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arxiv: 2604.23891 · v1 · submitted 2026-04-26 · 📡 eess.SP

Fluid Antenna Enabled Compact Ultra Massive Antenna Array for Satellite Communications

Tianyu Han , Yongxu Zhu , Gan Zheng , Pantelis-Daniel Arapoglou This is my paper

Pith reviewed 2026-05-08 05:22 UTC · model grok-4.3

classification 📡 eess.SP
keywords fluid antennacompact ultra-massive antenna arraysatellite communicationsinterference suppressionoutage probabilityergodic rateuplink transmissionmultiple access
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The pith

Fluid antenna arrays render satellite uplink signals deterministic, shifting focus to interference statistics with linear beamforming gains.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper proposes fluid-antenna-enabled compact ultra-massive arrays for satellite uplink networks where multiple ground users share one satellite. Multiple ports on each fluid antenna activate simultaneously using a single RF chain to coherently combine signal components and suppress inter-user interference. Closed-form expressions for signal power, interference power, and distributions are derived to obtain outage probability in unified form plus approximation and ergodic rate. With sufficiently compact configurations the received signal becomes deterministic so performance depends on interference statistics, and adding ports produces linear beamforming gain while CUMA outperforms maximum ratio combining under identified conditions. Non-orthogonal multiple access CUMA shows better wideband performance than orthogonal access.

Core claim

In a satellite CUMA network all ground users share the same satellite for uplink transmission and CUMA suppresses inter-user interference. Closed-form expressions for received signal power, interference power and their distributions are derived. Based on these the outage probability is obtained in unified form along with an accurate approximation and the ergodic rate is characterized. Conditions are identified under which CUMA outperforms maximum ratio combining. Notably with sufficiently compact fluid antenna configurations the received signal becomes deterministic indicating system performance is dominated by interference statistics. Moreover increasing the number of ports yields a linear,

What carries the argument

Compact ultra-massive antenna array (CUMA) built from fluid antenna systems (FAS) that activates multiple ports simultaneously for coherent combining with one RF chain per antenna.

If this is right

  • With compact configurations the received signal power becomes deterministic so performance is governed by interference statistics alone.
  • Increasing the number of activated ports produces linear beamforming gain in the effective SINR.
  • CUMA outperforms maximum ratio combining once the identified conditions on compactness and port count are met.
  • Closed-form outage probability and ergodic rate expressions enable direct system design without Monte-Carlo simulation.
  • Non-orthogonal multiple access CUMA delivers higher performance than orthogonal access under wideband satellite conditions.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same single-RF-chain coherent combining principle could lower hardware costs in terrestrial massive MIMO deployments facing similar interference.
  • Hardware prototypes could test how real mutual coupling modifies the deterministic-signal limit predicted by the analysis.
  • The linear gain with port count suggests the approach scales to larger arrays without proportional increase in RF hardware.
  • Integration with adaptive satellite beamforming might extend the interference suppression benefits to multi-satellite constellations.

Load-bearing premise

Fluid antennas can be made compact enough that multiple ports combine coherently without significant mutual coupling, phase errors or hardware impairments.

What would settle it

Hardware measurements on fluid antenna prototypes showing that received signal variance stays above zero even as physical spacing shrinks to very small values, or that SINR gains with added ports fall below linear scaling.

Figures

Figures reproduced from arXiv: 2604.23891 by Gan Zheng, Pantelis-Daniel Arapoglou, Tianyu Han, Yongxu Zhu.

Figure 1
Figure 1. Figure 1: A conceptual uplink satellite communications system, where the satellite employs multiple 1D fluid antennas to serve view at source ↗
Figure 3
Figure 3. Figure 3: Outage probability and average SINR against the fluid view at source ↗
Figure 4
Figure 4. Figure 4: Outage probability and average SINR against the view at source ↗
Figure 5
Figure 5. Figure 5: Outage probability and average SINR against the view at source ↗
Figure 6
Figure 6. Figure 6: CDF of the signal power αI and the interference power from user u˜, Yu˜, against the threshold, given different µ. K, increasing W is equivalent to decreases µ. In other words, it is not the larger FAS size that leads to the performance degradation, but rather the lower port density µ. Additionally, as the number of users U, increases, the performance of the typical user decreased. To further analyze the i… view at source ↗
Figure 9
Figure 9. Figure 9: Ergodic rate against the number of user U, given different port density µ when W = 3. µ changes, under different bandwidth B and U. Firstly, in both B = 10 MHz and B = 20 MHz, with the increase on µ, the average SNR is linearly increased. This is due to that µ is another measure on the number of port K, while the beamforming gain is linearly increased with K, as discussed in Section IV-A. Also, at low dens… view at source ↗
Figure 11
Figure 11. Figure 11: The beamforming gain for interference against the view at source ↗
read the original abstract

