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In the present study, a model for analyzing the performance of laser-based satellite uplinks that account for beam wander and atmospheric turbulence is presented. The developed scintillation index model is a continuation of the recent works considering the effects of non-Kolmogorov turbulence on the propagation of Gaussian beams across turbulent media. We specifically concentrate on the specific regime under the tilt-corrected beam wander. The on-axis scintillation index is developed assuming an isotropic non-Kolmogorov spectrum model where the power law exponent (PLE) effective value, α , is taken to be 3 < α < 4. Within that regime, effective irradiance models for tilt-corrected beam wander are presented, using the Gamma and Gamma-Gamma distributions. Closed-form expressions of the average bit error rate (BER) are derived to evaluate the performance of the optical communication system considering coherent detection of differential phase shift keying. The obtained numerical results show the impact of the Gaussian beam radius at the transmitter, the PLE effective value, and the zenith angle on the on-axis scintillation index and the average BER of the system under consideration. Due to beam wander, the link performance in terms of scintillation index and average BER depends on the beam radius at the transmitter. With tilt-corrected beam wander the link performance becomes best at an intermediate value of beam radius. With the non-Kolmogorov turbulent medium model, the PLE effective value has a major effect on the beam radius with the best performance. The obtained results guide the choice of the optimum beam radius for best performance for each PLE effective value. [ABSTRACT FROM AUTHOR]

The ionospheric post-sunset irregularities are responsible for the discrepancies in the received global navigation satellite system (GNSS) signals to fluctuate the phase and amplitude resulting in scintillations in the respective components. Ionospheric scintillations reduce the signal quality and alter the signal reception time inducing position errors which is not preferable for the precise position applications. The level of ionospheric amplitude scintillation, quantified by the amplitude scintillation index (S4), is analyzed during the year 2022, which accentuates the ascending phase of solar cycle 25. For this, we analyzed scintillation intensity and occurrence percentage at a low latitude Indian location in India by employing all the available frequencies of the global positioning system (GPS) constellation. The scintillation distribution for each month is also observed which reveals that the autumn equinox seasons has high scintillation occurrence compared to the vernal equinox seasons. The impact of the scintillation on the three civilian signals (L1, L2 and L5) of the GPS constellation is also observed in terms of the scintillation intensity distribution. The cross-correlation of the S4 index for these three signals reveals a strong correlation existing among them during strong scintillations whereas L2 and L5 signals portray a high correlation irrespective of signal intensities. In brief, the strong scintillation occurrence percentage is higher in the L5 signal compared to the L1 and L2 in contrast with weak scintillation, which is high in L1, followed by L2 and L5. Further, the analysis shows that the autumnal equinox has the highest percentage occurrence of strong scintillations (less than 10 % of the scintillation cases) compared to the vernal equinox whereas among solstice seasons June solstice presented the least scintillation occurrence at the location. The outcomes of this study instigate further analysis of scintillation occurrences from diverse GNSS frequencies covering diverse solar activity conditions for complementing the development of robust scintillation mitigation strategies across the low latitudes during the diverse scintillation conditions. [ABSTRACT FROM AUTHOR]

Turbulent vortices with uneven refractive indices and sizes affect the transmission quality of laser beams in seawater, diminishing the performance of underwater wireless optical communication systems. Currently, the phase screen simulation model constrains the range of turbulent vortex scales that can be analyzed, and the mutual restrictions of the phase screen parameters are not suitable for use on large-scale turbulent vortices. Referring to the formation process of turbulent vortices based on Kolmogorov's turbulence structure energy theory, this study abstractly models the process and simulates the ocean turbulence effect as a spherical bubble with turbulent refractive index fluctuations using the Monte Carlo method, which is verified by fitting the probability distribution function of the received light intensity. The influence of the turbulence bubble model's parameters on light intensity undulation and logarithmic intensity variance, as well as the relationship between logarithmic intensity variance and the equivalent structural constant, are then studied. An equivalent structural constant model of ocean turbulence represented by the bubble model's parameters is established, which link the theoretical values with simulation values of the transmission characteristics. The simulation results show that the spherical bubble model's simulation of ocean turbulence is effective and accurate; therefore, the model can provide an effective Monte Carlo simulation method for analyzing the impact of ocean turbulence channel parameters of the large-scale turbulent vortices on wireless underwater optical transmission characteristics. [ABSTRACT FROM AUTHOR]

The major problems in an underwater wireless optical communication (UWOC) link are turbulence induced fading and pointing errors. In this paper, we have investigated the bit error rate (BER) performance of spatially coupled low-density parity-check (SC-LDPC) coded horizontal UWOC link over a strong turbulent channel model with pointing errors. The performance of this link for different channel and code parameters has been studied using simulations. It has been observed that a rate 1/2 ARJA protograph based SC-LDPC code with graph lifting factor of 256 gives a coding gain of 47 dB at a BER of ${10}^{-4}$ for strong turbulence channel model with pointing errors. An analytical BER expression for an uncoded UWOC link under strong turbulence with pointing errors for the On-Off Keying modulation technique has been derived. A multidimensional protograph based extrinsic information transfer algorithm has been developed to obtain the decoding thresholds for different channel parameters and code rates. We have also studied the SC-LDPC coded vertical UWOC link performance for some specific strong turbulence channel parameters with pointing errors and observed that as the link length increases from $20 m$ to $40 m$ , the performance gap between the hypothetical and cascaded channel models increases from $1.1 dB$ to $5.5 dB$ .

