C-PASS: Center-Fed Pinching Antenna System

A novel architecture of the center-fed pinching antenna system (C-PASS) is proposed. In contrast to the conventional end-fed PASS, signals are fed from the center input ports and propagate towards both sides of the waveguide. By doing so, spatial-multiplexing gain can be achieved in a single waveguide. Based on the proposed C-PASS, closed-form expressions for the degree of freedom (DoF) and power scaling laws are derived. These theoretical results reveal that C-PASS can achieve \emph{twice} the DoF and an additional multiplexing gain of $\mathcal{O}(P_T \ln^4 N/N^2)$ compared to the conventional PASS, where $P_T$ and $N$ represent the transmit power and pinching antenna number, respectively. Numerical results are provided to demonstrate that substantial capacity improvements can be achieved through the enhanced DoF and multiplexing gain of the C-PASS.

💡 Research Summary

This paper introduces a novel antenna architecture called the Center-Fed Pinching Antenna System (C-PASS) and provides a comprehensive theoretical and numerical analysis of its performance advantages over the conventional End-Fed PASS.

The research is motivated by a fundamental limitation of existing End-Fed PASS designs. In such systems, all pinching antennas (PAs) are connected along a single serial waveguide path fed from one terminal. This “Single-Input Multiple-Radiation” topology inherently leads to severe rank deficiency in the channel matrix, restricting the system’s Degrees of Freedom (DoF) to just one, regardless of the number of antennas. This bottleneck limits multi-user support to inefficient time- or frequency-division schemes.

To overcome this, the authors propose the C-PASS architecture. Its core innovation is the use of a tunable waveguide T-junction power splitter placed at the center of the system. This splitter divides the input signal into two controlled portions that propagate in opposite directions along the waveguide—forward and backward. Each direction has its own set of N PAs (for a total of 2N PAs). This creates two quasi-independent spatial signal paths within a single physical waveguide.

The paper meticulously develops the mathematical signal model for C-PASS, describing the power splitting, propagation, and radiation processes at each PA. Using this model, a comparative analysis with the End-Fed PASS is conducted. A key theoretical result (Theorem 1) proves that the C-PASS achieves a DoF of 2, which is double the DoF (=1) of the End-Fed PASS. This is because the C-PASS’s effective channel matrix can achieve full rank under general conditions, enabling true spatial multiplexing.

Beyond DoF, the paper derives the power scaling laws for both architectures (Theorem 2). The analysis reveals that while both systems provide an array gain that scales as O(ln²N / N), the C-PASS offers an additional multiplexing gain that scales as O(P_T ln⁴N / N²), where P_T is transmit power and N is the number of PAs per direction. This multiplexing gain is critical for capacity at high SNR and is absent in the End-Fed PASS. The authors also identify optimal design parameters, such as setting the distance between the two input ports (L_in) to an odd multiple of a quarter waveguide wavelength to maximize the multiplexing capability.

Numerical simulations validate the theoretical findings, demonstrating that the C-PASS provides substantial capacity improvements over the conventional End-Fed PASS. The work concludes that by fundamentally altering the feeding structure, C-PASS successfully breaks the rank limitation of waveguide-based antenna systems, paving the way for more efficient and high-capacity future wireless communication links.