Search

Free space optical communication (FSO) has become a popular research direction due to its high bandwidth, easy deployment, and inherent security. Its channel state is unstable because of atmospheric environment, especially in long‐distance ground transmission system. Interleaving combined with forward error correction coding (FEC) have been utilised to improve the stability of system. However, there is little detailed experimental results of deep interleaving combined with FEC under different atmospheric turbulence intensity over long‐distance FSO system. A deep interleaving with FEC method was designed and implemented on a 7 km long online experiment. The performance of different depth interleaving is analysed under weak and strong atmospheric turbulence state. The experiment results show that deep interleaving performs better under weak turbulence for the larger correlation factor of the channel and the outage probability of the system with deep interleaving can be greatly reduced. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence causes transmitted light to fade randomly, which results in irradiance scintillation fluctuations in the received signal and significantly affects the quality of wireless optical communication systems. In this paper, we investigate the propagation characteristics of a monochromatic light-emitting diode (LED) light beam through weak-to-strong turbulence. Considering the spatial incoherence of a monochromatic LED light source, the emitted light field of a monochromatic LED light source is represented by a random field multiplied by a deterministic field that follows a Gaussian distribution. Then, based on the extended-Rytov theory, a closed expression for the irradiance scintillation index under weak-to-strong turbulence is derived. In addition, the expression for the fading probability governed by the Gamma–Gamma model is given. Finally, the effects of near-earth atmospheric refractive index structural parameters, signal propagation distances, and working light wavelengths on propagation characteristics of the LED-based VLC system are simulated and compared with those of the laser-based one. The results theoretically confirm that laser light sources are more susceptible to atmospheric turbulence along the propagation path than monochromatic LED light sources. The investigation in this paper can provide theoretical support for the design of visible light communication systems in practical applications. [ABSTRACT FROM AUTHOR]

We derive the analytical expressions for root-mean-square (rms) beam wander (BW) and relative BW of a twisted electromagnetic elliptical vortex (TEEV) beam propagating through non-Kolmogorov atmospheric turbulence with the help of the extended Huygens–Fresnel principle and the second-order moments of the Wigner distribution function (WDF). Our numerical findings demonstrate that the BW of a TEEV beam with a small ellipticity, a large topological charge as well as a small waist width and initial coherent length is less affected by the turbulence. It can be also found that the effect of turbulence with a larger outer scale of turbulence, a generalized exponent parameter, and a generalized structure parameter on BW is more obvious. It is interesting to find that the effect of atmospheric turbulence on BW for a TEEV beam can be effectively reduced by regulating jointly the symbols and sizes of the twisted factor and topological charge. Therefore, modulation of the structure parameters of a TEEV beam provides a new way to mitigate turbulence-induced beam wander. Our work will be useful for free-space optical communications, remote sensing, and lidar distance measurement. [ABSTRACT FROM AUTHOR]

In this paper, the design of an atmospheric turbulence simulation system for free-space optical (FSO) communication is proposed. The system can accurately simulate repeatable atmospheric turbulent fading channels. It has a dynamic execution range of 30 dB with an execution rate of 1 MHz. The execution accuracy of the system is higher than 0.1 dB. In addition, the reliable TCP/IP protocol is used for communication, which ensures the convenience and versatility of the system. Experiments are carried out to investigate the performance of the system, and the results prove its superiority. Hence, the atmospheric turbulence simulation system can effectively simulate the influence of atmospheric turbulence channels on FSO communication systems. [ABSTRACT FROM AUTHOR]

Adaptive free-space optical (FSO) communication links have been extensively studied in order to adapt to variable atmospheric channel environments due to factors such as atmospheric turbulence. As a supporting technology, an all-in-one BPSK/QPSK switchable transmission and reception method is proposed and experimentally demonstrated for adaptive modulation format switching in FSO links. The transmission and reception of both modulation formats are realized based on the same IQ modulator and single-photodetector coherent receiver. Simulation and experimental results show that the QPSK signal has a power penalty of about 3–4 dB compared to the BPSK signal with a BER of about 1 × 10−3. The basis for format switching is given according to the various atmospheric channel conditions. The proposed method provides a flexible and efficient solution for variable FSO communication environments to improve their performance. [ABSTRACT FROM AUTHOR]

Free-space optical (FSO) communication can be subject to various types of distortion and loss as the signal propagates through non-uniform media. In experiment and simulation, we demonstrate that the state of polarization and degree of polarization of light passed though underwater bubbles, causing turbulence, is preserved. Our experimental setup serves as an efficient, low cost alternative approach to long distance atmospheric or underwater testing. We compare our experimental results with those of simulations, in which we model underwater bubbles, and separately, atmospheric turbulence. Our findings suggest potential improvements in polarization based FSO communication schemes. [ABSTRACT FROM AUTHOR]

An entanglement-based continuous variable (CV) QKD scheme is proposed, performing information reconciliation over an entanglement-assisted link. The same entanglement generation source is used in both raw key transmission and information reconciliation. The entanglement generation source employs only low-cost devices operated in the C-band. The proposed CV-QKD scheme with information reconciliation over an entanglement-assisted link significantly outperforms the corresponding CV-QKD scheme with information reconciliation over an authenticated public channel. It also outperforms the CV-QKD scheme in which a classical free-space optical communication link is used to perform information reconciliation. An experimental demonstration over the free-space optical testbed established at the University of Arizona campus indicates that the proposed CV-QKD can operate in strong turbulence regimes. To improve the secret key rate performance further, adaptive optics is used. [ABSTRACT FROM AUTHOR]

Multi-plane light converters (MPLC) are a means of deconstructing a wavefront into constituent modes that are focused at specific spatial locations, and the reverse—that specific inputs result in controlled modal output. We have used a pair of MPLCs with 21 Hermite–Gaussian modes to represent a free-space optical connection. The effects of strong atmospheric turbulence (Cn2 = 10−13 m−2/3) are emulated using a micromirror array producing a time sequence of aberrating frames. The modal crosstalk between transmitter and receiver modes induced by the turbulence is presented by measuring the intensity in receiver channels for the same turbulence. Six receiver modes are used for optical communication channels with a rate of 137 Gbits/s displaying the benefits of single input multiple output (SIMO) operation for overcoming the deleterious effects of turbulence. [ABSTRACT FROM AUTHOR]