It is possible to determine the ionospheric scintillation index with a modified NovAtel GPStation-6 dual-frequency GPS/GLONASS receiver by measuring small-scale fluctuations in the total electron content of the ionosphere and calculating their standard deviation. Based on the analysis of the shortcomings of the GPStation-6 receiver, the algorithm for monitoring of small-scale fluctuations of the total electron content based on digital filtering is substantiated. Experimental results of measurement of small-scale fluctuations of the ionospheric total electron content and results of estimation of their root-mean-square deviation and ionospheric scintillation index are presented. [ABSTRACT FROM AUTHOR]

Based on the extended Huygens–Fresnel principle, we found that the effect of turbulence on a beam is equivalent to frequency domain filtering. From the viewpoint of a filter, a quantitative description of turbulent effect on a beam is proposed. As an example, the beam spread and the radial and longitudinal components of the scintillation index of a Gaussian beam are expressed analytically using the definition of the criterion. Compared with the existing results, these expressions are simpler and have explicit physical meaning. With the help of quantitative description, we also found that weaker turbulence strength and longer propagation distance will cause a stronger turbulence effect on a beam when their coherence lengths of turbulence are equal. The relationship between beam size and turbulence effect is nonlinear. When the spatial correlation length of Gaussian–Schell-model beam is shorter, the effect of turbulence on it is smaller, and this agrees with the existing research studies. [ABSTRACT FROM AUTHOR]

The intensity scintillation index (S4) exhibits a linear frequency dependence derived from a power law phase screen model, with the exponent determined by the phase spectral index (p). However, it is well known that a departure from linearity occurs as S4 increases and saturates under strong scatter conditions. Additionally, statistical errors over a finite time window can also cause the scintillation statistic to deviate from the power law dependence on frequency. This paper presents a parametric analysis on the deviation in the S4 frequency dependence from the power law form. This investigation focuses on the contribution of statistical uncertainty in S4 calculations to the deviation in the frequency dependence according to the selected parameters: S4 and Fresnel frequency (fF). The deviation is determined by comparing p estimates calculated from the power law relationship of S4 at different L‐band frequencies with those inferred from a model‐fitting method using a multi‐frequency global navigation satellite system scintillation data set. The results show the well‐known departure from linearity as S4 increases, and reveal increasing deviations with decreasing fF. The spectral analysis indicates that the variability at the lowest frequencies of intensity spectra contributes to the statistical errors in S4 calculations due to the finite time window, particularly when scintillations are dominated by low‐frequency contributions at low fF. Simulation results of scintillation realizations show much lower deviations with a 30‐min compared to 1‐min window. This suggests a 1‐min window can be too short for reliable S4 calculations due to statistical uncertainty, especially at low fF. Plain Language Summary: The S4 index is a widely used measure to quantify scintillation strength. The relationship between S4 values at different signal frequencies is theoretically expected to follow a linearity. However, this simple relationship is only valid under certain conditions, and the calculation of S4 with a finite time window can introduce uncertainty that causes deviations from the expected frequency dependence. In this study, we analyze the uncertainty in the relationship between S4 values at different frequencies, considering the specific scintillation conditions. Our observations confirm two main findings: (a) a well‐known departure from linearity at high levels of scintillation, and (b) deviations caused by statistical uncertainty due to large‐scale fluctuations over a finite time window for S4 calculations. We validate these findings through simulations that cover a wide range of scintillation scenarios. Key Points: Deviation/departure of S4 frequency dependence from the power law form was parametrically investigated via global navigation satellite system scintillation observationsIn addition to the well‐known departure at high S4, larger deviations in the S4 frequency dependence were observed at lower fFSimulation shows lower deviations with a 30‐min S4 window rather than 1‐min due to statistical uncertainty depending on the window length [ABSTRACT FROM AUTHOR]

