Effects Of Atmospheric Turbulence Research Articles

FDM FSO communication system with coherent heterodyne optical receiver under strong turbulent condition

We provide an analytical approach to analysis the performances of FDM FSO communication system with coherent heterodyne optical receiver under strong turbulent condition which is modeled as gamma–gamma distribution. Analysis is carried out to find the bit error rate and channel capacity with RF subcarrier modulation taking into consideration the effect of strong atmospheric turbulence. Results are evaluated numerically in terms of average CNR, BER, channel capacity and power penalty suffered by the system due to the effect of turbulence at a given BER for several system parameters like turbulence variance, link distance, local oscillator power, etc. It is observed that at a data rate of 10 Gbps with number of subcarrier 32, the power penalty at a BER of 10−9 is 12 dB for a link distance of 3.6 km and the performance improves gradually with increasing the number of RF subcarriers. It is also observed that, the overall performance improves with the improvement in both RF and optical local oscillator power.

Enhancing earth-to-satellite FSO system spectrum efficiency with adaptive M-ary PSK and SIMO in presence of scintillation and beam wander

In this paper, the performance of the free-space optical (FSO) system from ground-to-satellite is analyzed considering the combined effect of atmospheric turbulence and beam wandering employing M-ary phase-shift keying (MPSK). Key parameters of the vertical connection, such as satellite altitude, zenith angle, and beam size, are investigated. In order to improve the spectrum efficiency, an adaptive transmission approach is applied to ensure efficient channel capacity usage. The procedure depends on changing the modulation order of the MPSK scheme according to the instantaneous channel state and the acceptable bit error rate. Furthermore, a single-input multiple-output (SIMO) system with maximal ratio combining (MRC) diversity is used to mitigate the effects of atmospheric turbulence and beam wandering. Closed-form and asymptotic expressions are presented for the average bit error rate (BER) for a single-input single-output (SISO) and SIMO techniques. Moreover, closed-form expression is deducted for the upper bound bandwidth efficiency. The accuracy of the derived closed-form and asymptotic expressions are verified by adopting appropriate simulation cases and parameters. The results show that the optimal value of the beam size ratio (BSZ) is 1.6. Also, the results prove that the adaptive technique improves spectrum efficiency by more four times the non-adaptive method at the same BER threshold of 10-2. By using a 1×4 SIMO system, the same adapted spectrum efficiency is achieved with a reduction of 4 dB in SNR versus 1×2 SIMO system and 8 dB in SNR versus SISO system. A 1×4 SIMO system achieves a threshold bit error rate of 5 dB reduction in SNR compared to a 1×2 SIMO system and a 10 dB reduction in SNR compared to SISO system.

Analysis of adaptive multi-rate FSO/RF hybrid systems using Málaga-ℳ distribution model in turbulent channels

Precise modeling of the optical channels with different conditions of atmospheric turbulence is an important prerequisite for analyzing the performance of Free Space Optical (FSO) communication systems. Statistical models which have been proposed to describe the optical channels, have some limitations to precisely model the channel. In this paper, The ℳ distribution which is a relatively new and comprehensive model, introduced by the University of Malaga, has been used to model and evaluate FSO/RF hybrid systems under the effect of atmospheric turbulence of the channels for two modes of operation, classic adaptive rate transmission, and the combination of adaptive rate transmission and automatic repeat request.The simulation results have been compared with the experimental results which have been reported by other researchers. It seems, using ℳ distribution model, yields more consistency between design objectives and implementation results.

Performance analysis of amplify and forward parallel relaying free-space optical system over M distribution

We propose a parallel relaying coherent orthogonal frequency division multiplexing free-space optical (FSO) communication system model with amplify-and-forward channel-state-information-assisted relaying in an M distribution. Considering the combined effects of path loss, pointing error, and atmospheric turbulence, the closed-form expression of moment generating function, probability density function, and cumulative distribution function of the upper limit of equivalent signal-to-noise ratio are derived. Furthermore, the Meijer G closed-form expressions for outage probability and symbol error rate (SER) over M aggregated channel are obtained. The influences of turbulence intensity, the number of parallel paths, normalized beam bandwidth, normalized jitter error, and pointing error on the outage probability and SER of the proposed FSO system are analyzed by simulation. Simulation results indicate that controlling the normalized beam width to a moderate range and reducing the normalized jitter error can reduce the outage probability and improve system reliability. After the pointing error exceeds 7, the SER decline tends to be flat, which provides a theoretical reference for the precision of the pointing technology, the selection of the optical antenna transmission aperture and the receiver sensitivity index in engineering design.

