Intelligent Reflecting Surface Enhanced Multi-UAV NOMA Networks

Intelligent reflecting surface (IRS) enhanced multi-unmanned aerial vehicle (UAV) non-orthogonal multiple access (NOMA) networks are investigated. A new transmission framework is proposed, where multiple UAV-mounted base stations employ NOMA to serve multiple groups of ground users with the aid of an IRS. The three-dimensional (3D) placement and transmit power of UAVs, the reflection matrix of the IRS, and the NOMA decoding orders among users are jointly optimized for maximization of the sum rate of considered networks. To tackle the formulated mixed-integer non-convex optimization problem with coupled variables, a block coordinate descent (BCD)-based iterative algorithm is developed. Specifically, the original problem is decomposed into three subproblems, which are alternately solved by exploiting the penalty-based method and the successive convex approximation technique. The proposed BCD-based algorithm is demonstrated to be able to obtain a stationary point of the original problem with polynomial time complexity. Numerical results show that: 1) the proposed NOMA-IRS scheme for multi-UAV networks achieves a higher sum rate compared to the benchmark schemes, i.e., orthogonal multiple access (OMA)-IRS and NOMA without IRS; 2) the use of IRS is capable of providing performance gain for multi-UAV networks by both enhancing channel qualities of UAVs to their served users and mitigating the inter-UAV interference; and 3) optimizing the UAV placement can make the sum rate gain brought by NOMA more distinct due to the flexible decoding order design.

I. Introduction

With the rapid development of manufacturing technology and the continuous reduction of costs, unmanned aerial vehicles (UAVs), also known as drones, have received significant attention for potential use in civil applications, such as cargo delivery, search and rescue, and traffic monitoring [1], [2]. Among others, UAV-enabled communications is one of the most appealing applications. Equipped with communication devices, UAVs can act as aerial base stations (BSs) to provide wireless communication service in many practical scenarios (e.g., communication recovery after natural disasters or traffic offloading for temporary hotspots). Compared with conventional terrestrial communications, on the one hand, the air-to-ground (A2G) channel has a high probability of being dominated by line-of-sight (LoS) links [3], which helps to establish high data rates and reliable transmissions. On the other hand, the mobility of UAVs is controllable and can be exploited to improve the performance of communications. For instance, the UAV can fly closer to its intended ground user to achieve better channel conditions.