Analytical expressions for scintillation index (SI) and bit error rate (BER) of partially coherent twisted Gaussian beam (PCTGB) propagating through turbulent atmosphere are derived. Numerical simulation results indicate that the PCTGB has an advantage over the partially coherent Gaussian beam (PCGB) for reducing turbulence-induced scintillation in short distance transmission, and the advantage is further enhanced with the increase of twisted factor. However, the situation is reversed when the PCTGB propagates for longer distance. An interesting result is found that the sign of twisted factor has almost no effect on the on-axis SI, but it has a significant effect on off-axis SI of PCTGB, the PCTGB can greatly drop turbulence-induced scintillation when the sign is positive. We also find that twisted factor dominates the effects on BER of PCTGB when twisted factor is small during long distance transmission, but BER is mainly determined by initial coherence width when twisted factor is large enough. Our results are useful for improving the performance of satellite-ground laser communication system and detecting the parameters of atmospheric turbulence. [ABSTRACT FROM AUTHOR]

New experimental technique have been proposed to discuss the turbulence impact reduction using beam shaping technique. In first phase of experiments, turbulence Impacted Vortex beam shaping technique has been introduced for a propagating beam which is effected by Kolmogorov type lab based turbulence simulator using Pseudo Random Phase Plate (PRPP). The new technique is executed with a spatially filtered collimated Gaussian beam that is propagated through computer controlled rotating PRPP to introduce the lab based turbulence on the beam profile. The turbulence impacted beam is then propagated through Vortex Phase Plate (VPP) to shape that beam into different topological charged Laguerre Gaussian modes and further collimated using two lens based configuration. We have measured the scintillation index of the spatial intensity profile collected from CCD. Comparative studies have been done by placing the image plane in five different positions on the propagation path of the beam so that we can detect Collimated, diverging and converging outputs. Four topological charges of LG beams and Gaussian beam have been used to envisage results. [ABSTRACT FROM AUTHOR]

In this paper, we present a detailed investigation of the Generalized Bessel-Laguerre-Gaussian (GBLG) beam propagating in a maritime turbulent atmosphere. Based on the born approximation and the Rytov theory with the use of the Von Karman spectrum, the scintillation index of the GBLG beam is evaluated and examined numerically. The obtained results indicate that the comportment of the GBLG beam is influenced by the turbulent strength, the inner scale of the maritime turbulent atmosphere, and the input beam parameters. The scintillation indexes of Laguerre-Bessel-Gaussian, Laguerre-Gaussian, Bessel-Gaussian and Gaussian beams through the considered medium are deduced as particular cases from our study. [ABSTRACT FROM AUTHOR]

Compressible turbulence around an aircraft will cause irradiance fluctuations in the optical waves in optical communication between the aircraft and the outside. Turbulence can cause signal fluctuation or even interruption, and the exploration of the issue plays an important role in aero-optics. Based on the extended Rytov theory, we develop theoretical expressions of the scintillation index for plane and spherical waves propagating in moderate to strong compressible fluctuations by a near wall compressible turbulence refractive index spectrum. The numerical results show that the derived scintillation index is in good agreement with weak fluctuation and asymptotic theory approximation. The predicted scintillation index first increases sharply with the path length and reaches a peak, after which it falls back to saturation. During weak fluctuations, the scintillation index of long wavelength waves is smaller than that of short wavelength waves, but in moderate to strong turbulence, the scintillation index of short wavelength waves has an advantage over that of long wavelength waves. Furthermore, we provide physical explanations for the scintillation index trend.

The paper discusses the atmospheric turbulence effect on modulating retro-reflector (MRR) optical communication systems with theoretical analysis and experimental verification. The transmission model is proposed by the phase screen method. Based on this model, the scintillation indices (SI) are calculated, and the aperture averaging effect of the MRR terminal is analyzed. Furthermore, a test bed with 506 m transmission link is established. In the experiment, the SI values of the MRR link and the direct link are obtained and compared. Experimental data agree well with the numerical simulation results and verify the theoretical model. The theoretical results are useful for analyzing the influence of turbulence on MRR systems. [ABSTRACT FROM AUTHOR]

Due to the advantages of high transmission rate, lower power consumption, high security, etc., underwater wireless optical communication (UWOC) has been widely studied and considered as a potential technique for underwater communication. However, its performance is severely degraded by oceanic turbulence due to refractive index fluctuations, which is caused by the change of inhomogeneous ocean environment. Within our derived spatial power spectrum model under anisotropic oceanic turbulence, we conducted a detailed investigation for a spherical wave propagating in weak anisotropic turbulence in this paper. Based on the derived oceanic spectrum, we proposed a scintillation index model for spherical wave in anisotropic oceanic turbulence considering the aperture averaging effect at non-zero inner scale and limited outer scale. Besides, we analyze the aperture averaging scintillation index under the influence of channel parameters such as inner and outer scales. Simulation results reveal that the scintillation index increases with the increase of the outer scale, while the inner scale induces an opposite trend on the scintillation index. Moreover, the inner scale exhibits a larger impact than the outer scale on the UWOC system over weak oceanic turbulence. [ABSTRACT FROM AUTHOR]

