Scintillation Index Research Articles

Laser beam scintillations of LIDAR operating in weak oceanic turbulence

The formulation of light detection and ranging (LIDAR) systems is derived and examined for the scintillation index, evaluated on-axis, of laser beams in horizontal links in the ocean with weak turbulence by utilizing the Rytov method. These scintillation indices, obtained for the Gaussian beam which is collimated, the limits of plane and spherical waves, are depicted versus the source size, target size, and parameter of the normalized reflector size. It is found that the source size, target size, and normalized reflector size parameter, lessening the scintillation index evaluated on-axis, are approximately 0.44 cm, 56 × 10 − 4 c m , and 2.2, respectively. Additionally, by using these values that minimize the scintillation index, the variation of the scintillations is shown against the propagation distance, radius of reflector, temperature and salinity fluctuation effects, mean squared temperature, and turbulent kinetic energy dissipation rate per unit mass of fluid at various selected source size and radius of reflector values.

Experimental study on laser propagation characteristics in local air turbulence disturbed by coherent acoustic waves

The existence of atmospheric turbulence seriously restricts the application of laser technology. However, the acoustic wave will affect the air density during the propagation process, change the original structure of the air turbulence, and then affect the propagation of the laser in the air turbulence. To study the effect of disturbance of atmospheric turbulence by acoustic waves on laser propagation, the propagation characteristics of lasers in local atmospheric turbulence under coherent acoustic wave perturbation are investigated experimentally. First, a coherent acoustic wave generator and a closed local air turbulence simulation device are designed. The propagation experiments of laser in a closed local air turbulence environment disturbed by coherent acoustic waves are carried out. It is found that the coherent acoustic wave will interfere with the wave reflected from the inner wall of the closed local air turbulence simulation device, and the sound pressure forms a regular standing wave intensity distribution pattern. Moreover, the variations of laser beam drift fluctuation variance, scintillation index, and beam diameter with coherent acoustic wave frequency, power, and sound source array position are studied. The results show that the original structure of air turbulence will be changed in the process of coherent acoustic wave propagation, which will affect the beam drift fluctuation variance, scintillation index, and beam diameter changes of laser propagation in air turbulence. The beam drift fluctuation variance, scintillation index, and beam diameter change regularly with the change of the frequency and power of the sound source. Changes of the fluctuations with frequency are relatively gentle, whereas changes with power are severe. This paper explores the influence of coherent acoustic waves disturbing air turbulence on laser propagation characteristics, and the research results provide a basis for improving or disturbing laser propagation performance in atmospheric turbulent environments.

Performance of bit error rate of laser beams for LIDAR used in marine atmospheric weak turbulence

The performance of laser beams is investigated for light detection and ranging (LIDAR) systems in marine weak atmospheric turbulence by using the Rytov method. Formulation, on-axis, of the scintillation indices of LIDAR systems, which is obtained for plane, collimated Gaussian, and spherical beams, is appraised for average bit error rate (BER) by utilizing the log-normal distributed intensity. Values, obtained for these beams, are checked against source sizes, target sizes, normalized target size parameters, propagation lengths, and the average signal-to-noise ratio (SNR) for specified parameters. It is observed that the target size that minimizes the scintillation index is independent of the source size. The target size minimizing scintillation index is approximately 10 − 3 c m . Variations in source size and target size values, independently of each other, affect the scintillation index and the performance of marine LIDAR system links. The contribution of the outer scale turbulence to the deterioration is more evident than the inner scale turbulence.

Effect of system parameters on power scintillation of Ince Gaussian beam in turbulent atmosphere

The power scintillation of Ince Gaussian (IG) beam is investigated numerically based on the split step method. The effect of the IG beam modes on mitigating the effect of atmospheric turbulence is studied. Furthermore, the power scintillation indices of IG beams with different modes are compared with the fundamental Gaussian beam power scintillation indices. Moreover, the ability of different parameters, namely the beam waist, operating wavelength, and the receiver aperture, to reduce the scintillation is examined. The obtained results show that IG beams with even modes have the advantage over Gaussian beams for links longer than 2 km. Moreover, IG beams with even modes are able to reduce the scintillation more than the ones with odd modes. Yet, increasing the beam waist and the receiver radius reduces the scintillation of the beams with odd mode more than the beams with even mode. Our results contribute to the improvement of free-space optical communication systems operating in turbulent atmosphere.

Shot-noise-limited photon-counting precoding scheme for MIMO ultraviolet communication in atmospheric turbulence.

