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Multi-Mode Pinching-Antenna Systems: Polarization-Aware Full-Wave Modeling and Optimization

Dengke Wei, Runxin Zhang, Yulin Shao, Fen Hou, Shaodan Ma

Abstract

Millimeter-wave and terahertz communications face a fundamental challenge: overcoming severe path loss without sacrificing spectral efficiency. Pinching antenna systems (PASS) address this by bringing radiators physically close to users, yet existing frameworks treat the waveguide as a mere transmission line, overlooking its inherent multi-mode capabilities and the critical role of polarization. This paper develops the first polarization-aware, full-wave electromagnetic model for multi-mode PASS (MMPASS), capturing spatial radiation patterns, modal polarization states, and polarization matching efficiency from first principles. Leveraging this physically grounded model, we reveal fundamental trade-offs among waveguide attenuation, atmospheric absorption, and geometric spreading, yielding closed-form solutions for optimal PA placement and orientation in single-user scenarios. Extending to multi-user settings, we propose a modular optimization framework that integrates fractional programming with closed-form polarization updates, scaling gracefully to arbitrary numbers of waveguides, PAs, and users. Numerical results show that MMPASS achieves up to a 167% increase in spectral efficiency compared with single-mode PASS. Moreover, when comparing MMPASS with its polarization-ignorant counterpart, polarization awareness alone improves the sum rate by up to 23%. By bridging rigorous electromagnetic theory with scalable optimization, MMPASS establishes a physically complete and practically viable foundation for future high-frequency wireless networks.

Multi-Mode Pinching-Antenna Systems: Polarization-Aware Full-Wave Modeling and Optimization

Abstract

Millimeter-wave and terahertz communications face a fundamental challenge: overcoming severe path loss without sacrificing spectral efficiency. Pinching antenna systems (PASS) address this by bringing radiators physically close to users, yet existing frameworks treat the waveguide as a mere transmission line, overlooking its inherent multi-mode capabilities and the critical role of polarization. This paper develops the first polarization-aware, full-wave electromagnetic model for multi-mode PASS (MMPASS), capturing spatial radiation patterns, modal polarization states, and polarization matching efficiency from first principles. Leveraging this physically grounded model, we reveal fundamental trade-offs among waveguide attenuation, atmospheric absorption, and geometric spreading, yielding closed-form solutions for optimal PA placement and orientation in single-user scenarios. Extending to multi-user settings, we propose a modular optimization framework that integrates fractional programming with closed-form polarization updates, scaling gracefully to arbitrary numbers of waveguides, PAs, and users. Numerical results show that MMPASS achieves up to a 167% increase in spectral efficiency compared with single-mode PASS. Moreover, when comparing MMPASS with its polarization-ignorant counterpart, polarization awareness alone improves the sum rate by up to 23%. By bridging rigorous electromagnetic theory with scalable optimization, MMPASS establishes a physically complete and practically viable foundation for future high-frequency wireless networks.

Paper Structure

This paper contains 33 sections, 8 theorems, 73 equations, 12 figures.

Table of Contents

  1. Introduction
  2. System Model
  3. Spatial Configuration
  4. Waveguides (WG)
  5. PAs
  6. Users (UR)
  7. Signal Model in MMPASS
  8. Full-Wave Modeling of the MMPASS Channel
  9. Waveguide Propagation
  10. PA Radiation
  11. Polarization-Aware Reception
  12. Sum-Rate Maximization with MMPASS
  13. Single-PA Single-User Analysis
  14. Single-PA Two-User Analysis
  15. General Multi-PA Multi-User Optimization
  16. ...and 18 more sections

Key Result

Theorem 1

The channel coefficient for mode $q$ from the input of the $m$-th waveguide to the radiation point of the $n$-th PA on the same waveguide is given by $\blacktriangleleft$$\blacktriangleleft$

Figures (12)

  • Figure 1: $M$ waveguides are deployed along the $x$-axis, each hosting $N$ PAs. Every PA is equipped with $Q$ independently orientable ports (one per guided mode), enabling simultaneous multi-stream transmission over a single waveguide.
  • Figure 2: The relationships among GCS, LCS and LSCS, where $\mathbf{o}$, $\bar{\mathbf{o}} \,/\, \bm{\psi}_{m,n}^{(\mathrm{PA})}$, and $\bar{\bm{\psi}}$ are their respective origins.
  • Figure 3: Electric field distribution of the $\text{TE}_{1,0}$ mode in a rectangular waveguide.
  • Figure 4: Spatial distribution of the normalized radiated electric-field intensity (dB scale) for a dual-mode PA. The two ports generate spatially separated beams.
  • Figure 5: Single-port single-user channel gain versus transmit and receive orientations: (a) PA rotation with fixed receiver orientation; (b) receiver rotation with PA fixed at its optimal orientation. The optimal PA orientation derived in Lemma \ref{['thm:single_angle']} and the corresponding user polarization orientation derived in \ref{['eq:optimal_user_orientation_2D']} are also labeled in the figure.
  • ...and 7 more figures

Theorems & Definitions (14)

  • Definition 1: LCS
  • Definition 2: LSCS
  • Theorem 1: Waveguide-to-PA Channel Gain in MMPASS
  • Theorem 2: PA Radiation Model
  • proof
  • Corollary 3
  • Theorem 4: PA-to-User Channel Gain in MMPASS
  • Theorem 5: Polarization Matching Coefficient
  • Lemma 6: Optimal PA Orientation for Single User
  • proof
  • ...and 4 more