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

In this paper, we derive a general and exact closed-form expression of scintillation index (SI) for a Gaussian beam propagating through weak oceanic turbulence, based on the general oceanic turbulence optical power spectrum (OTOPS) and the Rytov theory. Our universal expression not only includes existing Rytov variances but also accounts for actual cases where the Kolmogorov microscale is non-zero. The correctness and accuracy of our derivation are verified through comparison with the published work under identical conditions. By utilizing our derived expressions, we analyze the impact of various beam, propagation and oceanic turbulence parameters on both SI and bit error rate (BER) performance of underwater wireless optical communication (UWOC) systems. Numerical results demonstrate that the relationship between the Kolmogorov microscale and SI is nonlinear. Additionally, considering that certain oceanic turbulence parameters are related to depth, we use temperature and salinity data from Argo buoy deployed in real oceans to investigate the dependence of SI on depth. Our findings will contribute to the design and optimization of UWOC systems. • We derived a general and exact closed-form SI expression for a Gaussian beam. • Our general results extend existing results to more actual scenarios. • The correctness and accuracy of our derivation are verified through comparisons. • We investigate the dependence of SI on depth by data from Argo buoy. [ABSTRACT FROM AUTHOR]

Laser Gaussian beam propagation is important in optical communications; the challenges of the measured constant parameters are varied angle wave propagation, the receiver plan in the slant path of laser Gaussian beam (G) in Kolmogorov gauges, and the effect of the laser in a random medium. At this point, atmospheric turbulence is reputable based on laser Gaussian beam propagation. Moreover, the space propagation of Gaussian beam waves of higher-order Gaussian beam modes has been expanded from the effects of the scintillation index and with different zenith angles between the laser propagator and the detector (receiver) in the ramp path. Furthermore, in the Cartesian coordinate system, the scintillation index in the Kolmogorov standard is evaluated, and it employs two methods; first, Huygens–Fresnel fundamental and, second, random phase screen for work synchronization. Additionally, to analyze the contour parameters of constant construction, the intensity and receiver field is found to compute the high turbulent modes in the atmospheric deposition, which depends on propagation distances. These result in significant reserves in the matlab computation period and serve the imitation for enhancement of the link transmission; therefore, the zenith angle and scintillation index are pretentious by slant beam, and they enhance the performance that makes the comparisons found with fixed structure constant parameters C 2 n and the propagation distance against the scintillation index. [ABSTRACT FROM AUTHOR]

Based on the residual turbulent scintillation theory, the Mie-scattering lidar can measure the intensity of atmospheric turbulence by detecting the light intensity scintillation index of the laser return signal. In order to evaluate and optimize the reliability of the Mie-scattering lidar system for detecting atmospheric turbulence, the appropriate parameters of the Mie-scattering lidar system are selected and optimized using the residual turbulent scintillation theory. Then, the Fourier transform method is employed to perform the numerical simulation of the phase screen of the laser light intensity transformation on the vertical transmission path of atmospheric turbulence. The phase screen simulation, low-frequency optimization, and scintillation index calculation methods are provided in detail, respectively. Based on the phase distribution of the laser beam, the scintillation index is obtained. Through the relationship between the scintillation index and the atmospheric turbulent refractive index structure constant, the atmospheric turbulence profile is inverted. The simulation results show that the atmospheric refractive index structure constant profile obtained by the iterative method is consistent with the input HV5/7 model below 6500 m, which has great guiding significance to carry out actual experiments to measure atmospheric turbulence using the Mie lidar. [ABSTRACT FROM AUTHOR]

This paper uses a numerical simulation method to evaluate the propagation performance of the partially coherent flat-topped vortex hollow (FTVH) beam and the Gaussian Schell-model (GSM) beam through atmospheric turbulence along slant links. The atmospheric turbulence is simulated by random phase screens, and the performance of the beam models is evaluated by using scintillation index and signal-to-noise (SNR) on the receiver plane with various simulation conditions and parameters. This paper has shown that there is an optimal coherence length for both the partially coherent FTVH beam and the GSM beam. In different parameter settings, the partially coherent FTVH beam has the best propagation performance at N = 1 M = 1. Compared to the GSM beam, the partially coherent FTVH beam obtains an obvious improvement in SNR. The results show that the partially coherent FTVH beam is able to improve the communication efficiency of an FSOC system. [ABSTRACT FROM AUTHOR]

The possibility of constructing a near-infrared atmospheric optical communication system based on a pair of media converters of a signal format 100Base-TX / 1000BASE-T to format 1000BASE-SX / LX with SFP transceivers DEM-310GT was experimentally investigated. The FAN-OUT TUBING FTB900 SN-Y4 fiber cable connector, coming from the receiving radiation collimator for matching with the DEM-310GT transceiver, was modified. The transmitting radiation collimator is supplemented by a spiral phase plate to form a vortex beam. The influence of atmospheric influence on the data rate is analyzed.

