Atmospheric Turbulence Conditions Research Articles

Performance of a free-space optical communication system employing receive diversity techniques in anisotropic atmospheric non-Kolmogorov turbulence

In this paper, bit error rate (BER) performance of a free-space optical communication (FSOC) system operating in anisotropic non-Kolmogorov weak turbulence is investigated together with the spatial diversity techniques. The spatial diversity techniques are implemented as maximum ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) and applied to the receiver. The propagating beam is the Gaussian beam wave, and the modulation scheme is binary phase-shift keying (BPSK). Results are obtained for various parameters such as the anisotropy factor, non-Kolmogorov power law exponent, photodetector responsivity, equivalent load resistor, electronic bandwidth, Gaussian beam radius, wavelength, propagation distance, and turbulence structure constant. It is found that the spatial diversity technique used at the receiver causes significant improvement in the performance of an FSOC system under the conditions of anisotropic non-Kolmogorov atmospheric turbulence. It is also observed that BER performance improves as the atmospheric turbulence becomes more anisotropic. Among the spatial diversity techniques, SC is inferior to EGC and EGC is inferior to MRC in terms of BER performance.

Performance Enhancement of FSO System using Modified Exponentiated Weibull Channel Model

Background: In free space optics (FSO) communication system, turbulence in the atmosphere leads to the intensity fluctuations of the received signal at the receiver end. Due to this reason, the performance of the free space optical communication system gets affected and results in fading of the received signal. Objectives: Performance improvement of FSO system using sophisticated channel modelling and fading mitigation techniques. Methods: This paper presents the performance improvement of the FSO system using a modified Exponentiated Weibull (MEW) channel model. Moreover, the performance of the FSO system is compared using different channel models such as Gamma-Gamma and Exponentiated Weibull. Results: The results in bit error rate (BER) with respect to different signal to noise ratio (SNR) are obtained using MEW, Gamma-Gamma, and Exponentiated Weibull channel models. In addition, BER is calculated for different aperture sizes such as 1.8, 5, and 13 mm using the proposed channel model to improve the performance of the FSO system. Similarly, the relay assisted technique is utilized to calculate the BER using the proposed channel model. Conclusion: An improvement of free space optical communication system is presented in terms of fading using channel modelling at different atmospheric turbulence conditions. An appropriate channel model is proposed for improving the performance of the FSO communication system using fading mitigation techniques. The proposed MEW channel model best describes the strong atmospheric turbulence induced fading. In addition, the performance of the free space optics system is improved using fading mitigation techniques such as aperture averaging and relay assisted FSO system.

Pulse Jamming in Aperture-Averaged FSO Receiver Over Exponentiated Weibull Fading Channel

In this paper, we study the pulse jamming effect thoroughly in aperture-averaged free space optical (FSO) systems. The FSO fading channels are considered to be Exponentiated Weibull (EW) distributed due to its excellent fit for all aperture sizes over the ranges of atmospheric turbulence conditions. It is inferred from the theoretical analysis that random jamming behaviour is impulsive and it reveals a destructive property over aperture-averaged FSO receivers. Analytical expressions of the average bit error rate (ABER) and outage probability (OP) are formulated for the considered FSO system in the presence of random jammer. The effects of pulse jamming on ABER and OP performances are shown. Additionally, the asymptotic ABER analysis is performed in order to investigate the response of the considered FSO system in high transmit power region and to analyze the diversity order under the pulse jamming effect. Pointing error impairments, being another important performance degrading parameter of the FSO communication system, are studied along with the pulse jamming effect. Also, asynchronous jamming effect over the ABER performances is described by both analytically and graphically.

Optimal Prediction of Atmospheric Turbulence by Means of the Weather Research and Forecasting Model

