Free Space Optical Performance Research Articles

Joint estimation model for FSO channel parameters and performance evaluation based on CNNs.

Free space optical (FSO) communication systems experience turbulence-induced fading. As a possible solution, adaptive transmission, which adjusts transmitter parameters based on instantaneous channel state information (CSI), can be used. Most of the existing channel estimation methods ignore the impact of detection noise at the receiver, which will lead to additional estimation errors. In this paper, a joint estimation model based on convolutional neural networks (CNNs) is proposed to estimate detection noise and turbulence fading parameters. We obtained turbulence channel simulation data sets considering the background of detection noise based on the edge probability distribution function of the receive signal. The training of the CNN estimator is carried out through maximum pooling, adaptive learning rate, and regularization, ultimately accurately estimating channel characteristics based on the optimal output results of the network. The simulation results show that the proposed CNN joint estimator performs better in high-detection-noise environments compared with traditional maximum likelihood estimators, and it has better generalization ability in different real atmospheric environments.

A comprehensive survey of free-space optical communication – modulation schemes, advantages, challenges and mitigations

Abstract The demand for large bandwidth and high data rates in communication systems has become the main cause of the upgrade of traditional networks into free space optical (FSO) technology. FSO technology has gained significant popularity due to its easy deployment, high data rates, abundant bandwidth, enhanced security, and license-free spectrum utilization. However, the performance of FSO communication systems can be affected by certain limiting factors, such as changes in weather conditions during data transmission. To overcome these challenges and improve FSO performance, various modulation techniques are employed. This article presents a concise overview of the FSO communication system, highlighting different modulation techniques used to enhance its performance, as well as discussing its advantages, applications, and existing challenges. Some advanced modulation formats which are recently introduced in the field of FSO communication such as QPSK, DP-QPSK, QAM, and OFDM are also made part of this paper.

Asymptotic Ergodic Capacity Analysis for FSO Communication between Mobile Platforms in Maritime Environments

With the rapid development of wireless communication technologies, free-space optical (FSO) communication has gained much attention in recent years. The FSO technology, which is usually used for communication in terrestrial environments, is expanding its regime to communication in maritime environments. In this study, we analyze the ergodic capacity of FSO communication between mobile platforms in maritime environments under foggy conditions, pathloss, and pointing error. More specifically, based on the moment expressions, we derive the closed-form expression of asymptotic ergodic capacity using conventional detection techniques (i.e., heterodyne and intensity modulation/direct detection). The derived analytical results are cross-verified with simulation results via Monte Carlo simulations. The results show that the combined effects of fog with pathloss and pointing error degrade the FSO performance and that it is important to apply the appropriate detection technique according to the communication environment.

A Survey of Hybrid Free Space Optics (FSO) Communication Networks to Achieve 5G Connectivity for Backhauling.

Increased capacity, higher data rate, decreased latency, and better service quality are examples of the primary objectives or needs that must be catered to in the near future, i.e., fifth-generation (5G) and beyond. To fulfil these needs, cellular network design must be drastically improved. The 5G cellular network design, huge multiple-input multiple-output (MIMO) technology, and device-to-device communication are all highlighted in this comprehensive study. Hence, free-space optics (FSO) is a promising solution to address this field. However, FSO standalone is insufficient during turbulent weather conditions. FSO systems possess some limitations, such as being able to be disturbed by any interference between sender and receiver such as a flying bird and a tree, as it requires line-of-sight (LOS) connectivity. Moreover, it is sensitive to weather conditions; the FSO performance significantly decreases in bad weather conditions such as fog and snow; those factors deteriorate the performance of FSO. This paper conducts a systematic survey on the existing projects in the same area of research such as the hybrid FSO/Radio frequency (RF) communication system by listing each technique used for each model to achieve optimum performance in terms of data rate and Bit Error Rate (BER) to be implemented in 5G networks.

Novel Approximate Distribution of the Sum of Lognormal-Rician Turbulence Channels With Pointing Errors and Applications in MIMO FSO Links

In this paper, an approximate closed-form probability density function expression for the sum of lognormal-Rician turbulence channels with Rayleigh pointing errors is developed. The results of Kolmogorov-Smirnov goodness-of-fit statistical tests show that the proposed approximation is highly accurate across a wide range of channel conditions. Also, the analysis of approximation error is presented in detail, and it indicates that a more efficient approximation can be achieved for larger coherence parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$r$</tex-math></inline-formula> and smaller variance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sigma _{z}^{2}$</tex-math></inline-formula> . To reveal the importance of proposed approximation, the closed-form expressions for the ergodic capacity, outage probability, and bit-error rate are derived in terms of Meijer’s G-function. The performance of multiple-input multiple-output (MIMO) free-space optical (FSO) systems with equal gain combining (EGC) diversity technique are analyzed in detail under different scenarios, including the number of transmit and receive apertures, turbulence channels, and presence of pointing errors. It is observed that MIMO technology can offer a significant improvement in FSO performance when compared with the single-input single-output (SISO) systems. The ergodic capacity and BER performance at high signal-to-noise ratio are also obtained to provide further insights. Numerical results demonstrate the accuracy of the proposed approach.

