Unmanned aerial vehicles (UAVs) have emerged as a promising solution to provide wireless data access for ground users in various applications (e.g., in emergency situations). This paper considers a UAV-enabled wireless network, in which multiple UAVs are deployed as aerial base stations (BSs) to serve users distributed on the ground. Different from prior works that ignore UAVs' backhaul connections, we practically consider that these UAVs are connected to the core network through a ground gateway node via rate-limited multi-hop wireless backhauls. We also consider that the air-to-ground (A2G) access links from UAVs to users and the air-to-air (A2A) backhaul links among UAVs are operated over orthogonal frequency bands. Under this setup, we aim to maximize the common (or minimum) throughput among all the ground users in the downlink of this network subject to the flow conservation constraints at the UAVs, by optimizing the UAVs' deployment locations, jointly with the bandwidth and power allocation of both the access and backhaul links. However, the common throughput maximization is a non-convex optimization problem that is difficult to be solved optimally. To tackle this issue, we use the techniques of alternating optimization and successive convex programming (SCP) to obtain a locally optimal solution. Numerical results show that the proposed design significantly improves the common throughput among all ground users as compared to other benchmark schemes.Index Terms-Unmanned aerial vehicle (UAV), wireless networks, multi-hop backhauls, deployment optimization, bandwidth and power allocation. ). J. Xu is the corresponding author.in Internet-of-things (IoT) networks) [10]. Besides wireless communications, UAVs can also be used as aerial platforms for wireless power transfer (WPT) [11,12], wireless powered communication networks (WPCN) [13], and mobile edge computing (MEC) [14]. In the industry, various companies have launched their UAV-assisted wireless communication projects, and some preliminary prototypes include Facebook's Aquila [15] and Nokia's flying-cell (F-Cell) [16].As compared to conventional terrestrial wireless communications, UAV-assisted wireless communications have the following advantages. First, UAV-enabled aerial communication platforms can be quickly deployed on demand, and thus are cost-effective and suitable for emergency scenarios, e.g., when the terrestrial wireless infrastructures are damaged due to natural disasters. Next, the air-to-ground (A2G) wireless channels between UAVs and ground nodes normally have much stronger line-of-sight (LoS) links than conventional groundto-ground (G2G) wireless channels; as a result, the aerial BSs are expected to provide better wireless coverage and higher communication throughput than ground BSs. Furthermore, due to the fully controllable mobility in three-dimensional (3D) airspace, UAVs can adaptively change their locations over time for reducing the distances with intended ground users, so as to further improve the communication performance.In general,...
