Performance Of Free Space Optical Communication Research Articles

Phase screen prediction using deep phase network for FSO links.

Due to the presence of air turbulence in free-space optical (FSO) links, random fluctuations in wavefront phase and amplitude of the optical signal are reduced after it propagates through the air channel, which degrades the performance of free-space optical communication (FSOC) systems. Phase screen reflects the phase distortions resulting from air turbulence. Accordingly, accurate prediction with respect to phase screen is of significance for the FSOC. In this paper, we propose a phase screen prediction method based on the deep phase network (DPN). The advantages of the proposed method include strong robustness against air turbulence, low model depth, and fewer parameters as well as low complexity. The results reveal that our DPN enables desired inference accuracy and faster inference speed compared with the existing models, by combining the mean square deviation loss function with the pixel penalty terms. More concretely, the accuracy of phase screen prediction can reach up to 95%; further, the average time consumed to predict the phase screen is in the order of milliseconds only under various turbulence conditions. Also, our DPN outperforms the traditional Gerchberg-Saxton algorithm in convergence speed.

Performance Analysis of Free Space Optical Communication under Scintillation Effect

Free-Space Optical communication (FSO) offers unregulated bandwidth of up to 200 THz, security, higher speed, unlimited data rate and shortest installation time. However, weather attenuation, especially scintillation, has a massive impact on the performance of the FSO transmission channel. This paper evaluates link availability and effect of scintillation on FSO performance in terms of eye diagrams, Bit Error Rate (BER), Q-factor, and signal-to noise ratio. Our work used data collected from January 2018 to December 2021 by Tanzania Meteorological Authority (TMA) in Mwanza and Arusha regions for link availability and performance analysis. Simulation was performed to determine the FSO link availability, and we analyzed the scintillation effect on FSO performance using Submarine Laser Communication II and Hufnagel Valley (HV) day prediction models. It is observed that the HV day model performs better in predicting scintillation intensity compared to the former. In both regions, for the same transmission power of 20 dBm, the signal quality is better for a distance of 6 km where the BER value is found to be 10-8. These results suggest that, to achieve high-speed data transmissions, the maximum separating distance between FSO transceivers should not exceed 6 km.

Residual network-based aberration correction in a sensor-less adaptive optics system

The performance of free-space optical communication (FSOC) is often affected by atmospheric turbulence. The sensor-less adaptive optics (SLAO) system is an effective method for overcoming the effects of atmospheric turbulence. The performance of the control algorithm in the SLAO system directly determines whether the SLAO system can effectively correct wavefront aberrations. In this study, we introduce a residual network (ResNet) as a control algorithm to replace the traditional control algorithm. By lowering the number of iterations, this strategy enhances the real-time performance of the FSOC system. The final ResNet model can achieve an accuracy of 0.98 for training and 0.92 for testing. The simulation results show that stochastic parallel gradient descent (SPGD) algorithm takes 700 times longer and requires at least 500 iterations to achieve the same performance as ResNet. And we verify the feasibility of the ResNet model by setting up an experiment.

Orbital angular momentum spectra of twisted Laguerre-Gaussian Schell-model beams propagating in weak-to-strong Kolmogorov atmospheric turbulence.

The presence of atmospheric turbulence in a beam propagation path results in the spread of orbital angular momentum (OAM) modes of laser beams, limiting the performance of free-space optical communications with the utility of vortex beams. The knowledge of the effects of turbulence on the OAM spectrum (also named as spiral spectrum) is thus of utmost importance. However, most of the existing studies considering this effect are limited to the weak turbulence that is modeled as a random complex "screen" in the receiver plane. In this paper, the behavior of the OAM spectra of twisted Laguerre-Gaussian Schell-model (TLGSM) beams propagation through anisotropic Kolmogorov atmospheric turbulence is examined based on the extended Huygens-Fresnel integral which is considered to be applicable in weak-to-strong turbulence. The discrepancies of the OAM spectra between weak and strong turbulence are studied comparatively. The influences of the twist phase and the anisotropy of turbulence on the OAM spectra during propagation are investigated through numerical examples. Our results reveal that the twist phase plays a crucial role in determining the OAM spectra in turbulence, resisting the degeneration of the detection mode weight by appropriately choosing the twist factor, while the effects of the anisotropic factors of turbulence on the OAM spectra seem to be not obvious. Our findings can be applied to the analysis of OAM spectra of laser beams both in weak and strong turbulence.

