On the Blockage Effect in Pinching-Antenna Systems (PASS)

Technical Summary: On the Blockage Effect in Pinching-Antenna Systems (PASS)

Problem Statement
Pinching-antenna systems (PASS) represent a promising flexible antenna architecture for 6G wireless communications, capable of dynamically reconfiguring radiation points along a dielectric waveguide to establish favorable line-of-sight (LoS) links. However, the performance of PASS is critically challenged by random LoS blockage from surrounding obstacles (e.g., buildings, foliage, human bodies), particularly in high-frequency mmWave and Terahertz bands where diffraction is weak and attenuation is severe. Existing studies on PASS under blockage often rely on simplified or empirical assumptions that fail to capture the intrinsic spatial and geometric randomness of real-world obstacles, limiting the physical interpretability and applicability of their results in complex environments.

Methodology
This paper investigates the performance of PASS in the presence of random LoS blockage using a stochastic geometry framework. The authors model obstacles within a rectangular service area as a two-dimensional homogeneous Poisson Point Process (PPP). To characterize the blockage effect, two specific models are proposed:

1. Random-Height-and-Random-Radius (RHRR) Model: Obstacle heights follow a Gaussian distribution, and radii follow a uniform distribution, capturing the statistical heterogeneity of practical obstacles.
2. Deterministic-Height-and-Deterministic-Radius (DHDR) Model: A simplified counterpart assuming fixed obstacle dimensions, serving as a tractable abstraction for homogeneous environments.

The study derives closed-form analytical expressions for the LoS probability, outage probability, and ergodic rate. The analysis considers a downlink scenario where a single waveguide is mounted at a fixed height, and Pinching-Antennas (PAs) are dynamically activated at the position on the waveguide closest to the randomly located User Equipment (UE) to maximize channel gain. The performance is compared against Conventional Antenna Systems (CASS) with fixed-position antennas.

Key Contributions

• Novel Blockage Modeling: The paper introduces the RHRR and DHDR models to mathematically quantify LoS blockage in PASS, moving beyond simple exponential approximations to incorporate the random geometric dimensions and spatial distribution of obstacles.
• Analytical Derivations:
  • Closed-form expressions for LoS probability are derived for both RHRR and DHDR models.
  • Closed-form analytical and asymptotic expressions for outage probability are obtained, revealing that outage probability is positively correlated with obstacle spatial density, geometric dimensions, and service area width.
  • Analytical and approximate expressions for the ergodic rate are derived, showing a negative correlation with obstacle density and area width.
• Performance Bounds and Diversity Analysis: The analysis demonstrates that in the high-SNR regime, the outage probability of PASS reaches a lower bound determined by obstacle intensity rather than transmit power, resulting in a diversity order of zero. However, the ergodic rate exhibits linear growth with increasing SNR.
• Comparative Advantage: Theoretical proofs and simulations confirm that PASS consistently achieves lower outage probabilities and higher ergodic rates than CASS in obstructed environments, attributed to the ability of PASS to dynamically reposition PAs to avoid blockages.

Results
Numerical evaluations, validated against Monte Carlo simulations, yield the following insights:

• Obstacle Impact: Denser obstacle environments or statistically larger obstacles (greater height or radius) significantly increase the probability of LoS blockage, leading to higher outage probabilities and reduced ergodic rates.
• High-SNR Behavior: In the presence of random LoS blockage, increasing transmit power beyond a certain threshold yields negligible improvement in outage probability (diversity order remains zero) but continues to drive linear growth in ergodic rate.
• Area Width: The outage probability increases with the width of the service area, eventually approaching unity in very wide areas under fixed obstacle intensity, highlighting the dominance of spatially distributed obstacles in large-scale deployments.
• PASS vs. CASS: PASS outperforms CASS across all tested scenarios. The performance gap is attributed to the dynamic repositioning capability of PAs, which allows the system to select unblocked LoS paths more effectively than fixed antennas.
• Model Accuracy: The analytical results for both RHRR and DHDR models align closely with simulations. The DHDR model is shown to be a viable, less complex alternative to RHRR when obstacle height variability is low.

Significance
The paper claims that its systematic analysis provides a tractable and physically interpretable framework for characterizing LoS blockage in PASS. By bridging the gap between physical modeling and analytical tractability, the work offers fundamental insights for the design and optimization of blockage-resilient PASS. The findings underscore the necessity of accounting for obstacle randomness in the deployment of high-frequency wireless networks and validate the potential of PASS to maintain reliable connectivity in environments where conventional fixed antennas would suffer from severe performance degradation due to blockage.