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Phase-mismatched STAR-RIS with FAS-assisted RSMA Users

Farshad Rostami Ghadi, Kai-Kit Wong, Masoud Kaveh, F. Javier Lopez-Martinez, Yuanwei Liu, Chan-Byoung Chae, Ross Murch

TL;DR

This work addresses BS-to-two-user communication via a STAR-RIS with phase errors, where both users employ planar Fluid Antenna Systems (FAS) and RSMA is used to manage interference. It develops a realistic statistical model in which the equivalent FAS gain $g_{ ext{fas},u}$ is the maximum of correlated Gamma RVs, characterized through a multivariate Student-$t$ copula, and derives both outage probability and a heuristic average capacity. A von Mises phase-error model and a Jensen-based approximation enable tractable analysis, yielding compact OP expressions and practical AC estimates. Numerical results show that FAS substantially improves reliability and capacity compared to traditional antennas, and RSMA outperforms NOMA under phase imperfections, highlighting the robustness and potential of FAS-assisted STAR-RIS RSMA for next-generation networks.

Abstract

This paper considers communication between a base station (BS) to two users, each from one side of a simultaneously transmitting-reflecting reconfigurable intelligent surface (STAR-RIS) in the absence of a direct link. Rate-splitting multiple access (RSMA) strategy is employed and the STAR-RIS is subjected to phase errors. The users are equipped with a planar fluid antenna system (FAS) with position reconfigurability for spatial diversity. First, we derive the distribution of the equivalent channel gain at the FAS-equipped users, characterized by a t-distribution. We then obtain analytical expressions for the outage probability (OP) and average capacity (AC), with the latter obtained via a heuristic approach. Our findings highlight the potential of FAS to mitigate phase imperfections in STAR-RIS-assisted communications, significantly enhancing system performance compared to traditional antenna systems (TAS). Also, we quantify the impact of practical phase errors on system efficiency, emphasizing the importance of robust strategies for next-generation wireless networks.

Phase-mismatched STAR-RIS with FAS-assisted RSMA Users

TL;DR

This work addresses BS-to-two-user communication via a STAR-RIS with phase errors, where both users employ planar Fluid Antenna Systems (FAS) and RSMA is used to manage interference. It develops a realistic statistical model in which the equivalent FAS gain is the maximum of correlated Gamma RVs, characterized through a multivariate Student- copula, and derives both outage probability and a heuristic average capacity. A von Mises phase-error model and a Jensen-based approximation enable tractable analysis, yielding compact OP expressions and practical AC estimates. Numerical results show that FAS substantially improves reliability and capacity compared to traditional antennas, and RSMA outperforms NOMA under phase imperfections, highlighting the robustness and potential of FAS-assisted STAR-RIS RSMA for next-generation networks.

Abstract

This paper considers communication between a base station (BS) to two users, each from one side of a simultaneously transmitting-reflecting reconfigurable intelligent surface (STAR-RIS) in the absence of a direct link. Rate-splitting multiple access (RSMA) strategy is employed and the STAR-RIS is subjected to phase errors. The users are equipped with a planar fluid antenna system (FAS) with position reconfigurability for spatial diversity. First, we derive the distribution of the equivalent channel gain at the FAS-equipped users, characterized by a t-distribution. We then obtain analytical expressions for the outage probability (OP) and average capacity (AC), with the latter obtained via a heuristic approach. Our findings highlight the potential of FAS to mitigate phase imperfections in STAR-RIS-assisted communications, significantly enhancing system performance compared to traditional antenna systems (TAS). Also, we quantify the impact of practical phase errors on system efficiency, emphasizing the importance of robust strategies for next-generation wireless networks.

Paper Structure

This paper contains 13 sections, 4 theorems, 50 equations, 9 figures.

Table of Contents

  1. Introduction
  2. State-of-the-Art
  3. Motivation and Contributions
  4. Organization and Notations
  5. System Model
  6. Channel and Signal Model
  7. SINR Characterization
  8. Performance Analysis
  9. Statistical Characterization
  10. OP Analysis
  11. AC Analysis
  12. Numerical Results
  13. Conclusion

Key Result

Proposition 1

The CDF and PDF of the equivalent channel gain $g_{\mathrm{fas},u}$ for user $u$ considering phase errors in the FAS-aided STAR-RIS RSMA are given by eq-cdf and eq-pdf (see top of next page), where $t_{\nu_u}^{-1}\left(\cdot\right)$ is the inverse CDF (quantile function) of the univariate $t$-distri

Figures (9)

  • Figure 1: A FAS-aided STAR-RIS system with two users, one on each side.
  • Figure 2: The OP results of FAS-assisted STAR-RIS RSMA versus the average SNR $\overline{\gamma}$ with phase errors and ideal phases for $K=30$.
  • Figure 3: The OP results of FAS-assisted STAR-RIS versus the average SNR $\overline{\gamma}$ with phase errors and ideal phase for different multiple access scenarios when $K=30$.
  • Figure 4: The OP results of FAS-assisted STAR-RIS RSMA versus the number of STAR-RIS elements $K$ with phase errors and ideal phase for $\overline{\gamma}=50$ dB.
  • Figure 5: The OP results of FAS-assisted STAR-RIS RSMA versus the square root of reflection coefficient $\beta_\mathrm{r}$ with phase errors for $\overline{\gamma}=50$ dB and $K=55$.
  • ...and 4 more figures

Theorems & Definitions (10)

  • Proposition 1
  • proof
  • Proposition 2
  • proof
  • Corollary 1
  • proof
  • Remark 1
  • Proposition 3
  • proof
  • Remark 2