On the Performance of Pinching-Antenna Systems (PASS) Under Dynamic Channels with Blockages

Technical Summary: Performance of Pinching-Antenna Systems (PASS) Under Dynamic Channels with Blockages

Problem Statement
Pinching-Antenna Systems (PASS) represent a flexible antenna architecture utilizing dielectric waveguides and movable pinching antennas (PAs) to establish favorable Line-of-Sight (LoS) links. While PASS offers significant potential for adaptive beamforming and spatial diversity, its performance is fundamentally compromised by LoS blockages caused by obstacles such as walls, furniture, or human bodies. Existing research often relies on simplified probabilistic blockage models or assumes ideal LoS conditions, neglecting the complex interplay between deterministic obstacle-induced blockage and Non-Line-of-Sight (NLoS) scattering. Furthermore, the contribution of NLoS components in obstacle-rich environments (e.g., indoor settings, utility tunnels) remains underexplored, despite the possibility that strong NLoS scattering could sustain communication when LoS paths are obstructed. This paper addresses the gap in understanding how PASS performs when both LoS blockage effects and NLoS propagation are jointly considered under realistic geometric constraints.

Methodology
The authors propose a geometry-aware framework to model PASS performance in the presence of dynamic blockages.

• System Model: A downlink PASS scenario is defined with a single waveguide and a single PA serving a single User Equipment (UE). The PA can be activated at any position along the waveguide.
• Blockage Model: Instead of using probabilistic LoS indicators, the paper adopts a deterministic geometry-aware blockage model. Obstacles are modeled as cylinders. Based on the relative positions of the UE, the obstacle, and the waveguide, a specific "blockage region" on the waveguide is calculated where the PA cannot establish an LoS link to the UE.
• Channel Modeling: The composite channel gain incorporates three components:
  1. In-waveguide attenuation: Exponential power loss and phase shift as the signal travels along the waveguide.
  2. LoS Component: Modeled as a deterministic free-space path loss with phase shift, present only when the PA is outside the blockage region.
  3. NLoS Component: Modeled as the superposition of multiple scattering clusters, following a zero-mean complex Gaussian distribution.
• Fading Scenarios: The study analyzes two distinct channel conditions:
  • Rayleigh Fading: Occurs when the LoS path is blocked (PA is in the blockage region).
  • Rician Fading: Occurs when the LoS path is unblocked (PA is outside the blockage region), dominated by the LoS component with NLoS scattering.
• Performance Metrics: The paper derives closed-form expressions for Outage Probability and Ergodic Rate for both fading scenarios. It also investigates the optimal PA position by balancing spatial propagation loss against in-waveguide attenuation.

Key Contributions

1. Geometry-Aware Blockage Modeling: The paper establishes a direct link between physical obstacle locations and the PASS channel by defining a specific blockage region on the waveguide. This replaces abstract probabilistic models with a deterministic geometric approach.
2. Analytical Performance Expressions:
   • Closed-form expressions for outage probability are derived for both Rayleigh and Rician fading channels.
   • A closed-form expression for the ergodic rate is obtained for the Rayleigh case.
   • For the Rician case, a complete analytical expression and a tractable closed-form approximation for the ergodic rate are derived.
3. Optimal PA Deployment Criteria:
   • Analytical expressions for the endpoints of the blockage region are derived.
   • In the Rayleigh case, a closed-form solution for the optimal PA position is provided, which balances spatial path loss and waveguide attenuation.
   • In the Rician case, the paper proposes maximizing the average received Signal-to-Noise Ratio (SNR) as an effective criterion for determining the optimal PA position, avoiding complex optimization algorithms while maintaining near-optimal performance.
4. Insights on NLoS Scattering: The analysis reveals the dual nature of NLoS scattering: it can degrade outage performance by introducing randomness but can improve ergodic rate by providing additional power, particularly when the LoS component is weak or blocked.

Results and Findings
Numerical simulations validate the analytical derivations and reveal several critical insights:

• Impact of NLoS Scattering: NLoS scattering has a twofold effect. Under Rician fading, it can degrade outage performance due to increased channel fluctuations but simultaneously improve the ergodic rate. In the absence of an LoS path (Rayleigh case), sufficiently strong NLoS scattering can still support communication requirements.
• LoS Dominance: When the LoS component is strong (high Rician factor), the contribution of NLoS components to the achievable rate is negligible, and the system performance closely matches ideal LoS evaluations. However, ignoring NLoS propagation when the LoS component is weak leads to an underestimation of system performance.
• Optimal Positioning Trade-off: The optimal PA position is jointly determined by the environment geometry and the trade-off between spatial propagation loss and in-waveguide attenuation.
  • In scenarios with blockage, maintaining an LoS link by activating a PA further along the waveguide (despite higher waveguide attenuation) often yields superior performance compared to relying on NLoS propagation from a PA closer to the access point.
  • The transition between Rayleigh and Rician regimes can cause abrupt changes in ergodic rate (e.g., a jump from ~3 bps/Hz to ~16 bps/Hz) as the PA moves out of a blockage region.
• Coverage Patterns: The effective coverage area is not isotropic; it is segmented into "sectors" defined by the geometry of obstacles. High-rate regions correspond to locations where favorable propagation (LoS or weak blockage) is maintained.

Significance
The paper claims that properly addressing LoS blockage through geometry-aware modeling is vital for realizing the practical benefits of PASS. By explicitly incorporating both LoS and NLoS components, the study provides a more accurate performance analysis than existing LoS-only or probabilistic models. The derived deployment criteria offer practical guidelines for optimizing PA placement in complex, obstacle-rich environments, ensuring that the system can adapt to dynamic blockages while leveraging NLoS scattering when necessary. The work establishes a theoretical baseline for evaluating PASS in realistic scenarios where large-scale fading and geometric blockage are dominant factors.