Performance of Hybrid Satellite-UAV NOMA Systems Christina Gamal 1 , Kang An 2 , Xingwang Li 3 , Varun G. Menon 4 , Ragesh G. K. 5 , Mostafa M. Fouda 6 , Basem M. ElHalawany 1, * 1 Faculty of Engineering at Shoubra, Benha University, Egypt 2 Sixty-third Research Institute, National University of Defense Technology, Nanjing, China 3 Physics and Electronic Information Engineering, Henan Polytechnic University, China 4 SCMS School of Engineering and Technology, Ernakulam 683576, India 5 Indian Institute of Information Technology Kottayam, India 6 Department of Electrical and Computer Engineering, Idaho State University, Pocatello, ID, USA *Corresponding Author, Email: basem.mamdoh@feng.bu.edu.eg Abstract—This paper investigates the performance of non- orthogonal multiple access (NOMA) based hybrid satellite- unmanned aerial vehicle (UAV) systems, where a low Earth orbit (LEO) satellite communicates with the ground users via a decode and forward (DF) UAV relay. We investigate a two NOMA users system, where a far user (FU) and a near user (NU) are served by the UAV which is located at a certain height above the origin of the coverage circle. The channel between satellite and UAV is assumed to follow a Shadowed-Rician fading and the channels between UAV and users are assumed to follow a Nakagami-m fading. New closed- form expressions of the outage probabilities for the two users and the system are derived. Different from other work in literature, we take into consideration different parameters affecting the total link budget. Additionally, we propose an algorithm for minimizing the system outage probability. The mathematical analysis is verified by extensive representative Monte-Carlo (MC) simulations. Finally, simulations are provided to demonstrate the impact of important parameters on the considered system as well as the superiority of the NOMA scheme the over reference scheme. Index Terms—Outage Probability, Unmanned Aerial Vehicle, Satellite, Non-Orthogonal Multiple Access. I. I NTRODUCTION Recently, satellite communication (SatCom) has withdrawn an increasing research interest due to the several advantages offering over conventional terrestrial communication such as wide coverage area, covering harsh and isolated geographical regions where conventional wired or wireless communication can’t reach including maritime, deserts, and jungles. Moreover, SatCom serves well in disaster areas where the terrestrial networks are compromised. Additionally, SatCom can provide a wide range of flexible applications in the field of navigation, TV and Radio broadcasting services, Weather prediction and climate monitoring, Internet access, and satellite telephony [1]. On the other hand, SatCom networks face several challenges including operation cost [2], propagation delay [3], and signal degradation due to rain and atmospheric disturbances. Addition- ally, antenna-pointing errors angle caused by satellite perturba- tion or by the other side’s mobility may lead to communication outage [4]. Furthermore, the line-of-sight (LOS) link may be blocked by heavy shadowing or obstacles that retard communi- cation between the satellite and terrestrial users [5]. To combat such issues, hybrid satellite-terrestrial networks (HSTNs) based on relaying have been proposed in many literature [2], [5]– [7] to increase efficiency, and enhance the performance of the user whose direct link is unavailable or deteriorated. Satellites can be stationed in a variety of orbits including Low Earth orbit (LEO), medium Earth orbit (MEO), highly elliptical orbit (HEO), and geosynchronous orbit (GEO) [8]. Recently, LEO satellites constellation networks have withdrawn a great interest due to their small propagation delay, high data rate, and lower transmit power [9]. Consequently, we consider a LEO satellite setup in this work. On the other hand, unmanned aerial vehicles (UAVs) have been used as a wireless flying base station, mobile relay, or backhaul to improve the coverage, flexibility, and reliability of the network [1] in order to provide a variety of applications including reconnaissance, surveillance, disaster management, traffic control, healthcare, emergency search, military, agricul- ture, and communication relay [10], [11]. Thus, the combi- nation of a UAV and a satellite has the potential to provide a technological breakthrough for communication networks due to the UAV’s flexible mobility [12]–[14]. However, UAVs commu- nication have their own challenges including limited bandwidth and limited battery [9], which mandates the exploitation of spectrum and energy efficient techniques in both transmission and mobility. Non-orthogonal multiple access(NOMA) is one of the most promising and spectrum efficient techniques with a significant attention. NOMA improves the spectrum efficiency by serving more than one user at the same time with the same frequency resource [15], [16]. The power-domain NOMA (PD-NOMA) is the most widely used type NOMA at which a superposition of users’ signals is transmitted using different power levels. However, this type of non-orthogonality causes interference at the receiver. To cope with this, successive interference cancellation (SIC) is used at the receiver to successively decode users’ information one by one from the superimposing signal in the order of the received signal strength [17]. Improved spectrum efficiency, high connectivity, multiplexes many users, flexible power control method between strong and weak users, and low latency are all benefits of the NOMA scheme [18]. In contrast, the orthogonal multiple access (OMA) scheme is not the best choice to achieve the requirements for SatCom as OMA provides limit number of served users and reduced resource efficiency [4]. NOMA has been investigated for several network architectures and applications and proved to be an efficient technology [19]–[22]. 4963