Arrival Fluctuations Research Articles

Randomness of optical turbulence generated by Rayleigh–Bénard convection using intensity statistics

The experimental study of optical turbulence proves difficult due to challenges in generating controllable conditions in a laboratory environment. Confined water tanks that produce Rayleigh–Bénard (RB) convection are one method to generate optical turbulence using a controllable temperature gradient. It is of utmost concern to quantify the properties of the optical turbulence generated for characterization of other optical applications such as imaging, sensing, or communications. In this experimental study a Gaussian beam is propagated through a RB water tank where two intensity measurements are made at the receiver’s pupil and focal plane. The pupil and focal plane results include quantification of the intensity fluctuation distribution, scintillation distribution, and refractive index structure constant at various values of the temperature gradient. The angle of arrival fluctuations is also calculated at the focal plane to obtain a second estimate of C n 2. The pupil plane estimate for C n 2 using scintillation index and focal plane angle of arrival fluctuations is compared to preliminary predictions of C n 2 as a function of RB temperature gradient showing C n 2∼ΔT4/3. The outcomes of the study confirm that the RB process produces intensity fluctuations that follow gamma–gamma and log-normal probability density functions. Estimates of the refractive index structure constant C n 2 produce the same trends with different magnitudes when measured from the pupil and focal plane.

Analysis and Mitigating Methods for Jamming in the Optical Reconfigurable Intelligent Surfaces-Assisted Dual-Hop FSO Communication Systems

In this paper, we present a study investigating the impact of jamming in a Dual-Hop free-space optical (FSO) communication system assisted by reconfigurable intelligent surfaces (RIS) in the presence of a malicious jammer. We analyze the combined effects of atmospheric turbulence (AT), pointing error (PE), and angle of arrival (AoA) fluctuation of unmanned aerial vehicles (UAVs). Closed-form expressions for the overall average bit error rate (ABER) are derived while considering these impairments. To mitigate the jamming effect, we explore a Single-Input Multiple-Output (SIMO) FSO system and derive the end-to-end Average Bit Error Rate (ABER) under various jamming scenarios. Additionally, we conduct a comprehensive study by examining different placements of the malicious UAV jammer and RIS, drawing insightful conclusions on system performance. The analytically derived expressions are validated through Monte Carlo simulations.

Study of convective air turbulence based on the probability distribution function of angle of arrival fluctuations

This study investigates the probability density function (PDF) of angle of arrival (AoA) fluctuations of a laser beam propagating through convective air turbulence in two settings: indoor turbulence and atmospheric surface layer. The indoor turbulence is generated by an electric heater with a 50 cm × 100 cm surface area placed horizontally on the bottom. The atmospheric turbulence experiments are conducted over a level at 60 cm height from an asphalted area using a slightly divergent laser beam that propagates along 350 m. Image motion monitoring of four sub-apertures installed in front of the receiving telescope aperture is used to record the AoA fluctuations and their PDFs over the entrance pupil at the sub-aperture locations for both types of turbulence. The observations are made in two perpendicular directions to the propagation of the laser beam. The experimental PDFs of the AoA fluctuations are fitted with Gaussian, Lognormal, Weibull, and Gamma distribution functions. The results show that the Weibull and Gamma models are more consistent with the experimental data for the indoor convective air and atmospheric turbulence respectively. For the indoor convective turbulence, the full width at half maximum and the peak location of the measured PDFs increase with the heater temperature, as well as the goodness of fit, which saturates at high temperatures.

