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Security-Spectral Efficiency Tradeoff in STAR-RIS RSMA: A Max-Min Fairness Framework

Huiyun Xia, Yijie Mao, Sai Xu, Shuai Han, Hongbo Zhu

Abstract

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) enable full-space coverage but also expose wireless transmissions to security from multiple spatial directions. This paper investigates a STAR-RIS-assisted secure RSMA system where both internal and external eavesdroppers may coexist in the transmission and reflection regions. In such a scenario, the RSMA common stream simultaneously serves legitimate users, impairs external eavesdroppers, and avoids assisting internal eavesdroppers, leading to a challenging trade-off between spectral efficiency and confidentiality. To address this issue, we formulate a max-min fairness problem under secrecy constraints and develop an iterative algorithm to jointly optimize transmit beamforming and STAR-RIS phase shifts. Simulation results demonstrate that the proposed scheme improves spectral efficiency while maintaining confidentiality.

Security-Spectral Efficiency Tradeoff in STAR-RIS RSMA: A Max-Min Fairness Framework

Abstract

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) enable full-space coverage but also expose wireless transmissions to security from multiple spatial directions. This paper investigates a STAR-RIS-assisted secure RSMA system where both internal and external eavesdroppers may coexist in the transmission and reflection regions. In such a scenario, the RSMA common stream simultaneously serves legitimate users, impairs external eavesdroppers, and avoids assisting internal eavesdroppers, leading to a challenging trade-off between spectral efficiency and confidentiality. To address this issue, we formulate a max-min fairness problem under secrecy constraints and develop an iterative algorithm to jointly optimize transmit beamforming and STAR-RIS phase shifts. Simulation results demonstrate that the proposed scheme improves spectral efficiency while maintaining confidentiality.

Paper Structure

This paper contains 9 sections, 17 equations, 4 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model
  3. Problem Formulation and Optimization
  4. Problem Formulation
  5. Optimization Framework
  6. Optimization of phase shifts ${\bf\Theta}^l$
  7. Optimization of $\bf c$ and $\{{\bf w}_m,m\in\tilde{\mathcal{K}}\}$
  8. Numerical Results
  9. Conclusion

Figures (4)

  • Figure 1: Minimum user rate versus the secrecy threshold $r_{\rm E}$.
  • Figure 2: Minimum user rate versus the distribution radius $d$, $r_{\rm E}=0$dB.
  • Figure 3: Minimum user rate versus the number of STAR-RIS elements $N_{\rm S}$.
  • Figure 4: Minimum user rate vs the noise variance ratio $1/\sigma_{\rm E}^2$.