devices are distributed at hard-to-reach locations such as on top of trees, outside high-rise buildings, or even under bridges. In this case, cellular base stations can act as data collectors in IoT data collection networks. However, the limited energy storage capacity and communication capability hinder IoT devices from transmitting their data over long distances. Thus, IoT data collection networks have been demanding effective and flexible solutions for data acquisition.Recently, UAVs have been emerging as a promising approach to tackle the above challenges. In particular, when UAVs act as on-demand flying APs, thanks to their aerial superiority, they can establish good line-of-sight (LoS) links for the IoT nodes. This leads to better wireless communication channels, and thereby improving the quality of service (QoS) in comparison with traditional approaches, especially for IoT applications that are sensitive to delay and/or require stable communications [3]. In addition, in remote areas without access to terrestrial infrastructures, UAVs can provide a much more economic solution to collect IoT data than traditional approaches, e.g., long-range ground broadcasting stations or high-cost satellite communications. Another important advantage of UAVs is that they can be promptly established in emergency circumstances, in which the existing infrastructure is disrupted and incapable of receiving data from IoT devices [4]. Due to the flexibility, mobility, and low operational cost, UAVs have been being deployed as flying APs for some real-world projects, e.g., Google's Loon and Facebook's Aquila [5], [6].However, there are still some challenges that hinder the applications of UAVs in IoT data collection networks. In particular, unlike traditional solutions for collecting IoT data (e.g., deploying fixed APs), UAVs have limited energy resources supplied by batteries. When the UAVs' batteries are depleted, they must replenish their energy by flying back to the charging stations to charge or replace their batteries. It is worth noting that given a fixed working duration, the more time the energy replenishment process takes, the less time the UAVs can spend for collecting IoT data. Alternatively, the energy replenishment process is highly dynamic since it depends on the distance between the UAV and the charging stations. Therefore, optimizing energy usage and the energy replenishment process is critical to achieving high system performance, but very challenging in practice. Moreover, the UAVs often fly around to collect IoT data, while IoT nodes are statically allocated over different zones, and their sensing data are random depending on surrounding environments. To that end, optimizing operations of UAVs in different zones to