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Hybrid terrestrial-satellite (HTS) communication systems have gained a tremendous amount of interest recently due to the high demand for global high data rates. Conventional satellite communications operate in the conventional Ku (12 GHz) and Ka (26.5-40 GHz) radio-frequency bands for assessing the feeder link, between the ground gateway and the satellite. Nevertheless, with the aim to provide hundreds of Mbps of throughput per each user, free-space optical (FSO) feeder links have been proposed to fulfill these high data rates requirements. In this paper, we investigate the physical layer security performance for a hybrid very high throughput satellite communication system with an FSO feeder link. In particular, the satellite receives the incoming optical wave from an appropriate optical ground station, carrying the data symbols of N users through various optical apertures and combines them using the selection combining technique. Henceforth, the decoded and regenerated information signals of the N users are zero-forcing (ZF) precoded in order to cancel the interbeam interference at the end-users. The communication is performed under the presence of malicious eavesdroppers nodes at both hops. Statistical properties of the signal-to-noise ratio of the legitimate and wiretap links at each hop are derived, based on which the intercept probability metric is evaluated. The derived results show that above a certain number of optical apertures, the secrecy level is not improved further. Also, the system's secrecy is improved using ZF precoding compared to the no-precoding scenario for some specific nodes' positions. All the derived analytical expressions are validated through Monte Carlo simulations. INDEX TERMS Free-space optics, high-throughput communications, hybrid terrestrial-satellite systems, intercept probability, optical feeder links, physical layer security, zero-forcing precoding.

In this paper, the secrecy performance of a dual-hop mixed radio-frequency/underwater optical wireless communication (RF/UOWC) system is investigated. The considered system consists of one single antenna source node (S) communicating with one destination node (D), considered as the legitimate receiver, through the help of one amplify-and-forward (AF) relay node R equipped with multiple antennas for reception. Specifically, the relay receives the incoming signal from S via an RF link, applies maximal-ratio combining (MRC) technique, amplifies the output combined signal with a fixed gain, and then forwards it to D via an UOWC link. The transmission protocol is performed under the eavesdroppers' attempt to overhear the RF link (i.e., S −R). We derive an exact closed-form expression for the secrecy intercept probability (IP) in terms of the Fox's H-function, or in terms of the Meijer's G-function as a particular case. The derived secrecy performance metric is evaluated in terms of various channel and system parameters, and corroborated by Monte-Carlo simulation method. Our derived analytical formulas present an efficient tool to highlight the impact of some system and channel parameters on the secrecy performance, namely the number of relay antennas, number of eavesdropping nodes, relay gain, fading severity of RF links, and water turbulence severity of the UOWC link.

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In this paper, an improved numerical solver to evaluate the time-dependent radiative transfer equation (RTE) for underwater optical wireless communications (UOWC) is investigated. The RTE evaluates the optical path-loss of light wave in an underwater channel in terms of the inherent optical properties related to the environments, namely the absorption and scattering coefficients as well as the phase scattering function (PSF). The proposed numerical algorithm was improved based on the ones proposed in [1]-[4], by modifying the finite difference scheme proposed in [1] as well as an enhancement of the quadrature method proposed in [2] by involving a more accurate 7-points quadrature scheme in order to calculate the quadrature weight coefficients corresponding to the integral term of the RTE. Furthermore, the scattering angular discretization algorithm used in [3] and [4] was modified, based on which the receiver's field of view discretization was adapted correspondingly. Interestingly, the RTE solver has been applied to three volume scattering functions, namely: the single-term HG phase function, the two-term HG phase function [5], and the Fournier-Forand phase function [6], over Harbor-I and Harbor-II water types. Based on the normalized received power evaluated through the proposed algorithm, the bit error rate performance of the UOWC system is investigated in terms of system and channel parameters. The enhanced algorithm gives a tightly close performance to its Monte Carlo counterpart improved based on the simulations provided in [7], by adjusting the numerical cumulative distribution function computation method as well as optimizing the number of scattering angles. Matlab codes for the proposed RTE solver are presented in [8].INDEX TERMS Absorption, finite difference equation, inherent optical properties, numerical resolution, phase scattering functions, quadrature method, radiative transfer equation (RTE), scattering, underwater optical wireless communication (UOWC).

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