The propagation of variously structured vortex beams of wavelength 1530 nm through a random distorting medium was experimentally investigated. An effect of the aerosol barrier on the scintillation index of the beam was analyzed. The possibility of the correlation detection of the presence of a vortex phase under the beam wandering caused by a flow of warm air was experimentally investigated.

Purpose: With relation to radio astronomy, ionospheric scintillation of discrete cosmic radio sources is an undesirable effect. At the same time, ionospheric scintillations carry information on the ionospheric turbulence and are a separate object of study. From observations at the URAN-4 radio telescope of the power cosmic radio sources (3C144, 3C274, 3C405, and 3C461) at 20 and 25 MHz, the ionospheric scintillation parameters: index, characteristic period and spectral index were estimated. Correlation between spectral index measured from scintillation time spectrum and spectral index of space spectrum of irregularities have allowed considering the first of them as an index of ionospheric irregularity structure changing. Peculiarities of the ionospheric scintillation spectral index behavior compared with ionospheric scintillation index are analyzed for several time intervals. Design/methodology/approach: For analyzing the ionospheric scintillation data, the statistical methods were used. Findings: Estimations of ionospheric scintillation parameters were obtained during 1998–2007. It has been shown that spectral index mean values for each radio source depend on height at which it is observed that was not found for scintillation index. Comparison of yearly mean trends does not show the correlation of spectral index with the solar and geomagnetic activity indices (i.e. with the F10.7–flow of solar radiation at the wave length of 10.7 cm and Ap -index). The daily-seasonal dependence is clearly seen for the monthly mean values. In general, the maximum spectral index values were revealed at night, but small increase was also observed in day-time during 9–11 hours. For different time intervals, the behavior of ionospheric scintillation parameters obtained for the radio source been observed at greater angles of elevation (3C405 and 3C461) remarkably differ from that for the radio sources observed at lower angles of elevation. Conclusions: Using the data obtained as a result of long-term monitoring of the group of discrete cosmic radio sources at the URAN-4 radio telescope made it possible to investigate the behavior of spectral indices of ionospheric scintillations over a large time interval. The peculiarities found in the ionospheric scintillation spectrum index behavior require further analysis for explaining within the limits of present conceptions of radio wave propagation in such an irregularity medium as ionosphere.

Optical communication is a promising strategy for deep space exploration but it is susceptible to coronal solar wind turbulence impairments during superior solar conjunction. The variance of amplitude fluctuations caused by coronal turbulence on optical waves propagation is proposed in this paper. Both the generalized non-Kolmogorov coronal turbulence spectrum and the aperture averaging effect are taken into account. The analytic expression of the bit error rate (BER) for free-space optical (FSO) link is then derived based on Gamma-Gamma distribution model under the weak-to-moderate coronal turbulence channels. Numerical evaluations results demonstrate that coronal turbulence with small non-Kolmogorov spectral index, large outer scale, and inner scale has more potential to deduce amplitude fluctuations. With the increment of antenna radius, the amplitude fluctuations decrease obviously. In addition, the variation tendencies of these parameters can further result in small BER. Therefore, the link performance will be improved by decreasing the optical wavelength and enlarging the antenna radius. Our proposed amplitude fluctuations model with aperture averaging effect has potential application for the future deep space FSO communication.

The performance of free space optical communication (FSOC) systems is severely degraded by certain atmospheric conditions prevalent in places where they are deployed, in spite of their numerous advantages. In clear weather conditions, the random fluctuation in the atmosphere’s refractive index causes substantial scintillation losses to transmitted optical signals. It is therefore imperative to estimate the potential losses due to atmospheric turbulence in locations where FSOC links are to be deployed. This will provide the necessary fade margin for FSOC systems so that designed links withstand such atmospheric disturbances. In this paper, statistical analysis of wind speed data collected for various cities of South Africa is used for calculating the corresponding refractive index structure parameter (Cn2). These Cn2 values, as well as the zero inner scale and infinite outer scale model and finite inner and finite outer scale model, are used in computing the scintillation indices not exceeding 50%, 99%, 99.9%, and 99.99% of the time for the investigated locations. The Lognormal and Gamma–gamma distribution models are then employed for the computational analysis of the irradiance fluctuations and channel characteristics while considering the effect of pointing errors for weak and moderate to strong turbulence regimes. Finally, derived mathematical expressions for outage probabilities and bit error rate (BER) performances for FSOC links, employing various intensity modulation and direct detection (IM/DD) schemes, are presented.

