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Flexible-Antenna Systems: A Pinching-Antenna Perspective

Zhiguo Ding, Robert Schober, H. Vincent Poor

arXiv:2412.02376v1cs.ITeess.SP

TL;DR

The paper studies how pinching antennas reconfigure wireless channels while addressing large-scale path loss and same-signal constraints among antennas on one waveguide. It develops analytical and simulation results for single- and multi-antenna configurations, including NOMA and MISO interference channels, showing strong LoS links and achievable interference-channel upper bounds under identified conditions.

  • Problem

    Existing flexible-antenna systems have limited ability to combat large-scale path loss, while multiple pinching antennas on one waveguide must be fed with the same signal.

  • Method

    The paper analyzes pinching-antenna systems with single and multiple antennas and waveguides, applies NOMA to same-signal transmissions, and studies channel reconfiguration for MISO interference channels.

  • Results

    Pinching antennas create strong LoS links, reduce large-scale path loss, and can achieve the MISO interference-channel performance upper bound when the deployment supports achievability.

  • Takeaways & Limitations

    Pinching antennas provide a flexible-antenna approach whose channel reconfiguration can support NOMA and make an otherwise generally unattainable interference-channel upper bound achievable in suitable deployments.

  • Takeaways & Limitations

    The analysis omits waveguide propagation loss, so its results are upper bounds on achievable pinching-antenna performance.

Abstract

from arXiv · show

Flexible-antenna systems have recently received significant research interest due to their capability to reconfigure wireless channels intelligently. This paper focuses on a new type of flexible-antenna technology, termed pinching antennas, which can be realized by applying small dielectric particles on a waveguide. Analytical results are first developed for the simple case with a single pinching antenna and a single waveguide, where the unique feature of the pinching-antenna system to create strong line-of-sight links and mitigate large-scale path loss is demonstrated. An advantageous feature of pinching-antenna systems is that multiple pinching antennas can be activated on a single waveguide at no extra cost; however, they must be fed with the same signal. This feature motivates the application of non-orthogonal multiple access (NOMA), and analytical results are provided to demonstrate the superior performance of NOMA-assisted pinching-antenna systems. Finally, the case with multiple pinching antennas and multiple waveguides is studied, which resembles a classical multiple-input single-input (MISO) interference channel. By exploiting the capability of pinching antennas to reconfigure the wireless channel, it is revealed that a performance upper bound on the interference channel becomes achievable, where the achievability conditions are also identified. Computer simulation results are presented to verify the developed analytical results and demonstrate the superior performance of pinching-antenna systems.

I. INTRODUCTION

Pinching antennas reconfigure wireless channels by placing dielectric particles on waveguides, enabling strong line-of-sight links and reduced large-scale path loss. The paper develops analytical designs and evaluates single- and multi-antenna configurations, including NOMA and multi-waveguide interference channels.

  • Motivation: Flexible-antenna systems reconfigure effective wireless channel gains, but existing RIS/IRS, fluid, and movable antennas have limited ability to combat large-scale path loss.RIS/IRS systems can suffer double attenuation, while fluid and movable antennas remain limited in addressing large-scale path loss.
  • Pinching-Antenna Concept: Pinching antennas are activated by applying small dielectric particles to a dielectric waveguide, and their locations can be flexibly adjusted over a large scale.This flexibility can place an antenna close to a target receiver to create or strengthen a line-of-sight link.
  • Pinching-Antenna Concept: Additional pinches can resize the antenna system, and multiple pinching antennas can be applied to one or multiple waveguides in a flexible, low-cost manner.This capability provides a possible route toward MIMO implementation.
  • Single-Antenna Analysis: For a single pinching antenna and waveguide, the paper derives closed-form ergodic sum-rate expressions and shows strong LoS links can mitigate large-scale path loss.The performance gain over conventional antennas is affected by the area in which users are deployed.
  • Multiple Antennas: With multiple antennas on one waveguide, transmit power per antenna decreases, yet TDMA analysis shows users’ data rates increase monotonically with antenna count.Multiple antennas are therefore analytically beneficial in the considered TDMA-assisted system.
  • Multiuser Transmission: Because antennas on one waveguide share the same signal, user signals must be superimposed, motivating NOMA for simultaneous multiuser transmission.The multi-waveguide case is treated as a MISO interference channel, where channel reconfiguration can make the performance upper bound achievable under identified conditions.
  • Scope and Assumptions: The analysis assumes LoS links and omits waveguide propagation loss, so reported performance can be an upper bound and NLoS performance remains outside scope.The omitted propagation loss is much smaller than free-space path loss in the cited 28-GHz example.
  • Performance Comparison: The pinching-antenna sum rate is always larger than that of a conventional antenna, and the high-SNR sum-rate difference increases monotonically with D/d.The comparison is based on the stated analytical result for the considered systems.