Performance Analysis of Free Space Optical Networks Using the Beta-Average Recursive Estimator

The present paper aims to investigate and analyze the performance of a point-to-point optical network under a free-space optical (FSO) communication system. Free-space optical communication is a sophisticated technique that has been employed as a channel model of optical wireless communication to transfer large amounts of data at high speeds. However, two important issues, that could affect the source of the FSO link, have been added. The first one is related to chromatic dispersion which is attributed to the chirping phenomenon of the optical signal; the second issue concerns the secondary jamming power that can be generated by some Radio Frequencies from base stations. It is important to note that the effects of atmospheric turbulence, misalignment fading, and atmospheric attenuation as well as geometric losses were also taken into account. The purpose of this research work is to overcome these effects, in order to increase the data transmission rate from 1.25 to 50 Gbps in the proposed system. Consequently, a very efficient solution is suggested to correct these problems and to make the FSO link more reliable using the Beta-Average Recursive Estimator. In addition, the numerical results obtained are presented for the purpose of validating our proposal through the evaluation of the performance of a transmission link in terms of the bit error rate and Q-factor. In the end, the resulting conclusions are listed, explained and discussed.

Low-complexity parallel real-valued weight adaptive digital combining algorithm for coherent FSO communications employing modes diversity reception under atmospheric turbulence channel

Diversity reception is an effective technology to mitigate the atmospheric turbulence effect on the performance of the coherent free-space optical (FSO) communications. Furthermore, considering high-speed optical communications and the state-of-art clock frequency of state-of-the-art field-programmable gate array (FPGA) or application-specific integrated circuits (ASIC), reducing the computational complexity of digital combining algorithm and analyzing the performance of parallel structure algorithm has become important research issues for diversity reception system. In this paper, we experimentally investigate the performance of a low complexity digital combining algorithm (CA) based on adaptive filter By replacing complex weight with real-valued weight, the number of real multiplications required in digital CA can be reduced by 62.5%. real-valued weight CA also ensuring the similar BER and outage probability performance in terms of serial structure under three different turbulence strengths. For parallel structure algorithm, the performance of real-valued weight CA is superior to that of complex-valued weight CA.

Parameter estimation of orbital angular momentum based continuous-variable quantum key distribution

In principle, the number of orbital angular momentum (OAM) eigenstates is unbounded; therefore, OAM multiplexing could greatly increase the transmission rate. Compared with optical fibers, free-space channels fluctuate randomly over time due to the effects of atmospheric turbulence. Free-space channel estimation is necessary. Here, we study the channel parameter estimation of continuous-variable quantum key distribution (CVQKD) based on OAM, especially the influence of atmospheric turbulence on transmittance and excess noise. By comparing the secret key rate in the case of single link and multiplexing, the impact of the estimated parameters on the performance of the CVQKD system is evaluated. Simulation results show that the value of the estimated parameter will affect the secret key rate.

Performance of real-time adaptive optics compensation in a turbulent channel with high-dimensional spatial-mode encoding.

The orbital angular momentum (OAM) of photons is a promising degree of freedom for high-dimensional quantum key distribution (QKD). However, effectively mitigating the adverse effects of atmospheric turbulence is a persistent challenge in OAM QKD systems operating over free-space communication channels. In contrast to previous works focusing on correcting static simulated turbulence, we investigate the performance of OAM QKD in real atmospheric turbulence with real-time adaptive optics (AO) correction. We show that even though our AO system provides a limited correction, it is possible to mitigate the errors induced by weak turbulence and establish a secure channel. The crosstalk induced by turbulence and the performance of AO systems is investigated in two configurations: a lab-scale link with controllable turbulence, and a 340 m long cross-campus link with dynamic atmospheric turbulence. Our experimental results suggest that an advanced AO system with fine beam tracking, reliable beam stabilization, precise wavefront sensing, and accurate wavefront correction is necessary to adequately correct turbulence-induced error. We also propose and demonstrate different solutions to improve the performance of OAM QKD with turbulence, which could enable the possibility of OAM encoding in strong turbulence.

