Placement Optimization for UAV-Enabled Wireless Networks with Multi-Hop Backhauls in Urban Environments

In surveillance or search scenarios, exploiting unmanned aerial vehicles (UAVs) as relays to provide wireless data access for task-oriented ground robots (GRs) with remote base station have emerged as a promising application. This paper considers a UAV-enabled wireless network, where communication links could be line-of-sight (LoS) and non-line-of-sight (NLoS) due to obstacles in urban environments. Existing works typically adopted the free-space path loss model or the statistical channel model, which either ignored the impact of obstacles or assumed uniformly distributed obstacles and therefore might fail in practical NLoS scenarios. In this paper, taking the information of randomly distributed obstacles in environments into consideration, we aim to optimize the placement for the UAV-enabled multi-hop network to transfer more data collected by GRs and minimize the time delay in data transmission while satisfying the required communication quality. By reconstructing this complex non-convex optimization problem into two subprob-lems and solving them alternatively, we propose the multi-hop UAVs placement (mUP) method to get the solution, which contains the air-to-ground network formation (ATG-NF) algorithm and the communication quality-aware UAV placement (CQA-UP) algorithm. Simulation results show that in four types of typical urban environments or with different numbers of UAVs, the proposed mUP method achieves substantial performance gains in terms of communication quality and task performance compared to other placement approaches based on statistical channel models. We further discuss the robustness of the mUP method towards terrain measurement error.