Atmospheric Turbulence Conditions Research Articles

Enhancing Free Space Optical System Performance through Fog and Atmospheric Turbulence using Power Optimization

Free Space Optical (FSO) communication is gaining traction as a pivotal technology for next-generation communication systems, offering extremely high data rates, unlicensed bandwidth, and rapid transmission capabilities. However, its performance is significantly hindered by atmospheric factors such as turbulence and fog. This paper presents a comprehensive model for FSO communication designed to optimize performance under various atmospheric conditions. We assess channel capacity across a spectrum of weak to strong atmospheric turbulence using a Gamma-Gamma channel distribution. To mitigate channel losses, our system employs Wavelength Division Multiplexing (WDM) and multi-beam Multiple Input Multiple Output (MIMO) technologies. Results indicate that the integration of diversity techniques and WDM substantially enhances system performance in adverse weather conditions. Furthermore, power optimization is achieved through the implementation of optical amplifiers and feedback mechanisms from the receiver to the transmitter to adjust the transmitter power in accordance with received Bit Error Rate (BER). The proposed power-optimized WDM MIMO system demonstrates a remarkable BER of 1.48884e-15, while extending the transmission link distance to 2500 meters with a Q factor of 21.5 even under strong atmospheric turbulence conditions.

Mode Division Multiplexing Free Space Optical Communication System Performance with Aperture Averaging in Atmospheric Turbulence and Various Weather Conditions

Due to the serious challenge for mobile operators as a result of spectrum congestion in radio frequency spectrum, free space optical (FSO) communication has been considered a timeous alternative especially for future generation high capacity wireless networks. Although FSO offers advantages such as huge data carrying capacity among others, it suffers from the drawback of atmospheric turbulence and adverse weather conditions. As a mitigation to this problem, mode division multiplexing (MDM) has been another research direction. This paper proposes the design and numerical simulation of a hybrid modified duobinary return to zero (MDRZ) MDM free space optical communication system, which to the best of the authors’ knowledge has not been a subject of investigation. The performance metrics used are the bit error rate, quality factor and received optical power. Two modes, HG00 and HG01 were investigated and compared. It has been observed that the BER, Q factor and received power degrades as we increase the beam divergence, FSO transmission distance and data rate. Moreover, the performance gap between clear and moderate weather conditions is more for HG00 than for HG01 for the same considered data rates. The aperture averaging has been shown to be a strong mitigation strategy especially in foggy conditions.

Restoration of atmospheric turbulence-degraded images based on multi-frame information.

Atmospheric turbulence introduces random disturbances that degrade and distort images of observed targets as light propagates through the atmosphere. Although numerous algorithms have been developed to restore images degraded by turbulence, most of these algorithms lack sufficient generalization and are limited to specific application scenarios or fixed atmospheric turbulence intensities. In this paper, we propose an Atmospheric Turbulence Restoration Network (ATRN), a two-stage algorithm based on multi-frame information fusion. The first stage employs Transformers to comprehensively mitigate atmospheric turbulence, while the second stage performs multi-frame information fusion. Our approach effectively captures and processes both temporal and spatial information, extracting potentially clear image details from neighboring frames across multiple scenes and integrating this information to enhance restoration. The experimental results demonstrate that our algorithm has strong generalization capabilities across various atmospheric turbulence conditions, outperforming existing methods in restoring atmospheric turbulence-degraded images.

Evaluation of landing procedures for a high-altitude platform with skid-type landing gear based on pilot-in-the-loop simulations

AbstractIn the context of the project HAP, the German Aerospace Center (DLR) is currently developing a solar-powered high-altitude platform. The underlying vehicle is a fixed-wing aircraft that is supposed to be stationed in the stratosphere for 30 days. Due to, among others, low achievable rates of descent, the use of skids as landing gear and its high susceptibility to wind, landing this aircraft is a very challenging task. Hence, it requires a landing procedure specifically tailored to the aircraft’s particularities. Furthermore, this procedure needs to be easy to follow by pilots especially in adverse atmospheric conditions. This paper deals with a pilot-in-the-loop simulation campaign conducted to assess a landing procedure developed for the high-altitude platform in earlier works. Within this campaign, the pilots are supposed to land the aircraft following this developed procedure in atmospheric turbulence conditions. In addition, they also land the aircraft following a second procedure, which is based on a conventional landing. In doing so, the altitude at which a flare is performed and/or the propellers are shut down is varied. Finally, the novel procedure’s overall feasibility from a piloting point of view and its potential to reduce the risks during landing are assessed. The results show that the procedure proves to reduce the risk of inadvertent ground contact of the aircraft payload compartment, which is associated with serious damage to aircraft structure and payload. However, since the novel procedure is challenging for the pilots, some improvements to the procedure are proposed.

