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In free-space optical communication (FSOC) systems, atmospheric turbulence can bring about power fluctuations in receiver ends, restricting channel capacity. Relay techniques can divide a long FSOC link into several short links to mitigate the fading events caused by atmospheric turbulence. This paper proposes a Reinforcement Learning-based Relay Selection (RLRS) method based on Deep Q-Network (DQN) in a FSOC system with multiple transceivers, whose aim is to enhance the average channel capacity of the system. Malaga turbulence is studied in this paper. The presence of handover loss is also considered. The relay nodes serve in decode-and-forward (DF). Simulation results demonstrate that the RLRS algorithm outperforms the conventional greedy algorithm, which implies that the RLRS algorithm may be utilized in practical FSOC systems. [ABSTRACT FROM AUTHOR]

In this work, we have carried out a performance analysis of the Decode and Forward (DF), dual-hop hybrid Radio Frequency-Free Space Optical Communication (RF-(RF/FSO)) link. The Source to Relay (S → R) link is considered as an RF link, and Relay to Destination (R → D) link is a parallel RF and FSO link. The selection combining technique is employed at the destination. The RF links are modeled by Nakagami-m distribution, and the FSO link is modeled by and Malaga channel model with pointing errors. For the FSO link, both Intensity Modulation/Direct Detection (IM/DD) and Heterodyne detection schemes are considered. The closed form expressions of Probability Distribution Function (PDF), Cumulative Distribution Function (CDF), and Moment Generating Function (MGF) of end-to-end SNR are derived in form of Meijer's G function. Utilizing the above results, The expressions for outage probability, bit error rate (BER), and ergodic capacity have been obtained. The numerical results are compared for different strengths of fading of RF links, the strength of atmospheric turbulence, the severity of pointing errors, and detection techniques. The numerical results closely match with Monte-Carlo simulations which validate the analytical work. The RF-(RF/FSO) link's performance is found to be better than the equivalent RF-FSO link. The link (S → R or R → D) experiencing the stronger fading dominates the overall link performance. [ABSTRACT FROM AUTHOR]

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In this paper, we analyze a combined terrestrial-underwater optical communication link for providing high-speed optical connectivity between onshore and submerge systems. For this purpose, different transmission signaling schemes were employed to obtain performance results in terms of average bit error rate (ABER). In this sense, from the starting point of a known conditional bit-error-rate (CBER) in the absence of turbulence, the behavior of the entire system is obtained by applying an amplify-and-forward (AF) based dual-hop system: The first link is a terrestrial free-space optical (FSO) system assuming a Málaga distributed turbulence and, the second one, is an underwater FSO system with a Weibull channel model. To obtain performance results, a semi-analytical simulation procedure is applied, using a hyper-exponential fitting technique previously proposed by the authors and leading to BER closed-form expressions and high-accuracy numerical results. [ABSTRACT FROM AUTHOR]

Based on the probability density function of the composite channel considering Malaga atmospheric turbulence, pointing error and atmospheric path loss, the asymptotic expressions of channel capacity and outage probability for binary phase shift keying (BPSK) modulation system are derived using Meijer G function. The numerical results show that under the same intensity of turbulence and path loss condition, the optimal performance of channel capacity and outage probability can be obtained by adjusting the jitter standard deviation and beam waist radius when the receiver radius is fixed. Meanwhile, the influence of weather conditions on channel capacity is greater than that of the beam waist radius and jitter standard deviation for the same turbulence distribution. It is illustrated that the optimal system performance can be obtained when w z / a = 5 and σ s / a = 1.5 in sunny weather. The channel capacity increases slowly at small signal-to-noise ratio under moderate–strong turbulence, and the stronger the fluctuation variance is, the greater the influence of fog on atmospheric communication. [ABSTRACT FROM AUTHOR]

We investigate the influences of the phase noise induced by the demodulation module on the performance of subcarrier M-ary phase-shift keying (MPSK) optical communication system over turbulence channel. Novel asymptotic expression of average symbol error probability (SEP) is derived based on the Meijer's G function and Fourier series over Malaga turbulence. The Malaga channel model used in our study can be potentially applied to the performance analysis problems involving other turbulence channels. The influences of the phase noise and its truncation error on the SEP performance are analyzed under different turbulence conditions, phase noise deviation, and modulation order. It is illustrated that the phase noise significantly degrades the performance of the system over turbulence channel. The more truncated number of Fourier series is required to get the minimum truncation error when the phase noise is lower. Under turbulence conditions, the appearance of symbol error rate floor depends predominantly on the phase noise. [ABSTRACT FROM AUTHOR]