Photon counting is a promising solution to detecting low-power optical signals for ultraviolet (UV) communications in the forthcoming sixth-generation (6G) network. Different from the conventional additive white Gaussian noise (AWGN) model, the discrete signal-dependent Poisson shot noise poses challenges to the signal processing of photon-counting systems. In this paper, a joint design of precoder and equalizer is proposed for photon-counting multiple-input multiple-output (PhC-MIMO) UV systems. To circumvent the impasse arising from the signal-dependent shot noise, we propose an alternating optimization algorithm based on the minimum mean squared error (MMSE) criterion. The algorithm decomposes the joint design into convex subproblems solved in an alternating manner, and guarantees at least a stationary point solution. Numerical results corroborate that the proposed system exhibits robustness to turbulence fading and offers high throughput while mitigating the adverse effect of background radiation noise. Specifically, the 32 × 8 system can achieve a bit error rate (BER) of 10-5 at the signal energy of -154.0 dBJ per bit under strong Gamma-Gamma turbulence with the scintillation index (S.I.) of 3.

Ionospheric scintillation characteristics from GPS observations over Malaysian region after the 2011 Valentine’s day solar flare

Abstract Ionospheric scintillations due to plasma irregularities can severely affect the modern dynamic and technological systems whose operations rely on satellite-based navigation systems. We investigate the occurrence of ionospheric scintillation in the equatorial and low latitude region over Malaysia after the 2011 Valentine’s Day solar flare. A network of three Global Ionospheric Scintillation and Total Electron Content Monitor (GISTM) GSV4004B receivers with increasing latitudes from the magnetic equator were used to monitor ionospheric TEC, rate of change of TEC index (ROTI), and amplitude (S4) as well as phase (σ φ) scintillation indices. The results show a simultaneous sudden rise in S4 and σ φ along with a significant depletion of TEC at all three locations. However, the largest enhancement of scintillation indices accompanying a substantial TEC depletion is observed at the farthest low latitude station (UNIMAS) from the equator with values around 0.5, 0.3 rad, and 1 TECU, respectively. The corresponding values at the near-equatorial station (Langkawi; 0.4, 0.2 rad, and 3 TECU) and intermediate station (UKM; 0.45, 0.3 rad, and 5 TECU) are examined along with ROTI variations, confirming the simultaneous occurrence of kilometer-scale and sub kilometer scale irregularities during 17 and 18 February 2011. The radiation effects of the solar flare on the ionosphere were prominently recognized at the local nighttime hours (around 14:00 to 17:00 UT) coinciding with the equatorial prereversal enhancement (PRE) time to seed the equatorial plasma bubbles (EPBs) enhancement that resulted in ionospheric irregularities over the low latitudes. The significant TEC depletion seen in the signals from selected GPS satellites (PRNs 11, 19, 23, and 32) suggests plausible degradation in the performance of GPS-based services over the Malaysian region. The study provides an effective understanding of the post-flare ionospheric irregularities during an episode of minor geomagnetic storm period and aligns with the efforts for mitigating the scintillation effects in space-based radio services over low latitudes.

A New Approach for Low-Latitude Ionospheric Scintillation Prediction

The prediction of ionospheric scintillation is a current research topic. Data-oriented models have been proposed since there is not a mathematical model able to simulate the complex ionospheric mechanisms for the prediction of scintillation. Data-oriented models employ machine learning algorithms that are trained with known, post-mortem data, and then perform predictions from new data. An ensemble method based on sampling boosting applied for a set of decision trees is employed for the prediction of scintillation in a single low-latitude location in Brazil. The prediction was performed considering two classes, occurrence or absence of scintillation. The method uses as input temporal series of the scintillation index S4, the total electron content (TEC), and some geomagnetic and solar indexes for that location. Data encompasses the summer months of the previous solar cycle years (2010-2018), and it was split into mutually-exclusive training and validation/test sets. Prediction performance was promising, showing a potential to be developed for operational use. Data limitations related to time series extension, or also to a balanced distribution of samples/instances covering both classes, since scintillation occurrences are usually scarce, can be further developed as new data are available.

Novel Simulation and Analysis of Mie-Scattering Lidar for Detecting Atmospheric Turbulence Based on Non-Kolmogorov Turbulence Power Spectrum Model.

