In this paper, we study the physical layer security of free-space optical (FSO) communications under different eavesdropping scenarios. More specifically, the secrecy performance of FSO communication employing intensity modulation/direct detection detection is analyzed for the wellestablished Málaga channels. Three different realistic scenarios of eavesdropping are considered by assuming different placement locations for the eavesdropper in the paper. Novel expressions for the average secrecy capacity (ASC) and secrecy outage probability (SOP) are derived for the considered scenarios, and useful insights are also provided through asymptotic analysis. The results show: (i) When the eavesdropper is placed near the transmitter, atmospheric condition imposes a less significant impact on secrecy performance; (ii) Certain level of correlation can potentially enhance the secrecy performance for FSO communications; (iii) The correlation imposes opposite impacts on the ASC and SOP of FSO communications; and the secrecy performance metrics exhibit a non-monotonic impact with the increase of correlation; (v) When the correlation of the FSO links is too small or too large (i.e., the correlation parameter around 0 or 1), the correlation plays a more significant impact on secrecy performance; and (vi) The asymptotic slope of the SOP is 0.5 for all eavesdropping scenarios under practical FSO channels.
Intelligent reflecting surface (IRS) is an emerging key technology for the fifth-generation (5G) and beyond wireless communication systems to provide more robust and reliable communication links. In this letter, we propose a mixed dual-hop freespace optical (FSO)-radio frequency (RF) communication system that serves the end user via a decode-and-forward (DF) relay employing hybrid automatic repeat request (H-ARQ) protocols on both hops. Novel closed-form expressions of the probability density function (PDF) and cumulative density function (CDF) of the equivalent end-to-end signal-to-noise ratio (SNR) are computed for the considered system. Utilizing the obtained statistics, we derive the outage probability (OP) and packet error rate (PER) of the proposed system by considering generalized detection techniques on the source-to-relay (S-R) link with H-ARQ protocol and IRS having phase error. We obtain useful insights into the system performance through the asymptotic analysis which aids to compute the diversity gain. The derived analytical results are validated using Monte Carlo simulation.
In release 16 of the third-generation project partnership, it has been suggested that hybrid-automatic repeat request (H-ARQ) protocols can be used to ensure robust communications. In this paper, we incorporate H-ARQ in the free-space optical (FSO) communication systems to improve their performance and communication link reliability. We derive novel, to the best of our knowledge, closed-form expressions for the outage probability of the FSO systems under the combined impact of gamma–gamma distributed atmospheric turbulence and pointing errors (PEs) for heterodyne detection and intensity modulation and direct detection techniques. This is accomplished using various H-ARQ protocols, such as, at least once, chase combing, and incremental redundancy (INR). Further, using outage probability expressions, we calculate throughput for the FSO systems using H-ARQ protocols. Useful insights into the FSO system performance are obtained through the asymptotic analysis for the considered H-ARQ protocols and detection techniques. It is observed through the numerical results that H-ARQ with INR performs better than the other considered H-ARQ protocols. We also draw a comparison between the outage performance of an FSO system with boresight PE and without boresight PE.
Free space optical (FSO) communication is a promising candidate for the next generation (5G and beyond) wireless communication systems, due to its merits (i.e. low latency, high data rate, and license-free band, among others). However, atmospheric turbulence (AT) as well as pointing error (PE) are two of the main challenges with FSO communication that affect its performance. Here, the exact closed-form expression of the average secrecy capacity and secrecy outage probability under the composite effect of AT and non-zero boresight PEs is evaluated. For all the regimes of the AT (weak to strong), a generalised Malaga distribution is used to model the channel fading gain of the FSO link. The expressions are generalised and valid for all turbulence, and are applicable for intensity modulation direct detection as well as heterodyne detection techniques.
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