Due to the advantages of high transmission rate, lower power consumption, high security, etc., underwater wireless optical communication (UWOC) has been widely studied and considered as a potential technique for underwater communication. However, its performance is severely degraded by oceanic turbulence due to refractive index fluctuations, which is caused by the change of inhomogeneous ocean environment. Within our derived spatial power spectrum model under anisotropic oceanic turbulence, we conducted a detailed investigation for a spherical wave propagating in weak anisotropic turbulence in this paper. Based on the derived oceanic spectrum, we proposed a scintillation index model for spherical wave in anisotropic oceanic turbulence considering the aperture averaging effect at non-zero inner scale and limited outer scale. Besides, we analyze the aperture averaging scintillation index under the influence of channel parameters such as inner and outer scales. Simulation results reveal that the scintillation index increases with the increase of the outer scale, while the inner scale induces an opposite trend on the scintillation index. Moreover, the inner scale exhibits a larger impact than the outer scale on the UWOC system over weak oceanic turbulence.

Summary: Electromagnetic waves propagating in random mediums are susceptible to attenuation and fading effects. Therefore, to asses systems performances, it is required to determine the field intensity distribution. Extensive researches were carried out under the assumption of weak fluctuations conditions, in which case the Born and Rytov solutions are valid, and it is fairly established that the resultant distributions agree well with experimental data. In particular, the Rician distribution was shown to fit measurements data from a spacecraft in the event of solar superior conjunction. However, the problem under strong fluctuations is more challenging. In this case, multiple scattering effects need to be included. Numerous atmospheric‐based distributions were introduced, but similar results for deep space communications are still scarce. In this paper, by considering double scattering processes, a novel intensity distribution model is introduced which agrees with the Rician distribution under weak fluctuations and which extends to strong fluctuations conditions. Moreover, the distribution is considered for the applications of deep space communications. [ABSTRACT FROM AUTHOR]

Under the background that the Earth's aerosphere contains Kolmogorov and non-Kolmogorov turbulence, it is significant to research the combined influence of these two kinds of atmospherical turbulence on laser-satellite communication. In this paper, based on the power spectra of refractive-index fluctuations for non-Kolmogorov turbulence in the free troposphere and stratosphere, using the extended Rytov theory, the scintillation indices of the spherical wave in the free troposphere and the stratosphere are derived, respectively, which are valid in all regimes of turbulent fluctuations. On this basis, using a three-layer altitude-dependent turbulent spectrum model for vertical/slant path describing the variations of turbulent statistical characteristics with altitude in the aerosphere, which is more accurate than the two-layer model, we present the scintillation index of a spherical wave propagating through Kolmogorov and non-Kolmogorov turbulence along laser-satellite communication uplink at large zenith angles and estimate the combined influence of Kolmogorov and non-Kolmogorov turbulence on the scintillation index. It is noteworthy that this expression is also valid in all regimes of turbulent fluctuations. [ABSTRACT FROM AUTHOR]

Ionospheric scintillation refers to rapid and random fluctuations in radio frequency signal intensity and phase, which occurs more frequently and severely at high latitudes under strong solar and geomagnetic activity. As one of the most challenging error sources affecting Global Navigation Satellite System (GNSS), scintillation can significantly degrade the performance of GNSS receivers, thereby leading to increased positioning errors. This study analyzes Global Positioning System (GPS) scintillation data recorded by two ionospheric scintillation monitoring receivers operational, respectively, in the Arctic and northern Canada during a geomagnetic storm in 2019. A novel approach is proposed to calculate 1-s scintillation indices. The 1-s receiver tracking error variances are then estimated, which are further used to mitigate the high latitude scintillation effects on GPS Precise Point Positioning. Results show that the 1-s scintillation indices can describe the signal fluctuations under scintillation more accurately. With the mitigation approach, the 3D positioning error is greatly reduced under scintillation analyzed in this study. Additionally, the 1-s tracking error variance achieves a better performance in scintillation mitigation compared with the previous approach which exploits 1-min tracking error variance estimated by the commonly used 1-min scintillation indices. This work is relevant for a better understanding of the high latitude scintillation effects on GNSS and is also beneficial for developing scintillation mitigation tools for GNSS positioning. [ABSTRACT FROM AUTHOR]

Based on the residual turbulent scintillation theory, the Mie-scattering lidar can measure the intensity of atmospheric turbulence by detecting the light intensity scintillation index of the laser return signal. In order to evaluate and optimize the reliability of the Mie-scattering lidar system for detecting atmospheric turbulence, the appropriate parameters of the Mie-scattering lidar system are selected and optimized using the residual turbulent scintillation theory. Then, the Fourier transform method is employed to perform the numerical simulation of the phase screen of the laser light intensity transformation on the vertical transmission path of atmospheric turbulence. The phase screen simulation, low-frequency optimization, and scintillation index calculation methods are provided in detail, respectively. Based on the phase distribution of the laser beam, the scintillation index is obtained. Through the relationship between the scintillation index and the atmospheric turbulent refractive index structure constant, the atmospheric turbulence profile is inverted. The simulation results show that the atmospheric refractive index structure constant profile obtained by the iterative method is consistent with the input HV5/7 model below 6500 m, which has great guiding significance to carry out actual experiments to measure atmospheric turbulence using the Mie lidar.