The Mie-scattering lidar can detect atmospheric turbulence intensity by using the return signals of Gaussian beams at different heights. The power spectrum method and Zernike polynomial method are used to simulate the non-Kolmogorov turbulent phase plate, respectively, and the power spectrum method with faster running speed is selected for the subsequent simulation. In order to verify the possibility of detecting atmospheric turbulence by the Mie-scattering lidar, some numerical simulations are carried out. The power spectrum method is used to simulate the propagation of the Gaussian beam from the Mie-scattering lidar in a vertical path. The propagation characteristics of the Gaussian beam using a non-Kolmogorov turbulence model are obtained by analyzing the intensity distribution and spot drift effect. The simulation results show that the scintillation index of simulation is consistent with the theoretical value trend, and the accuracy is very high, indicating that the method of atmospheric turbulence detection using Mie-scattering lidar is effective. The simulation plays a guiding role for the subsequent experimental platform construction and equipment design.

Nowcasting of Amplitude Ionospheric Scintillation Based on Machine Learning Techniques

Ionospheric scintillation is a phenomenon that can compromise and even make the operation of some space-based systems unfeasible. In this context, it is important to develop tools capable of predicting its occurrence. However, modeling this phenomenon is quite complex due to the influence of several other aspects, such as the geomagnetic and solar activities, the seasons, and the geographic location. Therefore, the main objective of this article was to develop short-term predictive models about amplitude ionospheric scintillation through machine learning techniques. The dataset used was built considering information related to geomagnetic, solar, and interplanetary activities, the phenomenon’s temporal and geographic dependence, and the ionosphere’s state. To predict the value of the scintillation index <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$S_{4}$</tex-math></inline-formula> 30 min in advance, six models were used, based on three algorithms, the artificial neural network, the extreme gradient boosting, and the random forest. The results indicated a very satisfactory prediction capacity since a coefficient of determination of 0.87 was achieved by the lower performance model. Additionally, the results demonstrated the usefulness of the considered dataset and the feature selection approach in the model’s development phase, which led to better models in accord with some statistical tests performed.

Optical wireless communication system performance in natural water turbulence of any strength.

The recently introduced power spectrum model for natural water turbulence, i.e., that at any average temperature, average salinity, and stratification [J. Opt. Soc. Am. A37, 1614 (2020)JOAOD61084-752910.1364/JOSAA.399150], is extended from weak to moderate-to-strong regimes with the help of the spatial filtering approach. Based on the extended spectrum, the expressions for the scintillation index (SI) are obtained, and based on its signal-to-noise ratio and bit error rate of the underwater wireless optical communication (UWOC) system with the on-off-keying modulation and gamma-gamma irradiance distribution model, the analysis is performed. The obtained results are compared with those derived from the widely used Nikishov and Nikishov spectrum. It is shown that the natural water turbulence results in the SI for plane (spherical) waves attaining higher maxima values at shorter propagation distances, about 20 m (40 m) with respect to 30 m (50 m) of Nikishovs turbulence. Therefore, it predicts a stronger degradation of the UWOC system performance in weak and moderate turbulence regimes.

Adaptive symbol-by-symbol decision feedback threshold detection receiver to suppress oceanic turbulence scintillation effect

In this paper, an adaptive symbol-by-symbol decision feedback (DFB) threshold detection receiver is proposed that can suppress the scintillation effect of oceanic turbulence in underwater wireless optical communication (UWOC) system. The DFB receiver has a simple structure and can estimate the instantaneous channel state information (CSI) through the previously detected signal after adding pilot-symbols to the transmission data header. It thereby adaptively adjusts the decision threshold of the UWOC system receiver. The DFB receiver is independent of perfect CSI (PCSI). The channel model and transmitted signal power information are not essential for it. Moreover, the channel bandwidth can accommodate the inserted pilot-symbols. Using the gamma–gamma oceanic turbulence model, the performance of the DFB receiver under intensity fluctuation scintillation index σI2=0.15 and σI2=1.60 with different signal-to-noise ratios (SNRs) γ and different pilot-symbol lengths L was simulated. The results showed that the performance of the proposed DFB receiver was highly similar to the performance of the ideal receiver with PCSI known in advance. When σI2=1.60, γ=33dB, L=10, the bit error rate performance of the decision threshold TDFB of the DFB receiver was the same as that of the decision threshold TPCSI of the PCSI receiver, and the SNR gain loss was 0 dB. Furthermore, a wavelet denoising method was designed to improve the DFB receiver performance. When the sampling frequency of the DFB receiver was 25 MHz, σI2=1.60, γ=24dB, L=10, the proposed wavelet denoising preprocessing could obtain a SNR gain of 2.1 dB. These findings suggest that the proposed DFB receiver can promote the development of real-time applications of UWOC system.

