Performance Analysis of Artificial Noise-Aided Physical Layer Security for Mixed FSO-RF Non-Terrestrial Networks

This paper analyzes the physical layer security (PLS) of a mixed free-space optics (FSO)–radio frequency (RF) non-terrestrial network (NTN) employing artificial noise (AN). Unlike conventional RF-based PLS, we show that AN in mixed FSO–RF NTNs exhibits a cross-link coupling effect, where FSO pointing errors, geometric loss, and error propagation significantly influence the effectiveness of AN injected only on the RF hop. Closed-form expressions for bit error rate (BER), secrecy capacity, and secrecy outage probability (SOP) are derived under realistic NTN conditions. Numerical results reveal several key findings. First, without AN, the secrecy capacity remains zero regardless of transmit power, while AN creates a clear activation threshold enabling positive secrecy. Second, the optimal AN power-allocation factor is non-monotonic and depends on transmit power. Third, increasing the number of high altitude platform (HAP) antennas substantially reduces SOP by more than an order of magnitude. Finally, rain attenuation creates critical degradation regions where secure transmission is possible only under high AN power. These results demonstrate that AN plays an essential and uniquely different role in securing multilayer NTN architectures.