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Pinching Antenna Systems (PASS): Enabling Reconfigurable and Controllable Wireless Channels -- A Comprehensive Survey

This paper provides a comprehensive survey of Pinching Antenna Systems (PASS), a novel 6G technology that enhances wireless channel quality by dynamically positioning antennas along waveguides, while reviewing its applications in coverage, data rates, security, sensing, and energy efficiency alongside its potential and limitations.

Original authors: Elmehdi Illi, Marwa Qaraqe

Published 2026-04-07

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Elmehdi Illi, Marwa Qaraqe

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to have a conversation with a friend in a crowded, noisy room full of pillars and walls. In the old days of wireless technology (like 4G and early 5G), your phone and the cell tower were stuck in fixed positions, like two people standing still trying to shout over the noise. If a wall blocked the direct path between you, the signal would bounce off walls, getting weaker and weaker, or get lost entirely.

To fix this, engineers invented Pinching Antenna Systems (PASS). Think of PASS not as a single loudspeaker, but as a long, flexible garden hose that carries water (your data) from the main tap (the cell tower) all the way to your friend's side of the room.

Here is how the paper explains this technology in simple terms:

1. The "Garden Hose" Analogy

In a traditional system, the cell tower has a bunch of antennas stuck in one spot. In PASS, the tower connects to a long, thin cable (called a waveguide) that can run along a ceiling, a wall, or even a building's exterior.

Along this long cable, there are tiny little "leaks" or "spigots" called Pinching Antennas.

The Magic: You can slide these spigots to any spot along the hose.
The Benefit: If your friend is hiding behind a pillar, you don't need to shout louder. Instead, you just slide a spigot to a spot right next to them. Now, the signal doesn't have to travel far or bounce off walls; it just pops out right next to them.

2. Why is this better than the old ways?

The paper compares PASS to two other "smart" technologies that came before it:

vs. Movable Antennas (The "Dancing Antenna"): Imagine an antenna that can dance around a small stage. It's great, but it can only move a few feet. If your friend is on the other side of a huge building, the dancing antenna can't reach them. PASS is like a hose that stretches for tens of meters, so you can reach anyone, anywhere.
vs. Reconfigurable Intelligent Surfaces (The "Magic Mirror"): Imagine a giant mirror on the wall that reflects your voice to your friend. It works, but the sound has to travel to the mirror and back to your friend, losing half its energy twice. PASS is like a hose that goes directly to your friend, saving all that energy.

3. What can PASS actually do?

The researchers surveyed in the paper found that this "hose" technology is a Swiss Army knife for the future of 6G networks:

Super Coverage: It can reach into dark corners, basements, or behind thick walls where signals usually die.
Faster Speeds: Because the signal is stronger and clearer, you can download movies or stream 8K video instantly.
Stealth Mode (Security): Imagine you want to send a secret message to your friend, but a spy is listening nearby. With PASS, you can aim the "spigot" so precisely that the signal is loud for your friend but barely a whisper for the spy. It's like whispering directly into your friend's ear while the spy is standing across the room.
Sensing (Radar): You can use the hose to "feel" the room. By sending out signals and listening for echoes, the system can detect if a car is approaching or if a person is moving, even if they are hidden behind a wall.
Energy Saving: Because the signal doesn't have to travel far or bounce around, the tower doesn't need to blast as much power. It's like using a garden hose to water a specific plant instead of turning on a sprinkler for the whole lawn.

4. The Challenges (The "But...")

The paper also points out that this isn't perfect yet:

Friction in the Hose: As the signal travels down the long cable, it loses a little bit of strength (attenuation), especially at very high speeds (frequencies). Engineers need to make better "hoses" so the signal stays strong.
Moving Parts: Sliding the antennas requires motors or mechanical parts. If the network needs to change positions every second to keep up with moving cars, the motors might get tired or slow.
Complexity: Figuring out exactly where to put every single spigot to get the best result is a very hard math problem. The paper suggests using Artificial Intelligence (AI) to solve this puzzle quickly.