Optical Satellite Eavesdropping

In recent years, satellite communication (SatCom) systems have been widely used for navigation, broadcasting application, disaster recovery, weather sensing, and even spying on the Earth. As the number of satellites is highly increasing and with the radical revolution in wireless technology, eavesdropping on SatCom will be possible in next-generation networks. In this context, we introduce the satellite eavesdropping approach, where an eavesdropping spacecraft can intercept optical communications established between a low Earth orbit satellite and a high altitude platform station (HAPS). Specifically, we propose two practical eavesdropping scenarios for satellite-to-HAPS (downlink) and HAPS-to-satellite (uplink) optical communications, where the attacker spacecraft can eavesdrop on the transmitted signal or the received signal. To quantify the secrecy performance of the scenarios, the average secrecy capacity and secrecy outage probability expressions are derived and validated with Monte Carlo simulations. Moreover, the secrecy throughput of the proposed models is investigated. We observe that turbulence-induced fading significantly impacts the secrecy performance of free-space optical communication.

Quantum-correlation-based free-space optical link with an active reflector

We present a quantum-correlation-based free-space optical(FSO) link over 250 m using an outdoor active reflector 125 m from the transceiver station. The performance of free-space optical communication can be significantly degraded by atmospheric turbulence effects, such as beam wander and signal fluctuations. We used a 660 nm tracking laser to reduce atmospheric effects, by analyzing the fast beam wander and slow temporal beam drift, using this information to correct the quantum channel alignment of the 810 nm signal photons. In this work, the active reflector consisted of a mirror, a 6-axis hexapod stage, and a long-range wireless bridge. The slow drift of the beam path due to outdoor temperature changes was steered and controlled using wireless optical feedback between the receiver units and the active reflector. Our work provides useful knowledge for improved control of beam paths in outdoor conditions, which can be developed to ensure high quality quantum information transfer in real-world scenarios, such as an unmanned FSO link for urban quantum communication or retro-reflective quantum communication links.

Effects of Atmospheric Turbulence on Optical Wireless Communication in NEOM Smart City

The foundation of any smart city requires an innovative and robust communication infrastructure. Many research communities envision free-space optical communication (FSO) as a promising backbone technology for the services and applications provided by such cities. However, the channel through which the FSO signal travels is the atmosphere. Therefore, the FSO performance is limited by the local weather conditions. The variation in meteorological variables leads to variations of the refractive index along the transmission path. These index inhomogeneities (i.e., atmospheric turbulence) can significantly degrade the performance of FSO systems. Thus, a practical implementation of the FSO link must carefully consider the atmospheric turbulence effect. This paper aims to investigate the feasibility of FSO communication for NEOM, a promising smart city in Saudi Arabia. We study the effect of weather conditions on FSO links using the micrometeorology model, taking into account actual weather data. The FSO performance in winter and summer was compared in terms of the bit error rate, signal-to-noise ratio (SNR), link availability, and transmission distance. The study shows that the atmospheric turbulence strength is moderate and strong in winter and summer, respectively. The temperature has the biggest impact on the FSO system when compared to the other meteorological elements included in this study. Furthermore, at transmission distances less than 300 m, atmospheric turbulence does not significantly affect the FSO for the operating wavelength of 1550 nm. Furthermore, it has been shown that at transmission distances greater than 300 m, the SNR in summer is more than 18% higher than in winter. The findings of this research enable understanding of the effect of turbulence caused by NEOM weather on the FSO link, thus assisting engineers in establishing a reliable FSO backbone link by adjusting the relevant parameters.

Near-surface atmospheric turbulence profile measuring technology based on an airship-mounted laser communication system.

Atmospheric turbulence is an important factor affecting the transmission performance of free-space optical communications (FSOC), especially in the near-surface where the atmospheric turbulence characteristics are complex and variable. In this paper, we study the real-time measurement technique of a near-surface atmospheric turbulence profile of an airship-borne laser communication system based on the principle of light intensity scintillation. Aiming at the influence of an avalanche photon diode detector system noise and environmental factors such as background light and platform vibration on the measurement results, a noise-canceling scintillation index calculation method, combined with a wavelet threshold denoising method, is proposed to improve the accuracy of atmospheric turbulence profile measurements. We build a communication distance of 12 km airship-borne laser communication experiment and carry out a real-time measurement of turbulence profile under 1 km near the ground without affecting the laser communication rate of 2.5 Gbps data transmission. The experimental results show that the atmospheric turbulence profile measured in real time follows the same trend as the theoretical simulation curve of the Hufnagel-Valley model, and the jitter of the measured values after denoising is significantly smaller than that of the measured values without denoising. The research results provide technical guidance and data support to promote the development of space laser communication and adaptive optics.