Satellites provide seamless coverage and are critical for emergency communications during natural disasters. However, their performance is constrained by limited spectrum and high deployment cost. To address these issues, we propose a fluid antenna system (FAS)-based solution that enables dynamic signal adaptation. Building on this concept, a compact ultra-massive antenna array (CUMA) is introduced, where multiple ports are simultaneously activated to coherently combine signal components. This design mitigates interference while reducing cost, as each fluid antenna requires only a single RF chain yet achieves significant improvement in the received signal-to-interference-plus-noise ratio (SINR). We consider a satellite CUMA network where all ground users share the same satellite for uplink transmission, and CUMA is employed to suppress inter-user interference. Closed-form expressions for the received signal power, interference power, and their distributions are derived. Based on these results, the outage probability is obtained in a unified form along with an accurate approximation, and the ergodic rate is characterized. Our analysis identifies the conditions under which CUMA outperforms maximum ratio combining in satellite systems. Notably, with sufficiently compact fluid antenna configurations, the received signal becomes deterministic, indicating that system performance is dominated by interference statistics. Moreover, increasing the number of ports yields a linear beamforming gain. Numerical results further compare orthogonal and non-orthogonal multiple access CUMA, showing that the latter achieves superior performance under wideband conditions.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper proposes a fluid antenna system (FAS)-based compact ultra-massive antenna array (CUMA) for satellite uplink communications, where multiple ports are activated simultaneously on a single RF chain per antenna to coherently combine signals and suppress inter-user interference in shared-spectrum scenarios. It derives closed-form expressions for received signal power, interference power, and their distributions; obtains outage probability in unified form with an accurate approximation; and characterizes ergodic rate. The analysis claims that sufficiently compact FAS configurations render the received signal deterministic (performance then interference-dominated), with linear beamforming gain from increasing port count, and identifies conditions where CUMA outperforms maximum ratio combining. Numerical results compare orthogonal and non-orthogonal multiple access CUMA, favoring NOMA under wideband conditions.

Significance. If the derivations and ideal-model conclusions hold, the work offers analytical tools for low-cost massive arrays in satellite systems via fluid antennas, providing closed-form outage and rate expressions that enable design insights beyond simulation. The deterministic-signal regime under compactness is a potentially useful observation for interference-limited satellite uplinks. Credit is due for the unified outage form and the OMA/NOMA comparisons.

major comments (3)
  1. [§4] §4 (signal and interference power derivations): The received-signal model treats simultaneously activated ports as experiencing identical channel realizations with perfect phase coherence, yielding a deterministic signal power. No equation or subsection quantifies the required port spacing relative to wavelength or incorporates a mutual coupling matrix/phase-error term; this assumption is load-bearing for the claim that performance becomes interference-statistics-dominated and for the linear port-count gain.
  2. [§5] §5 (outage probability and ergodic rate): The closed-form outage expression and approximation, as well as the rate characterization, are obtained directly from the distributions under the ideal coherent-combining model. Any deviation from perfect coherence (e.g., coupling-induced amplitude/phase mismatch) would reintroduce signal randomness, altering the interference-dominance conclusion and requiring re-derivation of the unified outage form.
  3. [Numerical results] Numerical results section (comparison to MRC and NOMA/OMA): The simulations validate the analytical expressions but contain no sensitivity sweeps over port spacing, coupling coefficients, or phase-error variance; without these, the robustness of the deterministic-signal and linear-gain claims cannot be assessed against realistic hardware impairments.
minor comments (2)
  1. The abstract states that CUMA 'mitigates interference while reducing cost' but the text does not explicitly compare hardware cost or RF-chain count against a conventional massive MIMO baseline with the same number of elements.
  2. Notation for the number of activated ports and the compactness parameter could be introduced earlier and used consistently in the derivations to improve readability.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. We address each major comment point by point below, offering clarifications on the modeling assumptions while proposing targeted revisions to enhance the manuscript's transparency regarding the ideal coherent-combining regime.

read point-by-point responses

  1. Referee: §4 (signal and interference power derivations): The received-signal model treats simultaneously activated ports as experiencing identical channel realizations with perfect phase coherence, yielding a deterministic signal power. No equation or subsection quantifies the required port spacing relative to wavelength or incorporates a mutual coupling matrix/phase-error term; this assumption is load-bearing for the claim that performance becomes interference-statistics-dominated and for the linear port-count gain.