The Big Picture

The paper concludes that Pinching Antenna Systems are a game-changer for the future of wireless internet (6G). Instead of just building bigger towers or using more power, we can build "smart cables" that bring the signal right to the user's doorstep.

It's the difference between trying to shout across a canyon (old tech) and having a long, flexible tube that delivers a message right into the listener's hand (PASS). It promises faster internet, better security, and the ability to "see" through walls, all while using less energy.

1. Problem Statement

The evolution toward 6G wireless networks demands unprecedented data rates, ultra-reliable low-latency communication (URLLC), and integrated sensing and communication (ISAC). However, these goals are constrained by:

Spectrum Scarcity: The need for higher spectral efficiency within existing frequency bands.
Uncontrollable Channels: Traditional propagation channels suffer from stochastic losses (fading, shadowing) and Line-of-Sight (LoS) blockages due to obstacles.
Limitations of Existing Solutions:
- MIMO/mMIMO: Cannot fundamentally alter a non-LoS channel due to fixed spatial constraints at the transceiver.
- Reconfigurable Intelligent Surfaces (RIS): Can create virtual LoS but suffer from double free-space path loss (FSPL) attenuation and require massive numbers of elements for significant gain.
- Movable Antennas (MAs): Offer position flexibility but are limited to small scales (few wavelengths), making them ineffective against large-scale LoS blockages.

The Core Problem: There is a critical need for a technology that can dynamically reconfigure the wireless channel to overcome LoS blockages, reduce path loss, and enhance security without the double-FSPL penalty of RIS or the spatial limitations of MAs.

2. Methodology and Technical Framework

The paper provides a comprehensive survey of Pinching Antenna Systems (PASS), a novel architecture recently prototyped by NTT DOCOMO.

A. System Architecture

PASS consists of three main components:

Base Station (BS): Equipped with Radio-Frequency (RF) chains.
Dielectric Waveguide: A low-loss, long dielectric cable (often installed on building facades or ceilings) connected to the BS. It guides electromagnetic waves via total internal reflection.
Pinching Antennas (PAs): Small dielectric elements mechanically "pinched" (attached) to the waveguide at custom positions. They create controlled "leakage" points to radiate signals into free space.

B. Propagation Model

The signal propagation in PASS is decomposed into two stages:

In-Waveguide Propagation: The signal travels from the feed point to the PA. It suffers from attenuation ( $\alpha$ ) and phase shift ( $\beta$ ) dependent on the distance traveled. The power radiated by a specific PA depends on the coupling coefficient and the remaining power in the waveguide (governed by preceding PAs).
Free-Space Propagation: Once radiated, the signal undergoes standard FSPL and small-scale fading.

Key Advantage: By placing PAs close to users or in LoS positions, PASS drastically reduces the effective FSPL compared to a distant BS or a double-hop RIS link.

C. Architectural Variants

The survey categorizes PASS designs into:

Standard PASS: Single or multiple waveguides fed by dedicated RF chains.
Segmented Waveguide (SWAN): Waveguides divided into isolated segments with independent feed points to reduce in-waveguide attenuation and inter-antenna interference.
Center-Fed PASS (C-PASS): Signals fed from intermediate ports, splitting energy in forward and backward directions to double degrees of freedom.
Wireless-Fed PASS (Wi-PASS): Uses wireless backhaul to feed the waveguide, eliminating wired constraints.

3. Key Contributions and Surveyed Domains

The paper systematically reviews research across six major domains, comparing PASS against traditional MIMO, RIS, and MAs:

A. Extended Coverage and Data Rate

Optimization: Research focuses on optimizing PA locations and beamforming vectors to maximize sum-rate or ergodic capacity.
Techniques: Methods include Alternating Optimization (AO), Grid Search, and learning-based approaches (Graph Neural Networks, Deep Reinforcement Learning) to handle non-convex optimization problems.
Results: PASS demonstrates superior performance in NLoS scenarios by establishing virtual LoS links and reducing path loss, significantly outperforming fixed antenna arrays.

