One Friendly UAV against Multiple Mobile Eves in HAP Networks: A Novel Physical Layer Secure Communication Scheme

High altitude platforms (HAPs) are emerging as the critical enabler for the next generation communication system, due to their broad coverage, long endurance and high payload. However, the open and broadcast nature of wireless links makes HAP system susceptible to information eavesdropping. This paper investigates the secure downlink transmission for an HAP network, where one unmanned aerial vehicle (UAV) jammer is utilized against multiple mobile eavesdroppers (Eves) with imperfect location information. We aim to maximize the minimum average secrecy rate (MASR) by jointly optimizing subcarrier scheduling, subcarrier power allocation, UAV trajectory and HAP deployment. To address the formulated mixed-integer non-convex optimization problem, we first derive the optimal HAP deployment and utilize an estimation error model to resolve the locational uncertainty of the Eves. Subsequently, a low-complexity iterative algorithm is proposed, which decomposes the problem into three manageable subproblems and iteratively solves them to obtain a near-optimal solution. The convergence and complexity of the algorithm are further analyzed. Numerical results demonstrate that the proposed scheme significantly improves not only the MASR but also the fairness between terminal devices, which outperforms the baselines without resource allocation and/or trajectory optimization.