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arxiv: 2512.00302 · v1 · submitted 2025-11-29 · 📡 eess.SP

FAS-RSMA: Can Fluid Antennas Elevate RSMA Performance?

Jinyuan Liu , Yong Liang Guan , Tuo Wu , Kai-Kit Wong , Bruno Clerckx This is my paper

Pith reviewed 2026-05-17 03:57 UTC · model grok-4.3

classification 📡 eess.SP
keywords fluid antenna systemsrate-splitting multiple accessoutage probabilityaverage capacityspatial correlationimperfect CSITSISO broadcast channel
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The pith

Fluid antennas combined with rate-splitting multiple access lower outage probability and raise average capacity in SISO broadcast channels.

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

This paper investigates whether fluid antenna systems can improve the performance of rate-splitting multiple access in practical multiuser SISO broadcast settings with imperfect channel state information. It develops an analytical framework using block correlation models to account for spatial dependence between antenna ports. Closed-form expressions for outage probability and average capacity demonstrate that dynamic port selection strengthens the weakest channel and mitigates interference effects. Simulations confirm gains over conventional fixed-antenna RSMA and NOMA schemes.

Core claim

The paper claims that FAS-RSMA, through adaptive port reconfiguration, provides clear performance gains by combining RSMA's interference management with FAS spatial diversity, resulting in lower outage probability and higher average capacity compared to conventional antenna systems and NOMA.

What carries the argument

Block-correlation models (constant block correlation and variable block correlation) that capture realistic spatial dependence among fluid antenna ports and enable derivation of closed-form performance metrics for outage probability and average capacity.

If this is right

  • Dynamic port reconfiguration strengthens the weakest effective channel and improves SINR through higher channel gains and lower relative noise impact.
  • Variable block correlation model matches simulation results more tightly than constant block correlation across all port configurations.
  • FAS-RSMA achieves lower outage probability and higher average capacity than both conventional fixed-antenna systems and NOMA.
  • The gains arise without altering the underlying RSMA signaling structure of common and private streams.

Where Pith is reading between the lines

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

  • The correlation-aware framework could be adapted to evaluate FAS with other multiple-access schemes that rely on imperfect CSIT.
  • Port reconfiguration may offer similar benefits in heterogeneous traffic scenarios beyond the SISO broadcast case examined here.
  • Real-world deployment would require testing whether the modeled spatial dependence holds under mobility and hardware constraints.

Load-bearing premise

The constant block correlation and variable block correlation models sufficiently capture realistic spatial dependence among fluid antenna ports in practical SISO broadcast channels.

What would settle it

Monte Carlo simulations or channel measurements using actual fluid antenna hardware that show no reduction in outage probability or increase in average capacity relative to fixed-antenna RSMA would refute the claimed gains.

Figures

Figures reproduced from arXiv: 2512.00302 by Bruno Clerckx, Jinyuan Liu, Kai-Kit Wong, Tuo Wu, Yong Liang Guan.

