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LoS Blockage in Pinching-Antenna Systems: Curse or Blessing?
Zhiguo Ding, H. Vincent Poor
TL;DR
The paper investigates how LoS blockage affects the performance advantage of pinching-antenna systems over conventional antennas. It develops analytical results for single-user and multi-user cases and evaluates the resulting gains using outage probability and ergodic data rate. LoS blockage increases the advantage of pinching antennas, particularly in multi-user settings where interference links can be suppressed.
Problem
The paper addresses the lack of formal analysis of how LoS blockage affects pinching-antenna performance relative to conventional antennas.
Method
The paper develops analytical results for single-user outage probability and multi-user ergodic data rate, supported by simulations.
Results
LoS blockage increases pinching antennas' performance gain over conventional antennas, especially in multi-user systems where interference can be effectively suppressed.
Takeaways & Limitations
LoS blockage is beneficial rather than uniformly harmful for pinching-antenna systems, with its strongest reported value in multi-user interference suppression.
Abstract
from arXiv · showhide
This letter is to investigate the impact of line-of-sight (LoS) blockage on pinching-antenna systems. Analytical results are developed for both single-user and multi-user cases to reveal that the presence of LoS blockage is beneficial for increasing the performance gain of pinching antennas over conventional antennas. This letter also reveals that LoS blockage is particularly useful in multi-user cases, where co-channel interference can be effectively suppressed by LoS blockage.
I. INTRODUCTION
Pinching-antenna systems are motivated by low cost, strong user-specific LoS connections, and flexible MIMO reconfiguration. The letter studies how LoS blockage affects their performance relative to conventional antennas, including single-user outage and multi-user interference.
- Pinching antennas use simple dielectric particles applied to waveguides, supporting a low-cost transmission technique.
- Activating an antenna near a user can create a strong LoS connection and substantially reduce the user's path loss.
- Pinching-antenna MIMO systems can be flexibly reconfigured by adding or removing antennas.
- Prior analytical results reported significant performance gains over conventional antennas, while later work studied low-complexity activation and array gain.
- The letter develops single-user outage analysis and multi-user analysis to investigate whether LoS blockage increases pinching antennas' gain over conventional antennas.
II. SYSTEM MODEL
The system model considers a downlink with one user-centered waveguide per user and one activated pinching antenna nearest each user. LoS blockage is represented by an indicator that removes blocked links, with environment-dependent blockage models and optional waveguide loss in simulations.
- The downlink has M single-antenna users in a rectangular service area divided into M identical rectangles by parallel waveguides.
- Each user is uniformly distributed in the rectangle centered on its waveguide, and the nearest pinching antenna on that waveguide is activated.
- The received channel uses an effective gain ˜h_mk = α_mk h_mk, where α_mk equals zero for a blocked link and one otherwise.
- LoS blockage is modeled through environment-dependent blockage parameters, including a model for ultra-dense indoor environments.
- The model omits waveguide propagation loss in the main expression but includes it in the simulation section.
A. System Design I
System Design I uses zero-forcing precoding when the effective channel matrix is full-rank. If LoS blockage makes the matrix rank-deficient, the design switches to a low-complexity alternative.
- Zero-forcing precoding expresses the received signals in matrix form and uses channel-matrix inversion.
- When the precoding matrix is full-rank, normalization produces an effective channel gain and a corresponding user data-rate expression.
- If LoS blockage makes the precoding matrix rank-deficient, System Design I uses the low-complexity design described in the following subsection.
B. System Design II
System Design II assigns each pinching antenna to one user, avoiding channel-matrix inversion. This reduces implementation complexity, especially when the base station can rely on user locations instead of perfect channel state information.
- Each pinching antenna serves a single user, with p_mm = 1 and p_mk = 0 for k ≠ m.
- The one-antenna-per-user assignment simplifies each user's data-rate expression.
- Unlike Design I, Design II avoids channel-matrix inversion and can require only user locations when NLoS paths or small-scale fading are present.
III. PERFORMANCE ANALYSIS FOR THE CASE OF M = 1
For a single user, the paper derives outage-probability expressions and shows that pinching antennas outperform conventional antennas, with the gain increasing with service-area length D_L.
- Single-user model: The single-user analysis uses outage probability to compare pinching antennas with conventional antennas under LoS blockage.The user and pinching-antenna coordinates are simplified, and the user is assumed uniformly distributed in the service area.
