Energy-harvesting relay-assisted STAR-RIS-enhanced vehicular NOMA networks

Abstract

The integration of simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) with non-orthogonal multiple access (NOMA) technology represents an effective approach for enabling massive device connectivity and achieving 360° network coverage. Under the NOMA scheme, the communication performance of weak users can be significantly enhanced, thereby improving user fairness; however, this often comes at the cost of performance degradation for strong users. Therefore, an energy harvesting relay-assisted STAR-RIS enhanced vehicular NOMA network is considered in this paper. Specifically, the paper provides a detailed analysis of the signal-to-noise ratio (SNR) at the near and far vehicles within the STAR-RIS-assisted vehicular NOMA network. The channel gain from the base station to the near vehicle via STAR-RIS and the composite channel gain between the base station and the far vehicle are approximated using Gamma distributions, with the accuracy of these approximations validated through Monte Carlo simulations. Based on the end-to-end SNR and the statistical characteristics of the channels, closed-form approximate expressions for the outage probabilities of near and far vehicles are rigorously derived, where the direct communication link exists between the far vehicle and the base station, while the energy-harvesting relay provides an auxiliary communication for the near vehicle. All analytical results are validated through simulations. The numerical and simulation results show that, without increasing the number of STAR-RIS elements, the outage performance of the near and far vehicles can be effectively controlled by adjusting the power allocation at the base station and the reflection/transmission coefficients of the STAR-RIS elements. This approach promotes fairness among vehicle users. Moreover, comparison with the orthogonal multiple access (OMA) scheme demonstrates that NOMA achieves better fairness between vehicle users and significantly reduces the outage probability for the far vehicle. Additionally, the energy-harvesting relay helps alleviate the negative impact of NOMA on the near vehicle, thereby further enhancing its communication stability.