    Authors: We appreciate this observation. The derivations in §4 are developed under the limiting case of sufficiently compact FAS configurations, where port spacing approaches zero relative to the wavelength, causing the channels to the activated ports to become identical (perfect spatial correlation) and enabling coherent phase alignment. This leads to deterministic signal power as stated. We agree that an explicit quantification of the spacing threshold (e.g., via a minimum correlation coefficient derived from satellite channel models) would improve rigor. In revision, we will add a short paragraph in §4 referencing standard correlation functions for satellite links and stating the compactness condition under which the deterministic approximation holds. Mutual coupling and phase-error terms are omitted because the core contribution focuses on the ideal FAS-enabled coherent combining; their inclusion would require a substantially different model and is noted as future work. revision: partial

  2. Referee: §5 (outage probability and ergodic rate): The closed-form outage expression and approximation, as well as the rate characterization, are obtained directly from the distributions under the ideal coherent-combining model. Any deviation from perfect coherence (e.g., coupling-induced amplitude/phase mismatch) would reintroduce signal randomness, altering the interference-dominance conclusion and requiring re-derivation of the unified outage form.

    Authors: The closed-form results in §5 follow directly from the signal and interference distributions derived in §4 under the ideal model. We concur that relaxing perfect coherence would reintroduce randomness in the signal power and necessitate adjustments to the outage expression. To address this, the revised manuscript will include an explicit remark at the beginning of §5 reiterating that all expressions hold under the compactness conditions detailed in the updated §4. The unified outage form remains a valid analytical tool for the ideal case, which is the focus of the present work; extensions to imperfect coherence are left for subsequent studies. revision: partial

  3. Referee: Numerical results section (comparison to MRC and NOMA/OMA): The simulations validate the analytical expressions but contain no sensitivity sweeps over port spacing, coupling coefficients, or phase-error variance; without these, the robustness of the deterministic-signal and linear-gain claims cannot be assessed against realistic hardware impairments.

    Authors: The numerical results primarily serve to validate the closed-form expressions and to compare OMA versus NOMA performance under the proposed model. We acknowledge that sensitivity analysis to port spacing, coupling, and phase errors would help assess robustness. However, performing such sweeps while maintaining analytical tractability would require new approximations and extensive additional simulations that exceed the scope of the current contribution. In revision, we will add a paragraph in the numerical results section explicitly stating the ideal assumptions and identifying hardware-impairment sensitivity as an important avenue for future research. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivations follow standard modeling without reduction to inputs

full rationale

The paper's core derivations of received power, interference power, outage probability, and ergodic rate proceed from an explicit modeling choice: sufficiently compact fluid antenna ports experience a common channel coefficient enabling coherent combining. This leads to the deterministic-signal observation as a direct consequence of the stated compactness assumption rather than a fitted or self-referential result. No equations reduce a 'prediction' to a parameter fitted from the same data, and no load-bearing uniqueness theorem or ansatz is imported solely via self-citation. The analysis remains self-contained against external benchmarks such as standard SINR expressions in satellite uplink scenarios.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

The central claims rest on standard wireless channel and interference models plus the novel CUMA design concept; no free parameters or invented physical entities with independent evidence are stated in the abstract.

axioms (1)
  • domain assumption Standard assumptions of wireless channel modeling and interference statistics for satellite uplink links
    Invoked to obtain closed-form expressions for signal power, interference power, and their distributions.
invented entities (1)
  • Compact Ultra Massive Antenna Array (CUMA) no independent evidence
    purpose: To enable coherent signal combining across multiple fluid-antenna ports with a single RF chain for interference suppression
    New design introduced in the paper; no independent evidence outside this work is provided in the abstract.

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