Performance Optimization of Aerial Platform Communications in Coexistence Scenarios

Abstract

In recent years, the usage of an aerial platform, such as an unmanned aerial vehicle (UAV), has skyrocketed in a wide selection of applications due to mainly their low cost, small size, and high maneuverability. For instance, the wireless communication field is approaching the realization of the integrated UAV and conventional ground network for further improved communication performance. This is because the UAV-enabled communication is a promising technology, enjoying numerous traits such as the ability of on-demand deployment and the high likelihood of strong communication links. However, this coexistence poses significant challenges on the communication performance of both the UAV and the ground nodes. Thereby, it is necessary to study the parameters of the network to ensure harmonious coexistence. This thesis studies the performance of the UAV-enabled communication in various coexistence scenarios and applications. A UAV, acting as a communication platform, can coexist with different conventional ground serving nodes, such as WiFi access point (AP) and cellular base station (BS). In particular, four main coexistence topics or applications are considered in this thesis. The first topic considers software-defined coexisting UAV-mounted aerial BS (ABS) and WiFi AP, and investigates the queuing delay behavior via the ABS positioning and the AP traffic offloading. The subscribers (users) are divided into cellular subscribers (CSs) and WiFi subscribers (WSs). In such a scenario, it is challenging to decide the position of the ABS as well as the user association to the ABS and the AP. As such, it is beneficial to use an optimization technique to tune the corresponding system parameters and accordingly achieve the maximum possible data rate or minimum delay. It is found that there is an essential difference in the behavior of the ABS positioning between the objectives of data rate maximization and queuing delay minimization. The second topic considers a cellular network with a UAV-mounted aerial BS (ABS) coexisting with multiple terrestrial BSs (TBSs), where each BS is serving multiple users. Under the probabilistic channel based environment, the three-dimensional (3D) positioning for the ABS and the transmit power allocation for all the nodes are designed in the uplink (UL), downlink (DL), and combined UL and DL operations. In such scenario, interference between aerial and terrestrial links become a significantly limiting factor to the network performance. Therefore, optimizing the corresponding network parameters, such as the ABS position and transmit power, plays a vital role in reaching optimal performance. For instance, the study finds the optimal ABS altitude that addresses the trade-off between the desired signal power and interference. The third topic considers a UAV deployed as an ABS to assist a TBS, serving several users in a hotspot area, via user offloading. Considering unavailable knowledge of the users locations, the ABS is assumed to hover at the cell center (geometric center) above the TBS as a best-effort location for all the users. Taking into account the mutual interference between the aerial and terrestrial communication links due to spectrum reuse, the topic studies the impact of the ABS altitude and transmit power, as well as the offload portion, on the users DL sum-rate under the settings of line-of-sight (LoS) channel. The fourth topic studies a UAV-enabled wireless power transfer (WPT) system, where a rotary-wing UAV is dispatched to transfer radio frequency (RF) wireless energy to a group of energy receivers (ERs) at known locations. To provide uninterrupted long-term service, the UAV also harvests solar energy using solar panels. Due to limitations on size and weight, the UAV suffers limited energy in the on-board battery, which considerably confines the operation period and velocity. Using optimization is thus necessary to administer the instantaneous velocity of the UAV, and accordingly govern its propulsion power consumption, to efficiently consume the battery energy and preserve a smooth service. Considering the distance-dependent power received by the ERs, the optimization in this study suggests the optimal operation trajectory and cruising speed of the UAV for smooth and uninterrupted service.