Controlled turbulence simulators in the laboratory have been extensively employed to investigate turbulence effects on light propagation in the atmosphere, driven by some advanced optical engineering such as remote sensing, energy-delivery systems, and free-space optical communication systems. Many studies have achieved rich results on the optical turbulence intensity, scintillation index, and power spectral density characteristics of the light propagation path in the center of a turbulence simulator, but a comprehensive analysis of the optical turbulence characteristics for different spatial locations is still lacking. We simulate turbulence with air as the medium in a classical convective Rayleigh–Bénard turbulence simulator through high-resolution computational fluid dynamics methods, the three-dimensional refractive index distribution is obtained, and the optical properties are analyzed comprehensively. It is found that the hot and cold plumes and the large-scale circulation strongly influence the inhomogeneity of C n 2 in the turbulence tank, making it weak in the middle and strong near the boundary. The refractive index power spectral density at different heights is centrally symmetric, with the slope gradually deviating from the −5/3 scaling power with increasing distance from the central region. Under the log-log plot, the variation of the refractive index variance with height exhibits a three-segmented feature, showing in order: a stable region, a logarithmic profile, and a power-law profile, in the region close to the boundary. These results will contribute to the construction of a suitable turbulence simulator for optical engineering applications. [ABSTRACT FROM AUTHOR]

We derived the formula for the detection probability, signal-to-noise ratio (SNR), and average bit error rate (BER) for the signal orbital angular momentum (OAM) state carried via non-uniformly correlated high-order Bessel–Gaussian beam propagation in a turbulent atmosphere. The wavelength, receiver aperture, beam width, strength of the turbulent atmosphere, and topological charge effect on detection probability, SNR, and average BER of the signal OAM state were demonstrated numerically. The results show that the signal OAM state with low topological charge, a small receiver aperture, a narrow beam width, and a long wavelength can improve the performance of optical communications systems under conditions of weak atmospheric turbulence. Our results will be useful in long-distance free space optical (FSO) communications. [ABSTRACT FROM AUTHOR]

An enhanced transmission is presented in a multiple‐input‐multiple‐output (MIMO) dense‐wavelength division multiplexed (DWDM) free‐space‐optical (FSO) communication link using diversity coding techniques under the effect of turbulent weather phenomenon. The findings show good performance with an (8 channels × 2.5 Gbps data rate/channel) 20 Gbps 1500 m transmission distance. The bit‐error‐rate (BER), outage probability (OP), and signal‐to‐noise ratio (SNR) of the diversity combining techniques using maximum‐ratio combining (MRC), selection combining (SC), and equal‐gain combining (EGC) technique are evaluated in this work. The obtained results illustrate that Alamouti, space‐time coding (STC), space‐time block coding (STBC), space‐time trellis code (STTC), orthogonal STBC (O‐STBC), and quasi‐orthogonal STBC (QO‐STBC) on the minimum mean‐square‐error, and MRC are worth implementing on the DWDM‐FSO wireless communication systems. The mitigation of atmospheric turbulence is achieved using MIMO diversity combining techniques coding. The simulation results for diversity coding techniques using QO‐STBC/STTC and SC/MRC in the MIMO‐DWDM FSO communication system can improve BER performance, OP, and SNR. The MRC exhibits the lowest OP and BER when compared with the SC and EGC. The numerical results demonstrate that the FSO communication link using DWDM QO‐STBC/STTC improves the power penalty at both BER values under varying atmospheric turbulence conditions for ST, MT, and WT, in comparison to FSO systems without DWDM QO‐STBC/STTC diversity coding techniques. [ABSTRACT FROM AUTHOR]

In this paper, we propose, design, and evaluate a new hybrid terrestrial–underwater optical communication link for providing high-speed connectivity between land and underwater systems. A device based on an amplify-and-forward strategy is considered and used for the hybrid optical link. A performance analysis of the proposed hybrid system is then carried out, taking into account both the atmospheric and underwater channels and their respective degradation sources. Different networking scenarios and conditions are evaluated. To this end, the channel model of the terrestrial free-space optical (FSO) link is modeled using the Gamma–Gamma distribution, while the underwater optical link is modeled using the Weibull distribution. The former takes into account atmospheric and turbulence attenuation, geometric spread and pointing errors, while the latter takes into account underwater and turbulence attenuation and geometric spread. Accordingly, a new analytical closed-form expression for the bit error rate (BER), which depends on the cumulative distribution function of the holistic hybrid system, is derived. Analytical results show that pointing errors as well as atmospheric and oceanic turbulence seriously degrade the performance of the hybrid system. In addition, ocean turbulence leads to the occurrence of a BER floor in some scenarios. This is the first time that such a network is proposed and modeled under the assumption of critical channel impairments. [ABSTRACT FROM AUTHOR]

This paper proposes an adaptive bit-interleaved polar coded modulation (A-BIPCM) method based on minimum logarithmic upper bound weight (MLUW). It is designed to reduce the fading effects and long string of bit error interference caused by atmospheric turbulence in free-space optical communications (FSOC). To assess the effectiveness of this method across turbulent channels of varying intensities, we conducted an evaluation of the bit error rate (BER) performance of polar codes in turbulent channels. The results demonstrate significant performance improvements provided by the A-BIPCM method compared to conventional polar code encoding and decoding. Specifically, under weak, moderate, and strong turbulence conditions, the A-BIPCM method achieves performance gains of 0.96 dB, 1.66 dB, and 1.35 dB, respectively. Additionally, an experimental verification platform for FSOC employing intensity modulation direct detection (IM/DD) with an atmospheric turbulence simulation channel, is established in this work. When the optical power of the detector is −16 dBm, the traditional polar code encoding and decoding performance at BER = 2.36 × 10−5, whereas the A-BIPCM scheme exhibits a significantly higher performance at BER = 2.11 × 10−6. The BER has been improved by representing an order of magnitude. [ABSTRACT FROM AUTHOR]

The optical signal propagation used in satellite uplinks and downlinks is influenced by absorption, scattering, and changes in the atmospheric refractive index or turbulence, causing optical signal attenuation. A free space optics (FSO) communications system using coherent communication can improve the link sensitivity and reach higher distances. This article proposes a new architecture for the phase detector in an all-digital optical phase-locked loop (OPLL) for coherent optical detection. Firstly, the performance of the proposed phase detector is evaluated under Gaussian noise, where the best operation point is found for the OPLL working with two sample rates: 625 MSa/s and 10 GSa/s. The system analyses also take a non-negligible delay into account. Then, it will be evaluated and compared with an OPLL using an analog phase detector in the presence of atmospheric turbulence. Finally, in three different atmospheric turbulence conditions, the effect of wind speed on communication quality is investigated through the obtained bit error rate (BER) from the recovered data for a bit rate of 20 Gbps. The results show that the proposed digital phase detector can track a signal under longer feedback loop delays and fading signals. [ABSTRACT FROM AUTHOR]