Atmosphere turbulence effects on a multiple apertures free space optical communication systems

Abstract Wireless communication signals experienced fluctuations in intensity due to atmospheric turbulence. The presented work uses a multi-input-multi-output (MIMO) system to study the performance free-space optical (FSO) communication over a diverse atmospheric turbulence condition channel. The findings produced demonstrate how various channel models perform in FSO system at varying levels of channel turbulence. From 7.9 for the Weak turbulence to 45.3 for the strong turbulence, the turbulence attenuation rises linearly. The data could be transmitted via a link length of up to 1 km with strong turbulence. Strong turbulence could have higher Q-factor, yet is constrained by short distance (1 km) in comparison with weak turbulence, which has greater distance (3.8 km), therefore communication quality is lower. For moderate and weak turbulence, the maximum link is around 1.5 km and 3.8 km, respectively. This study is useful for designing a stable FSO system for regional environments suffer from frequent fluctuated weather turbulence such as the Middle East and South Asia.

Modelling the effects of a random medium on direct and reflected sound signals from a moving source

Randomnesses in acoustic propagation conditions affect the signal at a receiver placed at a distance from the source. Without any reflecting surface the random medium causes fluctuations in amplitude and phase of the received signal. In the presence of a wave reflecting surface, a version of the signal with other fluctuations is added at the receiver, with some amount of correlation between the direct signal and the reflected signal. A method is presented that in a single approach includes the random fluctuations of the received signals and the decorrelation effect for signal pairs. Numerical examples are calculated and validated in terms of mutual coherence functions by comparison with analytical solutions for different conditions of atmospheric turbulence. Exemplifying sounds with different source signals are produced for a moving source.

Enhanced frame synchronization and carrier recovery in coherent FSO communication: a pseudo-random and cyclic QPSK approach.

To alleviate the impact of atmospheric turbulence on receiver carrier recovery in free-space optical communication, this study introduces a novel frame synchronization and carrier recovery method. This method employs a blend of pseudorandom sequences and specific cyclic quadrature phase shift keying (QPSK) training sequences, facilitating frame synchronization and frequency offset (FO) estimation. The research designs a time-series metric curve to counteract the effect of side peaks, enhancing the accuracy of frequency offset estimation via QPSK spectrum features. Furthermore, the method incorporates a two-stage frequency offset estimation process, utilizing the inherent sign characteristics of the x and y polarization states. Applied to the quadrature amplitude modulation system under atmospheric turbulence conditions, the proposed algorithm demonstrates high precision under both strong and weak turbulence conditions. Furthermore, we confirmed the universality of the algorithm across multiple modulation formats.

Transformer neural networks for closed-loop adaptive optics using nonmodulated pyramid wavefront sensors

Context. The pyramid wavefront sensor (PyWFS) provides the required sensitivity for demanding future adaptive optics (AO) instruments. However, the PyWFS is highly nonlinear and requires the use of beam modulation to successfully close an AO loop under varying atmospheric turbulence conditions. This comes at the expense of a loss in sensitivity. Aims. We trained, analyzed, and compared the use of deep neural networks (NNs) as nonlinear estimators for the nonmodulated PyWFS, identifying the most suitable NN architecture for a reliable closed-loop AO. Methods. We developed a novel training strategy for NNs that seeks to accommodate for changes in residual statistics between open and closed loops, plus the addition of noise for robustness purposes. Through simulations, we tested and compared several deep NNs from classical to new convolutional neural networks (CNNs), plus the most recent transformer neural network (TNN; global context visual transformer, GCViT), first for an open loop and then for a closed loop. By identifying and properly retraining the most adequate deep neural net, we tested its simulated performance first in an open loop and then for closing an AO loop at a variety of noise and turbulence conditions. We finally tested the trained NN ability to close a real AO loop for an optical bench. Results. Using open-loop simulated data, we observe that a TNN (GCViT) largely surpasses any CNN in estimation accuracy in a wide range of turbulence conditions. Moreover, the TNN performs better in a simulated closed loop than CNNs, avoiding estimation issues at the pupil borders. When closing the loop at strong turbulence and low noise, the TNN using nonmodulated PyWFS data is able to close the loop, similar to a PyWFS with 12λ/D of modulation. When the noise is increased, only the TNN is able to close the loop, while the standard linear reconstructor fails even when a modulation is introduced. Using the GCViT, we closed a real AO loop in the optical bench and achieved a Strehl ratio between 0.28 and 0.77 for turbulence conditions corresponding to Fried parameters ranging from 6 to 20 cm, respectively. Conclusions. Through a variety of simulated and experimental results, we demonstrate that a TNN is the most suitable architecture for extending the dynamic range without sacrificing sensitivity for a nonmodulated PyWFS. It opens the path for using nonmodulated Pyramid WFSs in an unprecedented range of atmospheric and noise conditions.