Time jitter influence on the performance of gamma–gamma turbulence FSO links with various modulation schemes

ABSTRACTThe research area of optical wireless communication links has attracted significant interest over the last years due to the significant advantages offered by this kind of technology. However, the performance of FSO communication systems depends strongly on the effects, which are related to the atmosphere along the propagation path such FSO links are using. More specifically, the weather conditions and atmospheric turbulence effects can deteriorate considerably the performance characteristics. In this work, we are studying the joint influence of atmospheric turbulence and time jitter effects on the average BER of an FSO link by presenting a new approach for various modulation schemes. Thus new closed-form mathematical expressions are derived for accurate estimation of the BER performance of the optical wireless communication systems. Finally, using the obtained expressions and typical parameter values for FSO links, the numerical results are presented and then verified through Monte Carlo simulations.

Radio-optical line for SCADA systems with relay

Hybrid optical-radio communication systems provide high-speed access within the “last mile” and can act as highly reliable broadband channels of SCADA software and hardware systems. They use optical channels as the main communication lines, and radio channels as backup, since the transmission quality of atmospheric optical channels is largely determined by weather conditions. To reduce the effect of atmospheric turbulence, it is proposed to use the option of spatial diversity with line-of-sight channels and containing a repeater. When the signal-to-noise ratio decreases in the optical channel below the selected threshold, switching to the radio link is to be performed. The paper provides the results of modeling the operation of a hybrid radio-optical line in SCADA systems, in which a new switching system for optical and radio-frequency channels using a repeater is proposed. The dependences of the outage probability of the optical channel and the system as a whole are obtained.

Performance Analysis of a Multiple Subcarrier Modulated FSO Communication System using Direct Detection Optical Receiver under the Effect of Weak Atmospheric Turbulence

AbstractIn this paper, an analytical approach is presented to evaluate the bit error rate (BER) of a free space optical (FSO) link with radio frequency (RF) multiple subcarrier modulation, taking into account the effect of weak atmospheric turbulence considering a direct detection optical receiver followed by RF synchronous demodulator for each sub-channel. Analysis is carried out to find a closed-form expression for conditional BER at the output of the RF demodulators conditioned on a given value of atmospheric turbulence-induced fading and intermodulation distortion (IMD). The average BER for each sub-channel is then found by averaging the conditional BER over the probability density function of the atmospheric turbulence modeled as log-normal distribution. Degradations of BER due to atmospheric turbulence are evaluated for several values of system parameters like number of RF subcarrier, turbulence variance, link distance, data rate and power penalty suffered by the system due to atmospheric turbulence and IMD. For a given system bandwidth, it is found that the maximum power penalty occurs when the subcarrier number is around four and the performance gradually improves with increase in the number of subcarrier. For example, at a system bandwidth of 20 GHz with subcarrier number 4, the power penalty at a BER of 10–9 is found to be 30 dB for a link distance of 3.6 km, whereas the power penalty reduces to 13 dB when the number of subcarrier is increased to 32.

Coherence of vortex Bessel-like beams in a turbulent atmosphere.

Coherent properties of vortex conical waves propagating through a turbulent atmosphere are theoretically studied with the use of the analytical solution of an equation that describes the evolution of the second-order transverse mutual coherence function of an optical radiation field. The following parameters of vortex conical waves are considered: the degree of coherence, the coherence radius, the integral scale of the degree of coherence, and the integral scale of the squared degree of coherence. The effect of atmospheric turbulence on these coherence characteristics of vortex conical waves is analyzed at different values of their parameters. It turns out that the degree of coherence of a vortex conical wave, formed from a Gaussian beam while passing through a conical lens (axicon) and a spiral phase plate, at its optical axis, is almost independent of the initial radius of the Gaussian beam and the radius of the axicon aperture. In addition, all the coherence characteristics of vortex conical waves depend on the topological charge stronger than on the wave-vector component normal to the radiation propagation direction. A meter of the integral scale of the degree of coherence of vortex Bessel-like optical beams is shown to be a preferred sensor of optical radiation distortions in a turbulent atmosphere as compared to a meter of the coherence radius of such beams. A lower degree of coherence of vortex conical waves than of fundamental (vortex-free) conical waves in a turbulent atmosphere is proven with the use of the integral scale of the degree of coherence of these optical waves as a referent criterion.