PERFORMANCE ANALYSIS OF FSO LINK UNDER EXTREME WEATHER CONDITIONS

free space optics is currently a well-known technology for its use in the systematic transport of enormous volumes of data. It has proven to be a feasible technique for assisting existing networks with high data transfer safely across a moderate distance without the usage of fiber. Free space optics has emerged as the most viable option to the broadband market's current bottlenecks. However, certain external characteristics have a significant impact on the performance of free space optics. We calculate signal loss owing to environmental elements such as fog, snow, rain, blurring,and scintillation, as well as a detailed approach for reducing their impact on free space optical network performance. Signal quality has improved significantly using the provided methods.

Tropical Terrestrial Free Space Optical Link Performance Analysis

Free Space Optical (FSO) has a very good potential alternative solution to Radio Frequency (RF) to provide high terrestrial wireless communication. In tropical region, the only drawback of FSO is atmospheric attenuation due to mainly haze and rain. This study investigates the performance of FSO system over link distances that ranges between 9 km to 12 km as the average visibility in Kuala Lumpur, Malaysia is 10 km. Two models used to predict the atmospheric attenuation, namely Kruse model and modified Kruse model. These models are valid to predict the haze attenuation for visibilities that range between 9 to 12 km. The modified/corrected Kruse model shows some limitations in the prediction of haze attenuation over longer wavelength. However, the study indicates that attenuation due to haze is low, while air quality in Kuala Lumpur, Malaysia is acceptable. As the size of haze particles varies from 0.01 to 1 µm, the longer wavelength is preferable as 1550 nm to enhance FSO performance compared to 850 nm and 600 nm wavelengths. Thus, the longer wavelength is the optimal choice to enhance the FSO performance as it caused low scattering.

Optical wireless communication under the effect of low electric field

Abstract In this paper, the performance of free space optics (FSO) technology is investigated under the effect of external electric fields perturbations; standard parameters of the proposed system adopted by performing experimental setup measurements on the laser propagation axis and computer simulation using the “OptiSystem” program. The experimental results show linear relation between the electric field and the fluctuation of the output power As a result of the interference between the transmitted signal and the noise created by an electric field. The simulation results illustrate that when the transmitter divergence angle increased the Q-factor decreased, but not high enough to cause a complete blocking to the transmitted signal. The significance of these results, they help to understand the effect of electric discharges and Corona phenomenon of the electric field produced by the transmission line conductors nearby the optical communication systems where, (1 mrad) is the most effective divergence angle that reduces these effects.

Unified Performance Analysis of Reconfigurable Intelligent Surface Empowered Free-Space Optical Communications

Reconfigurable intelligent surface (RIS) is an excellent use case for line-of-sight (LOS) based technologies such as free-space optical (FSO) communications. In this paper, we analyze the performance of RIS-empowered FSO (RISE-FSO) systems by unifying Fisher-Snedecor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${\mathcal{F}}$ </tex-math></inline-formula> ), Gamma-Gamma ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cal {GG}$ </tex-math></inline-formula> ), and Malága ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cal {M}$ </tex-math></inline-formula> ) distributions for atmospheric turbulence with zero-boresight pointing errors over deterministic as well as random path-loss in foggy conditions with heterodyne detection (HD) and intensity modulation/direct detection (IM/DD) methods. By deriving the probability density function (PDF) and cumulative distribution function (CDF) of the direct-link (DL) with the statistical effect of atmospheric turbulence, pointing errors and random fog, we develop exact expressions of PDF and CDF of the resultant channel for the RISE-FSO system. Using the derived statistical results, we present exact expressions of outage probability, average bit-error-rate (BER), ergodic capacity, and moments of signal-to-noise ratio (SNR) for both DL-FSO and RISE-FSO systems. We also develop an asymptotic analysis of the outage probability and average BER and derive the diversity order of the considered systems. We validate the analytical expressions using Monte-Carlo simulations and demonstrate the performance scaling of the FSO system with the number of RIS elements for various turbulence channels, detection techniques, and weather conditions.