Propagation properties of Cosine‑hyperbolic Vortex Gaussian Beams propagating in turbulent atmosphere are investigated. In this context, the effect of topological charges and decentred parameters on the beam size and the kurtosis parameter is evaluated. It is obtained that, beams with bigger topological charges face additional beam spreading, as well as a decrease in the kurtosis parameter. On the other hand, as the decentred parameter increases, beams spread less in long communication links. Inversely, Kurtosis parameters increase especially as the decentred parameters is greater than 1.5. Up to this end, the presented results are critical for both the design and the performance improvement of the free-space optical communication systems. [ABSTRACT FROM AUTHOR]

The multi-hop amplify-and-forward free-space optical communication (FSOC) system is studied in random fog using the I-function, considering Gamma-Gamma atmospheric turbulence and Beckmann pointing error. Outage probability, average bit error rate and average ergodic channel capacity are obtained. Channel-state-information assisted relay performs better than fixed-gain relay under high transmitted power. Increasing the hop number significantly improves the performance. More hops are needed in medium fog than in light fog to achieve the same performance. In addition, on a single-hop link, the influence of fog channel on system performance is dominant, while atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have little effect on the system performance. However, on a multi-hop link, atmospheric turbulence intensity, normalized jitter standard deviation and normalized boresight error have serious effects on system performance. Compared with correcting the normalized boresight error, compensating the normalized jitter standard deviation greatly improves the multi-hop FSOC system performance. Furthermore, optimizing beam width can further improves the performance. To ensure good communication, the system should select a low-order modulation scheme. [ABSTRACT FROM AUTHOR]

The deployment of non-terrestrial networks (NTNs) is envisioned to achieve global coverage for 6G and beyond. In addition to space nodes, aerial NTN nodes such as high-altitude platform stations (HAPSs) and rotary-wing unmanned aerial vehicles (UAVs) could be deployed, based on the intended coverage and operational altitude requirements. NTN nodes have the potential to support both wireless access and backhauling. While the onboard base station provides wireless access for the end users, the backhauling link connects the airborne/space-borne base station to the core network. With its high data transmission capability comparable to fiber optics and its ability to operate in the interference-free optical spectrum, free space optical (FSO) communication is ideally suited to backhauling requirements in NTNs. In this paper, we present a comprehensive tutorial on airborne FSO backhauling. We first delve into the fundamentals of FSO signal transmission and discuss aspects such as geometrical loss, atmospheric attenuation, turbulence-induced fading, and pointing errors, all of which are critical for determining received signal levels and related link budget calculations. Then, we discuss the requirements of airborne backhaul system architectures, based on use cases. While single-layer backhaul systems are sufficient for providing coverage in rural areas, multi-layer designs are typically required to establish connectivity in urban areas, where line of sight (LoS) links are harder to maintain. We review physical layer design principles for FSO-based airborne links, discussing both intensity modulation/direct detection (IM/DD) and coherent modulation/coherent demodulation (CM/CD). Another critical design criteria for airborne backhauling is self-sustainability, which is further discussed in our paper. We conclude the paper by discussing current challenges and future research directions. In this context, we discuss reconfigurable intelligent surfaces (RIS) and spatial division multiplexing (SDM), for improved performance and an extended transmission range. We emphasize the importance of advanced handover techniques and scalability issues for practical implementation. We also highlight the growing role of artificial intelligence/machine learning (AI/ML) and their potential applications in the design and optimization of future FSO-based NTNs. [ABSTRACT FROM AUTHOR]

We investigate the evolution properties of a partially coherent Laguerre–Gaussian vector vortex (LGVV) beam through inhomogeneous atmospheric turbulence. Analytical formulae for the elements of a cross-spectral density matrix of a partially coherent LGVV beam propagating in turbulence are derived with the help of the extended Huygens–Fresnel principle. Our outcomes demonstrate that the normalized initial profile of a partially coherent beam with concentric dark rings gradually evolves into a Gaussian-like beam profile in turbulence. We also find that the beam is emitted at a large zenith angle and quickly converts to a Gaussian-like beam. Furthermore, it is also shown that a propagation beam with a large topological charge has a stronger ability to resist atmospheric turbulence. In order to confirm our numerical results, we combine the complex screen method and multi-phase screen method to simulate the propagation of a partially coherent LGVV beam in atmospheric turbulence. It is indicated that the simulation results are in good agreement with theoretical predictions. Our results will pave the way for the development of free-space optical communications and remote sensing. [ABSTRACT FROM AUTHOR]

We use the extended Huygens–Fresnel integral to analyze the propagation properties of a class of partially coherent beams with Laguerre non-uniform coherence properties (called Laguerre non-uniformly correlated beams) in free space and in a turbulent atmosphere. We focus on how different initial beam orders and coherence lengths affect the propagation behavior of the beams, such as the evolution of intensity, degree of coherence, propagation factor, and beam wander. Our results show that non-uniform coherence properties play a role in resisting the degrading effects of turbulence. Furthermore, adjusting the initial beam parameter of the non-uniform coherence structure, i.e., increasing the beam order and decreasing the coherence, can further improve the turbulence resistance of the beams. Our results have potential applications in free-space optical communications. [ABSTRACT FROM AUTHOR]

In dual-wavelength free-space optical communication systems, the wavefront distortion differences caused by the wavelength difference of the signal and beacon lights cannot be completely corrected. In this study, according to the method of geometric optics, the relationship between the wavefront phases of the signal and beacon lights is analyzed by establishing a mathematical model of the cross-correlation ratio of the arrival-angle fluctuation and phase relationship of different Gaussian-beam wavelengths under weak turbulence near the surface. The numerical results show that after Gaussian beams of different wavelengths propagate through the same atmospheric channel, the cross-correlation number of the arrival-angle fluctuation decreases with an increase in the wavelength difference, waist radius, and turbulence intensity, and increases with an increase in propagation distance. The beam-wavefront phase difference increases with an increasing wavelength difference. Finally, a wavefront distortion correction experiment of a dual-wavelength adaptive optical system verified the correctness of the wavefront phase-difference relationship between the signal light and beacon light established in this study. [ABSTRACT FROM AUTHOR]