Effects of Atmospheric Turbulence on High Cadence Observations of Stellar Occultations

In this paper we study the effects of systematic noise and atmospheric turbulence on image quality and signal-to-noise ratio for high cadence (∼20 fps) observations, in the context of fortuitous stellar occultation surveys. We performed an analysis of the spatial distribution of the weighted centroid of a reference star, in the X and Y direction (in the detector), and we also analyze the FWHM behavior to study the effect on the stellar profile. This information allowed us to find both the shift and the deformation of each image, which are the most significant effects of atmospheric turbulence on such images. Our observations were carried out at the 84 cm telescope of the San Pedro Mártir Observatory, located in Ensenada, Baja California, México. In order to isolate the atmospheric turbulence contribution on image quality and motion, the systematic noise due to telescope motion and other low frequency components were filtered-out from the distribution curve for the centroids. We used the APPHi pipeline as our analysis tool to study the effect of the size of the Region of Interest (ROIs) on energy loss in the photometry of the stars. We find for example that using ROI of 7 × 7 pixels in our study, the shifting and deformation of the images will produce an energy loss of ∼10%, for more than 9% of the observations, even after removing the systematic noise. Such energy loss can be comparable to the flux drop expected for some TNO occultations, thus leading to spurious detections. We also find that the algorithm developed to calculate an optimal ROI in the APPHi pipeline, makes a good compromise between avoiding the flux loss and keeping a reasonably small ROI, to reduce the data storage requirements and maximize the number of stars that can be simultaneously observed while limiting the overlap of ROIs for stars which are very close to each other.

Multimode communication through the turbulent atmosphere.

A central question in free-space optical communications is how to improve the transfer of information between a transmitter and a receiver. The capacity of the communication channel can be increased by multiplexing of independent modes using either: (1) the multiple-input-multiple-output (MIMO) approach, where communication is done with modes obtained from the singular value decomposition of the transfer matrix from the transmitter array to the receiver array, or (2) the orbital angular momentum (OAM) approach, which uses vortex beams that carry angular momenta. In both cases, the number of usable modes is limited by the finite aperture of the transmitter and receiver, and the effect of the turbulent atmosphere. The goal of this paper is twofold: first, we show that the MIMO and OAM multiplexing schemes are closely related. Specifically, in the case of circular apertures, the leading singular vectors of the transfer matrix, which are useful for communication, are essentially the same as the commonly used Laguerre-Gauss vortex beams, provided these have a special radius that depends on the wavelength, the distance from the transmitter to the receiver, and the ratio of the radii of their apertures. Second, we characterize the effect of atmospheric turbulence on the communication modes using the phase screen method put in the mathematical framework of beam propagation in random media.

Effect of Atmospheric Turbulence on the Striking Accuracy of Guided Bombs

This paper presents the test results of a simulation of an air-to-surface guided bomb drop in a turbulent atmosphere. The guided bomb was developed from a practice bomb built and upgraded by the Air Force Institute of Technology. The paper presents the test results of a numerical simulation of an air-to-surface guided bomb drop ran in a proprietary software environment. The numerical simulation inputs included aerodynamic characteristics calculated with PRODAS software and verified by wind tunnel tests. The stochastic components of atmospheric turbulence were simulated with a stochastic process model proposed by Shinozuki. Examples of the guided bomb drop simulation results are given in the paper. The effect of atmospheric turbulence parameters, i.e. standard deviation, σ and turbulence scale, Lw on the striking accuracy and ground impact scatter, are also shown.

Effect of atmospheric turbulence on timing instability for partially reciprocal two-way optical time transfer links

In this paper we analyze the limits of optical time transfer through atmospheric turbulence and relate those predictions to timing uncertainty analysis using the Allan timing variance (TVAR). The power spectrum of timing uncertainty due to atmospheric turbulence is expressed with the help of Taylor's frozen flow hypothesis, identifying a ${f}^{\ensuremath{-}8/3}$ and ${f}^{\ensuremath{-}2/3}$ power-law behavior for uncorrelated and partially correlated turbulence, respectively. The scaling of each power law is related to the geometry of the link and the turbulence profile. The power-law slopes are used to calculate two TVAR scaling coefficients relevant to turbulence timing noise, ${c}_{5/3}$ and ${c}_{\ensuremath{-}1/3}$, which can be applied to time-transfer analysis of timing data affected by turbulent fluctuations. Examples of a 2-km horizontal partially overlapping two-way link estimate the atmospheric contribution to timing fluctuations to be below $10\phantom{\rule{4pt}{0ex}}\mathrm{fs}$, while a two-way link to a medium-Earth-orbit satellite experiences timing fluctuations on the order of $2\phantom{\rule{4pt}{0ex}}\mathrm{fs}$. Comparison of turbulence theory to a recent two-way optical time transfer experiment shows good agreement with the expected power-law behavior and scaling factors.