Figure 1
Figure 1. Figure 1: Multi-user downlink FAS-RSMA communication system. where P denotes the total transmission power, tc and tu represent the power allocation factors for the common stream and the u-th user’s private stream, respectively. sc and sp,u are the common and private data streams, respectively. To meet the total power constraint, we have tc + PU u=1 tu = 1. Then, the received signal of the u-th user at the n-th port1… view at source ↗
Figure 2
Figure 2. Figure 2: Outage probability comparison between different correlation models with different parameter configurations 0 1 2 3 4 5 6 Average SNR (dB) 10-5 10-4 10-3 10-2 10-1 100 Outage probability FAS-RSMA, User 1, N=10 FAS-RSMA, User 2, N=10 FAS-RSMA, User 1, N=5 FAS-RSMA, User 2, N=5 TAS-RSMA, User 1 TAS-RSMA, User 2 FAS-RSMA, Simulation TAS-RSMA, Simulation [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Performance of outage probability comparison between FAS and TAS of RSMA system. configuration, results are presented for both user 1 and user 2. For both N values, the FAS-NOMA curves of user 1 and user 2 essentially overlap. This behavior stems from the current power allocation: the power split between the two NOMA users is not highly asymmetric, so although user 1 is assigned slightly more power, in the… view at source ↗
Figure 4
Figure 4. Figure 4: Performance of outage probability comparison between RSMA and NOMA. OP as a function of the common power allocation factor tc under a fixed private power allocation factor (tp,1, tp,2) = [PITH_FULL_IMAGE:figures/full_fig_p010_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Outage probability versus power allocation factors in FAS–RSMA. In (a), the private power fractions are tied to tp,1 = 0.6(1 − tc) and tp,2 = 0.4(1 − tc). In (b), the common power allocation is fixed to tc = 0.7 and the private power allocation varies. 0 1 2 3 4 5 6 Average SNR (dB) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Average capacity (bps/Hz) FAS-RSMA Common FAS-RSMA Private User One FAS-RSMA Private User Two… view at source ↗
Figure 6
Figure 6. Figure 6: Performance of average capacity for common and private messages of FAS-RSMA and TAS-RSMA. 0 1 2 3 4 5 6 Average SNR (dB) 0.5 1 1.5 2 2.5 3 Average sum capacity (bps/Hz) VBC FAS-RSMA CBC FAS-RSMA, d=0.95 CBC FAS-RSMA, d=0.97 TAS-RSMA Simulation, FAS-RSMA Simulation, TAS-RSMA N=10 N=5 [PITH_FULL_IMAGE:figures/full_fig_p011_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Performance of average sum capacity between different correlation models. V. CONCLUSION This paper has developed a correlation-aware analytical framework for the FAS-RSMA system. FAS-driven dynamic port reconfiguration enables spatial selectivity for RSMA, strengthening the weakest user link and boosting SINR. Using block-correlation modeling with both CBC and VBC models, we derived closed-form expressions… view at source ↗
read the original abstract

As 6G networks demand massive connectivity and stronger interference control, rate-splitting multiple access (RSMA) is attractive because it superposes a common stream and user-private streams and remains effective under imperfect CSIT and heterogeneous traffic. In practical multiuser deployments, two considerations arise: the common stream decoding constraint imposed by the weakest user, and residual inter-user interference can remain non-negligible, particularly in single-input single-output (SISO) broadcast settings and under an imperfect CSIT scenario. Motivated by prior advances of RSMA research, we investigate a complementary mechanism-fluid antenna systems (FAS), with dynamic port reconfiguration supplies adaptive spatial selectivity without altering the RSMA signaling structure. Can FAS help alleviate these considerations and enhance RSMA performance? We develop a tractable correlation-aware analytical framework based on block-correlation models, including constant block correlation (CBC) and variable block correlation (VBC), to capture realistic spatial dependence among ports. Closed-form expressions are derived for outage probability (OP) and average capacity (AC), revealing how port reconfiguration strengthens the weakest effective channel and improves SINR through higher channel gains and lower relative noise impact. Monte Carlo simulations verify the analysis and show that VBC matches simulations more tightly than CBC across all port configurations. Finally, FAS-RSMA provides clear gains over conventional antenna systems and NOMA, achieving lower OP and higher AC by combining RSMA interference management with FAS spatial diversity.

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

2 major / 2 minor

Summary. The manuscript investigates integrating fluid antenna systems (FAS) with rate-splitting multiple access (RSMA) in SISO broadcast channels to improve outage probability and average capacity under imperfect CSIT. It introduces a correlation-aware framework based on constant block correlation (CBC) and variable block correlation (VBC) models, derives closed-form expressions for OP and AC, verifies them via Monte Carlo simulations (with VBC matching more closely), and reports performance gains over fixed-antenna RSMA and NOMA through enhanced spatial diversity and interference management.

