Improving Physical Layer Security Using UAV-Enabled Mobile Relaying

Mobile relaying has aroused great interest in wireless communications recently, thanks to the rapid development and evolvement of unmanned aerial vehicles. This letter establishes the utility of mobile relaying in facilitating secure wireless communications. In particular, we consider transmit optimization in a four-node (source, destination, buffer-aided mobile relay, and eavesdropper) channel setup, wherein we aim at maximizing the secrecy rate of this system. However, the secrecy rate maximization problem is nonconvex and intractable to solve. To circumvent the nonconvexity, we exploit the difference-of-concave (DC) program to develop an iterative algorithm, which is proven to have Karush-Kuhn-Tucker point convergence guarantee. The algorithm conducts a water-filling-based solution in each DC iteration, and thus is computationally efficient to implement. In addition, for a given special case, we show that each DC iteration could yield a closed-form solution, which further reduces the computational complexity. Simulation results demonstrate that the proposed mobile relaying scheme could significantly outperform the static relaying scheme in terms of secrecy enhancement.