Spiral spectrum of anomalous vortex beams propagating in a weakly turbulent atmosphere

ABSTRACTWe study the spiral spectrum of anomalous vortex beams propagating through a turbulent atmosphere. Based on the Huygens–Fresnel integral and the Rytov approximation, the integral expression and then the analytical expression for the spiral spectrum of anomalous vortex beams in the weakly turbulent atmosphere are derived. The capacity of wireless optical links using the anomalous vortex beam is obtained. It is found that the spiral spectrum of the anomalous vortex beam is less affected by turbulence than that of the Laguerre-Gaussian beam. And thus, the information capacity of wireless optical links using the anomalous vortex beam is larger than that using the Laguerre-Gaussian beam. The influence of beam order, wavelength, topological charge, propagation distance, refractive index structure constant and the radius of receiver aperture on spiral spectrum is investigated. These results contribute to reduce the disturbing effects of atmospheric turbulence on the orbital angular momentum of the vortex beam.

Research on the key technology of turbulence suppression for atmospheric optical laser communication based on OFDM

Based on the analysis of the two main problems of atmospheric channel in free space optics (FSO), aiming at the problem of laser atmospheric channel especially the problems of frequency selective fading and multipath effect in complex turbulent environment, the suppression method is proposed based on OFDM turbulence effect, the FSO-OFDM system is built, the baseband model of this system and the signal of multi-carrier modulation and demodulation method are studied. First of all, this article systematically analyzes the mechanism of turbulence effect of atmospheric channel, and discusses the model of plane wave laser communication system under the influence of atmospheric turbulence, the space optical communication system model of Gaussian beam of a logarithmic normal turbulent channel is established under the influence of atmospheric turbulence, the probability density function of light intensity is deduced, the methods for analyzing the effects of various atmospheric turbulence effects on system performance using the signal-to-noise ratio probability density function; the OFDM multi-carrier modulation scheme of FSO-OFDM system is designed, the baseband mode model of FSO-OFDM system is constructed, and the modulation and demodulation principle of its signal is studied using this model. Finally, FSO-OFDM system is realized by using MATLAB, and the FSO communication system under the multipath interference is simulated, experiments on bit error rate characteristics under different guard intervals are performed. It is confirmed that the FSO-OFDM system has a strong ability to resist multipath interference and frequency selective fading, as well as good bit error rate (BER) performance. It can effectively solve the problem of intersymbol interference and the reliable link, and has a very broad application prospect and using value.

Effect of turbulence and noise on ultraviolet and mid-infrared spectrum in optical wireless communications

Ultraviolet and mid-infrared (MIR) constitute two extreme ends of the optical wireless communication band. Recently, there has been a rise in increased interest for research in these two spectral bands. For successful deployment of these technologies, the performance of these two spectrum bands needs to be thoroughly analyzed. We consider turbulence and noise effects over these spectral bands. Specifically, this paper considers separate and combined effects of atmospheric turbulence and various noise sources to model the channel impairment. The results show that the system performances are extremely sensitive to the choice of channel model. It is found that the relatively unused MIR spectrum offers excellent characteristics for outdoor optical communications in the presence of atmospheric turbulence.

Performance study of multi-hop coherent OFDM FSO system over M distribution model

The performance of multi-hop coherent orthogonal frequency division multiplexing (OFDM) free space optical (FSO) system is studied by using quadrature phase shift keying (QPSK) modulation in this paper. A generalized model called M distribution is selected, which is suitable for all categories of turbulence ranging from weak to strong and characterizes other existing statistical models of atmospheric turbulence induced fading as its special case. The system uses decode and forward (DF) relay protocol between the transmitter and receiver of the relay auxiliary link. Considering the joint attenuation effects of atmospheric turbulence, path loss and pointing error on the atmospheric channel fading model, we derive the Meijer G closed-form expressions of outage probability and symbol error rate (SER). Furthermore, the effects of key factors, such as relay link length, the number of relay nodes and subcarriers on the outage and SER performance of OFDM FSO system are analyzed through simulations. This work lays a theoretical foundation for the practical application of the relay system.