Performance Evaluations for RIS-Assisted GF-NOMA in Satellite-Aerial-Terrestrial Integrated Networks

Satellite-terrestrial integrated network is regarded as an effective solution to address the issues of network coverage blind spots in remote areas and uneven distribution of network resources. However, the limited spectrum resources fail to meet the ever-growing user demands, and satellite-to-ground communications are susceptible to obstruction and shadowing effects, which compromise signal transmission quality. To this end, this paper investigates a satellite-aerial-terrestrial integrated network (SATIN) enhanced by reconfigurable intelligent surfaces (RIS) and unmanned aerial vehicles (UAV) to bolster communication reliability and coverage capabilities. To reduce signaling overhead and transmission delay, the satellite employs a grant-free non-orthogonal multiple access (GF-NOMA) protocol for user communications. Considering that co-channel interference (CCI) is inevitable in practical communication scenarios, exact analytical expressions for the outage probabilities of near user (UN) and far user (UF) are derived under which the shadowed-Rician fading model and Nakagami-<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> fading model are employed for the satellite link and terrestrial link, respectively. Furthermore, an expression for the system outage probability and energy efficiency (EE) have been provided to facilitate further evaluation of the system performance. Simulation results demonstrate that increasing the number of RIS reflecting elements effectively mitigates the adverse impact of CCI on system outage performance. Even when satellite links are more severely affected by shadowing effects, the proposed system maintains excellent outage performance, verifying its robustness and applicability in complex environments. Additionally, to achieve optimal system EE, it is crucial to dynamically adjust the configuration parameters of number of RIS reflecting elements and transmit power of UAV based on specific application scenarios.