###### Abstract  
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 [PERSON]'s G and [PERSON]'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. refe: paper's article's contribution  
A Mixed FSO/RF Integrated Satellite-High Altitude Platform Relaying Networks for Multiple Terrestrial Users with Presence of Eavesdropper: A Secrecy Performance  
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deployment, etc., compared to its RF counterpart [5]. Despite these unique features, the FSO link usually suffers from atmospheric turbulence due to variation in refractive index as the result of inhomogeneity in atmospheric temperature and pressure that can degrade the system performance [6]. In addition, the link is highly prone to pointing error caused by building sways, leading to misalignment between the transmitter and receiver [7]. As a directional link, it requires line-of-sight (LOS) between the transmitter and receiver, which may be stochastically blocked by the moving, surrounding objects, such as birds, airplanes, etc. [8]. High altitude platforms (HAPs) have attracted immense attention in the research community due to their inherit advantages from both the terrestrial and satellite communication systems [9]. They have been suggested as a future candidate for communication system to offer efficient broadband wireless access at low cost [10]. As flying aerial platforms, such as airships, planes, and balloons; they are usually deployed in stratosphere at altitude between 17 to 22 km above the earth's surface, where there is existence of strong LOS between the satellite and HAP for efficient wireless transmission [11]. According to International Telecommunication Union (ITU), a larger footprint of 100 km radius has been suggested to act as HAPs, and this can replace various terrestrial base stations [12]. Regardless of the communication link, satellite communication system link usually experiences a masking effect due to unavailability of LOS between the satellite and user equipment. This occurs as a result of obstacles and shadowing, leading to poor quality of services and long delay propagation, especially in the case of geostationary satellites [13]. To overcome this effect, an integrated satellite-high altitude platform (ISHAP) mixed FSO/RF relaying scheme has been proposed in literature, where the RF communication can be used in conjunction with the FSO counterpart.