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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 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.

In this paper, we study the performance analysis of free-space optical (FSO) system employing a constant-power variable-rate adaptive subcarrier intensity modulation (SIM) and utilizing phase-shift keying (PSK) over the Málaga atmospheric turbulence with pointing errors. According to a target bit-error rate (BER) and the instantaneous signal-to-noise ratio (SNR), the adaptive strategy adopts the subcarrier PSK modulation order. More precisely, we develop new analytical expressions for the average BER, the upper bound on ergodic channel capacity for non-adaptive modulation, and the attainable spectral efficiency (SE) of an adaptive SIM-FSO communication system. According to the simulation results, it is possible to achieve an increase in SE under weak-to-strong turbulence conditions without the need to increase the average optical power transmitted or compromise the BER requirements.

Traditional optical spatial modulation techniques all rely on the premise that the system receiving end can obtain accurate channel state information.The optical differential spatial modulation (ODSM) system effectively avoids complex channel estimation, but related research only analyzes the system performance under a single turbulent state, and ignores the effects of factors such as pointing errors and path loss in actual free space optical communication systems.Therefore, the Málaga turbulence channel, which could characterize all atmospheric turbulence states, was used to derive the upper bound expression and diversity order of the bit error rate of the ODSM system under the atmospheric joint effect such as turbulence, pointing error, and path loss.The analysis results show that compared with other optical spatial modulation schemes, the ODSM system avoids complex channel estimation, making the system no longer affected by channel estimation errors, thus possessing higher anti-interference and stability.The bit error rate performance of the ODSM system decreases as the intensity of turbulence and pointing error increase.By optimizing parameters such as the number of optical antennas, the number of photodetectors, and the modulation signal order, the bit error rate performance of the ODSM system can be further improved.

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]

The underlay cognitive radio-based hybrid radio frequency/free-space optical (RF/FSO) systems have emerged as a promising technology due to their ability to eliminate spectrum scarcity and spectrum under-utilization problems. The physical layer security of such a network with a primary user, a secondary source, a secondary receiver, and an eavesdropper is therefore examined in this work. In this network, secret communication occurs between two reliable secondary peers over the RF and FSO links simultaneously, and the eavesdropper can only overhear the RF link. In particular, the maximum transmits power limitation at the secondary user as well as the permissible interference power restriction at the primary user are also taken into consideration. All the RF and FSO links are modeled with $\alpha$-$\mu$ fading and Málaga turbulence with link blockage and pointing error impairments. At the receiver, the selection combining diversity technique is utilized to select the signal with the best electrical signal-to-ratio (SNR). Furthermore, to examine the secrecy performance taking into account the effects of each system parameter, closed-form expressions for the secrecy outage probability and effective secrecy throughput are derived. The resultant expressions are finally verified by Monte-Carlo simulations.

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This work deals with the secrecy performance analysis of a dual-hop RF-FSO DF relaying network composed of a source, a relay, a destination, and an eavesdropper. We assume the eavesdropper is located close to the destination and overhears the relay’s transmitted optical signal. The RF and FSO links undergo ( $\alpha -\kappa -\mu $ )-shadowed fading and unified Málaga turbulence with pointing error. The secrecy performance of the mixed system is studied by deriving closed-form analytical expressions of secure outage probability (SOP), strictly positive secrecy capacity (SPSC), and intercept probability (IP). Besides, we also derive the asymptotic SOP, SPSC, and IP upon utilizing the unfolding of Meijer’s $G$ function where the electrical SNR of the FSO link tends to infinity. Finally, the Monte-Carlo simulation is performed to corroborate the analytical expressions. Our results illustrate that fading, shadowing, detection techniques (i.e. heterodyne detection (HD) and intensity modulation and direct detection (IM/DD)), atmospheric turbulence, and pointing error significantly affect the secrecy performance. In addition, better performance is obtained exploiting the HD technique at the destination relative to IM/DD technique.

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]

Performance analysis is presented for a decode-and-forward protocol based mixed radiofrequency (RF) and free-space optical (FSO) dual-hop transmission system with digital coherent detection. The RF path is modeled by Beaulieu-Xie fading, while the FSO hop is characterised by the Málaga (M) distributed turbulence with pointing errors. We first derive novel and exact analytical expressions for the cumulative distribution function (CDF), the probability density function and the moment generating function (MGF) of the overall signal-to-noise ratio by means of Meijer's G function, followed by the accurate infinite series expressions of the performance criterions, such as the outage probability, the average bit-error rate (BER) and the ergodic capacity (average channel capacity). Asymptotic analysis for the CDF, the MGF, the outage probability, the average BER, and the ergodic capacity is also provided. Monte Carlo simulations are performed to verify these derived expressions.

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 Space diversity of multiple-input multiple-output (MIMO) system combined with the adaptive subcarrier modulation technique can effectively reduce the atmospheric channel fading effects. Malaga turbulent probability density function is obtained based on the MeijerG function. The asymptotic expression of average bit error rate in non-adaptive and adaptive modulated MIMO wireless optical systems are derived respectively, and M-ary phase shift keying (MPSK) and M-ary differential phase shift keying (MDPSK) modulation are used in these two kinds of systems. Under moderate turbulence conditions, error performances are analyzed with numerical simulations between these two systems. The results show that the average bit error performance in the adaptive modulated system can be significantly better than the non-adaptive modulated system. But under the same conditions of modulation order and target bit error rate, error performance of non-adaptive modulation is easier to be affected by transmit and receive aperture number than adaptive modulation in MIMO system. [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]