III. USING MULTIPLE PINCHING ANTENNAS ON A SINGLE WAVEGUIDE

With multiple pinching antennas on one waveguide, antenna placement reconfigures both large-scale path loss and phase shifts, while the antennas share a common waveguide-fed signal.

  • Because antennas on one waveguide receive phase-shifted versions of the same signal, conventional independent-signal MISO strategies do not directly apply.
  • The model assumes equal sharing of total transmit power among the activated pinching antennas.The waveguide wavelength is related to the free-space wavelength through the effective refractive index.
  • Multiple pinching antennas provide additional reconfigurable degrees of freedom through their positions, affecting large-scale path loss and phase shifts.
  • Pinching antennas can be added to one or multiple waveguides flexibly and at low cost, enabling changes in antenna-system size.

A. OMA-Assisted Pinching-Antenna Systems

The OMA-assisted system serves users individually with multiple pinching antennas and uses their relocatability to approach a rate upper bound while retaining low hardware cost.

  • Under OMA, users are served individually, and multiple pinching antennas can improve each user’s data rate by positioning antennas near the closest waveguide point.
  • If antenna locations can satisfy the required phase-alignment condition, an upper bound on the user data rate becomes attainable.The condition requires the relevant phase terms to sum to an integer multiple of 2π.
  • The illustrated deployment uses one waveguide and two pinching antennas, with weak and strong users uniformly distributed in separate equal-sized squares.
  • A sequential location-search algorithm places the first antenna and then successive antennas along the waveguide while enforcing a guard distance to avoid coupling.
  • Unlike conventional antennas and other flexible antennas, pinching antennas can realize this upper bound through large-scale repositioning and require only one RF chain.

B. NOMA Assisted Pinching-Antenna Systems

Because multiple antennas on one waveguide share a signal, the paper applies power-domain NOMA and uses user scheduling to avoid requiring finely optimized antenna locations.

  • NOMA superimposes multiple users’ signals when multiple pinching antennas on a single waveguide serve users simultaneously.
  • Each pinching antenna is placed closest to its assigned user, while users are ordered by ascending channel strength for successive interference decoding.
  • Scheduling users far apart makes the antenna nearest each user dominant in its effective channel, reducing the need for fine-tuned placement.

2) The Case of M = N = 2:

For two users and two antennas, the analysis approximates ergodic rates under separated deployments and compares high-SNR NOMA and OMA performance under a matched-power assumption.

  • The Case of M = N = 2: For large D1, the two users’ separated square deployments permit simplified channel-gain expressions and preserve the assumed weak-to-strong channel ordering.
  • The Case of M = N = 2: The users’ ergodic data rates are evaluated from their uniform spatial distributions and then approximated in the high-SNR regime.
  • The Case of M = N = 2: A closed-form approximation for the NOMA-assisted ergodic sum rate is obtained for large D1 and high SNR.
  • The Case of M = N = 2: Under the stated large-D1 condition, the difference between the NOMA and OMA sum rates is guaranteed to be positive.
  • The Case of M = N = 2: For a fair comparison, the analysis assumes MP = Pm, matching total transmit power across NOMA and OMA.

IV. USING MULTIPLE PINCHING ANTENNAS ON MULTIPLE WAVEGUIDES

With multiple waveguides and one pinching antenna per waveguide, different waveguides can carry different signals, enabling multiuser MISO transmission modeled through configurable channels.

  • Multiple waveguides can be fed with different signals, unlike multiple antennas on one waveguide.This allows the received signal at each user to combine independently transmitted waveguide signals.
  • The channel between user U_m and waveguide k depends on the corresponding pinching-antenna configuration.
  • Beamforming coefficients p_m,k allocate user m’s signal across waveguides under an overall transmit-power constraint.
  • The received signal and SINR expressions account for propagation phase shifts along waveguides and from antennas to users.
  • The resulting system resembles a conventional multiuser MISO interference channel, with the special case M = K = 2 analyzed in detail.

A. Existing Results for Two-User Interference Channels

The two-user interference-channel benchmark exposes a trade-off between strengthening intended signals and suppressing interference, while pinching antennas can reconfigure channels to approach the resulting upper bound.

  • Optimizing two users’ SINRs is inherently coupled because improving one user can degrade the other user’s performance.
  • MRC maximizes intended-user signal strength but leaves interference uncontrolled, whereas ZF suppresses cochannel interference without maximizing intended signal strength.
  • The interference-channel SINR upper bound corresponds to each user occupying the whole bandwidth without the other user’s interference.
  • Upper-bound conditions: Phase matching aligns each user’s beamforming coefficient with its intended channel, maximizing the desired-signal numerator.
  • Upper-bound conditions: Orthogonality removes inter-user interference from each SINR denominator.
  • Upper-bound conditions: Because antenna locations configure the channels, moving pinching antennas can make the upper bound achievable when both conditions hold on a micrometer scale.The bound changes with antenna locations but remains nearly unchanged for micrometer-scale movements.

