⚡ electrical engineering

RSMA Technique for Multi-User Downlink Single-Waveguide Multi-Pinching Antenna Systems

This paper proposes a multi-user downlink framework that integrates Rate-Splitting Multiple Access (RSMA) with a single-waveguide, multi-pinching antenna system to achieve high sum-rates, improved fairness, and lower bit error rates while maintaining low hardware complexity and a single RF chain.

Original authors: Harsh Raj, Mallena Vardhan, Keshav Singh, Sudip Biswas

Published 2026-06-09

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CC BY 4.0

Original authors: Harsh Raj, Mallena Vardhan, Keshav Singh, Sudip Biswas

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 shout a message to a group of friends scattered across a large, noisy park. In a traditional setup, you might need a separate megaphone and a dedicated speaker for each friend, which is expensive and bulky.

This paper proposes a smarter, cheaper way to do it using a new technology called Pinching Antenna Systems (PASS), combined with a clever communication strategy called Rate-Splitting Multiple Access (RSMA).

Here is the breakdown of how it works, using simple analogies:

1. The Hardware: The "Magic Hose" (PASS)

Instead of using many separate antennas, imagine a single, long, flexible hose (a waveguide) running along the ground.

The Pinching: Along this hose, there are small "pinch points." When you squeeze (or "pinch") the hose at a specific spot, a little bit of the water (the signal) leaks out and sprays into the air.
The Flexibility: The cool part is that you can move these pinch points anywhere along the hose. If your friends move, you can slide the pinch points closer to them to get a stronger spray.
The Benefit: You only need one water pump (a single radio-frequency chain) to feed the whole hose, but you can create multiple spray points. This saves a lot of money and hardware compared to having a separate pump for every friend.

2. The Strategy: The "Group Chat" (RSMA)

Now, imagine you have a message that is partly the same for everyone and partly unique for each person.

The Common Message: This is like a group announcement (e.g., "Everyone, meet at the big oak tree!"). Everyone needs to hear this clearly.
The Private Messages: These are personal notes (e.g., "John, bring your hat," "Sarah, bring your umbrella").
The Problem: If you shout everything at once, the private notes might get drowned out by the group announcement or by other people's private notes.
The RSMA Solution: The paper uses a technique called RSMA. Think of it as a smart shouting order:
1. First, everyone listens for the Group Announcement. They all agree on what it says, even if there is some background noise.
2. Once they understand the group message, they "cancel it out" in their brains (like turning down the volume on the group chat).
3. Now, with the group noise gone, they can clearly hear their own Private Note.

3. The Optimization: "Finding the Sweet Spot"

The paper's main job was to figure out two things to make this system work perfectly:

Where to place the pinch points: Should the hose be squeezed near John or near Sarah? The paper uses a computer algorithm to slide the pinch points to the exact spots that give the best signal to everyone.
How much power to use: How loud should the group announcement be compared to the private notes? If the group message is too quiet, no one hears it. If it's too loud, it drowns out the private notes. The algorithm balances this perfectly.

4. The Results: What Did They Find?

The researchers ran simulations (computer tests) to see how well this idea works. They found:

Faster Speeds: The system can send more data to everyone at once compared to older methods.
Fairness: Even if one friend is far away and another is close, the system adjusts so that everyone gets a decent message, not just the person closest to the hose.
Fewer Mistakes: The messages arrive with fewer errors (lower "Bit Error Rate"), meaning the information is more reliable.
Convergence: The computer algorithm quickly figured out the best positions and power levels, stabilizing after just a few tries.

Summary

In short, this paper shows that by using a single flexible hose with movable pinch points (PASS) and a smart "group chat then private note" strategy (RSMA), we can talk to many people at once efficiently. It's cheaper, uses less energy, and ensures everyone gets their message clearly, even in a crowded environment.

Technical Summary: RSMA Technique for Multi-User Downlink Single-Waveguide Multi-Pinching Antenna Systems

Problem Statement
Next-generation wireless networks require high spectral efficiency and massive connectivity with reduced hardware complexity. While Pinching Antenna Systems (PASS) offer a promising solution by dynamically radiating signals from flexible positions along a single waveguide using a single radio-frequency (RF) chain, managing multi-user interference in such architectures remains a critical challenge. Existing studies on PASS primarily focus on channel modeling and beamforming, leaving a gap in the integration of advanced interference management techniques. This paper addresses the need for an efficient framework to manage interference and optimize performance in a multi-user downlink scenario where multiple pinching antennas are deployed on a single shared waveguide.

