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IEEE Access

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

Anh M. Nguyen, Minh Q. Vu, Ngoc The Dang

Published Jan 1, 2026• 1 citation
Publisher

Abstract

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.

DOI
10.1109/access.2026.3658255

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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.
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 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 secrecy (PLS) performance of a hybrid free space optical and radio frequency (FSO/RF) communication system under a modified selection combining scheme. The transmission scheme takes into account secrecy performance as well as diversity gain and ease of implementation. The effects of FSO link, namely the FSO atmospheric turbulence and the FSO receiver pointing error, are included in the analysis while the power amplifier (PA) inefficiency for the RF transmission is considered to have more realistic understandings on the system performance. The exact analytical expressions for the performance indicators including the average secrecy capacity (ASC) and secrecy outage probability (SOP) of the investigated mixed FSO/RF system are derived. The asymptotic SOP analysis reveals useful insights into the performance of the investigated mixed system. Analytical and simulation results are presented to evaluate the PLS performance of the proposed mixed system as well as to compare the performance of other hybrid systems with different setups.
This paper presents a study of the physical layer security performance of a mixed radio frequency-free space optical (RF-FSO) system with a wireless-powered friendly jammer. The RF links undergo the Nakagami-m fading, and the FSO link experiences the Exponentiated Weibull (EW) distribution. A two-hop decode-and-forward (DF) relay is present in the system. A single-antenna eavesdropper located on the RF link attempts to eavesdrop on the channel transmission information. There is a nearby multi-antenna jammer that can be charged wirelessly, and a save-then-transmit (ST) protocol is introduced in the jammer. The closed expressions for the secrecy outage probability (SOP) and the average secrecy capacity (ASC) of the mixed RF-FSO system are derived, and the correctness of the expressions is verified using the Monte-Carlo method. The influence of various key factors on the secrecy performance of the system is analyzed by simulations. The results show that increasing the average interference noise ratio, the number of interferer antennas, the time block allocation factor, and the size of the receiving aperture has a significant effect on the secrecy performance. This study provides a new system structure and a good theoretical basis for evaluating the physical layer security performance of mixed RF-FSO systems.
This paper presents research on the physical layer security performance of a multi-user mixed RF/FSO system based on optimal user interference. In this system model, the RF link experiences Rayleigh fading, and the FSO link follows the Fischer-Snedecor F distribution. The system adopts a double-hop-decode-and-forward (DF) relay scheme. We also consider the effect of directivity errors in the FSO link and assume the presence of an illegal eavesdropper with a single antenna near the RF link. The source node controls the energy collection and information forwarding using a multi-user structure based on simultaneous wireless information and power transfer (SWIPT). We select the optimal user to jam the eavesdropper's communication. We derive closed-form expressions for the mixed RF/FSO communication system's secrecy outage probability (SOP) and average secrecy capacity (ASC). Monte Carlo simulations are performed to verify the accuracy of these expressions. By formulating and simulating the simulation system, the impact of various important factors on the mixed system's physical layer security (PLS) is analyzed. The analysis indicates that increasing the number of antennas and interference signal-to-noise ratio (SNR) of the optimal user, the time allocation factor and energy conversion efficiency, and the improvement in the quality of atmospheric channels with improved weather will significantly enhance this system's PLS.
In this paper, we analyze the confidentiality of a hybrid radio frequency (RF)/free-space optical (FSO) system with regard to physical layer security (PLS). In this system, signals are transmitted between the source and destination using RF and FSO links, with the destination employing the maximal-ratio combining (MRC) scheme. A non-cooperative target (NCT) is assumed to have eavesdropping capabilities for RF and FSO signals in both collusion and non-collusion strategies. The Nakagami-m distribution models fading RF links, while FSO links are characterized by the Málaga (M) distribution. Exact closed-form expressions for the system’s secrecy outage probability (SOP) and effective secrecy throughput (EST) are derived based on the generalized Meijer G-function with two variables. Asymptotic expressions for the SOP are also obtained under high-signal-to-noise-ratio (SNR) regimes. These conclusions are validated through Monte Carlo simulations. The superiority of the hybrid RF/FSO system in improving the communication security of a single link is confirmed in its comparison with conventional means of RF communication.