We investigate the physical layer security of decode-and-forward-relayed free space optics (FSO)/radio frequency (RF) communication system. In this network, the eavesdropper exists after relay node and overhears RF transmission.Further, FSO being a line-of-sight transmission is assumed to be secure from eavesdroppers. Here, we have the Gamma-Gamma (ΓΓ) distribution for FSO link and generalized − distribution for RF link. The security for information transmission to the legitimate user in the presence of an eavesdropper is measured in terms of secrecy capacity and secrecy outage probability. Deriving the probability density function and cumulative distribution function of end-to-end signal-to-noise ratio, the closed-form expressions for security parameters are achieved. The numerical analysis of the proposed system is done under the influence of atmospheric turbulence effects and various fading conditions. The results have been verified through simulation. KEYWORDSdual hop, FSO/RF system, physical layer security, secrecy capacity, secrecy outage probability Int J Commun Syst. 2018;31:e3468.wileyonlinelibrary.com/journal/dac to confuse eavesdropper. They provide secure transmission but require higher transmitting signal power to generate artificial noise. Moreover, the design of beamformer is a bit difficult due to computational complexity. The situation when main link condition is worse than the eavesdropper link creates hassle in security analysis of RF system. This limitation can be improved by using the multiple-input multiple-output (MIMO), 5 multiple-input and single-output (MISO), 6 and single-input and multiple-output (SIMO) 7 systems. However, due to high cost and size limitations, the designing of the network system with multiple antennas becomes difficult. The free space optics (FSO) has emerged as an effective solution to provide secured transmission with high data rate because it is a line-of-sight (LoS) transmission. 8 However, it has a limited transmission distance of the order of few kilometers only. It is a license-free transmission, but atmospheric turbulence effects (ie, smoke, snow, and fog) are main factors in deteriorating the performance of an FSO system. The PLS analysis of single hop FSO system investigated in 9 in which the secrecy performance was evaluated for data transmission consisting of single source and destination in the presence of an eavesdropper.Because of the complementary properties of FSO and RF communications, mixed FSO/RF systems have gained the attention of various researchers. These systems are more economical and save unnecessary modifications in existing system networks. They also avoid significant interference as both communications operate on different frequency bands. 10 It has been found that RF link can be a wiretapping link in mixed FSO/RF systems. Hence, the secure and legitimate transmission via mixed systems remains impossible before applying PLS. Recently, trade-off analysis on security-reliability is investigated for multiuser SIMO based RF/FSO relay networks...
The accurate estimation of underwater Visible Light Communication (VLC) channel conditions is challenging due to its widespread attenuation and scattering effects. The channel attenuation is a linear function of frequency and causes exponential signal power loss whereas due to the scattering effect, numerous photons are statistically generated as light beams strike water molecules and there arise security concerns. Assuming realistic underwater conditions, this paper investigates the security performance of a typical Non-Orthogonal Multiple Access (NOMA)-assisted underwater VLC system. It consists of a Floating Vehicle Transmitter (FVT), equipped with multiple Light Emitting Diodes (LEDs) to transmit the signal to two legitimate near-end and far-end Underwater Vehicles (UVs) in presence of an active/passive eavesdropper. The Channel State Information (CSI) of each transmitting link is estimated with the use of a Minimum Mean Square Error (MMSE) technique. Furthermore, we propose a LED selection mechanism to select an LED that can achieve the highest secrecy rate defined under the constraints of known and unknown CSI of legitimate and/or eavesdropping links. Using the Successive Interference Cancellation (SIC) technique, a novel closed-form secrecy outage probability expressions for the conventional single-LED and multi-LED NOMA-VLC links for both known and unknown CSI scenarios is derived. The security performance of the proposed multi-LED NOMA-VLC system is compared with the conventional single-LED NOMA-VLC system under the effect of air bubbles for both fresh and salty water. Finally, we verify the validity of the numerical results through Monte-carlo simulation analysis.INDEX TERMS Non-orthogonal multiple access, physical layer security, secrecy capacity, secrecy outage probability and underwater visible light communication.
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