Symbol-Level Physical Layer Security Design in Space-Air-Ground Integrated Networks

In the down-link secure space-air-ground integrated network (SAGIN) with practical discrete modulation symbols, we consider the joint design of the power of satellite and unmanned aerial vehicle (UAV) swarm beamforming to combat a formidable eavesdropper intercepting both the satellite-to-UAV and UAV-to-ground communication links. Given the challenge posed by the discrete symbols-based joint optimization problem, the security mechanism for SAGIN is developed through alternating optimization. Specifically, the optimal power of satellite is first determined with a fixed UAV beamformer. Based on this power, we propose a pair of solution approaches: convex optimization-based iteration (CObI) and heuristics-based semi-analytic iteration (HbSAI). The former focuses on achieving high performance while the latter emphasizes efficiency. The effectiveness of both CObI and HbSAI methods is validated through simulation results, demonstrating their ability to enhance the achievable secrecy rate.