Performance evaluation and investigation of diffraction optical elements effect on bit error rate of free space optics and performance investigation of space uplink wireless optical communication under varying atmospheric turbulence conditions

AbstractSeveral other optical antenna topologies have been developed and implemented throughout the years. These topologies include a variety of optical components, including the axicon optical element, dual‐secondary mirror, cone reflecting mirror, prism beam slier, and beam‐splitter/beam combiner. In contrast, the secondary reflecting mirror causes an obscuration loss that must be compensated for by reducing the transmission power in an optical antenna design. In order to address this issue in space optical communication, the present research helps to develop an enhanced two diffractive optical elements (DOEs) technology however the data presented therein only shows that DOEs may boost transmission power efficiency, which is insufficient for system designers. Though On‐Off Keying (OOK) is widely used in optical communication systems at the moment, the proposed research include DOEs into an OOK space uplink optical. The proposed research uses numerical simulation to explore how much a space uplink OOK system's bit error rate (BER) may be improved by using DOEs and adjusting fundamental parameters. The proposed BER model takes environmental factors like wind and detector noise into account. Using this theoretical model, the present work helps to investigate the effect of DOEs on the BER versus fundamental parameter characteristic curves in space uplink optical communication. Based on the findings, the DOEs structure has the potential to significantly enhance the BER performance of space uplink optical communication systems, especially at high obscuration ratios. When the obscuration ratio is 0.25, 0.167, or 0.125 and the transmission power is 1 W, for instance, the DOEs may improve the BER by a factor of two or one order of magnitude or less when the parameters are changed to the typical parameter values as specified. Results increase by a factor of six, three, and two orders of magnitude, respectively, when transmitting at 5 W. The results show that DOEs can significantly enhance the BER performance, especially at high obscuration ratios. The findings suggest that integrating DOEs into the optical subsystem is a straightforward approach to improving the performance of space uplink optical communication systems.

Multi-line-of-sight Hartmann-Shack wavefront sensing based on image segmentation and K-means sorting

Multi-line-of-sight wavefront sensing, crucial for next-generation astronomy and laser applications, often increases system complexity by adding sensors. This research introduces, to the best of our knowledge, a novel method for multi-line-of-sight Hartmann-Shack wavefront sensing by using a single sensor, addressing challenges in centroid estimation and classification under atmospheric turbulence. This method contrasts with existing techniques that rely on multiple sensors, thereby reducing system complexity. Innovations include combining edge detection and peak extraction for precise centroid calculation, improved k-means clustering for robust centroid classification, and a centroid filling algorithm for subapertures with light loss. The method’s effectiveness was confirmed through simulations for a five-line-of-sight system and experimental setup for two-line and three-line-of-sight systems, demonstrating its potential in real atmospheric aberration correction conditions. Experimental findings indicate that, when implemented in a closed-loop configuration, the method significantly reduces wavefront residuals from 1 λ to 0.1 λ under authentic atmospheric turbulence conditions. Correspondingly, the quality of the far-field spot is enhanced by a factor of 2 to 4. These outcomes collectively highlight the method’s robust capability in enhancing optical system performance in environments characterized by genuine atmospheric turbulence.

Multiplexed orbital angular momentum beams demultiplexing using hybrid optical-electronic convolutional neural network

Advancements in optical communications have increasingly focused on leveraging spatial-structured beams such as orbital angular momentum (OAM) beams for high-capacity data transmission. Conventional electronic convolutional neural networks exhibit constraints in efficiently demultiplexing OAM signals. Here, we introduce a hybrid optical-electronic convolutional neural network that is capable of completing Fourier optics convolution and realizing intensity-recognition-based demultiplexing of multiplexed OAM beams under variable simulated atmospheric turbulent conditions. The core part of our demultiplexing system includes a 4F optics system employing a Fourier optics convolution layer. This optical spatial-filtering-based convolutional neural network is utilized to realize the training and demultiplexing of the 4-bit OAM-coded signals under simulated atmospheric turbulent conditions. The current system shows a demultiplexing accuracy of 72.84% under strong turbulence scenarios with 3.2 times faster training time than all electronic convolutional neural networks.

