Secure Transmission in Space-Air-Ground Integrated Networks Assisted by Reconfigurable Holographic Surface

This paper proposes a secure space-air-ground in tegrated network (SAGIN) transmission scheme in which an unmanned aerial vehicle (UAV) equipped with reconfigurable holographic surfaces (RHS) antennas to tackle the challenges of ultra-long distances in ground-space (G2S) links and eaves dropping risks in wireless channels. Leveraging the holographic beamforming principle, the UAV can relay confidential G2S signals while transmitting interference as artificial noise toward eavesdroppers to counter interception. The closed-form expressions of the secrecy transmission rate for SAGIN are first derived. Then, an optimization strategy is designed under onboard energy consumption constraints, which jointly optimizes the beam radiation amplitude, 3D trajectory and power allocation, aiming to maximize the average secrecy rate (ASR) while satisfying the quality of service for legitimate users. A block coordinate descent algorithm is employed to decouple the original non convex problem into four subproblems, and feasible solutions are obtained through successive convex approximation. Simulation results validate the advantages of the proposed scheme, with the ASR significantly outperforming baseline schemes, such as those without holographic beamforming, horizon-only trajectory deployment, and without power control. The variations of the ASR with RHS element size and UAV transmit power are also analyzed. This research provides new theoretical insights and technical solutions for secure communication in SAGIN.