Adaptive optics technology and urban horizontal link laser communication system

The wavefront distortion caused by atmospheric turbulence seriously affects the performance of free-space optical communication (FSOC) system. Adaptive optics (AO) technology has been proven to be an effective method to suppress atmospheric turbulence. Two laser communication terminals equipped with AO system are designed and manufactured, which are used in the horizontal link near the ground. In the laboratory simulation, 100-Gbps data transmission can be realized under moderate turbulence (D / r0 < 10), the bit error rate (BER) is better than 1 × 10 − 6, and the measured single-mode fiber coupling efficiency is 42% to 48%. 10-Gbps data transmission can be realized under strong turbulence (D / r0 > 10), and the BER is better than 1 × 10 − 6. The BER is high in the practical communication test under the urban horizontal link of 1 km (D / r0 = 10), the BER is 1 × 10 − 3, and the coupling efficiency is 12% to 20%. Finally, to achieve the goal of 100-Gbps FSOC for near ground and long distance, based on the experimental performance of the existing two sets of laser communication system, the solution of laser communication system is proposed, which provides a reference for relevant researchers.

Using Machine Learning Algorithms for Accurate Received Optical Power Prediction of an FSO Link over a Maritime Environment

The performance prediction of an optical communications link over maritime environments has been extensively researched over the last two decades. The various atmospheric phenomena and turbulence effects have been thoroughly explored, and long-term measurements have allowed for the construction of simple empirical models. The aim of this work is to demonstrate the prediction accuracy of various machine learning (ML) algorithms for a free-space optical communication (FSO) link performance, with respect to real time, non-linear atmospheric conditions. A large data set of received signal strength indicators (RSSI) for a laser communications link has been collected and analyzed against seven local atmospheric parameters (i.e., wind speed, pressure, temperature, humidity, dew point, solar flux and air-sea temperature difference). The k-nearest-neighbors (KNN), tree-based methods-decision trees, random forest and gradient boosting- and artificial neural networks (ANN) have been employed and compared among each other using the root mean square error (RMSE) and the coefficient of determination (R2) of each model as the primary performance indices. The regression analysis revealed an excellent fit for all ML models, indicative of their ability to offer a significant improvement in FSO performance modeling as compared to traditional regression models. The best-performing R2 model found to be the ANN approach (0.94867), while random forests achieved the most optimal RMSE result (7.37).

Few-Mode Fiber-Based Free-Space Optical Communication With Nonzero Boresight Pointing Errors

Bit-error rate (BER) performance of free-space optical communication (FSO) systems is compromised by pointing errors. Few-mode fiber (FMF) based receiver emerges as a promising solution for FSO, because FMF can obtain high coupling efficiency in the presence of pointing errors. The high coupling efficiency allows FMF based system to achieve improved BER performance. In this letter, based on a simplified mode demultiplexer model, we propose a method to use the coupling efficiencies of each mode of FMF to characterize the BER of the FMF based FSO system having nonzero boresight pointing errors. We find that the boresight displacement can significantly deteriorate the BER performance, and the jitter variance can reduce the impact of boresight within a suitable range.

Estimating the Performance of Free Space Optical Communication in Rain Weather Conditions Using Various Models and Modified Duobinary Return to Zero Technique

Free space optical communication (FSO) system has remarkably gained importance of late due to the various advantages associated with it. However, before deployment of FSO communication system, an understanding of the laser beam scintillation due to the atmospheric as well as other weather-induced losses need to be understood. This paper mainly focuses on simulative performance analysis of an FSO system under rain conditions using Optisystems commercial software and rainfall data. Thus, the available models which are the Suriza, Carbonnea, Japan and Samir models were used to determine the rainfall-induced attenuation for a given rainfall rate. Unlike in the previous articles that only used on off keying as the modulation scheme at the transmitter, we have used modified duobinary return to zero (MDRZ) modulation. Further, to improve the MDRZ-based FSO performance in rain conditions, we have used the single-input multiple-output (SIMO) concept with equal gain combining (EGC) at the receiver and again proved that the improvement in BER, Q factor and received optical power suggests that MDRZ-based SIMO FSO communication results in a remarkable improvement.