Seasonal Variations And Forecasting In Wholesale Prices Of Okra In Surat Market Of Gujarat, India

The analysis of prices and market arrivals over time is important for formulating a sound agricultural price policy. Fluctuations in market arrivals largely contribute to price instability. In order to devise the appropriate ways and means for reducing the price fluctuations of agricultural commodities, there is a need to have a thorough understanding of the price behavior over time.Forecasting the price of agricultural commodities, presents some unique challenges such as data quality issues, weather aberrations, high fluctuations, price variations across neighboring marketplaces, etc. On the demand side, the instability in the prices of agricultural commodities is influenced by a number of factors such as annual variation in production, low price elasticity of demand and seasonality of agricultural production.The study relied upon the secondary time series data on monthly market prices and arrivals of Okra collected from the Agricultural Produce Market Committee, Surat. Analysis was performed on the monthly and annual time series data on wholesale price to develop reliable forecast for 2022. Month wise Seasonal indices of Okra in the Surat market showed that the seasonal price indices were above average from November to March while below average from May to October when market arrivals are more. The seasonal pattern showed that prices declined from April to October and reached the lowest point in May and reached peak in December. Seasonal fluctuations were observed both in market arrivals as well as prices of Okra. In the present investigation, various Seasonal, Non-seasonal, and Seasonal ARIMA (Box-Jenkins) models were developed to measure the forecast accuracy. The best model was chosen on the basis of the least values of Schwarz Bayesian criteria (SBC) and Mean absolute percentage error (MAPE). After performing a series of diagnostic tests, it was observed that N-BIC (12.417) and MAPE (22.88) were the least for SARIMA (0,0,2) (0,1,1)12 model. It came out to be the most representative model for the price of Okra in the Surat market. The model can be used for reaching dependable price forecasts. The quantification of these aspects in the vegetables is an immediate need to formulate effective policies to make prices stable thereby safeguard the interest of the farmers as well as the consumer.

Exploring the Relationship between Stochastic Events and Patterns of International Tourism in Kenya

Purpose: The purpose of this research was to investigate the fluctuations in international tourist arrivals in Kenya and identify the underlying factors contributing to these trends. The study aimed to gain a deeper understanding of the impact of stochastic events on tourist arrivals.
 Methodology: The research focused on international tourist arrivals in Kenya across seven geographical areas. It utilized annual time series data from 1980 to 2020 and examined various variables, including tourism arrivals, terrorism, political instability, conferences, and natural disasters. Descriptive statistics, the Kruskal-Wallis test, Lepage change point detection, and a Vector Autoregressive Model (VAR) with Granger causality tests were employed to analyze the data.
 Findings: The findings revealed significant fluctuations in preferred destinations, source markets, purpose of visits, and length of stay among international tourists in Kenya over the study period. Major stochastic events were observed to coincide with significant changes in net arrivals, source markets, or destinations. Notably, terrorism, conferences, and the combined impact of all factors had a significant influence on net arrivals. Political instability, pandemics, and natural disasters were also found to affect international tourism arrivals.
 Recommendations: Based on the results, policymakers are advised to prioritize safety and security measures to mitigate the adverse effects of pandemics and political instability on the tourism industry in Kenya. Additionally, the study recommends the development of targeted marketing strategies to attract resilient source markets. Furthermore, promoting sustainable tourism practices is essential to mitigate the long-term impact of negative events on the industry. These recommendations aim to enhance the resilience and growth of the tourism sector in Kenya.

Performance of HQAM/XQAM Laser Communication System in Anisotropic Non-Kolmogorov Ground–HAP–Satellite Uplink

Satellite laser communication can achieve high-speed, high-precision, and high-security broadband communication without being constrained by the electromagnetic spectrum, which has attracted attention. So, this paper proposes the use of a high-altitude platform (HAP) under anisotropic non-Kolmogorov turbulence to improve the communication performance of the system. Cross quadrature amplitude modulation (XQAM) and hexagon quadrature amplitude modulation (HQAM) are applied to the ground–HAP–satellite (G-H-S) laser communication system. Considering the combined effects of uplink light intensity scintillation, beam wander, and the angle of arrival fluctuation, the G-H-S system’s bit error rate (BER) closure expression is derived under the EW distribution. Simultaneously, the relationship between the G-H-S system’s signal-to-noise ratio (SNR) and BER under different anisotropic factor u values is simulated and compared with the traditional ground–satellite (G-S) system. The results show that the communication performance of the G-H-S system with HQAM modulation is better. In addition, the effects of the zenith angle, receiving aperture, transmitter beam radius, and beam divergence angle on the BER performance of the system are also studied. Finally, the correctness of the analysis results is verified via Monte Carlo simulation. This research will benefit the design and optimization of satellite laser communication systems.