B. Power Efficiency

Power Minimization: Studies optimize PA placement to minimize total transmit power while meeting QoS constraints.
Energy Efficiency (EE): Trade-offs between Spectral Efficiency (SE) and EE are analyzed. Centralized deployments (single waveguide) often show higher EE due to fewer RF chains, while distributed (multi-waveguide) offers higher SE.
Mechanical Cost: Recent works incorporate the energy cost of moving PAs into the optimization objective.

C. Physical Layer Security (PLS)

Confidentiality: PASS leverages flexible positioning to create constructive signal superposition at legitimate users and destructive interference at eavesdroppers.
Covert Communication: PASS can hide transmission existence by optimizing PA positions to stay within "forbidden zones" of wardens or by using sensing to track adversaries.
Results: PASS achieves lower Secrecy Outage Probability (SOP) than fixed arrays, even against smart eavesdroppers.

D. Multiple Access Schemes

NOMA & RSMA: The survey details how PASS integrates with Non-Orthogonal Multiple Access (NOMA) and Rate-Splitting Multiple Access (RSMA).
Mechanism: Flexible PA positioning allows for better channel separation and interference management, improving sum-rates and fairness compared to uniform arrays.

E. Integrated Sensing and Communication (ISAC)

Dual Functionality: PASS uses the same waveguide infrastructure for both communication and radar sensing.
Performance: By placing PAs closer to targets, PASS improves the Cramér-Rao Bound (CRB) for target localization and reduces sensing outage probability, outperforming conventional Uniform Linear Arrays (ULAs).

F. Cutting-Edge Technologies (Federated Learning)

Straggler Mitigation: PASS is proposed to improve channel quality for "straggler" clients in Federated Learning (FL) networks, reducing training latency and improving model accuracy.

4. Key Results and Findings

Path Loss Reduction: PASS significantly mitigates FSPL by placing radiating elements near the user, offering a distinct advantage over RIS (which suffers double path loss) and MAs (limited range).
LoS Resilience: PASS effectively overcomes LoS blockages by physically moving the antenna to a clear line-of-sight, a capability MAs lack due to their limited movement range.
Security: The ability to create channel dissimilarity between legitimate users and eavesdroppers via pinching beamforming provides robust physical-layer security.
Trade-offs:
- Attenuation: In-waveguide attenuation becomes significant at higher frequencies (mmWave/THz), requiring robust waveguide materials.
- Complexity: Mechanical movement of PAs introduces latency and energy costs; hybrid schemes (activating subsets of PAs) are suggested to balance performance and cost.
- Hardware: Multi-waveguide setups require multiple RF chains, increasing hardware costs compared to single-waveguide RIS.

5. Significance and Future Directions

This survey is significant as it is one of the first comprehensive reviews dedicated solely to PASS, bridging the gap between theoretical electromagnetic models and practical 6G system design.

Future Research Directions Identified:

Hybrid RIS-PASS: Combining RIS and PASS to dynamically align phase shifts and antenna positions for maximum gain.
High-Frequency Extension: Investigating PASS performance at mmWave and Terahertz bands, addressing the challenge of increased waveguide attenuation.
Real-Time Positioning: Developing low-complexity, adaptive algorithms for real-time PA repositioning in dynamic environments without incurring excessive mechanical costs.
Channel Estimation: Addressing the rank-deficiency issues in channel estimation for large-scale PASS arrays and developing unified pilot signaling for hybrid PASS/MIMO systems.
Standardization: Integrating PASS concepts into 3GPP standards for 5G-Advanced and 6G.

In conclusion, the paper establishes PASS as a transformative technology for 6G, offering a unique blend of channel reconfigurability, path-loss mitigation, and security that complements or surpasses existing solutions like RIS and MIMO.