Energy Efficiency and Hover Time Optimization in UAV-Based HetNets

Abstract

In this dissertation, we investigate the performance of downlink transmission link in a three-tier HetNet. Our goal here is to enhance the edge capacity of a macro cell by deploying unmanned aerial vehicles (UAVs) as flying base stations and small cells (SCs) for improving the capacity of indoor users in scenarios like temporary hotspot regions or during disaster situations where the terrestrial network is either insufficient or out of service. UAVs are energy-constrained devices with a limited flight time, which makes it vital that we utilize that energy in the most optimum manner. Our approach here is to formulate this as a two layer optimization scheme, where we first optimize the power consumption of each tier for enhancing the system energy efficiency under the minimum QoS requirement, which is followed by optimizing the average hover time of UAVs. We can find the solution to these nonlinear constrained optimization problems by first utilizing the Lagrange multipliers method and then implementing sub-gradient method for obtaining convergence. The results show that through optimal power allocation, the system EE improves significantly in comparison to when maximum power is allocated to ground users. The hover time optimization results in increased flight time of UAVs thus providing service for longer durations.