Now it is indisputable fact that the Earth's aerosphere contains Kolmogorov and non-Kolmogorov turbulence, which has been confirmed by increasing experimental evidences and theoretical investigations. Based on this, the scientific community conducts investigations on the combined influence of Kolmogorov turbulence and non-Kolmogorov one on optical wave propagating in the aerosphere. In this paper, based on the further additional measurements, a three-layer altitude-dependent turbulent spectrum model of refractive-index fluctuations for vertical/slant path is proposed to describe the variations of turbulent statistical characteristics with altitude in the aerosphere. This model that is more accurate than the two-layer model is used to estimate the performance of laser-satellite communication system for satellite-to-ground link. Using the extended Rytov theory, the scintillation index of a plant wave propagating through Kolmogorov and non-Kolmogorov turbulence along laser-satellite communication downlink at large zenith angles is presented. It is noteworthy that this expression is also valid in all regimes of turbulent fluctuations. [ABSTRACT FROM AUTHOR]

The scintillation index and bit error rate (BER) of a Gaussian beam propagating in a weakly turbulent soft tissue are formulated and analysed numerically. The scintillation indices are plotted against half of the measured slope in the range of power-law scaling at different tissue parameters, such as the random variations in the refractive index of the tissue, outer scale of the tissue turbulence and the tissue length between the optical source and the detector. Moreover, BERs of Gaussian beams against the signal to noise ratio (SNR) are examined for different tissue parameters. Our graphical results show that the scintillation index and BER increase with larger outer scales, longer tissue lengths and larger random variations in the refractive index of the tissue. In comparison with the spherical wave propagation, it was found that Gaussian beam yields larger scintillation index and BER values. [ABSTRACT FROM AUTHOR]

Purpose: Variations of ionospheric scintillation index of the power cosmic radio sources (3С144, 3С274, 3С405, and 3С461) observed for the year-to-day time intervals with the radio telescope at the decameter wavelengths are analyzed. Design/methodology/approach: The statistical methods were used for the ionospheric scintillation data analysis. Findings: The ionospheric scintillation index had been estimated during 1998–2007. Annual average values correlated very largely with the solar activity indices (sunspot numbers and flux at the wavelength of 10.7 cm). Daily-seasonal dependence is characteristic of monthly mean values. Daily mean values show significant variability, in particular, related to geomagnetic activity. Conclusions: Behavior of the ionospheric scintillation index of the power cosmic radio sources shows different character for different time intervals.

The oceanic turbulence refraction power spectrum model with a function of the eddy diffusivity ratio is adopted. By using the Rytov method, the scintillation index (SI) is derived using the aperture averaging technique for Gaussian beam under weak to strong oceanic turbulence. Variations of the SI in weak to strong oceanic turbulence are evaluated versus link length and the oceanic turbulence parameters such as the ratio of temperature to salinity contributions to the refractive index spectrum, the rate dissipation of the mean squared temperature, and the rate of dissipation of kinetic energy per unit mass of fluid. Considering the eddy diffusivity ratio of salinity to temperature, the effects of the ratio of temperature to salinity contributions to the refractive index spectrum on the performance of underwater wireless optical communication (UWOC) systems in weak to strong turbulence are analysed. [ABSTRACT FROM AUTHOR]

The laser beam blink propagation for Super Lorentz Gaussian (SLG) beam was calculated in free space (FSO) by using the Huygens Fresnel approximation method. In addition, the Scintillation index was articulated and evaluated BER (bit error rate) up to several kilometres for different parameters. Scintillation index variations in plan receiver point were treated in all states without turbulence in the atmosphere and using dual models. Our results were shown to be compatible with the results of earlier studies on Gaussian beam scintillation and propagation. Moreover, the Kolmogorov-spectrum function reduced the blink fluctuation. Also, the outcomes indicate that the small beam width of the dual SLG beam will bring a far distance by utilizing the same source size of the Gaussian beam. Better fallouts were achieved with decreased scintillation index that applied and changed a dual mode with a fixed structure constant factor. A low scintillation index, intensity, and receiver field were obtained. To this end, the scintillation index affects the circumstances depending on the magnitude of turbulence and control of the beam propagation factors. [ABSTRACT FROM AUTHOR]

