Satellite-aerial-terrestrial network (SATN) is considered as a promising architecture for sixth-generation (6G) wireless communication networks to achieve seamless coverage, flexible wireless access, and high data rate. Moreover, non-orthogonal multiple access (NOMA), and reconfigurable intelligent surface (RIS) can significantly increase spectrum and energy efficiency. Recently, the integration of these two technologies and SATN has attracted a lot of attention both in academia and industry. This survey provides a comprehensive overview of RIS-empowered SATN with NOMA. In particular, the rudimentary knowledge of SATN, NOMA scheme, and RIS technology is presented. Then, the motivations for investigating the NOMA-RIS-assisted SATN are discussed. In addition, we introduce the three usage modes of RIS, two scenarios of NOMA-RIS, and the path loss model of NOMA-RIS-assisted SATN. Next, the system performance is analyzed for a case study. Besides, a comprehensive overview of resource allocation in NOMA-RIS-assisted SATN is provided, where theoretical and artificial intelligence-based methods are compared and analyzed. Moreover, physical layer security and covert communication are selected as two representative security techniques to be discussed in NOMA-RIS-aided SATN. Furthermore, the combination of other emerging technologies with NOMA-RIS-assisted SATN is investigated. Finally, this survey provides a detailed discussion of the main challenges and open issues that need to be deeply investigated from a practical point of view, including channel modeling, channel estimation, deployment strategies, and backhaul control.

In this paper, we investigate the outage behavior of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-empowered cognitive non-terrestrial vehicle network with non-orthogonal multiple access (NOMA), in which Rician fading channels and imperfect/perfect successive interference cancellation (ipSIC/pSIC) are considered. In the network, STAR-RIS is utilized to assist the transmission and reflection of the superposed signals from a secondary unmanned aerial vehicle. Under the interference constraints of primary vehicle users, we derive the exact expressions of outage probability for all secondary vehicle users to evaluate the system parameters. Moreover, to gain deeper insights, the asymptotic expressions of outage probability along with the diversity order are obtained. Then, from numerical results, we confirm the correctness of our theoretical analysis and discuss the influences of key parameters on system performance. Compared with STAR-RIS-orthogonal multiple access (OMA)/non-STAR-RIS scheme, the superiority of our scheme is verified.

Recently, non-orthogonal multiple access (NOMA)-enabled cognitive satellite-unmanned aerial vehicle (UAV)-terrestrial networks have attracted extensive attention for the advantages of enhancing spectrum efficiency and coping with the exponential growth of access users. In this paper, we propose a joint subchannel assignment and power allocation algorithm for NOMA-enabled cognitive satellite-UAV-terrestrial networks to further enhance the transmission performance, where imperfect channel state information is taken into consideration. Specifically, we formulate a mixed integer non-linear programming resource allocation problem to optimize the sum rate of the secondary network, in which the demands of the interference temperature constraint for primary users, the minimum transmission rate of each secondary user, the maximal transmitter power of the UAV, and the maximal number of secondary users that each subchannel can serve are satisfied. To tackle this tough non-convex problem, we first decouple it into two subproblems, namely, subchannel assignment and power allocation. Next, a heuristic subchannel assignment algorithm and a Taylor series and successive convex approximation-based power allocation algorithm are designed to solve the above subproblems, respectively. By alternately optimizing the sum rate of the secondary network, we finally solve the mixed integer non-linear programming optimization problem. Numerical results reveal the impacts of key parameters on system performance and indicate that our proposed scheme outperforms benchmarks in large-scale networks.

Cognitive radio and non-orthogonal multiple access are promising to improve the spectrum efficiency for the satellite-high altitude platform (HAP)-terrestrial networks. To achieve the optimal system performance and guarantee the quality of service, we formulate a constrained mixed integer nonlinear programming problem to maximize the sum rate of secondary network for the considered NOMA-enabled cognitive satellite-HAP-terrestrial network, while considering the constraints of the quality of service of primary users, the maximum power of HAP, and the number of secondary users in a NOMA group. To tackle the non-convex optimization problem, we decouple it into subchannel assignment and power assignment subproblems. Then, a greedy heuristic algorithm is proposed to assign subchannels to secondary users. Moreover, to solve the power allocation subproblem, we further design a power allocation algorithm by utilizing successive convex approximation, dual decomposition, and subgradient methods. On this basis, an iterative joint resource allocation algorithm is designed. Numerical simulations and comparisons with the orthogonal multiple access scheme are provided to verify the effectiveness of the proposed scheme.

With the increasing requirements for efficient utilization of satellite resources, both non-orthogonal multiple access (NOMA) scheme and cognitive radio (CR) have been recognized as one of the promising solutions, particularly, for the enhancement of spectrum efficiency. This letter investigates the performance of NOMA-based integrated satellite-terrestrial relay network (ISTRN) in the presence of multiple primary users (PUs) under spectrum sharing environment. Specifically, we derive the closed-form expressions of outage probability (OP) and ergodic capacity (EC) of the considered system under the interference constraint imposed by multiple adjacent PUs. To get further insights, the asymptotic analysis for OP at high signal-to-noise ratios (SNRs) is also obtained. Numerical results are provided to verify our analysis and reveal the impacts of key parameters on system performance.

In this paper, we investigate the performance of non-orthogonal multiple access (NOMA)-based overlay cognitive integrated satellite-terrestrial relay network with secondary network selection, where the selected secondary network accesses the licensed spectrum by assisting the communication of primary network with amplify-and-forward protocol. Besides, NOMA techniques are used to improve the spectrum efficiency. Specifically, the closed-form expressions of outage probability (OP) for both primary network and secondary network are obtained. To get inherent insights, the asymptotic analysis of OP is further derived at high signal-to-noise ratios. Moreover, the diversity orders of the both networks are further provided. Finally, Monte Carlo simulations are given to valuate our analysis, and the effects of key parameters on system performance are revealed. Through the comparison between orthogonal multiple access (OMA) scheme and NOMA scheme, the superiority of our proposed scheme is proved.