Analysis of Jamming Effects in IRS Assisted UAV Dual-Hop FSO Communication Systems

In this paper, we investigate the effects of jamming caused by a malicious unmanned aerial vehicle (UAV) on the performance of a free-space optical (FSO) communication system using a legitimate UAV as a relay and utilizing an intelligent reflecting surface (IRS) to improve the coverage. We statistically characterize the combined effects of the three impairments on the performance of the considered system: atmospheric turbulence (AT), pointing error (PE), and angle of arrival (AoA) fluctuation of both the UAVs. Incorporating the aforementioned effects, closed-form expressions of the overall average bit error rate (ABER) and outage probability (OP) for the considered system are derived. We also present a comprehensive study of the proposed system by considering different locations of the malicious UAV jammer and IRS and draw some interesting conclusions. Useful insights into the system performance are obtained through the asymptotic analysis. Furthermore, the analytically derived expressions are verified through Monte-Carlo simulations.

On the performance of all-optical RORIS dual hop UAV based FSO systems

In this work, we investigate the performance of an all-optical dual hop free-space optical (FSO) communication system using an unmanned aerial vehicle (UAV) as a relay and utilizing a reconfigurable optical intelligent reflecting surface (ROIRS) to improve the coverage. We statistically characterize the combined effects of the three impairments: atmospheric turbulence (AT), angle of arrival (AoA) fluctuation of UAV, and the geometric and misalignment loss (GML) caused by the fluctuations of the transmitter, ROIRS, and the position and orientation of the UAV. Incorporating the aforementioned effects and the non-orthogonality of the received beam at the UAV and the receiver, closed-form expressions of the overall outage probability and the average bit error rate for the considered system are derived. Furthermore, the analytically derived expressions are verified by Monte-Carlo simulations.

Metalens integrated receiver to reduce the effect of angle of arrival jitter in free space optical communication

In this paper, we propose a metalens integrated receiver for free space optical communication, which provides robust performance in the presence of angle of arrival fluctuations jitter. As a proof of concept, we show that our proposed receiver can tolerate up to a 2.5 mrad angle of arrival fluctuations with a detector radius of 20 µm before reaching a 3 dB power penalty. We design a meta unit cell to realize the optimized phase profile and present a quasi-3D full wave simulation of the proposed architecture. The system shows steady signal power and bit error rate in the presence of angle of arrival fluctuation.

Deep VULMAN: A deep reinforcement learning-enabled cyber vulnerability management framework

Cyber vulnerability management is a critical function of a cybersecurity operations center (CSOC) that helps protect organizations against cyber-attacks on their computer and network systems. Adversaries hold an asymmetric advantage over the CSOC, as the number of deficiencies in these systems is increasing at a significantly higher rate compared to the expansion rate of the security teams to mitigate them. The current approaches in cyber vulnerability management are deterministic and one-time decision-making methods, which do not consider future uncertainties when prioritizing and selecting vulnerabilities for mitigation. These approaches are also constrained by the sub-optimal distribution of resources, providing no flexibility to adjust their response to fluctuations in vulnerability arrivals. We propose a novel framework, Deep VULMAN, consisting of a deep reinforcement learning agent and an integer programming method to fill this gap in cyber vulnerability management process. Our sequential decision-making framework, first, determines the near-optimal amount of resources to be allocated for mitigation under uncertainty for a given system state, and then determines the optimal set of prioritized vulnerability instances for mitigation. Results show that our framework outperforms the current methods in prioritizing the selection of important organization-specific vulnerabilities, on both simulated and real-world vulnerability data, observed over a one-year period.