The propagation of a laser beam in an underwater environment gives rise to attenuation caused by absorption and scattering as well as fading caused by turbulence. These effects lead to a decrease in light power and fluctuations in light intensity. To account for variations in water velocity, flow direction, and distance in communication links, field measurements were conducted in rivers and ocean nearshore waters. Based on these measurements, a model for an underwater wireless optical channel was developed. Our study involves the measurement and analysis of light intensity fluctuation, loss of received light power, and the structure constant in various underwater regions. Additionally, the investigation examines the variation of the bit-error rate in relation to different signal-to-noise ratios. The results suggest that the river displays attributes of low turbulence, whereas the ocean's nearshore waters exhibit a moderate level of turbulence under the specified circumstances. As the velocity of the water increases, the scintillation index of the received optical signal also increases, indicating a higher degree of signal instability. When transmitting horizontally, the scintillation index of the received optical signal remains unaffected by the direction of water flow. Furthermore, as the distance between the transmitter and receiver increases, the scintillation index of the received optical signal intensifies, indicating a greater level of signal instability. The presence of the external interference factors can increase turbulence intensity. Within a depth range of 60 cm underwater, the turbulence intensity at different depths remains basically unchanged. [ABSTRACT FROM AUTHOR]

In laser satellite communications, studies of the scintillation index that are based on the three-layer altitude-dependent turbulence model have paid little attention to the different character of stratospheric turbulence from turbulence in the other two layers. This difference is a consequence of the special power spectrum with a power law exponent of 5. By adopting different wave structure functions at different altitudes in the extended Rytov theory, we analyze the scintillation index along the slant path in the regime of moderate-to-strong turbulence. Our results show that the weak fluctuation theory is limited to a smaller zenith angle, and the scintillation index has a larger maximum value in the focusing regime, when compared to the Kolmogorov model. We also find that the change of the scintillation index with the outer scale of the stratospheric turbulence only appears with moderate-to-strong turbulence, and it weakens with the increase of the turbulence outer scale. Moreover, the impact of the Gaussian beam radius on the scintillation index is different in the downlink and the uplink paths. We can approximate the downlink beam as a plane wave in most cases, but we must optimize the uplink beam to minimize the irradiance fluctuation. • Weak fluctuation theory of scintillation index is limited to a smaller zenith angle based on three-layer turbulence model. • Larger outer scale of stratospheric turbulence leads to a smaller maximum of scintillation index with strong turbulence. • Scintillation index in downlink satellite path is almost unchanged with different Gaussian beam radius in most cases. • There is always an optimal beam radius for uplink path to have lowest scintillation index in different turbulence strengths. [ABSTRACT FROM AUTHOR]

Optical communication is a promising strategy for deep space exploration but it is susceptible to coronal solar wind turbulence impairments during superior solar conjunction. The variance of amplitude fluctuations caused by coronal turbulence on optical waves propagation is proposed in this paper. Both the generalized non-Kolmogorov coronal turbulence spectrum and the aperture averaging effect are taken into account. The analytic expression of the bit error rate (BER) for free-space optical (FSO) link is then derived based on Gamma–Gamma distribution model under the weak-to-moderate coronal turbulence channels. Numerical evaluations results demonstrate that coronal turbulence with small non-Kolmogorov spectral index, large outer scale, and inner scale has more potential to deduce amplitude fluctuations. With the increment of antenna radius, the amplitude fluctuations decrease obviously. In addition, the variation tendencies of these parameters can further result in small BER. Therefore, the link performance will be improved by decreasing the optical wavelength and enlarging the antenna radius. Our proposed amplitude fluctuations model with aperture averaging effect has potential application for the future deep space FSO communication. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we characterize the performance of Intensity Modulation/Direct Detection (IM/DD) and coherent detection modulation techniques in free-space optical (FSO) communication system in terms of bit error rate (BER). The system of interest includes M-ary Pulse Position Modulation (M-PPM) in correspondence to IM/DD and M-ary Phase Shift Keying (M-PSK) and M-ary Quadrature Amplitude Modulation (M-QAM) with respect to coherent detection. The system is subjected to weak turbulence regime of log-normal channel with scintillation index less than 0.75. Coherent techniques involve the amount of signal pulses transmitted in a particular time as the major component of their generation, except IM/DD which is a result of photon intensity, i.e. the number of photons forming a pulse. It is worth mentioning that in previous studies, the performance of IM/DD techniques had been evaluated in terms of photon-count fluctuation statistics and thus the impact of fading strength of channel on signal intensity was not discussed anywhere. In this work, an objective equivalence has been achieved between these two kinds of detection by deriving the BER expression of M-PPM in terms of probability density function of signal intensity, while considering fading strength, scintillation index, thermal noise and outage probability as system parameters for numerical analysis. [ABSTRACT FROM AUTHOR]

Abstract We have studied the scintillation index of a double vortex beam propagating in strong turbulent atmosphere by numerical simulation. It is found that the on-axis scintillation index of double vortex beams is smaller than that of single vortex beams propagating in the atmospheric turbulence. Moreover, when the beam propagation distance exceeds a certain value, the on-axis scintillation indices of double vortex beams are smaller than that of Gaussian beams. [ABSTRACT FROM AUTHOR]