Capacity of UAV-Enabled Multicast Channel: Joint Trajectory Design and Power Allocation

摘要

This paper studies an unmanned aerial vehicle (UAV)-enabled multicastchannel, in which a UAV serves as a mobile transmitter to deliver commoninformation to a set of $K$ ground users. We aim to characterize the capacityof this channel over a finite UAV communication period, subject to its maximumspeed constraint and an average transmit power constraint. To achieve thecapacity, the UAV should use a sufficiently long code that spans over its wholecommunication period. Accordingly, the multicast channel capacity is achievedvia maximizing the minimum achievable time-averaged rates of the $K$ users, byjointly optimizing the UAV's trajectory and transmit power allocation overtime. However, this problem is non-convex and difficult to be solved optimally.To tackle this problem, we first consider a relaxed problem by ignoring themaximum UAV speed constraint, and obtain its globally optimal solution via theLagrange dual method. The optimal solution reveals that the UAV should hoverabove a finite number of ground locations, with the optimal hovering durationand transmit power at each location. Next, based on such amulti-location-hovering solution, we present a successive hover-and-flytrajectory design and obtain the corresponding optimal transmit powerallocation for the case with the maximum UAV speed constraint. Numericalresults show that our proposed joint UAV trajectory and transmit poweroptimization significantly improves the achievable rate of the UAV-enabledmulticast channel, and also greatly outperforms the conventional multicastchannel with a fixed-location transmitter.