Hybrid optical communications for supporting the Palapa Ring network

In order to connecting villages in rural area with a city, a hybrid optical communication is proposed. The Optical fiber communications that are implemented as a backbone on the Palapa Ring network has an opportunity to be expanded to reach rural area through the implementation of free-space optical communications (FSO) as the optical relaying networks (ORN). The major problem of FSO as the ORN is turbulence effects induce an optical propagation. In this research, FSO as the ORN implements novel technique to improve the performance of hybrid FSO where the amplification of signal is produced through serial configuration of optical amplifiers in cascaded meanwhile to reduce the noise in the end of receiver, optical band pass filter is implemented. The simulation of hybrid FSO is performed in Optisystem 7.0 where ORN is implemented in two nodes. The length of optical fiber connection to FSO is set to be constant at 75 km. The range of optical propagation in FSO is set at 1–9.4 m. The results of simulation show that at the range of 1–8 Km of optical propagation under atmospheric turbulence, range values for SNR and BER are achieved at 46.23–26.61 dB and 6.7x10-17–1.5x10-6, respectively.

Exact distributions for bit error rate and channel capacity in free‐space optical communication

Bit error rate (BER) and channel capacity are two important metrics to assess the performance of free-space optical communication (FSOC) systems. Due to the fading of the optical signal owing to the atmospheric effects, these two quantities behave as random variables. Most of the studies in this direction have focused on the calculation of only the average of these quantities. However, since the complete information about a random variable is encoded in its distribution, it is more informative to examine the latter itself. In this work, the authors derive exact probability density function (PDF) expressions for the BER and the channel capacity for an arbitrary irradiance model. In particular, they investigate these exact results for log-normal, gamma-gamma, and K distributions. For the BER analysis, they focus on the binary phase shift keying and quadrature phase shift keying modulation schemes. The authors' analytical reults are found to be in conformity with Monte Carlo simulations. The exact PDFs of the BER and the channel capacity reveal that there are several instances when the average is unable to capture the actual behaviour of these quantities, and therefore one must be careful in drawing conclusions based on the first moment only.

The Effects of Power Control on Free-Space Optical Communications during Snowfall and Rainfall

Free-space optical (FSO) communication requires a line-of-sight connection between a transmitter and a receiver in which the information signal is modulated by an optical carrier that propagates in free space. The FSO channel is greatly affected by weather conditions such as fog, rain, and snow. In the literature, several adaptive techniques, such as power control (PC), have been suggested to mitigate channel link degradations. In this paper, we investigate the effects of snow and rain attenuation on the bit error rate (BER) of the FSO system using two types of modulations, the on-off keying (OOK) modulation and the pulse-position modulation (16-PPM). The effect of PC on the performance of FSO communications is also examined in this study. We evaluated the system’s performance with two types of snow, wet snow and dry snow, as well as with different rain regions. Results show that PC improves the BER of the FSO system; a high rate of improvement is found for wet snow and rain. PC has almost no effect with dry snow because of the high attenuation and the limitations on transmitted power. The BER for 16-PPM is better than that for OOK modulation.

Theoretical analysis of simultaneously improving the light coupling efficiency and bandwidth between two separated grating couplers using integrated distributed Bragg reflectors

The performance of free space optical (FSO) communication system is limited by atmospheric turbulent extremely. Adaptive optics (AO) is the significant method to overcome the atmosphere disturbance. Especially, for the strong scintillation effect, the sensor-less AO system plays a major role for compensation. In this paper, a modified artificial fish school (MAFS) algorithm is proposed to compensate the aberrations in the sensor-less AO system. Both the static and dynamic aberrations compensations are analyzed and the performance of FSO communication before and after aberrations compensations is compared. In addition, MAFS algorithm is compared with artificial fish school (AFS) algorithm, stochastic parallel gradient descent (SPGD) algorithm and simulated annealing (SA) algorithm. It is shown that the MAFS algorithm has a higher convergence speed than SPGD algorithm and SA algorithm, and reaches the better convergence value than AFS algorithm, SPGD algorithm and SA algorithm. The sensor-less AO system with MAFS algorithm effectively increases the coupling efficiency at the receiving terminal with fewer numbers of iterations. In conclusion, the MAFS algorithm has great significance for sensor-less AO system to compensate atmospheric turbulence in FSO communication system.

Potentials of radial partially coherent beams in free-space optical communication: a numerical investigation.

