Exploiting UAV as NOMA based Relay for Coverage Extension Syeda Kanwal Zaidi 1 , Syed Faraz Hasan 1 , Xiang Gui 1 , Nazmul Siddique 2 , and Salmiah Ahmad 3 1 School of Engineering and Advanced Technology, Massey University, New Zealand 2 School of Computing and Intelligent Systems, Ulster University, UK 3 Department of Electrical and Computer Engineering, King Abdul Aziz University, Jeddah, KSA Correspondance: k.zaidi@massey.ac.nz Abstract—Unmanned aerial vehicles (UAVs) aided commu- nication has acquired research interest in many civilian and military applications. The use of UAV as base stations and as aerial relays to improve coverage of existing cellular networks is prevalent in current literature. Along with this, a few studies have proposed the use of non-orthogonal multiple access (NOMA) in UAV communications. In this paper, we propose a network where a ground user and an aerial UAV relay is accessed using NOMA, where the UAV acts as decode-and-forward (DF) relay to extend the coverage of source. The performance of the proposed model is shown by evaluating outage behaviour for different transmit power and fading environments with Monte Carlo simulations. System throughput of proposed network appears to be better than orthogonal multiple access (OMA) based equivalent network. The results show that with an adequate height of the UAV NOMA based relay, quality of service (QoS) of cell edge user is satisfactory. Index Terms—Unmanned aerial vehicles (UAVs), non- orthogonal multiple access (NOMA), relaying I. I NTRODUCTION Unmanned Aerial vehicles (UAVs), commonly known as drones, have gained popularity in the last few years in a wide range of applications including parcel delivery, online stream- ing, traffic monitoring, disaster management, agriculture and farming. In most of the scenarios, these UAVs are deployed as low altitude platforms (LAPs) with the ability of autonomously or remotely operated, with the aim of providing UAV-assisted communication. Recently, UAVs have received significant recognition by academia, research industry and government bodies to aid wireless communications by providing ubiqui- tous coverage and promote information dissemination [1]. For future wireless communications, these low cost devices with high mobility are expected to provide wireless connectivity in several use cases for example, as a substitute of fixed base station (BS) in case of natural disasters, as an adhoc base station to offload data in crowded networks, as an on demand relay to extend the coverage of existing network when there is severe shadowing between users and the base station. All these mentioned scenarios are applicable to beyond fifth-generation (B5G) networks and UAVs stand as as strong candidate to be utilised in B5G networks. Non-orthogonal multiple access (NOMA) is an essential technology for the radio access design of 5G cellular networks. Power-domain NOMA works on the concept of assigning vary- ing power levels to users within the same frequency and time block. Unlike serving the users in orthogonal resource block in orthogonal multiple access (OMA), several research works demonstrated that NOMA improves the spectral efficiency and throughput of the system by serving users non-orthogonally [2]. In NOMA, the users are served together based on their distinct channel conditions and different quality of service (QoS) requirements. Since UAVs have distinct communication characteristics than ground users which are mentioned in Section II, NOMA appears to be a great fit to access ground users and UAV together. This research paper highlights the existing research works where UAV communication has been considered to aid existing cellular networks for improved cov- erage. Furthermore, a comprehensive overview of the existing works on NOMA based UAV communication is also given. We introduce the concept of ground-aerial NOMA where a ground user and a UAV are served together non-orthogonally based on their unique channel characteristics. In the proposed network, UAV acts as a relay to extend the coverage of source and serve a cell-edge user. With Monte Carlo simulations, we evaluate the performance of the network by evaluating outage behaviour at serviced ground users. We also compare the system throughput of the proposed system with OMA based network and show that our proposed network is superior in performance than OMA based network. II. UAVS IN WIRELESS COMMUNICATION The ever increasing network connected devices such smart- phones, smart-tablets, internet-of-things connected sensors emboss a huge network traffic on wireless networks which then needs to be fueled up with advance technologies to cater the data traffic demand. There has been several new research ventures introduced for example, full duplexing, multiple input multiple output (MIMO), device-to device (D2D) communica- tions, milli meter wave communication, NOMA to meet B5G network goals. To meet the similar objectives, UAVs have also been introduced in several research works to improve the network coverage and provide seamless connectivity to the devices where infrastructure is not present or been demol- ished because of natural disasters. In [1], a high level UAV- aided network architecture is explained to highlight the main channel characteristics, design considerations and available 978-1-5386-3531-5/17/$31.00 c  2017 IEEE