Methodology
The authors propose a system model integrating Rate-Splitting Multiple Access (RSMA) into a Single-Input Single-Output (SISO) single-waveguide architecture equipped with multiple pinching antennas.

System Model: A base station (BS) utilizes $N$ pinching antennas along a dielectric waveguide to serve $M$ single-antenna users. The antennas extract electromagnetic energy from the guided wave to radiate signals, establishing near-field Line-of-Sight (LoS) links. The effective channel for each user is a coherent combination of contributions from all active pinching antennas.
Transmission Strategy: The system employs a single-layer RSMA scheme. Each user's message is split into a common stream ( $s_c$ ) and a private stream ( $s_m$ ). The common stream is decoded by all users, while private streams are decoded individually after Successive Interference Cancellation (SIC) removes the common signal.
Optimization Framework: The core objective is to maximize the system sum-rate by jointly optimizing two variables:
1. Transmit Power Allocation: Distributing power between the common stream and private streams for each user.
2. Antenna Placement: Determining the optimal physical locations of the pinching antennas along the waveguide.
This joint optimization is formulated as a non-convex problem due to the coupled dependence of Signal-to-Interference-plus-Noise Ratio (SINR) on both power variables and antenna positions. To solve this, the authors propose an Alternating Optimization (AO) framework:
- Antenna Position Optimization: A grid-based greedy search algorithm is used. The waveguide is discretized into candidate locations, and antennas are placed to minimize propagation distances to users while satisfying minimum spacing constraints ( $\lambda/2$ ).
- Power Allocation Optimization: For fixed antenna positions, the problem is addressed using Successive Convex Approximation (SCA). The non-convex rate expressions are decomposed into Difference-of-Convex (DC) forms. The interference terms are linearized using first-order Taylor expansion, transforming the subproblem into a convex optimization solvable via standard tools (e.g., CVX).

Key Contributions

Framework Development: The paper develops an RSMA-enabled transmission framework specifically for a single-waveguide architecture with multiple pinching antennas, establishing a comprehensive multi-user signal and channel model.
Joint Optimization Formulation: A fairness-constrained sum-rate maximization problem is formulated to jointly optimize transmit power allocation and pinching antenna placement.
Algorithmic Solution: The authors propose an AO framework to solve the resulting non-convex problem. This includes an SCA-based approach for power allocation (utilizing DC decomposition and Taylor linearization) and a grid-based greedy search for antenna placement.
Performance Validation: Numerical results demonstrate the efficacy of the proposed framework in improving sum-rate, ensuring user rate fairness, and reducing Bit Error Rate (BER).

Simulation Results
Simulations were conducted for a downlink scenario with $N=3$ antennas and $M=3$ users operating at 28 GHz.

Convergence: The proposed iterative algorithm demonstrated monotonic convergence of the objective function (sum-rate) to a stable point.
Antenna Placement: The optimization algorithm successfully positioned antennas closer to user locations, reducing free-space propagation loss and increasing received signal power.
Power Allocation: The results showed a dynamic allocation where a significant portion of power was assigned to the common stream to ensure all users could decode it, while private stream power was adjusted to maximize individual rates.
Fairness and Reliability: The system achieved balanced rates among users despite varying channel conditions, highlighting the fairness benefits of RSMA. Bit Error Rate (BER) performance improved with increasing Signal-to-Noise Ratio (SNR), with users having favorable channel conditions achieving lower BERs.

Significance and Claims
The paper claims that the proposed RSMA-enabled PASS framework effectively manages multi-user interference while preserving the low-cost and scalable characteristics of pinching antenna systems. By jointly optimizing antenna placement and power allocation, the framework enhances spectral efficiency and overall system reliability. The authors conclude that this integration offers a promising solution for dense, high-frequency wireless deployments, enabling strong near-field LoS links with a single RF chain and improved user-centric performance. The work highlights the potential of combining spatial radiation control (via PASS) with advanced interference management (via RSMA) for future wireless networks.