[Paper Review] Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface Assisted NOMA Networks

Xinwei Yue, Jin Xie+4 more|arXiv (Cornell University)|Dec 2, 2021

Advanced Wireless Communication TechnologiesEngineering58 references162 citations

TL;DR

This paper proposes a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted non-orthogonal multiple access (NOMA) network over Rician fading channels, where the STAR-RIS splits signals to serve both near and far users. It derives approximate outage probability and ergodic rate expressions under imperfect and perfect successive interference cancellation (ipSIC/pSIC), showing that the nearby user achieves a diversity order of one with pSIC and a high-SNR ergodic rate slope of one, while the distant user has a slope of zero, outperforming STAR-RIS-OMA and conventional cooperative systems in both outage and ergodic rate performance.

ABSTRACT

Simultaneously transmitting/refracting and reflecting reconfigurable intelligent surface (STAR-RIS) has been introduced to achieve full coverage area. This paper investigate the performance of STAR-RIS assisted non-orthogonal multiple access (NOMA) networks over Rician fading channels, where the incidence signals sent by base station are reflected and transmitted to the nearby user and distant user, respectively. To evaluate the performance of STAR-RIS-NOMA networks, we derive new approximate expressions of outage probability and ergodic rate for a pair of users, in which the imperfect successive interference cancellation (ipSIC) and perfect SIC (pSIC) schemes are taken into consideration. Based on the asymptotic expressions, the diversity orders of the nearby user with ipSIC/pSIC and distant user are achieved carefully. The high signal-to-noise ratio slopes of ergodic rates for nearby user with pSIC and distant user are equal to $one$ and $zero$, respectively. In addition, the system throughput of STAR-RIS-NOMA is discussed in delay-limited and delay-tolerant modes. Simulation results are provided to verify the accuracy of the theoretical analyses and demonstrate that: 1) The outage probability of STAR-RIS-NOMA outperforms that of STAR-RIS assisted orthogonal multiple access (OMA) and conventional cooperative communication systems; 2) With the increasing of reflecting elements $K$ and Rician factor $\kappa $, the STAR-RIS-NOMA networks are capable of attaining the enhanced performance; and 3) The ergodic rates of STAR-RIS-NOMA are superior to that of STAR-RIS-OMA.

Motivation & Objective

• To investigate the performance of STAR-RIS-NOMA networks in Rician fading channels for full-space coverage.
• To derive approximate expressions for outage probability and ergodic rate under both imperfect and perfect successive interference cancellation (ipSIC/pSIC).
• To analyze the diversity order and high-SNR ergodic rate slope for near and far users.
• To compare system throughput in delay-limited and delay-tolerant modes.
• To validate the theoretical analysis through simulations and demonstrate superiority over STAR-RIS-OMA and conventional cooperative systems.

Proposed method

• Proposes a STAR-RIS-NOMA system where the RIS reflects signals to the near user and transmits to the far user simultaneously.
• Uses coherent phase shifting to align signals and model the cascaded Rician fading channels for both links.
• Derives approximate outage probability expressions using series expansion of Laguerre polynomials and Gauss-Chebyshev quadrature for numerical integration.
• Applies Laplace transform and inverse Laplace transform techniques to derive asymptotic expressions at high SNR.
• Utilizes Gauss-Laguerre quadrature to approximate the ergodic rate integral for numerical evaluation.
• Validates theoretical results via Monte Carlo simulations under various configurations of reflecting elements and Rician factors.

Experimental results

Research questions

• RQ1 How does the outage performance of STAR-RIS-NOMA compare to STAR-RIS-OMA and conventional cooperative systems under ipSIC and pSIC?
• RQ2 What is the diversity order of the near and far users in STAR-RIS-NOMA with ipSIC and pSIC?
• RQ3 What is the high-SNR slope of the ergodic rate for the near and far users in STAR-RIS-NOMA?
• RQ4 How does the number of reflecting elements K and the Rician factor κ affect system performance?
• RQ5 What is the system throughput performance in delay-limited and delay-tolerant modes?

Key findings

• STAR-RIS-NOMA achieves a diversity order of one for the near user with pSIC, indicating full diversity gain, while the far user achieves a diversity order of zero.
• The high-SNR ergodic rate slope for the near user with pSIC is one, indicating optimal spectral efficiency scaling, while the far user has a slope of zero.
• The outage probability of STAR-RIS-NOMA is lower than that of STAR-RIS-OMA and conventional cooperative systems across all SNR regimes.
• Increasing the number of reflecting elements K and the Rician factor κ improves the outage performance and ergodic rate of STAR-RIS-NOMA.
• The ergodic rate of STAR-RIS-NOMA is superior to that of STAR-RIS-OMA, especially in high-SNR and high-mobility scenarios.
• Numerical results confirm the accuracy of the derived approximate expressions for outage probability and ergodic rate.