The performance of ground-based astronomical observations and free-space optical communication (FSOC) systems suffers from atmospheric turbulence and meteorological conditions. The a priori knowledge of atmospheric conditions several hours before observations allows the programming of astronomical observations (flexible scheduling) to be optimized. In this paper, we present a prediction study based on the Weather Research and Forecasting (WRF) model. It allows the prediction and characterization of a useful set of meteorological parameters relevant to atmospheric physics (e.g., pressure, temperature, relative humidity, wind speed, and direction). Predicted parameters are then injected into an optical turbulence (OT) model to compute the refractive index structure constant C n 2. We performed sets of simulations for Cerro Pachon Observatory in Chile, using the data from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR). The main goal is to quantify how accurately numerical weather prediction models can reproduce conditions over the complex terrain of the Cerro Pachon area. In order to produce a reliable forecast, meteorological prognostic skills need accurate representations of the physical parameterization options. Three widely used Planetary Boundary Layer (PBL) schemes and two Land Surface Models (LSM) were tested, analyzed, and compared in order to find the optimal WRF configuration. Predictions are compared to in situ measurements coming from balloon-borne radiosoundings. It is determined that the predicted C n 2 are in good agreement with the radiosoundings measurements with a mean relative error (MRE) under 6.4% at all altitudes when using balloon measurements to deduce some parameters such as the outer scale of turbulence L 0, which is used in the OT model. For a fully operational prediction, the MREs between the predictions and the measurements range from 1.4% to 8% according to the different ways to estimate the L 0 profiles. Seasonal statistics are also presented for different meteorological and turbulence parameters.

Algorithmic decoding of dense OAM signal constellations for optical communications in turbulence.

We demonstrate an optical detection and decoding strategy to increase the information rate and spectral efficiency of free-space laser communication links affected by turbulence by means of dense orbital angular momentum (OAM) modulation. Using three candidate receiver architectures-based on a Shack-Hartmann sensor, a Mode Sorter, and a complex conjugate projection scheme as a base case-we demonstrate an algorithmic classification system based on the received OAM spectra produced by these architectures. This classification scheme allows low-error-rate data transmission in turbulence using 16-OAM, 32-OAM, and 64-OAM symbol constellations, with OAM states between -20 and 20. We evaluate and compare their performance under weak to strong atmospheric turbulence conditions using an accuracy metric and confusion matrices.

Experimental investigation of LG beam propagating in actual atmospheric turbulence

The propagation properties of Laguerre-Gaussian (LG) beamare investigated experimentally in an actual turbulent atmosphere, including beam propagation, phase change, and singularity perturbation. The experimental results indicate that with the increase of propagation distance the beam gradually spread, the intensity distribution of LG beam is divergent and weakened, and the position of the singularity is unstable. In particular, we compare the propagation characteristics of the Gaussian beam and LG0,1 beam through the same atmospheric turbulence conditions. The experimental results show that the propagation stability of the LG beam is found to be better than that of the Gaussian beam.

Removing Atmospheric Turbulence via Deep Adversarial Learning

Restoring images degraded due to atmospheric turbulence is challenging as it consists of several distortions. Several deep learning methods have been proposed to minimize atmospheric distortions that consist of a single-stage deep network. However, we find that a single-stage deep network is insufficient to remove the mixture of distortions caused by atmospheric turbulence. We propose a two-stage deep adversarial network that minimizes atmospheric turbulence to mitigate this. The first stage reduces the geometrical distortion and the second stage minimizes the image blur. We improve our network by adding channel attention and a proposed sub-pixel mechanism, which utilizes the information between the channels and further reduces the atmospheric turbulence at the finer level. Unlike previous methods, our approach neither uses any prior knowledge about atmospheric turbulence conditions at inference time nor requires the fusion of multiple images to get a single restored image. Our final restoration models DT-GAN+ and DTD-GAN+ outperform the general state-of-the-art image-to-image translation models and baseline restoration models. We synthesize turbulent image datasets to train the restoration models. Additionally, we also curate a natural turbulent dataset from YouTube to show the generalisability of the proposed model. We perform extensive experiments on restored images by utilizing them for downstream tasks such as classification, pose estimation, semantic keypoint estimation, and depth estimation. We observe that our restored images outperform turbulent images in downstream tasks by a significant margin demonstrating the restoration model's applicability in real-world problems.

BER analysis of FSO system with Airy beam as carrier over exponentiated Weibull channel model

Average Bit Error Rate (BER) expression of free-space optical (FSO) communication links with Airy beam as signal carrier under weak atmospheric turbulence and on–off keying modulation scheme is derived based on scintillation index of Airy beam and Exponentiated Weibull channel model. The average BER has been evaluated at different transverse scale factors and exponential decay factors of Airy beam and link distances. And comparison of the average BER of FSO links with Airy beam and Gaussian beam as signal carrier has been carried out. The simulation results show that the average BER of FSO links with Airy beam as carrier decreases with the increase of mean signal to noise ratio and increases with the increase of transmission distance. When the transverse scale factor is about 1.5 cm, a lower average BER can be obtained. And the smaller the exponential decay factor is, the lower the average BER is. Under the same atmospheric turbulence condition, the average BER of FSO links with Airy beam as carrier is obviously better than that of FSO links with Gaussian beam as carrier. The results of this research have some significance for the application of Airy beam in FSO system.