Modelling and assessment of the arrival and departure process at the terminal area: A case study of Chennai international airport

Airport and terminal area traffic congestion is a growing concern of the air traffic management (ATM) system. A major step towards mitigating the impacts of congestion is understanding the fluctuating arrival and departure process at airports. Exploring the intrinsic time-varying nature of the arrival and departure process enhances this understanding, hence, is instrumental in formulating operational policies for managing airport capacity and delays. This paper presents a data-driven statistical framework to investigate, understand, and model the time-varying fluctuations in the aircraft arrival and departure process. We have considered one-month data obtained from Chennai international airport operations for this study. The dynamic evolution and fluctuation characteristics of arrivals and departures are conducted using Seasonality and Multifractal Detrended Fluctuation Analysis. Six theoretical probability distributions were used to fit the observed aircraft arrivals and departures at different temporal scales using the maximum likelihood estimation method. This study also develops a G1(t)/G2(t)/KFirst Come First Serve dynamic queuing model for a day’s arrival and departure process. The results suggest that arrival and departure processes exhibit weak but visible multifractal characteristics. Fluctuations are higher at a smaller temporal scale compared to a larger scale, and the probability distributions of arrival and departure are also time-varying. The framework has universal applicability, and the insights have important implications for operational policies on the air transportation workforce and infrastructure management.

Performance of Integrated Ground-Air-Space FSO Links Over Various Turbulent Environments

We analyze the outage probability of integrated ground-air-space free-space optical (FSO) communication links for different atmospheric turbulence channel models. The performance of ground to high altitude platforms (HAPS), HAPS to geostationay Earth orbit (GEO) satellite, HAPS to HAPS and ground to GEO satellite is investigated for various link configurations (downlink, horizontal, and uplink) and parameters such as zenith angle, channel state, horizontal and vertical deviations, altitude, beam waist, receiver aperture diameter, wind speed, and visibility. The atmospheric attenuation (the effects of fog, cloud and volcanic activities), atmospheric turbulence, angle of arrival (AOA) fluctuations and pointing error are included in the considered model. Closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) are obtained for Lognormal, Gamma and exponentiated Weibull distributed channel models. The benefit of using HAPS is shown by comparison of outage probabilities for direct ground to GEO satellite and HAPS assisted ground to GEO satellite links.

Research on uplink performance of MIMO terrestrial-satellite laser communication based on OSM-MBM joint modulation

This paper proposes a multiple-input multiple-output (MIMO) terrestrial-satellite laser communication uplink system based on optical spatial modulation with medium-based modulation (OSM-MBM) joint modulation. The Málaga(M) distribution is used to describe the combined effects of near-ground turbulent light intensity scintillation, beam wander and angle of arrival fluctuation. The closed expression of the bit error rate (BER) of the OSM-MBM joint modulation MIMO terrestrial-satellite laser communication uplink is derived. The simulation analyzes the influence of transmitting radius, receiving aperture, beam divergence, zenith angle and signal-to-noise ratio on the system error performance, and compares it with traditional modulation, media-based modulation and optical space modulation. Finally, the experiment is verified by Monte Carlo. This research provides a theoretical reference for the engineering realization of the modulation technology for the MIMO terrestrial-satellite laser communication uplink system.

Study of anisotropy of convective optical underwater turbulence and the effect of the mean water temperature in the presence of a varying temperature gradient on it