Significance. If the modeling assumptions hold, the work offers a tractable way to quantify how FAS port reconfiguration can strengthen the weakest-user channel and complement RSMA's common/private stream structure, yielding lower OP and higher AC in multiuser SISO settings. Strengths include the closed-form derivations, explicit baseline comparisons, and Monte Carlo verification that confirms internal consistency with the chosen models.

major comments (2)
  1. [§III] §III (Channel and Correlation Models): The CBC and VBC models are load-bearing for the SINR statistics and subsequent closed-form OP/AC expressions. These discretize ports and impose a specific block-correlation structure; the paper does not compare the assumed coefficients against measured covariances or continuous-position electromagnetic models of fluid antennas. If the structure overestimates effective diversity for the weakest user, the reported gains relative to NOMA (visible in the simulation figures) become unreliable.
  2. [§IV] §IV (Outage Probability Derivation): The closed-form OP for the common stream relies on the order statistics under the VBC model. Without a sensitivity study on the correlation coefficients or a comparison to more general covariance matrices, it is unclear whether the claimed strengthening of the weakest effective channel holds beyond the specific model assumptions.
minor comments (2)
  1. [Abstract] The abstract and introduction could more explicitly state that all quantitative gains are conditional on the block-correlation models.
  2. [Figures] Figure captions should specify the number of Monte Carlo realizations and the exact parameter values used for the correlation coefficients.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive and detailed comments on our manuscript. We address the major comments point by point below, indicating where revisions will be made to strengthen the presentation.

read point-by-point responses

  1. Referee: [§III] §III (Channel and Correlation Models): The CBC and VBC models are load-bearing for the SINR statistics and subsequent closed-form OP/AC expressions. These discretize ports and impose a specific block-correlation structure; the paper does not compare the assumed coefficients against measured covariances or continuous-position electromagnetic models of fluid antennas. If the structure overestimates effective diversity for the weakest user, the reported gains relative to NOMA (visible in the simulation figures) become unreliable.

    Authors: The CBC and VBC models were selected to enable tractable closed-form derivations while capturing the dominant block-wise spatial correlation effects that arise in fluid antenna port selection. These structures are consistent with modeling approaches used in prior FAS literature for analytical purposes. We agree that direct benchmarking against measured covariance matrices or full continuous-position electromagnetic simulations would provide additional context; such data are not incorporated here because they are not publicly standardized and would require dedicated experimental campaigns. The Monte Carlo results in the paper confirm consistency between analysis and simulation under the stated models. In the revision we will expand the discussion in §III to justify the model choice with references to electromagnetic principles of reconfigurable antennas and to explicitly note the model-specific nature of the reported gains. revision: partial

  2. Referee: [§IV] §IV (Outage Probability Derivation): The closed-form OP for the common stream relies on the order statistics under the VBC model. Without a sensitivity study on the correlation coefficients or a comparison to more general covariance matrices, it is unclear whether the claimed strengthening of the weakest effective channel holds beyond the specific model assumptions.

    Authors: The derivations in §IV are developed under the VBC model precisely because it allows variable intra-block correlations that better reflect realistic port reconfiguration. We concur that a sensitivity study would clarify robustness. In the revised manuscript we will add numerical results that vary the correlation coefficients over representative ranges and recompute the outage probability for the common stream, thereby showing that the improvement in the weakest-user effective channel persists. We will also include a short discussion on how the qualitative conclusions extend to more general covariance structures. revision: yes

standing simulated objections not resolved
  • Direct comparison of the assumed correlation coefficients to measured covariances obtained from physical fluid-antenna prototypes or to full-wave continuous-position electromagnetic models, as this would require new experimental data collection outside the scope of the present theoretical analysis.

Circularity Check

0 steps flagged

No significant circularity; derivations are self-contained under stated channel models

full rationale

The paper introduces block-correlation models (CBC/VBC) as assumptions to capture port spatial dependence, then derives closed-form OP and AC expressions from standard SINR statistics under those models. Monte Carlo simulations are used only to verify internal consistency with the same models. No step reduces the target performance metrics to a fitted parameter or self-citation that is itself defined by the result; the correlation structures are external modeling choices, not outputs of the present analysis. The central claim of gains is therefore a direct consequence of the derivations under the stated assumptions rather than a tautology.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 0 invented entities

The central claim rests on domain-standard wireless channel assumptions plus two block-correlation models introduced for tractability; no new physical entities are postulated.

free parameters (1)
  • block correlation coefficients
    Parameters defining spatial dependence in CBC and VBC models that shape the closed-form OP and AC expressions.
axioms (1)
  • domain assumption Block-correlation models (CBC and VBC) accurately represent port-to-port dependence in fluid antenna systems.
    Invoked to obtain tractable closed-form expressions for outage probability and average capacity.

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