In this paper, an atmospheric structure constant C n 2 model is proposed for evaluating the channel turbulence degree of atmospheric laser communication. First, we derive a mathematical model for the correlation between the atmospheric coherence length r 0 , the isoplanatic angle θ 0 and C n 2 using the Hufnagel–Valley (HV) turbulence model. Then, we calculate the seven parameters of the HV model with the actual measured r 0 and θ 0 data as input quantities, so as to draw the C n 2 profile and the θ 0 profile. The experimental results show that the fitted average C n 2 contours and single-day C n 2 contours have superior fitting performance compared with our historical data, and the daily correlation coefficient between the single-day computed θ 0 contours and the measured θ 0 contours is up to 87%. This result verifies the feasibility of the proposed method. The results validate the feasibility of the proposed method and provide a new technical tool for the inversion of turbulence C n 2 profiles. [ABSTRACT FROM AUTHOR]

Free-space optical (FSO) communication has been studied for next-generation network systems. Because an FSO system establishes point-to-point communication links, maintaining alignment among the transceivers is a critical challenge. In addition, atmospheric turbulence causes significant signal loss in FSO vertical links. Even in clear weather conditions, transmitted optical signals suffer significant scintillation losses due to random variations. Thus, the effect of atmospheric turbulence should be considered in vertical links. In this paper, we analyze the relationship between pointing error and scintillation from the aspect of beam divergence angle. Furthermore, we propose an adaptive beam that optimizes its divergence angle according to the pointing error between the communicating optical transceivers to mitigate the effect of scintillation due to pointing error. We performed a beam divergence angle optimization and compared it with adaptive beamwidth. The proposed technique was demonstrated using simulations, which revealed an enhanced signal-to-noise ratio and the mitigation of the scintillation effect. The proposed technique would be useful in minimizing the scintillation effect in vertical FSO links. [ABSTRACT FROM AUTHOR]

We show that the focusing of a random electromagnetic beam by a lens gives rise to a scintillation index at the geometrical focus that generally differs from that of the incident beam. In the examples we present, focusing produces a significant increase of the index. This observation is of particular relevance for optical communication systems in which scintillation is a major cause of signal degradation. [ABSTRACT FROM AUTHOR]

This review paper discusses the complete evolution of free-space optical (FSO) communication, also known as unguided optical communication (UOC) technologies, all the way back to ancient man's fire to today's machine-learning-supported UOC systems. The principles, significance, and developments that have happened over the past several decades, as well as installation methodologies, technological limitations, and today's challenges of UOCs are presented. All the subsets of UOC: FSO communication, underwater optical wireless communication (UOWC), and visible light communication (VLC), with their technology/system developments, potential applications, and limitations are reviewed. The state-of-the-art developments/achievements in (i) FSO channel effects and their mitigation techniques; (ii) radio-over-FSO techniques; (iii) wavelength division multiplexing and sub-carrier multiplexing techniques; (iv) FSO for worldwide interoperability for microwave access applications; (v) space optical satellite communication (SOSC); (vi) UWOC; (vii) photoacoustic communication (PAC); (viii) light-fidelity; (ix) VLC; (x) vehicular VLC (V2LC); and (xi) optical camera communication are reviewed. In addition, the current developments on emerging technologies such as artificial intelligence (to improve the performance of UOC systems), energy harvesting (for the effective utilization of UOC channels), and near-future communication network scenarios (mandatory for secured broadband digital links) are covered. Finally, in brief, to achieve the full potential of UOC systems, challenges that require immediate research attention are summarized. [ABSTRACT FROM AUTHOR]

The performance of satellite-to-ground laser-based communication links is highly affected by atmospheric turbulence. Coherent detection with spatial diversity at the ground station receiver can mitigate the scintillation effects caused by atmospheric turbulence. Traditionally, the scintillation effects are modeled based on the Kolmogorov spectrum model. However, the experiments have indicated that scintillation effects on the laser beam propagation have non-Kolmogorov properties. Our goal in the present work is to analyze the average bit error rate (BER), outage probability (OP), and ergodic capacity of the satellite-to-ground heterodyne optical communication system with receiver spatial diversity. A differential phase-shift keying modulation technique is considered in this work. The propagated laser signal from the satellite to the ground station is assumed to be subjected to Málaga-distributed atmospheric turbulence. The atmospheric turbulence statistics are carried out based on the conventional Kolmogorov spectrum model and the three-layer altitude (TLA) non-Kolmogorov spectrum model. The performance of popular diversity combining techniques, namely, maximum ratio combining (MRC) and equal gain combining (EGC) techniques are analyzed. The statistical models of the MRC technique under the Málaga-distributed atmospheric channel model are obtained in analytical form expressions. The statistical models of the EGC technique under the Málaga-distributed atmospheric channel model are obtained via the fast Fourier transform representation of the characteristic function method. Based on these statistical models, average BER, OP, and ergodic capacity expressions for each type of diversity combining technique are derived. For the communication system under investigation, the performance of MRC and EGC multiple aperture receiver systems are compared to a single aperture receiver with the same total aperture area. These comparisons are carried out under the same conditions in terms of zenith angle and signal-to-noise ratio. The obtained results show that the performance of the optical communication system under investigation with MRC and EGC receivers can be improved by increasing the order of diversity. In addition, it is found that the difference in the performance between Kolmogorov and TLA non-Kolmogorov spectrum models is not significant at low zenith angles, while this difference increases as the zenith angle increases. All numerical results are verified by Monte-Carlo simulations. [ABSTRACT FROM AUTHOR]