Nonuniformly correlated partially coherent beams (PCBs) have extraordinary propagation properties, making it possible to further improve the performance of free-space optical communications. In this paper, a series of PCBs with varying degrees of coherence in the radial direction, academically called radial partially coherent beams (RPCBs), are considered. RPCBs with arbitrary coherence distributions can be created by adjusting the amplitude profile of a spatial modulation function imposed on a uniformly correlated phase screen. Since RPCBs cannot be well characterized by the coherence length, a modulation depth factor is introduced as an indicator of the overall distribution of coherence. By wave optics simulation, free-space and atmospheric propagation properties of RPCBs with (inverse) Gaussian and super-Gaussian coherence distributions are examined in comparison with conventional Gaussian Schell-model beams. Furthermore, the impacts of varying central coherent areas are studied. Simulation results reveal that under comparable overall coherence, beams with a highly coherent core and a less coherent margin exhibit a smaller beam spread and greater on-axis intensity, which is mainly due to the self-focusing phenomenon right after the beam exits the transmitter. Particularly, those RPCBs with super-Gaussian coherence distributions will repeatedly focus during propagation, resulting in even greater intensities. Additionally, RPCBs also have a considerable ability to reduce scintillation. And it is demonstrated that those properties have made RPCBs very effective in improving the mean signal-to-noise ratio of small optical receivers, especially in relatively short, weakly fluctuating links.

A novel trellis coded overlapping amplitude and pulse position modulation scheme for gamma-gamma channel free-space optical communication

A novel trellis coded-4×8 overlapping amplitude and pulse position modulation (TC-4×8AOPPM) scheme is proposed to enhance bit error rate (BER) performance of free-space optical communication (FSO) system. In addition, an uncoded AOPPM referential scheme is also designed. The schemes manage to decrease BER by designing gamma-gamma (GG) channel applicable decoding and demodulation methods. Simulation results of 8, 16 and 64-state TC-4×8AOPPM show 2.5–3.3 dB SNR gain against traditional TC-4×8AOPPM scheme respectively. Thus significant BER performance improvement is achieved and the reliability of the FSO system is also enhanced.

Hybrid Atmospheric Compensation in Free-Space Optical Communication

Since the direct-gradient (DG) method uses the Shack-Hartmann wave front sensor (SH-WFS), based on the phase-conjugation principle, for atmospheric compensation in free-space optical (FSO) communication, it cannot effectively correct high-order aberrations. While the stochastic parallel gradient descent (SPGD) can compensate the distorted wave front, it requires more calculations, which is sometimes undesirable for an FSO system. A hybrid compensation (HC) method is proposed by properly using the DG method and SPGD algorithm to improve the performance of FSO communication. Simulations show that this method can well compensate wave-front aberrations and upgrade the coupling efficiency with few computations, preferable correction results, and rapid convergence rate.

Optical communication in turbid and turbulent atmosphere

Free space optical-communication (FSO) has gained significant importance due to its unique features: large bandwidth, license free spectrum, high data rate, easy and quick deployability, less power and low mass requirement. However, the performance of FSO is degraded in the turbid and turbulent atmosphere, dramatically. Various techniques are proposed to cope with the turbid media and turbulence in atmosphere, e. g. aperture averaging, diversity, adaptive optics, modulation and coding and orbital angular momentum. However, in the above systems with point-to-point optical communication structure, there exist obvious drawbacks or they are complex and expensive, and thus difficult to use in practice. In this article, array-to-point optical communication (APOC) with good performance in turbid and turbulent atmosphere is demonstrated. The strongly disturbed communication channel can be expressed as a circular complex Gaussian transmission matrix, and the transmitted field is described as a linear combination of the fields coming from different and independent segments of the digital micro-mirror device (DMD), so that the cross terms are averaged on the surface of bucket detector. Instead, the contributions of all segments for each light field nearly becomes equally weighted. Turbulence and other noises are reduced for the incoherence with sampling matrix based on the second-order correlation which has widely been used in ghost imaging and LIDAR. Consequently, narrow-band optical filter is not required at the receiver. The decoding algorithm is a new signal processing strategy from information technology, compressed sensing, which discards low frequency components in sampling process and recovers the signal by conducting convex optimization. Numerical simulations and experiments with binary and multi-bits level signals are demonstrated to show that the bit error rate of the proposed method APOC is approximately 10-4-10-2, which is feasible for the optical communication in such complex communication channels. The communication rate, limited by the frequency of the DMD and the sampling rate of the receiver, could reach hundreds of kbit/s, and with improved technology a rate of Mbit/s should be attainable. This proposed APOC could realize optical communication in turbid and turbulent atmosphere and thus offers a very effective approach to promoting the implementation in military and rescue.