Coding techniques for diversity enhancement of dense wavelength division multiplexing MIMO‐FSO fault protection protocols systems over atmospheric turbulence channels

AbstractAn enhanced transmission is presented in a multiple‐input‐multiple‐output (MIMO) dense‐wavelength division multiplexed (DWDM) free‐space‐optical (FSO) communication link using diversity coding techniques under the effect of turbulent weather phenomenon. The findings show good performance with an (8 channels × 2.5 Gbps data rate/channel) 20 Gbps 1500 m transmission distance. The bit‐error‐rate (BER), outage probability (OP), and signal‐to‐noise ratio (SNR) of the diversity combining techniques using maximum‐ratio combining (MRC), selection combining (SC), and equal‐gain combining (EGC) technique are evaluated in this work. The obtained results illustrate that Alamouti, space‐time coding (STC), space‐time block coding (STBC), space‐time trellis code (STTC), orthogonal STBC (O‐STBC), and quasi‐orthogonal STBC (QO‐STBC) on the minimum mean‐square‐error, and MRC are worth implementing on the DWDM‐FSO wireless communication systems. The mitigation of atmospheric turbulence is achieved using MIMO diversity combining techniques coding. The simulation results for diversity coding techniques using QO‐STBC/STTC and SC/MRC in the MIMO‐DWDM FSO communication system can improve BER performance, OP, and SNR. The MRC exhibits the lowest OP and BER when compared with the SC and EGC. The numerical results demonstrate that the FSO communication link using DWDM QO‐STBC/STTC improves the power penalty at both BER values under varying atmospheric turbulence conditions for ST, MT, and WT, in comparison to FSO systems without DWDM QO‐STBC/STTC diversity coding techniques.

Performance of Orbital Angular Momentum Communication for a Non-Uniformly Correlated High-Order Bessel–Gaussian Beam in a Turbulent Atmosphere

We derived the formula for the detection probability, signal-to-noise ratio (SNR), and average bit error rate (BER) for the signal orbital angular momentum (OAM) state carried via non-uniformly correlated high-order Bessel–Gaussian beam propagation in a turbulent atmosphere. The wavelength, receiver aperture, beam width, strength of the turbulent atmosphere, and topological charge effect on detection probability, SNR, and average BER of the signal OAM state were demonstrated numerically. The results show that the signal OAM state with low topological charge, a small receiver aperture, a narrow beam width, and a long wavelength can improve the performance of optical communications systems under conditions of weak atmospheric turbulence. Our results will be useful in long-distance free space optical (FSO) communications.

Wind and structural loads data measured on parabolic trough solar collectors at an operational power plant

Wind loading is a primary contributor to structural design costs of concentrating solar-thermal power collectors, such as heliostats and parabolic troughs. These structures must resist the mechanical forces generated by turbulent wind, while the reflector surfaces must maintain optimal optical performance. Studying wind-driven loads at a full-scale, operational concentrating solar-thermal power plant provides insights into the wind impact on the solar collector field beyond the capabilities of wind tunnel tests or state-of-the-art simulations. We conducted comprehensive field measurements of the atmospheric turbulent wind conditions and the resulting structural wind loads on parabolic troughs at the Nevada Solar One plant over a two-year period. The measurement setup included meteorological masts and structural load sensors on four trough rows. Additionally, a lidar scanned the horizontal plane above the trough field. In this study, we describe the high-resolution dataset characterizing the complex flow field and resulting structural loads. This first-of-its-kind dataset will enhance the understanding of wind loading on collector structures and will help in designing the next-generation solar collectors and photovoltaic trackers.

Performance analysis of 160 Gbps-60 GHz OFDM-MIMO RoFSO transmission with WDM-PDM dual multiplexing

Abstract An orthogonal frequency division multiplexed-radio over free space optics (OFDM-RoFSO) communication system with dual multiplexing is proposed. Eight input data streams are transmitted simultaneously by using wavelength division multiplexing (WDM) at level 1 and polarization division multiplexing (PDM) at level 2. Input data streams 1–4 and 5–8 are assigned with same set of carrier frequencies i.e. 193.1 THz, 193.3 THz, 193.5 THz and 193.7 THz and separate polarization levels (X and Y polarized) before transmitting through RoFSO link. A total data rate of 160 Gbps (4 × 2 × 20 Gbps) is achieved in this design. With proposed system, channel performance is evaluated under the influence of atmospheric attenuation and turbulence conditions while measuring system BER, Q-factor, SNR and received power etc. The effect of channel crosstalk is analysed for WDM-PDM dual multiplexed design while considering single input single output (SISO) and multiple input multiple output (MIMO) FSO configurations. The overall analysis predicts that a higher transmission rate and improved capacity levels can easily be achieved with dual multiplexed system.