Dependencies of GPS Scintillation Indices on the Ionospheric Plasma Drift and Rate of Change of TEC Around the Dawn Sector of the Polar Ionosphere

AbstractThe dependencies of global positioning system (GPS) scintillation indices on ionospheric plasma flow and the rate of change of total electron content (TEC) around the dawn sector for the first time of the polar ionosphere are investigated. The phase scintillation index (σφ) derived from GPS measurements of the Canadian High Arctic Ionospheric Network (CHAIN) shows linear dependencies on both the plasma drift speed measured by the SuperDARN radar and on the rate of change of TEC estimated from the GPS receivers of CHAIN. However, the amplitude scintillation index (S4) does not show any dependence on the plasma flow or the rate of change of TEC. These results further support Wang et al. (2018), https://doi.org/10.1002/2017JA024805 at the noon sector. The dependence of the phase scintillation index on the plasma flow further evidences that the standard phase scintillation index is dominated by refractive variations due to the use of a fixed cut‐off frequency of 0.1 Hz while detrending the phase observable. The dependence of the phase scintillation index on the rate of change of TEC consolidates the dominance of refractive variations inside.

Structurally stable beams in the turbulent atmosphere: dark and antidark beams on incoherent background [Invited

We demonstrate analytically and verify numerically that recently discovered, and experimentally realized, partially coherent dark and antidark beams are structurally stable on propagation in a statistically homogeneous, isotropic random medium, such as the turbulent atmosphere. The dark/antidark beams defy diffraction in free space, and they manifest themselves as dark/bright notches/bumps against an incoherent background. The structure of a bump/notch remains invariant on propagation of the beam through the random medium, while the peak amplitude of the bump/notch decays with the propagation distance in the medium at a rate dependent on the strength of the medium turbulence. We also evaluate numerically the scintillation index of such beams and show that it is significantly lower than that of generic, low-coherence Gaussian Schell-model beams. The combination of structural stability and low scintillations makes partially coherent dark/antidark beams very promising candidates for information transfer and optical communications through atmospheric turbulence.

Atmospheric Effects on Satellite–Ground Free Space Uplink and Downlink Optical Transmissions

Free space optical (FSO) communications have the potential to be one of the most essential technologies for solving the high-bandwidth demands of communications between satellites and ground stations. In this study, we examine the impact of the atmosphere on satellite–ground FSO uplink and downlink communications. To consider diverse atmospheric conditions on both uplink and downlink, we derive FSO channel elements such as the fog attenuation coefficient, refractive index parameter, coherence length, turbulence model, and angle-of-arrival fluctuation. Unlike conventional work, we provide FSO channel analysis based on variations in the Fried parameter, zenith angle, scintillation index, and Rytov variance. Using simulation results from the optical settings, we examine the influence on channel performance of conditions such as atmospheric attenuation and intensity fluctuation. Based on this examination, we determine that 1550 nm is the preferred wavelength for both uplink and downlink FSO channels to mitigate the impact of turbulence and that larger receiver apertures lessen angle-of-arrival changes.

Orbital angular momentum for turbulence mitigation in long free-space optical communication links

The ability of laser beams carrying orbital angular momentum to reduce the scintillation level of Gaussian beam propagating in atmospheric turbulence is numerically investigated. The effect of topological charge, beam waist, operating wavelength and also the receiver radius on the scintillation indices is examined in detail. It is shown that the Gaussian beam carrying orbital angular momentum has a priority over a fundamental Gaussian beam in communication links longer than 2.5 Km. It is clear that Gaussian beams carrying orbital angular momentum in the cases when having a small beam waist and operating at a high wavelength are able seriously to reduce the scintillation index. Accordingly, the presented results will be significant for designing optical communication links that use laser beams carrying orbital angular momentum.

Wave optics simulations of plane-wave propagation in non-Kolmogorov turbulence

We derive limits for wave optics simulations of plane wave propagation in non-Kolmogorov turbulence using the split-step method and thin phase screens. These limits are used to inform two simulation campaigns where the relationship between volume turbulence strength and normalized intensity variance for various non-Kolmogorov power-laws. We find that simulations of smaller power laws are limited turbulence strengths with Rytov numbers of 7 when the simulation side-length sampling rate is 8192. Under these same conditions it is possible to simulate volume turbulence strength out to Rytov numbers of 12 for Kolmogorov power-laws and higher. We show that the peak scintillation and Rytov number where peak scintillation occur increases monotonically with power-law. Also, that if turbulence strength is fixed, the relationship between scintillation index and power-law depends on the operating regime. In weak turbulence the relationship is negative, and it is positive in stronger turbulence. This work also emphasizes the importance of properly scaling turbulence strength with comparing results with different power-laws and the influence and importance of defining inner and outer scales in these simulations.

