Optimal Deployment of Tethered Drones for Maximum Cellular Coverage in User Clusters

Unmanned aerial vehicle (UAV) assisted cellular communication is gaining significant interest recently. Although it offers several advantages over terrestrial communication, UAV communication suffers from two main shortcomings. The typical untethered UAV (uUAV) has a limited battery power supply and therefore limited flying time, and it needs an extra wireless backhaul link to connect users to the core network. In this paper, we propose the utilization of the tethered UAV (tUAV) to assist the cellular network, where the tether provides power supply and connects the tUAV to the core network through high capacity link. The tUAV however has a limited mobility due to the limited tether length. A stochastic geometry-based analysis is provided for the coverage probability of an UAV-assisted cellular network where the mobile users located within a circular hot-spot. For that setup, we analyze and compare two scenarios: (i) utilizing uUAV and (ii) utilizing tUAV, for offloading the terrestrial base station (TBS). We capture the aforementioned limitations of each of the uUAV and the tUAV in our analysis. A novel user association analysis is provided given the TBS and the UAV locations. Next, we study the optimal locations of the uUAV and the tUAV to maximize the coverage probability. Multiple useful insights are revealed. For instance, numerical results show that tUAVs outperform uUAVs when the tether length is above 75 m, given that the uUAV is available for 80% of the time due to its battery limitations.