Free-space optical communication with quasi-ring Airy vortex beam under limited-size receiving aperture and atmospheric turbulence.

Vortex beams carrying orbital angular momentum (OAM), which feature helical wavefronts, have been regarded as an alternative degree of freedom for free-space optical (FSO) communication systems. However, in practical applications, atmospheric turbulence and limited-size receiving aperture effects will cause OAM modal degradation and seriously reduce the received power. In this paper, by controlling the radial phase distribution of conventional OAM beams, quasi-ring Airy vortex beams (QRAVBs) are successfully generated in the experiments to increase the received power under the limited-size receiving aperture conditions. By employing 72-Gbit/s 16-ary quadrature amplitude modulation (16-QAM) discrete multi-tone (DMT) signals, we successfully demonstrate free-space data transmission with QRAVBs in the experiments. Moreover, the transmission performance of QRAVBs under atmospheric turbulence is also evaluated. Comparing with conventional OAM beam and Bessel beam, the obtained results show that QRAVBs can achieve higher received power and better BER performance under limited-size receiving aperture and atmospheric turbulence conditions.

Coherent beam combination of seven 1.5 µm fiber amplifiers through up to 1 km atmospheric turbulence: near- and far-field experimental analysis.

A laser testbed based on active coherent beam combination (CBC) of seven 1.5 µm, 3 W fiber amplifiers was developed for applications requiring high power such as power density deposition on targets or free space laser communication. For the first time to our knowledge, the frequency-tagging locking of optical coherence by single-detector electronic-frequency tagging technique was implemented in the field in real atmospheric turbulence conditions in a target-in-the-loop configuration. Successful combination was achieved after horizontal propagation of 311 m and 1 km, at 1.5 m above the ground, while the estimated average turbulence strength was Cn2∼4.10-14m-2/3. We present the CBC laser bench and an embedded near-field interferometer called PISTIL (PISton and TILt) able to measure the relative phase shift of each emitter. We show that this measurement can provide information on relative turbulence-induced phase variation of the combined laser beams. In particular, the far-field beam envelope wandering can be estimated through this diagnosis. Results are supported by an analytical model and confirmed by numerical post-analysis of measured far-field interference. This additional interferometer may improve CBC beam pointing through turbulence.

Performance of UAV-to-Ground FSO Communications with APD and Pointing Errors

Recently, a combination of unmanned aerial vehicles (UAVs) and free-space optics (FSO) has been investigated as a potential method for high data-rate front-haul communication links. The aim of this work was to address the performance of UAV-to-ground station-based FSO communications in terms of the symbol error rate (SER). The system proposes utilizing subcarrier intensity modulation and an avalanche photo-diode (APD) to combat the joint effects of atmospheric turbulence conditions and pointing error due to the UAV’s fluctuations. In the proposed system model, the FSO transmitter (Tx) is mounted on the UAV flying over the monitoring area, whereas the FSO receiver (Rx) is placed on either the ground or top of a high building. Unlike previous works related to this topic, we considered combined channel parameters that affect the system performance such as transmitted power, link loss, various atmospheric turbulence conditions, pointing error loss, and the total noise at the APD receiver. Numerical results have shown that, for the best system SER performance, the value of an average APD gain at the Rx can be selected, varying from 18 to 30, whereas the equivalent beam waist radius at the Tx should be in a range from 2 to 2.2 cm in order to decrease the effects from the UAV’s fluctuations.