The outdoor terrestrial terahertz (THz) communication links have recently attracted great research and commercial interest in response to the emerging bandwidth-hungry demands for extremely high-speed wireless data transmissions. However, their development is hindered by the random behavior of the atmospheric channel due to the molecular attenuation, adverse weather effects, and atmospheric turbulence (along with free space path loss (FSPL) and pointing errors) due to the stochastic misalignments between the transmitter and the receiver. Thus, in this work, we investigate the joint influence of these detrimental effects on both capacities, i.e., average (ergodic) and outage, of such a typical line of sight (LOS) THz communication link. Specifically, atmospheric turbulence-induced intensity fluctuations can be modeled by using either the suitable gamma or the well-known gamma–gamma distribution for weak and moderate to strong turbulence conditions, respectively. Additionally, weak to strong stochastic misalignment-induced intensity fluctuations, due to generalized pointing errors with non-zero boresight (NZB), are emulated by the appropriate Beckman distribution. Taking into additional consideration the unavoidable presence of FSPL and the different but realistic water vapor concentration values along with the influence of weather conditions, an outage performance analysis has been conducted. Considering the abovementioned significant effects, novel analytical mathematical expressions have been extracted for both average (ergodic) and outage capacity, which are critical metrics that first incorporate the total influence of all of the above significant effects on the THz links' performance. Through the derived expressions, proper analytical results verified by simulations are presented and demonstrate the validity of our analysis. It is notable that the derived expressions can accommodate realistic parameter values involved in all the above-mentioned major effects and link characteristics. In this context, they provide encouraging quantitative results and outcomes for both capacity metrics under investigation. The latter enables the design and the establishment of modern and future high-speed THz links, which are expected to cover longer propagation distances and thus become even more vulnerable to atmospheric turbulence effect. This is modeled and incorporated in our analysis and expressions contrary to most of the previous works in the open technical literature. [ABSTRACT FROM AUTHOR]

Free-space optical communication is restricted by random media-stimulated beam degradation. However, partially coherent structured beams modulated by the coherence structure can potentially mitigate this negative effect. By employing the extended Huygens–Fresnel integral, we provide an examination of the second-order statistical features of a common type of partly coherent structured beams, self-splitting structured beams, in a turbulent ocean. The implications of turbulence parameters relating to the ocean and beginning beam parameters corresponding to the progression of such beam propagation attributes are fully investigated. Our numerical outcomes show that, for turbulence with a low-dissipation kinetic energy rate per unit mass of fluid, small Kolmogorov inner scale, large relative strength of temperature to salinity undulations, and large dissipation rate of mean-square temperature has a greater negative effect on the structured beams. In addition, we suggest an effective approach, enhancing the order of the beam and reducing the coherence length of the beams, to lower the oceanic turbulence-induced negative effects, and thus have future extensive possibilities in free-space optical communication. [ABSTRACT FROM AUTHOR]

In this paper, a high-speed short-reach free space optical (FSO) transmission system is proposed for data center interconnect (DCI). This system supports the transmission of 400 Gbps over a single wavelength by employing spatial vertical cavity surface emitting lasers. Space division multiplexing along with polarization division multiplexing of the Laguerre–Gaussian (LG) modes (LG 00 and LG 01 ) are exploited to achieve higher capacity over FSO channel. Intensity modulation direct detection scheme is used in the proposed system for cost-efficiency. Least mean square equalization is applied to the received data signals to mitigate the FSO channel's impairments. The performance of the proposed system is evaluated by observing the bit error ratio (BER) for weak, moderate and strong turbulences at different weather dynamics. The effects of polarization induced crosstalk and aperture misalignment induced crosstalk on BER degradation are also investigated. FSO channel lengths of 700 m, 500 m and 150 m are achieved for weak, moderate and strong turbulences, respectively. The receiver sensitivities at forward error correction limit of 3.8 × 10 - 3 for each FSO channel length are compared with back to back configuration. A penalty of 2.5 dB is observed for strong turbulence whereas 2 dB and 0.6 dB penalties are obtained for moderate and weak turbulences, respectively. The proposed FSO-DCI could be used to provide high aggregate transmission capacity of 800 Gbps and 1.6 Tbps for future DCI by adding more LG modes or other wavelengths. [ABSTRACT FROM AUTHOR]

A laser beam propagating in the free space suffers numerous degradation effects. In the context of free space optical communications (FSOCs), this results in reduced availability of the link. This study provides a comprehensive comparison between six machine learning (ML) regression algorithms for modeling the refractive index structure parameter ( C n 2) . A single neural network (ANN), a random forest (RF), a decision tree (DT), a gradient boosting regressor (GBR), a k-nearest neighbors (KNN) and a deep neural network (DNN) model are applied to estimate C n 2 from experimentally measured macroscopic meteorological parameters obtained from several devices installed at the Naval Postgraduate School (NPS) campus over a period of 11 months. The data set was divided into four quarters and the performance of each algorithm in every quarter was determined based on the R2 and the RMSE metric. The corresponding RMSE were 0.091 for ANN, 0.064 for RF, 0.075 for GBR, 0.073 for KNN, 0.083 for DT and 0.085 for DNN. The second part of the study investigated the influence of atmospheric turbulence in the availability of a notional FSOC link, by calculating the outage probability (Pout) assuming a gamma gamma (GG) modeled turbulent channel. A threshold value of 99% availability was assumed for the link to be functional. A DNN classification algorithm was then developed to model the link status (On-Off) based on the previously mentioned meteorological parameters. [ABSTRACT FROM AUTHOR]

For a serial relaying underwater wireless optical communication (UWOC) system with ON‐OFF keying modulation, we theoretically evaluate the optimal power allocation techniques in order to minimise the end‐to‐end bit error rate (BER), subject to transmission power constraints. At first, we evaluate the end‐to‐end BER with respect to all degrading effects of the UWOC channel, namely scattering, absorption, and turbulence‐induced fading and then develop a closed‐form BER expression as a function of transceiver parameters and water type. The optimal power allocation methods are obtained using the perfect channel state information available at the receiver (CSIR) and transmitter (CSIT) for both detect‐and‐forward (DF) and amplify‐and‐forward (AF) serial relaying systems. For each relaying method, we consider a dual‐hop UWOC system and determine optimal power allocation to minimise the BER. Afterwards, the optimal power allocation in a multi‐hop system is obtained to minimise the end‐to‐end BER. Compared to the equal power allocation, our results illustrate that UWOC relaying systems with optimal power allocation can significantly improve the end‐to‐end BER and expand the communication link. For instance, the proposed power allocation method for the DF and AF relay node in a 60 m single relay system improves the system performance at the BER of 10−5 by 2.5 and 1.8 dB compared to the equal power allocation, respectively. [ABSTRACT FROM AUTHOR]