2) A Special Case Which Guarantees Constraints in (37) and (38):

A special two-user deployment demonstrates how antenna placement can satisfy the upper-bound constraints, with searchable phase-related values covering an achievable range.

  • When users lie on the x-axis and waveguides are placed as specified, antenna locations can be selected to satisfy the orthogonality constraint.
  • The relevant function is monotonic, so adjusting a pinching-antenna coordinate can realize every value within its stated range.
  • For sufficiently separated users, the achievable range becomes much larger than the wavelength, supporting feasible constraint satisfaction.
  • Search-based design: Algorithm 1 enumerates candidate antenna locations and selects the pair maximizing the minimum two-user SINR after obtaining ZF beamforming vectors.
  • Multiple feasible integer choices exist even when users are not located on a line parallel to the waveguides.

3) A Search-Based Algorithm to Approach the Upper Bound:

Simulations evaluate single- and multiple-antenna configurations, showing gains from flexible placement and NOMA, while also revealing a sum-rate–weak-user-rate trade-off.

  • Search-based algorithm: The search-based algorithm enumerates candidate antenna locations to find configurations that approach the interference-channel upper bound using ZF or MRC.
  • Simulation results: In the single-antenna case, pinching antennas achieve higher ergodic sum rates than conventional antennas by reducing users’ path losses.
  • Simulation results: The pinching-antenna gain over conventional antennas increases as the deployment area or rectangle’s long side grows.With fixed rectangle width, users’ path losses through pinching antennas remain unchanged as the long side increases.
  • Simulation results: Increasing the number of pinching antennas significantly increases performance gain over conventional antennas, and the search algorithm achieves the upper bound in (16).
  • NOMA results: NOMA increases sum rate relative to benchmark schemes and can outperform OMA, especially when users’ channel conditions differ more.
  • NOMA results: NOMA’s sum-rate gain favors the strong user, while the weak user’s data rate decreases, particularly at high SNR.The weak user treats the partner’s signal as noise, causing its high-SNR rate to approach a constant.

C. The Multi-Pinching-Antenna Multi-Waveguide Case

The multi-pinching-antenna, multi-waveguide system is studied as a two-user, two-waveguide MISO interference channel. Pinching-antenna placement can attain the interference-channel upper bound in some deployments, while low-complexity placement and beamforming can preserve performance.

  • System model: The considered case activates two pinching antennas on two waveguides to serve two users, forming a two-user MISO interference-channel setting.The system uses N = K = M = 2.
  • Performance: Exhaustive antenna-location search shows that pinching antennas outperform conventional-antenna benchmarks in the considered deployment.The benchmarks include MRC, ZF, and an upper bound based on conventional-antenna locations.
  • Low-complexity design: A search restricted to locations near associated users achieves the same performance as the upper bound for the deployment considered in Fig. 10.The restricted search focuses on locations close to ψPin_m.
  • Low-complexity design: The two antenna-location searches in Fig. 10 achieve the same performance, indicating that optimal pinching-antenna locations are very close to ψPin_m.This observation motivates placing antennas next to associated users before applying ZF or MRC.
  • Achievability conditions: Upper-bound achievability depends on user/waveguide deployment: it is always achieved in the separated-rectangle case but not in every same-square realization.For users uniformly distributed within the same square, one random realization achieves the bound and another does not.
  • Conclusion: The paper concludes that reconfigurable pinching antennas can achieve the interference-channel performance upper bound, while the general MISO case remains future work.The studied MISO setting is the special case M = N = K = 2.

APPENDIX A PROOF FOR LEMMA 2

The appendix proves Lemma 2 by comparing conventional and pinching-antenna performance through an analytically defined difference. Monotonicity and limiting values establish that this difference is nonnegative.

  • Proof strategy: The proof begins from the conventional-antenna ergodic sum-rate expression and develops an upper bound for analytically comparing systems.The upper bound is motivated by the difficulty of obtaining the exact sum rate.
  • Proof strategy: A high-SNR approximation is used to obtain insight into the performance difference between conventional and pinching-antenna systems.The approximation relies on the high-SNR assumption.
  • Analytical reduction: The performance difference between pinching and conventional antennas is expressed analytically using auxiliary functions such as g3(x).The proof reduces the desired inequality to properties of these functions.
  • Monotonicity argument: Showing that g3(x) is monotonically increasing and satisfies g3(0) = 0 establishes g3(x) ≥ 0 for x ≥ 0.The proof uses derivative calculations and limits to establish monotonicity.
  • Conclusion: The nonnegative auxiliary-function result completes the proof that the pinching-antenna performance gain over the conventional system is always positive.This conclusion follows from the two limits and the monotonicity of g3(x).
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