Cooperative UAV Cluster-Assisted Terrestrial Cellular Networks for Ubiquitous Coverage

Unmanned aerial vehicles (UAVs), featured by flexible configuration, robust deployment, and line-of-sight links, has a great potential to provide ubiquitous wireless coverage and high-speed transmission. In this paper, we aim to analyze the coverage performance of UAV-assisted terrestrial cellular networks, where partially energy-harvesting-powered caching UAVs are randomly deployed in the 3-D space with a minimum and maximum altitude, i.e., <inline-formula> <tex-math notation="LaTeX">$H_{l}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$H_{h}$ </tex-math></inline-formula>. A novel cooperative UAV clustering scheme is proposed to offload ground mobile terminals (GMTs) from ground cellular base stations to cooperative UAV clusters. A cooperative UAV cluster is developed within a cylinder with projection centered on a GMT, based on their energy states, the cached contents, and the cell loads. With tractable Poisson point process and Gamma approximation, explicit expressions for the successful transmission probabilities are obtained. A theoretical analysis reveals that the cooperative probability of a UAV and the offloading probability of a GMT have bell-shaped relation with respect to the radius of the cylinder and the cache hit probability (the matching probability of a content request and content cache). Numerical results are provided to demonstrate the impacts of the system parameters on the cooperative UAV cluster. The results also give the optimal average altitude (<inline-formula> <tex-math notation="LaTeX">${H_{l}+H_{h}}/{2}$ </tex-math></inline-formula>) and altitude difference (<inline-formula> <tex-math notation="LaTeX">$H_{h}-H_{l}$ </tex-math></inline-formula>) in maximizing the coverage performance with the proposed cooperative transmission scheme.