Data centers are crucial to the growth of cloud computing. Next-generation data center networks (DCNs) will rely heavily on optical technology. Here, we have investigated a bidirectional wavelength-division-multiplexed (WDM) free space optical communication (FSO) system for deployment in optical wireless DCNs. The system was evaluated for symmetric 10 Gbps 16—quadrature amplitude modulation (16-QAM) intensity-modulated orthogonal frequency-division multiplexing (OFDM) downstream signals and 10 Gbps on-off keying (OOK) upstream signals, respectively. The transmission of optical signals over an FSO link is demonstrated using a gamma–gamma channel model. According to the bit error rate (BER) results obtained for each WDM signal, the bidirectional WDM-FSO transmission could achieve 320 Gbps over 1000 m free space transmission length. The results show that the proposed FSO topology offers an excellent alternative to fiber-based optical interconnects in DCNs, allowing for high data rate bidirectional transmission. [ABSTRACT FROM AUTHOR]

We proposed the statistical misalignment model and the power-efficient configuration of transceivers for bi-directional multi-input and multi-output (MIMO) based vertical free space optical (FSO) links. Spatial diversity based MIMO FSO systems could be used to mitigate atmospheric fading issues. However, the increased number of channels can cause additional pointing error in pointing, acquisition and tracking (PAT) systems. The statistical misalignment model for detecting misalignment error is derived from the multiple transceivers. For the bi-directional characteristics of non-terrestrial back-haul networks, transmission performance is down-leveled to the worse in the asymmetric MIMO configuration of transceivers. The symmetric structure can mitigate the effect of increased pointing error to improve transmission performance. The proposed technique can be applied to the design of power-efficient FSO systems for non-terrestrial wireless back-haul networks. [ABSTRACT FROM AUTHOR]

By analyzing the effect of the degree of polarization (DoP) of the partially coherent Gaussian Schell-model (GSM) beam on a heterodyne system of an uplink path and a downlink path, we developed an innovative and noteworthy theory according to which σsy (signal beam waist radius in the y direction component) and δSyy (coherence length of the signal beam in the yy direction component) had a more significant impact on heterodyne efficiency and DoP than the turbulence term on uplink and downlink paths. Namely, the DoP and heterodyne efficiency of an uplink path are higher than that of a downlink path when σsy > 0.02 m or δSyy ≥ 0.03 m. This innovative rule provides an efficient way for increasing the heterodyne efficiency of a signal beam propagating along an uplink or a downlink path channel in satellite-ground communication links in free-space optical heterodyne detection communication. [ABSTRACT FROM AUTHOR]

We propose a single laser source-based free space optical link for transmission of bidirectional data of three different channels between modular data centers, where each channel has a data rate of 20 Gbps. The downlink channels are transmitted using a combination of differential quadrature phase shift keying (DQPSK) and pulse amplitude modulation (PAM). The uplink channels are transmitted using On-Off keying (OOK). The optical signals are transmitted over a 500-m free space optical (FSO) link based on Gamma–Gamma channel model. A regenerative wavelength converter is used to generate multiple optical pulsed carriers from the received downlink optical signals. The performance of the proposed architecture is evaluated under medium turbulence condition, which represents the FSO channel in a data center scenario. Our simulation results show that the proposed architecture gives acceptable bit-error-rate (BER) results. The architecture can be used as a potential substitute to fiber-based interconnects in a data center or a general point link in a passive optical network (PON), enabling high data-rate and cost-efficient transmission. [ABSTRACT FROM AUTHOR]

This paper introduces a novel free space optics (FSO) communication system for future-generation high-speed networks. The proposed system integrates orbital angular momentum (OAM) modes with an optical code division multiple access (OCDMA) technique. Two OAM beams are used ( L G 0 , 0 and L G 0 , 10 ), each of which is used for transmitting three independent channels. Each channel is assigned by fixed right shift (FRS) codes and carries 10 Gbps of information data. The performance of the proposed model is evaluated under different foggy and dust storm conditions. Furthermore, the performance of two cities with different geographical locations, Alexandria city in Egypt and Srinagar city in India, is investigated to demonstrate its ability to be implemented in future generations. Bit error rate (BER), eye diagrams, received optical power (ROP), and channel capacity are used for studying the performance of the proposed system. The observed simulation results show successful transmission of 60 Gbps overall capacity with the longest propagation FSO range for Alexandria city, which is 1400 m. Because dust storms have a large attenuation when compared to different foggy conditions, the proposed model had the shortest propagation range of 315 m under low dust (LD), 105 m under moderate dust (MD), and 40 m under heavy dust (HD). Furthermore, the cloudy weather conditions that affect Srinagar city, which is considered a hilly area, make our suggested model achieve 1000 m. [ABSTRACT FROM AUTHOR]

Data receiving frontends using avalanche photodiodes are used in optical free-space communications for their effective sensitivity, large detection area, and uncomplex operation. Precise control of the high voltage necessary to trigger the avalanche effect inside the photodiode depends on the semiconductor's excess noise factor, temperature, received signal power, background light, and also the subsequent thermal noise behavior of the transimpedance amplifier. Several prerequisites must be regarded and are explained in this document. We focus on the application of using avalanche photodiodes as data receivers for the on/off-keying of modulated bit streams with a 50% duty cycle. Also, experimental verification of the performance of the receiver with background light is demonstrated. [ABSTRACT FROM AUTHOR]

In this paper we investigate the application of free space optical (FSO) communications, energy harvesting, and unmanned aerial vehicles (UAVs) as key technology enablers of a cost-efficient backhaul/fronthaul framework for 5G and beyond (5G+) networks. This novel approach is motivated by several facts. First, the UAVs, acting as relay nodes, represent an easy-to-deploy and adaptive network that can provide line-of-sight between the base stations and the gateways connected to the core network. Second, FSO communications offer high data rates between the UAVs and the network nodes, while avoiding any potential interference with the 5G radio access networks. Third, energy harvesting in the optical domain has the potential to extend the UAVs' battery life. Nevertheless, the presence of atmospheric turbulence, atmospheric attenuation, and pointing errors in the FSO links severely degrades their performance. For this reason an accurate yet tractable modelling framework is required to fully understand whether an UAV-FSO backhaul/fronthaul network with energy harvesting can be applied. To this end, we consider a composite channel attenuation model that includes the effect of turbulence fading, pointing errors, and atmospheric attenuation. Using this model, we derive analytical closed-form expressions of the average harvested energy as a function of the FSO link parameters. These expressions can be used to improve energy harvesting efficiency in FSO link design. We have applied our proposed expressions to evaluate the energy harvested in vertical FSO links for a variety of real scenarios under a modified on-off keying (OOK) scheme optimized for energy harvesting. From the simulations carried out in this paper, we demonstrate that significant values of harvested energy can be obtained. Such performance enhancement can complement the existing deployment charging stations. [ABSTRACT FROM AUTHOR]