Phase-Noise Characterization in Stable Optical Frequency Transfer over Free Space and Fiber Link Testbeds

Time and frequency metrology depends on stable oscillators in both radio-frequency and optical domains. With the increased complexity of the highly precise oscillators also came the demand for delivering the oscillators’ harmonic signals between delocalized sites for comparison, aggregation, or other purposes. Besides the traditional optical fiber networks, free-space optical links present an alternative tool for disseminating stable sources’ output. We present a pilot experiment of phase-coherent optical frequency transfer using a free-space optical link testbed. The experiment performed on a 30 m long link demonstrates the phase-noise parameters in a free-space optical channel under atmospheric turbulence conditions, and it studies the impact of active MEMS mirror stabilization of the received optical wave positioning on the resulting transfer’s performance. Our results indicate that a well-configured MEMS mirror beam stabilization significantly enhances fractional frequency stability, achieving the−14th-order level for integration times over 30 s.

Analysis of Tip/Tilt Compensation of Beam Wandering for Space Laser Communication

Laser communication has been considered as a novel method for earth observation satellites with generation of high data volume. It offers faster data transmission speeds compared to conventional radio frequency (RF) communication due to the short wavelength and narrow beam divergence. However, laser beams are refracted due to atmospheric turbulence between the ground and the satellite. Refracted laser beams, upon reaching the receiver, result in angle-of-arrival (AoA) fluctuation, inducing image dancing and wavefront distortion. These phenomena hinder signal acquisition and lead to signal loss in the course of laser communication. So, precise alignment between the transmitter and receiver is essential to guarantee effective and reliable laser communication, which is achieved by pointing, acquisition, and tracking (PAT) system. In this study, we simulate the effectiveness of tip/tilt compensation for more efficient laser communication in the satellite-ground downlink. By compensating for low-order terms using tip/tilt mirror, we verify the alleviation of AoA fluctuations under both weak and strong atmospheric turbulence conditions. And the performance of tip/tilt correction is analyzed in terms of the AoA fluctuation and collected power on the detector.

An All-Digital Optical Phase-Locked Loop Suitable for Satellite Downlinks

The optical signal propagation used in satellite uplinks and downlinks is influenced by absorption, scattering, and changes in the atmospheric refractive index or turbulence, causing optical signal attenuation. A free space optics (FSO) communications system using coherent communication can improve the link sensitivity and reach higher distances. This article proposes a new architecture for the phase detector in an all-digital optical phase-locked loop (OPLL) for coherent optical detection. Firstly, the performance of the proposed phase detector is evaluated under Gaussian noise, where the best operation point is found for the OPLL working with two sample rates: 625 MSa/s and 10 GSa/s. The system analyses also take a non-negligible delay into account. Then, it will be evaluated and compared with an OPLL using an analog phase detector in the presence of atmospheric turbulence. Finally, in three different atmospheric turbulence conditions, the effect of wind speed on communication quality is investigated through the obtained bit error rate (BER) from the recovered data for a bit rate of 20 Gbps. The results show that the proposed digital phase detector can track a signal under longer feedback loop delays and fading signals.

On the capacity of a SIM-based cooperative NLOS UVC system with best relay selection

Ultraviolet communication (UVC) is a promising technology due to its ability to operate in non-line-of-sight (NLOS) mode thereby eliminating the pointing acquisition and tracking (PAT) requirement as needed by infrared and visible light communications. However, NLOS UVC suffers from very high attenuation and turbulence-induced fading when operated over a long distance. Due to these limitations, the existing literature on the NLOS UVC is mostly restricted to short-distance communications only. Therefore, this paper addresses these challenges by proposing an outdoor subcarrier-intensity-modulation (SIM) based multi-relay cooperative communication system employing the best relay selection and decode-and-forward (DF) relaying protocol. The turbulence-induced fading is modelled using lognormal distribution under weak atmospheric turbulence conditions. We derive novel closed-form analytical expressions of outage probability and ergodic capacity. Correctness of the derived analytical expressions is validated through numerical simulations.

Performance evaluation of UAV-assisted FSO link in generalized Malaga distributed atmospheric turbulence conditions

Performance evaluation of UAV-assisted FSO link in generalized Malaga distributed atmospheric turbulence conditions