SPRINT, system parameters recurrent invasive tracking: a fast and least-cost online calibration strategy for adaptive optics

ABSTRACT Future large adaptive telescopes will trigger new constraints for the calibration of adaptive optics (AO) systems equipped with pre-focal deformable mirrors (DMs). The image of the DM actuator grid, as seen by the wavefront sensor (WFS), may evolve during the operations because of the flexures of the opto-mechanical components present in the optical path. This will result in a degraded AO performance that will affect the scientific operation. To overcome this challenge, it will be necessary to regularly monitor and compensate for these DM/WFS mis-registrations, either by physically realigning some optical components or by updating the control matrix of the system. In this paper, we present a new strategy to track mis-registrations using a pseudo-synthetic model of the AO system. The method is based on an invasive approach where signals are acquired on-sky, before or during the scientific operations, and fed to the model to extract the mis-registration parameters. We introduce a method to compute the most sensitive modes to these mis-registrations that allows us to reduce the number of degrees of freedom required by the algorithm and to minimize the effect on the scientific performance. We demonstrate that, by using only a few of these well-selected signals, the method provides very good accuracy for parameter estimation, well under the targeted accuracy, and has a negligible effect on the scientific path. In addition, the method appears to be very robust to varying operating conditions of noise and atmospheric turbulence and it performs equally for both pyramid and Shack–Hartmann WFSs.

Modelling of atmospheric optical turbulence with the Weather Research and Forecasting model at the Ali observatory, Tibet

ABSTRACT In this study, we make use of a meso-scale numerical model to obtain the parameters of atmospheric optical turbulence at the Ali observatory above the Tibetan Plateau. These parameters are essential for ground-based optical observations and some adaptive optics techniques. We present the characterization of the vertical distributions of atmospheric optical turbulence Cn2 and the main integral parameters, such as coherence time τ0, isoplanatic angle θ0 and seeing ε0 derived from Cn2. Meanwhile, the measurement of Cn2 obtained with a radiosonde instrument is used to quantify the model performance on reconstructing the optical turbulence above the site. This shows a high level of agreement and provides a reliable tool for astronomical site testing. The results presented in this paper demonstrate the good behaviour of the atmospheric optical turbulence condition at Ali, particularly during the summer half of the year. We find yearly median values of seeing ε0 = 0.47 arcsec, isoplanatic angle θ0 = 4.35 arcsec and atmospheric coherence time τ0 = 5.52 ms in 2016, and we also provide independent confirmation of the potential of the Ali site in adaptive optics. Moreover, it has been proven that meso-scale models can provide reliable estimations of atmospheric optical turbulence conditions above an astronomical site on the Tibetan Plateau.

LDPC code with Dynamically adjusted LLR under FSO turbulence channel

With the development and progress of space technology, more and more scholars begin to study FSO communication. The free-space optical communication takes the atmosphere as the medium, and the signals are susceptible to atmospheric turbulence. As an important channel code, LDPC code is applicable to almost all channels. In order to obtain better communication performance, we propose dynamically adjusted log-likelihood ratio (LLR) as the input of LDPC decoder in this paper. Compared with the traditional LLR input, the simulation results show that the LDPC code based on dynamically adjusted LLR has stronger anti-turbulence ability and better error performance under different atmospheric turbulence conditions. Coherent communication system with PM-QPSK modulation format under FSO turbulence channel is utilized in our work.

Performance of Vehicular Visible Light Communications under the Effects of Atmospheric Turbulence with Aperture Averaging.

In this paper, we investigate the performance of a vehicular visible light communications (VVLC) link with a non-collimated and incoherent light source (a light-emitting diode) as the transmitter (Tx), and two different optical receiver (Rx) types (a camera and photodiode (PD)) under atmospheric turbulence (AT) conditions with aperture averaging (AA). First, we present simulation results indicating performance improvements in the signal-to-noise ratio (SNR) under AT with AA with increasing size of the optical concentrator. Experimental investigations demonstrate the potency of AA in mitigating the induced signal fading due to the weak to moderate AT regimes in a VVLC system. The experimental results obtained with AA show that the link’s performance was stable in terms of the average SNR and the peak SNR for the PD and camera-based Rx links, respectively with <1 dB SNR penalty for both Rxs, as the strength of AT increases compared with the link with no AT.