The deployment of unmanned aerial vehicles (UAVs) for free space optical communications is an attractive solution for forwarding the vital health information of victims from a flood-stricken area to neighboring ground base stations during rescue operations. A critical challenge to this is maintaining an acceptable signal quality between the ground base station and UAV-based free space optics relay. This is largely unattainable due to rapid UAV propeller and body movements, which result in fluctuations in the beam alignment and frequent link failures. To address this issue, linearly polarized Laguerre–Gaussian modes were leveraged for spatial mode diversity to prevent link failures over a 400 m link. Spatial mode diversity successfully improved the bit error rate by 38% to 55%. This was due to a 10% to 19% increase in the predominant mode power from spatial mode diversity. The time-varying channel matrix indicated the presence of nonlinear deterministic chaos. This opens up new possibilities for research on state-space reconstruction of the channel matrix. [ABSTRACT FROM AUTHOR]

In this work, multi input and single output optical wireless communication (OWC) links are analyzed, assuming the channel experiences fading due to atmospheric turbulence. OOK signaling and Alamouti-type space-time coding with intensity modulation and direct detection is assumed for weak to strong turbulence conditions modeled with the gamma–gamma distribution and closed form analytical expressions for their outage probability are derived for the first time, to the best of our knowledge. Finally, the corresponding numerical results are obtained, through the obtained mathematical expressions. Performance of a multi input and single output channel is analyzed for its outage probability as a function of power to noise ratio and normalized threshold. Three different cases for strong, medium, and weak turbulences are investigated. [ABSTRACT FROM AUTHOR]

Hermite–Gaussian (HG) beams have significant potential to improve the capacity of free-space optical communication (FSOC). The influence of pointing error on the propagation characteristics of an HG beam cannot be ignored in the FSOC system. Although the average irradiance of the HG beam under a small pointing error from the FSOC tracking mechanism has been investigated through Taylor series approximation, here, we propose that the average irradiance of the HG beam under an arbitrary magnitude pointing error can be deduced through a statistical averaging method. We firstly found that the average irradiance profile of an HG beam finally changes into an approximately Gaussian shape with the increase in pointing error and propagation distance and a larger beam waist at the transmitter could mitigate the profile change. The correlation coefficient between deduced theoretical expression and Monte Carlo simulation reaches 0.999. Additionally, the effective spot size, location of the local extreme value, average received power and signal-to-noise ratio (SNR) loss for an HG beam under pointing error were theoretically deduced and analyzed for the first time. We found that the effective spot size of the higher-order HG beam experiences less broadening under the pointing error than that of the lower-order HG beam. The fundamental theoretical expressions of average irradiance for an HG beam under pointing error have provided effective guidance for analyzing the propagation characteristics and link performance. [ABSTRACT FROM AUTHOR]

The performance of free space optical communication (FSOC) systems is severely degraded by certain atmospheric conditions prevalent in places where they are deployed, in spite of their numerous advantages. In clear weather conditions, the random fluctuation in the atmosphere's refractive index causes substantial scintillation losses to transmitted optical signals. It is therefore imperative to estimate the potential losses due to atmospheric turbulence in locations where FSOC links are to be deployed. This will provide the necessary fade margin for FSOC systems so that designed links withstand such atmospheric disturbances. In this paper, statistical analysis of wind speed data collected for various cities of South Africa is used for calculating the corresponding refractive index structure parameter ( C n 2 ). These C n 2 values, as well as the zero inner scale and infinite outer scale model and finite inner and finite outer scale model, are used in computing the scintillation indices not exceeding 50%, 99%, 99.9%, and 99.99% of the time for the investigated locations. The Lognormal and Gamma–gamma distribution models are then employed for the computational analysis of the irradiance fluctuations and channel characteristics while considering the effect of pointing errors for weak and moderate to strong turbulence regimes. Finally, derived mathematical expressions for outage probabilities and bit error rate (BER) performances for FSOC links, employing various intensity modulation and direct detection (IM/DD) schemes, are presented. [ABSTRACT FROM AUTHOR]

Non-Kolmogorov turbulence has been widely observed in free-space optical communication links and should be used to evaluate the system performance. We calculated the wavefront residual variance in the condition of the non-Kolmogorov turbulence model and deduced the mathematical expression of the probability density function (PDF) for the coupling efficiency (CE) of a single-mode fiber (SMF). The PDF was simulated, and the results showed its robustness and rationality. Moreover, an experiment was set up to verify the PDF with experimental distribution. The correlation coefficients are above 0.95 in all cases, which means the statistical model of the CE fitted the experimental distribution well. [ABSTRACT FROM AUTHOR]

Free-space optics (FSO) communication enjoys desirable modulation rates at unexploited frequency bands, however, its application is hindered by atmospheric turbulence which causes phase shifting in laser links. Although a single deformable mirror (DM) adaptive optics (AO) system is a good solution, its performance remains unsatisfactory as the proportion of tilts aberrations becomes relatively high. This condition happens when the incident angle of the laser beam for the optical receiver dynamically shifts. To tackle this problem, we introduce a fast steering mirror (FSM), DM cascaded AO architecture, based upon which we also propose an atmospheric turbulence compensation algorithm. In this paper, we compare the compensation ability of FSM and DM towards tilts aberrations. Furthermore, we gain model matrices for FSM and DM from testbed and simulatively verify the effectiveness of our work. For a Kolmogorov theory-based atmospheric turbulence disturbed incident laser beam where the tilt components take up 80 % of the total proportion of wavefront aberrations, our proposed architecture compensates the input wavefront to a residual wavefront root mean square (RMS) of 1 16 wavelength, compared to 1 6 wavelength for single DM architecture. The study intends to overcome atmospheric turbulence and has the potential to guide the development of future FSO communications. [ABSTRACT FROM AUTHOR]

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]

