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Security and privacy are two main dominant features of any communication system. In this paper, physical layer security of free space optical communication system using chaotic modulation scheme i.e., differential chaos shift keying (DCSK) is analyzed, where eavesdropper is actively present near the receiver and interfering between the transmission of secret messages from a transmitter to the receiver. In this manuscript, we have derived analytical expressions for the average secrecy capacity and secrecy outage probability which is used as a metric for secrecy performance analysis. The channel characterization is carried out using gamma–gamma model for weak-to-strong turbulence conditions. The effect of physical layer parameters like transmission link length, spreading length, etc. are considered for evaluating the security performance of the system. Numerical analysis is carried out and graphical results are presented. The results depicted that a very good average secrecy capacity can be achieved even in the presence of eavesdropper, however, it requires a tradeoff between high signal-to-noise ratio of main channel and large values of spreading factor. The proposed system is very promising for the future secured communication systems.

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This paper explores the performance of a single input multiple output (SIMO) free space optical (FSO) communication system using differential chaos shift keying (DCSK) with diversity combining techniques. Specifically, the paper presents the use of maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) diversity techniques for the DCSK-FSO system. Adding chaos to FSO systems using diversity can significantly improve the reliability and the system performance, making them a promising option for various 5G/6G wireless applications. The inherent properties of chaos, such as security, wide band nature, and immunity to fading, make it an attractive option for FSO communication. The Gamma-Gamma turbulence model was used to characterize the FSO link, and the system's performance was analyzed in terms of bit error probability across varying atmospheric turbulence levels, link lengths, and numbers of receivers. The results indicate that under the same system conditions, MRC/EGC performs 2-4 dB better in terms of error probability than the SC diversity technique. Furthermore, the SIMO-DCSK-FSO system presents a substantial improvement in diversity gain, with around 30 dB for moderate turbulence and 37 dB for strong turbulence.