery task and solved using a tailored off-grid approximate message passing (OG-AMP) algorithm. The imaging and synchronization modules are coupled within a closed-loop alternative optimization framework, where improved imaging enables more accurate phase refinement, and vice versa. Numerical results show that the proposed framework achieves accurate synchronization and imaging under phase errors. Compared with conventional approaches, it shows superior robustness and accuracy.

NLOS-Aided Joint OTA Synchronization and Off-Grid Imaging for Distributed MIMO Systems

Abstract

Distributed multiple-input multiple-output (MIMO) architectures enable large-scale integrated sensing and communication (ISAC) by providing high spatial resolution and robustness through spatial diversity. However, practical phase-coherent sensing is challenged by phase synchronization errors and modeling mismatch caused by grid discretization. Existing over-the-air (OTA) synchronization methods typically treat synchronization and sensing tasks separately, which may lead to inaccurate phase alignment when multipath components are used for imaging. In this paper, we propose a non-line-of-sight (NLOS)-aided joint OTA synchronization and off-grid imaging framework for distributed MIMO ISAC systems. First, a line-of-sight (LOS)-assisted coarse synchronization is performed to establish initial phase coherence across distributed links. Subsequently, an iterative refinement stage exploits reconstructed NLOS components obtained from imaging results. By modeling off-grid effects via a first-order Taylor expansion, we transform measurements with nonlinear off-grid offset into an augmented linear model with jointly sparse reflectivity and off-set variables. The imaging problem is reformulated as a structured sparse recovery task and solved using a tailored off-grid approximate message passing (OG-AMP) algorithm. The imaging and synchronization modules are coupled within a closed-loop alternative optimization framework, where improved imaging enables more accurate phase refinement, and vice versa. Numerical results show that the proposed framework achieves accurate synchronization and imaging under phase errors. Compared with conventional approaches, it shows superior robustness and accuracy.
Paper Structure (40 sections, 61 equations, 9 figures)

This paper contains 40 sections, 61 equations, 9 figures.

Table of Contents

  1. Introduction
  2. Related Works
  3. Contributions
  4. System Model
  5. Environment Setting
  6. Grid-Based Signal Model
  7. Off-Grid Approximation Model
  8. OTA Synchronization Algorithm
  9. Coarse Synchronization
  10. Synchronization Refinement
  11. OG-AMP Algorithm for Imaging
  12. Problem Formulation
  13. Approximate Message Passing
  14. From FN ${\rm F}_{\hat{y}}$ to VN $\tilde{\bm x}$
  15. From VN $\tilde{\bm x}$ to FN ${\rm F}_x$
  16. ...and 25 more sections

Figures (9)

  • Figure 1: A joint OTA synchronization and imaging scenario in disributed MIMO systems.
  • Figure 2: A 2D pixel grid, with targets randomly distributed on the grid, exhibits the off-grid effect shown by the red arrow.
  • Figure 3: A sketch of the proposed joint OTA phase synchronization and off-Grid imaging method which iterates between imaging and synchronization during the AO stage.
  • Figure 4: The factor graph of the OG-AMP algorithm, where squares represent FNs and circles represent VNs.
  • Figure 5: The imaging results of the GAMP method and proposed OG-AMP method. (a)-(d): The performance degradation of GAMP by POs and off-grid offsets. (e)-(f): Effectiveness of OG-AMP iteration and gradient update. (g)-(h): Effectiveness of AO iteration.
  • ...and 4 more figures