Free Space Optics (FSO) technology enabling next-generation near-Earth communication is prone to severe propagation losses due to atmospheric-turbulence-induced fading and Mie scattering (clouds). As an alternative to the real-time evaluation of the weather effects over optical signal, a state-of-the-art laboratory testbed for verification of slant APD-based (Avalanche Photodiode) FSO links in laboratory conditions is proposed. In particular, a hardware channel emulator representing an FSO channel by means of fiber-coupled Variable Optical Attenuator (VOA) controlled by driver board and software is utilized. While atmospheric scintillation data are generated based on Radiosonde Observation (RAOB) databases combined with a statistical design approach, cloud attenuation is introduced using Mie theory together with empirical Log-Normal modeling. The estimation of atmospheric-turbulence-induced losses within the emulated optical downlink is done with an FSO IM/DD prototype (Intensity Modulation/Direct Detection) relying on two different data throughputs using a transmitter with external and internal modulation. Moreover, the receiver under-test is a high-speed 10 Gbps APD photodetector with integrated Transimpedance Amplifier (TIA) typically installed in OGSs (Optical Ground Stations) for LEO/GEO satellite communication. The overall testbed performance is addressed by a BER tester and a digital oscilloscope, providing BER graphs and eye diagrams that prove the applied approach for testing APD-TIA in the presence of weather-based disruptions. Furthermore, the testbed benefits from the used beam camera that measures the quality of the generated FSO beam. [ABSTRACT FROM AUTHOR]

Free-space optical (FSO) communication possesses the advantages of high security, large information capacity, high-speed transmission, small terminal size, low power consumption, easy and flexible deployment, etc. It is a promising communication technique, and could be an alternative to optical fiber communication and traditional radio frequency (RF) communication. One of principal challenges that FSO faces is the susceptibility of the atmospheric channel, which is turbulent in nature. After the optical signals propagate over the atmospheric channel, their wavefronts suffer from deleterious perturbation, thus resulting in degradation in the performance of the FSO. This knowledge with respect to FSO links helps to optimize the system design and reduce the adverse effects of atmospheric turbulence. The atmospheric coherence length of FSO links reflects the atmospheric turbulence effects, and it is one of the key parameters of FSO systems. Measuring the atmospheric coherence length, as a result, is of great interest as well as importance to the FSO community. In this paper, several methods associated with atmospheric coherence length measurement are reviewed. [ABSTRACT FROM AUTHOR]

In this work, we investigate the impact of the transmit aperture selection technique to alleviate the effect of jamming on a free space optical (FSO) communication system. We consider the FSO system to suffer from a jamming signal and Gaussian noise concurrently. The analysis in this paper is conducted on the assumption that all wireless optical links follow the negative exponential distribution with pointing error. A general N × 1 FSO system is studied, which allows the transmitter to select a link with the maximum channel gain to transmit the information. We derive a closed-form expression of the bit error rate (BER) of the considered FSO system in the presence of a jammer. We extract the analytical coding gain and diversity order from the asymptotic behavior of the derived BER. We show that the transmit aperture selection allows the FSO system corrupted by a jammer to attain a diversity order of 0.5 N, equal to that of a multiple-input single-output FSO system, while appreciably reducing the power requirements of (N − 1) transmitters. The theoretical analysis is verified by simulations. We also make a quantitative comparison with repetitive coding (RC) and optical space shift keying (OSSK) and show that the transmit aperture selection has a 3 dB advantage over RC; moreover, for a 2 × 1 system, it provides twice the diversity offered by OSSK. The tenacity of aperture selection is also studied numerically for Gamma-Gamma fading FSO channels. [ABSTRACT FROM AUTHOR]

An in-flight unmanned aerial vehicle (UAV) free-space optical (FSO) communication channel model is proposed by considering the beam deviation of the UAV under different motion states and the phase distortion caused by atmospheric turbulences. The influence of the different motion states and turbulences on the communication quality is evaluated through phase screen and Monte Carlo methods. When the average bit error rate (BER) is 10−5, the signal-to-noise ratio (SNR) should be increased from 13 dB to 20 dB when the tilt angle of the UAV increases from 0 to 5 mrad. An SNR of up to 20 dB is required when the variance of the wind σ α 2 is 2 mrad. The performance of the in-flight UAV FSO link can be effectively improved through spatial diversity receiving technology. The BER of lower than 10−5 can be obtained just with an SNR of 13 dB if the spatial diversity array with four receivers is used. [ABSTRACT FROM AUTHOR]

Featured Application: In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of the in-phase and quadrature signal branches based on the Q value of the eye diagram with less computation. The proposed ASOP scheme can contribute to the frequency offset estimation and phase estimation of free-space optical communication (FSO) systems with turbulence disturbance. Thus, it is hoped that the ASOP scheme we proposed will see the practical application of coherent FSO communication. The Gram–Schmidt orthogonalization procedure (GSOP) and Löwdin symmetric orthogonalization procedure (SYOP) are the two mainstream algorithms for the compensation of phase mismatch in an imperfect optical 90° hybrid. In this paper, we put forward an algorithm switching orthogonalization procedure (ASOP) according to the quality of in-phase and quadrature signals based on the Q value of the eye diagram with less computation. If the quality of the in-phase and quadrature signals has a significant difference, we use the GSOP and select the signal branch with better quality as the initial reference vector for orthogonalization. If they are of about the same quality, then we use the SYOP. We present computer simulations for a coherent free-space optical (FSO) quadrature phase-shift keying (QPSK) communication system and demonstrate the system improvement that can be achieved using the ASOP. Finally, we also show that the proposed ASOP scheme can contribute to the frequency offset and phase estimation of the FSO system in the environment of atmospheric turbulence. [ABSTRACT FROM AUTHOR]

Free space optical communication (FSO) is widely deployed to transmit high data rates for rapid communication traffic increase. Asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) modulation is a very efficient FSO communication technique in terms of transmitted optical power. However, its performance is limited by atmospheric turbulence. When the channel includes strong turbulence or is non-deterministic, the bit error rate (BER) increases. To reach optimal performance, the ACO-OFDM decoder needs to know accurate channel state information (CSI). We propose novel detection using different deep learning (DL) algorithms. Our DL models are compared with minimum mean square error (MMSE) detection methods in different turbulent channels and improve performance especially for non-stationary and non-deterministic channels. Our models yield performance very close to that of the MMSE estimator when the channel is characterized by weak or strong turbulence and is stationary. However, when the channel is non-stationary and variable, our DL model succeeds in improving the performance of the system and decreasing the signal to noise ratio (SNR) by more than 8 dB compared to that of the MMSE estimator, and it succeeds in recovering the received data without needing to know accurate CSI. Our DL decoders also show notable speed and energy efficiency improvement. [ABSTRACT FROM AUTHOR]