High-altitude platform (HAP) stations are pivotal in non-terrestrial networks, enhancing communication capabilities and extending cost-effective network access to rural or remote areas. HAP-assisted free-space optical (FSO) communications provide a promising solution for improving data rates. To safeguard against eavesdropping, especially during emergencies, we propose a physical-layer security mechanism to enhance the control signaling resilience in disaster response and network failure detection. We investigate the secrecy performance of an integrated HAP-based FSO and unmanned aerial vehicle (UAV)-enabled radio frequency (RF) downlink system using the decode-and-forward relaying protocol. While optical links inherently provide better security, our focus is on the eavesdropping threats to the RF link. We derive novel and exact analytical and asymptotic closed-form expressions for the secrecy outage probability (SOP) and the probability of strictly positive secrecy capacity (PSPSC). Our results reveal the significant impact of atmospheric turbulence, RF fading, pointing errors, and optical detection technologies on the overall secrecy performance, providing valuable insights for designing secure mixed FSO and RF downlink communication systems.
High-altitude platform (HAP) stations are pivotal in non-terrestrial networks, enhancing communication capabilities and extending cost-effective network access to rural or remote areas. HAP-assisted free-space optical (FSO) communications provide a promising solution for improving data rates. To safeguard against eavesdropping, especially during emergencies, we propose a physical-layer security mechanism to enhance the control signaling resilience in disaster response and network failure detection. We investigate the secrecy performance of an integrated HAP-based FSO and unmanned aerial vehicle (UAV)-enabled radio frequency (RF) downlink system using the decode-and-forward relaying protocol. While optical links inherently provide better security, our focus is on the eavesdropping threats to the RF link. We derive novel and exact analytical and asymptotic closed-form expressions for the secrecy outage probability (SOP) and the probability of strictly positive secrecy capacity (PSPSC). Our results reveal the significant impact of atmospheric turbulence, RF fading, pointing errors, and optical detection technologies on the overall secrecy performance, providing valuable insights for designing secure mixed FSO and RF downlink communication systems.
In this paper, the secrecy performance of a mixed free space optical (FSO)/radio frequency (RF) integrated satellite-high altitude platform (HAP) relaying networks for terrestrial multiusers with the existence of an eavesdropper is investigated. In this network, FSO is adopted to establish the link between the satellite and HAP for which it experiences Gamma-Gamma distributions under different detection schemes (i.e., heterodyne and intensity modulation direct detection). The transmission between the amplify-and-forward (AF) relaying HAP and terrestrial multiusers is through the RF and is modeled as shadowed-Rician fading distribution. Owning to broadcasting nature of RF link, it is assumed that an eavesdropper attempts to intercept the users’ confidential message, and the eavesdropper link is subjected to Rician distributions. Specifically, the closed-form expression for the system equivalent end-to-end cumulative distribution function is derived by exploiting the Meijer’s G and Fox’s H functions. Based on this expression, the exact closed-form expressions of the system connection outage probability, secrecy outage probability, and strictly positive secrecy capacity are obtained under the different detection schemes at HAP. Moreover, the asymptotic analyze of the system secrecy outage probability is provided to obtain more physical insights. Furthermore, the accuracy of all the derived analytical closed-form expressions is verified through the Monte-Carlo simulations. In addition, the impact of atmospheric turbulence, pointing errors, shadowing severity parameters, and Rician factor are thoroughly evaluated. Under the same system conditions, the results depict that heterodyne detection outperforms the intensity modulation direct detection.
In this work, we propose a new physical layer security framework for optical space networks. More precisely, we consider two practical eavesdropping scenarios: free-space optical (FSO) eavesdropping in the space and FSO eavesdropping in the air. In the former, we assume that a high altitude platform station (HAPS) is trying to capture the confidential information from the low earth orbit (LEO) satellite, whereas in the latter, an unmanned aerial vehicle (UAV) eavesdropper is trying to intercept the confidential information from the HAPS node. To quantify the overall performance of both scenarios, we obtain closed-form secrecy outage probability (SOP) and probability of positive secrecy capacity (PPSC) expressions and validate with Monte Carlo simulations. Furthermore, we provide important design guidelines that can be helpful in the design of secure non-terrestrial networks.
In this work, we propose a new physical layer security framework for optical space networks. More precisely, we consider two practical eavesdropping scenarios: free-space optical (FSO) eavesdropping in the space and FSO eavesdropping in the air. In the former, we assume that a high altitude platform station (HAPS) is trying to capture the confidential information from the low earth orbit (LEO) satellite, whereas in the latter, an unmanned aerial vehicle (UAV) eavesdropper is trying to intercept the confidential information from the HAPS node. To quantify the overall performance of both scenarios, we obtain closed-form secrecy outage probability (SOP) and probability of positive secrecy capacity (PPSC) expressions and validate with Monte Carlo simulations. Furthermore, we provide important design guidelines that can be helpful in the design of secure non-terrestrial networks.
We investigate the physical-layer security performance for a wireless-powered relaying mixed free-space optical-radio frequency (FSO-RF) system. In this system, a source transmits optical information to a relay (R), which decodes the received signal with a decode-and-forward protocol, converts it into an RF signal, and forwards it to a destination in the presence of an eavesdropper. Furthermore, we also assume that R has no extra power supply, and it only relies on harvesting energy from a power beacon to forward the information. Assuming that the FSO and all of the RF channels experience gamma-gamma and Rayleigh fading, the closed-form analytical expressions for secrecy outage probability and the probability of strictly positive secrecy capacity are derived. In addition, the effects of atmospheric turbulence, pointing errors, and detection techniques on the system performance are analyzed. Finally, we perform Monte Carlo simulations to verify the accuracy of the derived expressions.
In this paper, the secrecy performance of the physical layer security (PLS) of the hybrid free-space optical/radio frequency (FSO/RF) communication system is analyzed. The transmission protocol of the considered system is performed under the eavesdropper's attempt to overhear the RF link between the transmitter and legitimate receiver of the hybrid system. The FSO link is characterized by Málaga-M distribution while the RF links are modeled by Nakagami-m distribution. The two practical eavesdropping modes considered in this paper include: colluding and non-colluding. Exact closed-form expressions for the system's secrecy outage probability (SOP), the asymptotic of the SOP (SOP∞), the probability of strictly positive secrecy capacity (SPSC), the average secrecy capacity (ASC), and the asymptotic of the ASC (ASC∞), are specifically derived under the influence of both eavesdropper modes. Our derived analytical expressions present an efficient tool to investigate the impact of some channel parameters on the secrecy performance, namely the fading severity of the RF links, atmospheric turbulence severity, pointing error of the FSO link, number of eavesdroppers, and the power of the eavesdropper links. The results show that the increase of the eavesdroppers' number under both modes profoundly degrades the considered system secrecy performance. The accuracy of the numerical results obtained is validated by Monte-Carlo simulations.
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.