In this paper, the anisotropy of optical convective underwater turbulence is investigated in terms of the variance of angle of arrival (AOA) fluctuations of a narrow laser beam propagating through it in different sections of the medium. The collimated laser beam with a wavelength 532 nm and a diameter 1 cm, which passes through a convective underwater turbulence. The turbulence is generated in a water tank with dimensions of 20 cm × 36 cm × 20 cm, which is installed on a flat surface electrical heater. During the experiments, the mean water temperature (MWT) can be changed from room temperature to 34 ∘C by increasing the heater temperature. The use of the heater also generates a temperature gradient in the medium. The laser beam propagates along a horizontal path with a length of 20 cm inside the tank at different altitudes from the heater source, as well as at different distances from one of the side walls of the turbulent medium. After passing the laser beam through the turbulent medium, the fluctuations of the AOA components in the vertical and horizontal directions are measured. From the time series of the measured AOA fluctuations, their variances are determined. The anisotropy of the medium is investigated by comparing the variance of AOA components measured in the vertical and horizontal directions. We show that the variances of both of vertical and horizontal components of the AOA fluctuations are increased with the MWT, and they are saturated at higher MWTs. In addition, different anisotropic behaviors are observed for the variances of the measured AOA fluctuations at the vicinity of the lateral wall and upper surface of the water. At the vicinity of the lateral wall the variances of the AOA fluctuations in the horizontal component are larger, but at the vicinity of the upper surface the variances of the AOA fluctuations in the vertical component are dominant. This behavior may be caused by the change of the convection motion direction in the turbulent fluid.

A study on angle of arrival fluctuations in a temperature-, salinity- and sweetness-induced underwater optical turbulence

In this paper, we report the measurement of fluctuations of two components of Angle of Arrival (AoA) of a narrow laser beam propagating through an underwater turbulence induced by a mixture of hot and cold waters having inhomogeneity at small scales, and consider the effect of the water mean temperature on the AoA fluctuations. The same studies on the AoA fluctuations are presented when a mixture of two volumes of water with different salinity and sweetness are used. The results show that the AoA fluctuations increase by increasing the difference in initial temperatures of the hot and cold waters. Comparison of the variances of the AoA fluctuations for two different propagation lengths, reveals that it is greater for the longer path. In addition, comparison of the AoA fluctuations in two directions for different initial temperatures of the cold and hot waters shows that the underwater turbulence is an anisotropic medium whose anisotropy increases with the mean temperature of the water. In the experiments with the mixture of two volumes of water with different salinity and sweetness, the AoA fluctuations are not seriously affected by the water salinity, while for the sweetness we see a small but interesting response.

Kalman filter spot centroid detection algorithm in optical wireless communication

Atmospheric turbulence creates a light spot on the focal plane of the receiving end scintillation and angle of arrival fluctuation in optical wireless communication. The combination of a spot centroid detection algorithm and Kalman filter (KF) is proposed for spot tracking. The theoretical calculated value of the spot circle fit is filtered to obtain an optimal estimate of the spot center. The root mean square error of the detected spot centroids is used as an evaluation metric to verify the conventional algorithm and KF algorithm for the real-time tracking of the laser spot at transmission distances of 1.3, 4.1, and 10.3 km. The results show that the root mean square error of the spot in the pitch direction decreases from 0.740, 2.537, and 7.256 pixels to 0.683, 1.792, and 4.756 pixels, respectively, whereas the root mean square error of the spot in the horizontal direction decreases from 0.481, 2.131, and 5.932 pixels to 0.463, 1.471, and 4.362 pixels, respectively, compared with the conventional algorithm at three transmission distances. The envelope fluctuation of the baseband signal at the receiving end is significantly improved using KF algorithm compared with the conventional algorithm, which improves the stability of the optical wireless communication link.

Research on the performance of SIMO RC-OFDM system with exponential Weibull distribution in the downlink satellite–terrestrial laser communication

In this paper, a repetition code (RC) coherent orthogonal frequency division multiplexing (OFDM) single input multiple output (SIMO) system based on the exponential Weibull distribution model is proposed for downlink satellite–terrestrial laser communication. The closed form expression of bit error rate (BER) of the SIMO RC-OFDM system is derived by considering the joint effect of downlink intensity scintillation and angle of arrival fluctuation (AOA). The relationship between the BER and signal-to-noise ratio (SNR) of uncoded system and SIMO RC-OFDM system is compared under weak, medium and strong atmosphere turbulence, and the influence of the number of receiver antennas on the SIMO RC-OFDM system is studied. At the same time, under the condition of weak turbulence, the influence of important indicators such as zenith angle, receiving aperture, optimal beam divergence angle, and optimal transmission radius on the satellite–terrestrial laser communication downlink SIMO RC-OFDM system is analyzed. The Monte Carlo (MC) principle uses randomness to solve integral problems, and the obtained results are verified by MC simulations. This work provides a theoretical basis for research on coding technology in satellite–terrestrial laser downlink communication SIMO systems.

