Delay-tolerant UAV-assisted Communication: Online Trajectory Design and User Association

UAV-assisted communication is a promising paradigm to enhance the performance of wireless networks. In this paper, we consider the network randomness in the delay-tolerant UAV-assisted communication systems. We aim to optimize both the UAV trajectory and user association in an online manner to achieve a finite queueing delay at the users' buffers with the minimum energy consumption of the UAVs. We first formulate our problem as a stochastic dynamic programming problem that integrates the future expected energy consumption and delay into its objective function and constraints, respectively. To deal with the inaccurate knowledge of future user demand and mobility, we apply the Lyapunov optimization technique and convert it into a series of online joint UAV trajectory and user association optimization problems. However, each online problem is a non-convex non-linear integer programming problem, which is very challenging to solve in general. As such, we first eliminate the coupling relationship of user association variables in each online problem. By exploiting the special graph structure of the problem, we can decompose the joint problem into inner optimization and outer optimization problems. Then, we propose the online UAV mobility and user association selection (UMUAS) scheme, which includes two steps to solve the inner and outer problems. Simulation results show that our online UMUAS scheme achieves the near-optimal performance of the energy-delay tradeoff, which also has a 44.2% improvement over the greedy scheme and a 51.3% improvement over the interference-free scheme. Moreover, it achieves the highest energy efficiency among the three benchmark schemes.