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In the presence of mobile ground users, it is imperative to optimize the placement of unmanned aerial vehicle-base stations (UAV-BSs) in a UAV-assisted communication network. Moreover, the placement update interval is also a crucial parameter that impacts the network performance; hence, it needs to be optimized. Our work aims to jointly optimize the mobile users’ association with the UAV-BSs, resource allocation, UAV-BS placement, and the update interval. We propose to divide the above optimization into two phases: phase 1 and phase 2. In phase 1, the user association, resource allocation, and UAV-BS placement are optimized, whereas the update interval is optimized in phase 2. Two different frameworks, namely, max sum rate and max min rate, are utilized for the joint optimization. Specifically, in phase 1 of the max sum rate framework, the objective is to maximize the sum rate of the users, whereas, in phase 1 of the max min rate framework, the worst-off user rate is maximized. In phase 2, the update interval is optimized by minimizing the total UAV-BS flight time and maximizing the user coverage probability. Further, the analytical expression for the coverage probability of the mobile users is also derived. A sequential approach is proposed to solve phase 1 and phase 2 jointly. We prove that the convergence of the sequential approach is guaranteed. It is observed that, in the max sum rate framework, the average update interval is independent of the number of UAV-BSs. However, in the max min rate framework, the average update interval is dependent on the number of UAV-BSs. The proposed work is also compared with a benchmark approach wherein the update interval is not optimized. It has been shown that, unlike the proposed work, the benchmark approach cannot adapt to the desired priority of service time and coverage probability. |