- Pinching-antenna outage: At high SNR, the pinching-antenna outage probability is approximated from the dominant term in the analysis.The approximation follows from the high-SNR behavior of the relevant integral terms.
- Performance comparison: The performance gain of pinching antennas over conventional antennas is expressed through the difference between their outage probabilities.The comparison is developed using the conventional-antenna outage expression and an inequality for the resulting terms.
- Performance comparison: At high SNR, pinching antennas achieve strictly smaller outage probability than conventional antennas under the blockage model in (2).This result is stated as Corollary 1.
- Effect of service-area length: The outage-probability performance gain is a monotonically increasing function of D_L.Increasing D_L reduces the relevant integral term, producing the stated monotonicity.
Pinching Antennas in Ultra-Dense Networks
For ultra-dense indoor networks, the paper adopts a blockage model with squared distance in the exponent and derives closed-form outage expressions. The resulting comparison again shows a D_L-increasing performance gain for pinching antennas.
- Blockage model: In ultra-dense indoor networks, LoS blockage can be modeled with squared distance in the exponent instead of a square-root form.This alternative model simplifies the outage-probability analysis.
- Outage analysis: The outage probability for pinching-antenna systems under the alternative blockage model is given in closed form.The expression is obtained by rewriting the outage probability and applying algebraic manipulations.
- Outage analysis: The error function Φ(·) appears in the closed-form outage expression.The paper explicitly identifies Φ(·) as the error function.
- Performance comparison: Under the squared-distance blockage model, conventional antennas have strictly larger outage probability than pinching antennas.The paper states that this agrees with Corollary 1.
- Effect of service-area length: The performance gain of pinching antennas over conventional antennas is a monotonically increasing function of D_L.The new result confirms the same monotonic relationship found under the earlier blockage model.
IV. PERFORMANCE ANALYSIS FOR THE CASE OF M > 1
For multi-user pinching-antenna systems, the analysis shows that LoS blockage can suppress co-channel interference and produce gains over conventional antennas that grow with SNR. A two-user high-SNR corollary establishes an unbounded performance gain under the stated special case.
- Multi-user interference suppression: LoS blockage is useful for suppressing co-channel interference in multi-user pinching-antenna systems.Interfering antennas are typically farther from a user, making their links more likely to be blocked.
- Analytical setup: The analysis uses ergodic data rate as the performance metric and focuses on Design II for illustrating multi-user interference effects.
- Conventional baseline: The conventional-antenna ergodic data rate is bounded at high SNR and cannot increase through transmit-power increases alone.
- Analytical approximation: Under M = 2, ym = βm, and blockage model (3), Lemma 3 provides a high-SNR approximation for U1’s ergodic data rate.The special case places each user directly underneath its associated waveguide.
- Performance gain: For the two-user case, the high-SNR performance gain of pinching antennas over conventional antennas is unbounded.
V. NUMERICAL STUDIES
Numerical studies evaluate ergodic rates and outage probabilities under specified blockage and propagation-loss settings. They show larger multi-user gains for pinching antennas, increasing gain with SNR, and close agreement between analytical and simulation results.
- Simulation settings: Case II includes both LoS blockage and waveguide propagation loss of 0.08 dB/m in the simulations.The simulations use noise power −90 dBm, d = 3 m, and fc = 28 GHz.
- Numerical comparisons: The multi-user performance gain of pinching-antenna systems over conventional systems is more significant than in the single-user case.The multi-user evaluation uses ergodic sum rate, while the single-user figures include ergodic data rate and outage probability.
- SNR dependence: The performance gain of pinching-antenna systems increases monotonically with SNR in the multi-user case.This numerical trend confirms Corollary 2.
- Design comparison: Design II may lose performance relative to Design I but still provides a significant gain over conventional antenna systems.
- Analytical validation: The analytical curves in Figs. 2 and 4 perfectly match the simulation-based curves.The comparison verifies the accuracy of the developed analytical results.
VI. CONCLUSIONS
The letter concludes that LoS blockage increases the performance advantage of pinching antennas over conventional antennas, especially in multi-user systems. In those systems, blockage can effectively suppress co-channel interference.
- Conclusion: LoS blockage is beneficial for increasing the performance gain of pinching antennas over conventional antennas.
- Conclusion: LoS blockage is particularly useful in multi-user cases because co-channel interference can be effectively suppressed.