Scintillation index for the optical wave in the vertical oceanic link with anisotropic tilt angle.

The influence of the ocean depth and anisotropic tilt angle on vertical underwater wireless optical communication (UWOC) systems is considered in this study. We propose a power spectrum model of oceanic turbulence with an anisotropic tilt angle for the first time. Thereafter, the expression of the scintillation index is derived for a spherical wave propagating over anisotropic oceanic turbulence in the vertical link. In addition, considering the temperature and salinity, relevant data of the Atlantic and Pacific oceans at different depths are selected to study further the effect of ocean depth on the scintillation index. The results indicate that the scintillation index strongly depends on the ocean depth and anisotropic tilt angle. Moreover, the scintillation index is also related to other parameters, such as temperature and salinity, kinematic viscosity, the anisotropic factor, optical wavelength, and propagation distance. The presented results can be beneficial in designing optical wireless communication systems in the ocean environment.

Propagation characteristics study of radially polarized Gaussian vortex beam in ocean turbulence

Based on the extended Huygens–Fresnel principle, a semi analytical expression of the radially polarized Gaussian vortex (RPGV) beam in ocean turbulence has been deduced. The influence of topological charge, propagation distance, laser wavelength, rate of dissipation of the mean-squared temperature χT, ratio of temperature or salinity contributions to the turbulence ω, rate of dissipation of kinetic energy per unit mass of fluid ε on the propagation characteristics are discussed in detail. The results show that the beams with higher topological charges can mitigate ocean turbulence effect. The scintillation index increases with the increase of propagation distance and laser wavelength and the decrease of topological charge. When χT is less than1×10−9K2/s, scintillation index increases with an increase of χT. When χT is larger than 1×10−9K2/s, saturation phenomenon of the scintillation index will appear. When propagation distance is less than 40 m, scintillation index increases with an increase of ω. However, when propagation distance is larger than 40 m, scintillation index increases with the decrease of ω. The propagation distance is very sensitive to the degree of polarization (DOP). The degree of polarization becomes worse with the increase of propagation distance, χT and ω and the decrease of topological charge and ε.

Correlation analysis of normalized ROTI and S4 as observed during different geomagnetic conditions of the 2013 September equinox over the stations within EIA African sector

It has been a concern of space scientists and engineers for many decades to have a well-defined proxy to characterize the fluctuations experienced by radio-based signals propagating through the ionosphere. Scintillation indices were deduced using the signal intensity to show the level of fluctuations in phase and amplitude of radio signal passing through the ionosphere. Subsequently, more than two decades now, the rate of change of Total Electron Content (TEC) index (ROTI) was as well used to characterize the level of fluctuations associated to ionospheric irregularities on radio signals. This paper examines the level of relationship between the rate of change of TEC index (ROTI) and the ionospheric amplitude scintillation (S4) within the belt of equatorial ionization anomaly of West African sector. The study was carried out using the Scintillation Network Decision Aid (SCINDA) Global Positioning System (GPS) data obtained from ground-based stations located at Dakar (Senegal) and Cape Verde. The comparative analysis of ROTI and S4 was done with normalized ROTI. The results obtained have shown that the magnitude of normalized ROTI and S4 corresponds very well, higher in Cape Verde compared to Dakar. It was observed that the correlation coefficient could be ∼0.6 when all the satellites are used. However, the relationship increases with individual satellite in which the coefficient of correlation could be up to ∼0.97 and R2 ∼ 0.94. It can be concluded that the relationship between the S4 and ROTI depends on the location and the level of ionospheric disturbance condition as observed in this work.

Analysis of aperture averaging effect and communication system performance of wireless optical channels with from weak to strong turbulence in natural turbid water

The aperture averaging technique can alleviate the effects of optical signal scintillation on the performance of underwater wireless optical communication (UWOC) systems. This paper derives a simplified formula that allows for an accurate and efficient analysis of the aperture averaging effect. Based on the OTOPS (oceanic turbulence optical power spectrum) model, the analytical formulas of the scintillation index from weak to strong turbulent channels for the optical signals of Gaussian, spherical and plane waves with the aperture averaging effect, are obtained. Then, the simple approximation expressions are deduced, which are appropriate to evaluate the performances of UOWC systems with the receiving aperture greater than 0.05 m and the link length not larger than 100 m, and simultaneously, they are proved by theoretical analyses and numerical processes.