Trajectory and Power Optimization for Buffer-Assisted Amplify-and-Forward UAV Relay

This paper proposes a buffer-assisted amplify-and-forward (AF) unmanned aerial vehicle (UAV) relay for communication between two ground nodes without a direct link. These communications are essential in disaster rescue areas where fairness plays a significant role. To achieve fairness, we aim to maximize the minimum information rate. The traditional way of prefixed scheduling the time slots to transfer and receive at the UAV does not guarantee that the communication system uses high signal-to-noise ratio (SNR) communication links instead of low SNR links. Therefore, we choose to employ a buffer at the UAV to store the information and transfer it to the destination nodes in the high SNR links. The pairing of time slots is necessary here since we are using a buffer, and we do not know in which time slot data is transmitted to the destination node after it is received at the UAV. Consequently, we formulate a fairness maximization problem by jointly optimizing the trajectory and power control. Unfortunately, this formulation results in a non-convex problem. We propose a solution based on the principles of the minorize-maximize (MM) algorithm and linear programming relaxation techniques to solve the fairness problem and pairing of time slots. Numerical results demonstrate that the trajectory, power control, and paired slots favor the UAV and ground nodes to communicate in the high SNR channel links, thus maintaining fairness.