Completion Time Minimization for Multiantenna UAV-Enabled Multicasting With Rank-Two Multicast Beamforming

UAV information dissemination or multicast is a critical use case for UAV communication systems. However, UAVs' size and load limitations limit their onboard energy and endurance time. In this paper, we consider a multi-antenna unmanned aerial vehicle (UAV) enabled multicasting system, where a multi-antenna UAV is dispatched to transmit a shared file to a set of ground terminals (GTs). Our objective is to minimize mission completion time while satisfying the UAV speed and transmission power constraints, as well as guaranteeing the successful file recovery probability at the GTs. To address this issue, we introduce rank-two multicast beamforming and propose an online optimization scheme. The proposed scheme consists of two steps: 1) Before the UAV takes off, the open-loop rank-two beamforming without the channel state information (CSI) is used to obtain the initial planning, including the horizontal auxiliary distance parameter, UAV trajectory, UAV multicast trajectory segments (MTSs) on which the UAV will transmit multicast information, the number of multicasting packets to be transmitted at each MTS and mission completion time which is the upper bound. 2) Based on step 1, the UAV optimizes the rank-two multicast beamforming vectors using real-time CSI during flight to reduce the number of multicasting packets that need to be transmitted at each MTS, further decreasing mission completion time. Finally, numerical results indicate that the proposed scheme can significantly reduce mission completion time compared to the single antenna scheme.