Unmanned aerial vehicles (UAVs) have played an important role in air-ground integration network. Especially in Internet of Things (IoT) services, UAV equipped with communication equipments is widely adopted as a mobile base station (BS) for data collection from IoT devices on the ground. In this paper, we consider an air-ground network in which the UAV flies straightly to collect information from the IoT devices in a 2-D plane based on the CSMA/CA protocol. Due to UAV's continuous mobility, the communication durations of devices in different locations with UAV are not only time-limited, but also vary from each other. To analyze the throughput performance of uplink multiple access control (MAC) protocol, we propose a new analysis model to deal with the communications heterogeneity in the network. Firstly, we divide the devices in the coverage into different clusters according to their communication durations. Then, a quitting probability indicating the probability that a device quits the UAV's coverage at each time slot is clarified. A modified three-dimensional Markov chain model adopting the quitting probability and cluster division is developed for the performance analysis. Besides, we also propose a modified CSMA/CA protocol which fully considers the heterogeneity of the access time and adaptively allocates the time resource among the devices in different clusters. Finally, the effects of retry limit, initial contention window size, the density of the devices, UAV's speed and coverage area are discussed in the simulation section. Index Terms Air-Ground integration network, CSMA/CA, IoT, Markov chain model, UAV I. INTRODUCTION Wireless communication with unmanned aerial vehicles (UAVs) has been a popular technology in air-ground integration network for military, public, and emergency applications [1]. Compared with the terrestrial communication system, UAV-based air-ground network is not only easy for deployment, but also has better wireless channels [2], [3]. Therefore, numerous UAVrelated wireless communication systems have been developed [4]-[7]. One typical application is the UAV-based air-ground IoT system [8], in which UAV is deployed as a mobile BS flying over the IoT devices to collect the sensor information [9], [10]. In this way, the UAV-based network gets rid of the complex routing design and greatly improves the efficiency of information collection. However, there are still some challenges in analyzing and designing the UAV-based data collection system. One of the challenges is that it is difficult to model the UAV-based communication system compared with the traditional static network due to UAV's mobility. Generally, the fixed-wing UAV is usually adopted in data collection scenario for its long endurance. This characteristic requires the UAV to fly continuously without hovering, causing the time-varying channels between the transceivers. In this case, the devices can only access the UAV when the UAV is close to them while the wireless links break if the UAV flies away, resulting in the limited com...