Co-Movement between Tourist Arrivals of Inbound Tourism Markets in South Korea: Applying the Dynamic Copula Method Using Secondary Time Series Data

Tourism demand is severely affected by unpredicted events, which has prompted scholars to examine ways of predicting the effects of positive and negative shocks on tourism, to ensure a sustainable tourism industry. The purpose of this study was to investigate if non-linear dependence structures exist between tourist flows into South Korea from five major source countries, as South Korea has undergone fluctuations in tourist arrivals due to diverse circumstances and has complex relations with tourism source countries. Additionally, the study examines the structures of extreme tail dependence, which is indicated in the case of unexpected events, and identifies how co-movements vary over time through dynamic copula–GARCH (generalized autoregressive conditional heteroskedasticity) tests. The secondary time series data for the 2005–2019 period of tourist arrivals to Korea were derived from the Korea Tourism Knowledge and Information System for testing the copula models. The copula estimations indicate significant dependencies among all market pairs as well as the strongest dependence between China and Taiwan. Moreover, extreme tail dependence structures show co-movements for four pairs of tourism markets in only negative shocks, for five pairs in both positive and negative conditions, but no co-movement in the China–Taiwan pair. Finally, the dynamic dependence structures reveal that the China–Taiwan dependence is higher than the other time-varying dependence structures, implying that the two markets complement each other.

Dependence Structure between Tourist Arrivals of Inbound Tourism Markets inSouth Korea

Tourism demand is severely affected by unpredicted events. The purpose of this study was to investigate if non-linear dependence structures exist between tourist flows into South Korea from five major source countries, as South Korea has undergone fluctuations in tourist arrivals due to diverse circumstances and has complex relations with tourism source countries. Additionally, the study examines the structures of extreme tail dependence, which is indicated in the case of unexpected events, and identifies how co-movements vary over time through dynamic copula-GARCH (Generalized Autoregressive Conditional Heteroskedasticity) tests. The copula estimations indicate significant dependencies among all market pairs as well as the strongest dependence between China and Taiwan. Moreover, extreme tail dependence structures show co-movements for four pairs of tourism markets in only negative shocks, for five pairs in both positive and negative conditions, but no co-movement in the China-Taiwan pair. Finally, the dynamic dependence structures reveal that the China-Taiwan dependence is higher than the other time-varying dependence structures, implying that the two markets complement each other.

Developing climate services for Caribbean tourism: a comparative analysis of climate push and pull influences using climate indices

ABSTRACT Climate indices have a long history of use to combine multi-faceted climate information for tourism resource evaluation. Traditionally, indices have been used to assess tourists’ sensitivity to destination climatic pull factors, not tourists’ sensitivity to source market climate as a push factor for seasonality-driven markets. This study addresses this gap by using tourism climate indices to assess the influence of climatic push and pull factors for seasonal fluctuations in arrivals to Antigua and Barbuda, Barbados, and Saint Lucia, from the province of Ontario, Canada (from January 2008 to December 2017). Building on the conceptual foundation of the Holiday Climate Index:Beach (HCI:Beach), this study uses an optimization algorithm to develop two indices: (1) an optimized in-situ index that estimates the climatic pull-factor of the destination, and (2) an optimized ex-situ index that estimates the climatic push-factor from the source market. Findings reveal the optimized ex-situ (push) index explains 83% (R 2 = 0.830) of the variability in total monthly arrivals from Ontario and has greater predictive accuracy than the in-situ (pull) index. The research advances understanding of climatic influences on Caribbean tourism arrivals and provides the foundation for new seasonal forecast-based Climate Services (CS) for destination managers and marketers.