Developing certification procedures for quiet supersonic aircraft using shaped sonic boom predictions through atmospheric turbulence

New sonic boom noise certification procedures for quiet supersonic aircraft are in development under the International Civil Aviation Organization for a proposed standard that could enable supersonic overland flight. Although these low-boom aircraft do not exist yet, the procedures can be developed and evaluated using existing empirical and simulation datasets. While prior analyses used N-wave datasets to exercise various proposed procedures, these evaluations were limited because the effects of atmospheric turbulence on N-waves do not necessarily generalize to shaped sonic booms. The analysis is therefore extended to include shaped sonic boom predictions through several atmospheric turbulence conditions, such as different atmospheric boundary layer heights, using three different numerical methods. Six noise metrics (PL, ASEL, BSEL, DSEL, ESEL, and ISBAP) are calculated for the ground waveform predictions and compared to metrics for predictions performed under quiescent standard day conditions. Methods for validating prediction tools or correcting for the atmospheric effects are discussed, and ideas for robust methods for implementation in certification procedures are proposed.

Free Space Optics, Channel Models and Hybrid Modulation Schemes: A Review

Free space optical (FSO) communication system is a wireless communication technology which possess advantages such as huge license free bandwidth and high data carrying capacity among others. It has proved to be a viable solution towards the radio frequency spectrum crunch and hence has been applied in cellular back haul, earth to satellite links, deep space and defense among others. However, FSO performance is adversely affected by atmospheric turbulence and weather conditions. Different channel models are used to estimate the performance of FSO communication system. Meanwhile, different mitigation techniques have been proposed and analyzed over the years for improvement of FSO communication. Of late, hybrid modulation schemes have proved to be another research direction. In this article, we review the FSO channel models as well as hybrid modulation schemes that have been recently introduced to further improve the FSO performance. The conditional probability of error for the commonly used FSO modulation schemes are presented. Finally, the expressions for the bit error for the hybrid modulation schemes are also given.

Increasing the Link‐distance of Free‐space Quantum Coherent Communication with Large Area Detectors

We report a large area photo-diode based homodyne detector for free-space quantum coherent communication. The detector's performance is studied in terms of detection bandwidth and electronic noise for shot-noise limited quantum signal detection. Using large area photo-diodes increases signal collection efficiency from turbulent atmospheric channels, in comparison with typical fibre based free-space homodyne detectors. Under identical atmospheric turbulence and receiver aperture conditions, our homodyne detector based on 1mm diameter photo-diode experiences 0dB loss due to turbulence while a 10um fibre based detector experiences 13.5dB of signal loss over a 700km free-space link, at 90 degree elevation angle.

Performance analysis of MRR FSO communication system under Gamma–Gamma​ turbulence channel with pointing error

The performance of modulating retro-reflector (MRR) free space optical (FSO) communication system has been studied before for various atmospheric turbulence conditions. However, the effect of pointing error (PE) on the MRR FSO communication system has not been studied yet. In this work, we investigate the PE impact on the MRR FSO communication system. Specifically, we obtain the probability density function (PDF) of the system in closed form expression regarding Gamma–Gamma turbulence channel and Rayleigh PE. Using this PDF, we derive the statistical characteristics closed form expressions of the received optical signal-to-noise ratio (SNR) including PDF, cumulative distribution function (CDF), moments and moment generating function (MGF). Relying on the obtained analytical results, we derive the system average bit error rate (BER), outage probability, ergodic capacity (EC) and amount of fading (AF) performances in closed forms. The closed form expressions are given in terms of Meijer’s G-function with the exception of obtaining the moments of the received optical SNR and AF in terms of elementary functions. For the given weather, PE and system conditions, we optimize the beam divergence angle to minimize the system average BER. Based on this, we suggest controlling of the beam divergence angle with the transmission distance according to the optimum beam divergence angle to improve the system BER performance. Finally we demonstrate the system performances according to the optimum beam divergence angle.

Level Crossing Rate and Average Fade Duration of Satellite-to-UAV FSO Channels

This paper investigates the second-order statistics of the level crossing rate (LCR) and average fade duration (AFD) for the optical wireless channels between a satellite and a vehicle. Specifically, the novel expressions of LCR and AFD for the low earth orbit (LEO) satellite-to-unmanned aerial vehicles (UAVs) laser links are derived in the presence of the generalized misalignments caused by the satellite vibration together with the UAV hovering, which is modeled by a four-parameter Beckmann distribution, and the atmospheric turbulence conditions. The analysis can also be applied to other kinds of vehicles, which, in fact, require simpler channel models. Several numerical examples are presented to quantitatively demonstrate the impact of both atmospheric turbulence and pointing errors on the LCR and AFD. The Monte Carlo simulations are performed to validate the correctness of the theoretical derivations.