Enhanced spectrum slicing: wavelength division multiplexing approach for mitigating atmospheric attenuation in optical communication

Over last decades, the free space optics (FSO) emerged as most prominent way of communication over radio frequency communication and microwave communication. The FSO works in the same way optical fiber cable (OFC) network works with the only difference that the optical beams were transmitted through the free space and not by using the OFC core i.e. the glass fiber. The presence of mist, fog, rain and clouds in the atmosphere directly affects the performance of FSO and the power of propagating signal is degraded significantly. Since the wavelength of propagating beam is comparable to size of fog particles, they mainly lead to atmospheric attenuation. To mitigate the impact of atmospheric attenuation on signals, the present study develops a spectrum slicing (SS)-wavelength division multiplexing (WDM) system with Pre and Post Amplification. The performance of the proposed SS-WDM-Pre and SS-WDM-Post scheme is analyzed and then compared with the traditional SS-WDM model in the Opti-system in terms of Q-factor and BER values. The analysis of the proposed and traditional scheme is done under varying attenuation and power signals during summer, winter, autumn and spring season.The comparative analysis proved that the performance of the proposed SS-WDM-Post and SS-WDM-Pre-model is more effective and efficient when compared with the traditional SS-WDM model in terms of Q-Factor and BER during all 4 seasons.

FSO performance analysis of a metro city in different atmospheric conditions

Abstract In this article, a free-space-optics (FSO) model for a metro city based on real-time visibility data under different weather conditions is presented. Here, the relation between atmospheric attenuation and visibility value and its impact on the FSO performance is carried out. Also, the hybrid-SIM modulation technique is applied for the bit error rate (BER) improvement of the system. Here, Bhubaneswar (the capital of Odisha, India) is considered for the above analysis. The total attenuation, BER, and link margin are calculated for the FSO performance evaluation of Bhubaneswar city and these are simulated using Matlab. The simulation results show that the proposed hybrid-SIM modulation technique achieves about 2.9 dB of SNR improvement than the existing BPSK-SIM technique. Also, from the link margin analysis, it is observed that the sunny day is the most suitable for FSO link transmission. These results can be helpful to decide the FSO parameters and the FSO setup can be designed for any city.

Free Space Optics, Channel Models and Hybrid Modulation Schemes: A Review

Free space optical (FSO) communication system is a wireless communication technology which possess advantages such as huge license free bandwidth and high data carrying capacity among others. It has proved to be a viable solution towards the radio frequency spectrum crunch and hence has been applied in cellular back haul, earth to satellite links, deep space and defense among others. However, FSO performance is adversely affected by atmospheric turbulence and weather conditions. Different channel models are used to estimate the performance of FSO communication system. Meanwhile, different mitigation techniques have been proposed and analyzed over the years for improvement of FSO communication. Of late, hybrid modulation schemes have proved to be another research direction. In this article, we review the FSO channel models as well as hybrid modulation schemes that have been recently introduced to further improve the FSO performance. The conditional probability of error for the commonly used FSO modulation schemes are presented. Finally, the expressions for the bit error for the hybrid modulation schemes are also given.

Effect of fog and scintillation on performance of vertical free‐space optical link from Earth to low Earth orbit satellite

SummaryThis paper studies the effects of fog and scintillation attenuations on the performance of the vertical free‐space optical (FSO) link from the Earth to low Earth orbit (LEO) satellite. FSO link performance was investigated under different foggy conditions and turbulence by calculating the power received, achieved data rate, and link margin. Different parameters were studied such as the wavelength, the transmitted power, the divergence angle, and the diameter of receiving optical antenna. The results show that the FSO performance can be improved using 1,550‐nm wavelength, increasing the transmitted power to a range of 5 mW and reducing the divergence angle to a range of 1 μrad.

Unified Finite Series Approximation of FSO Performance Over Strong Turbulence Combined With Various Pointing Error Conditions

In this paper, we investigate both the bit error rate (BER) and outage performance of free-space optical (FSO) links over strong turbulence combined with various pointing error conditions. Considering atmospheric turbulence and pointing errors as main factors that deteriorate the quality of an optical link, we obtain a unified finite series approximation of the composite probability density function, which embraces generalized pointing error models. This approximation leads to new unified formulas for the BER and outage capacity of an FSO link, which account for the two possible detection mechanisms of intensity modulation/direct detection and heterodyne detection. Selected simulation results confirm that the newly derived approximations can give precise predictions of both the average BER and the outage capacity of FSO communication that are generally applicable to all environments.

Atmospheric Turbulence Suppression Algorithm Based on APD Adaptive Gain Control for FSO Links of 5G Floating Base Stations

The data transmission between 5G floating base stations will be an important application area of the free space optical (FSO) link, but the problem of atmospheric disturbance will directly affect the application effect. An atmospheric turbulence suppression algorithm based on avalanche photodiode adaptive gain control (APDAGC) for the laser transmission terminal was proposed to suppress atmospheric turbulence. The given design mechanism and experimental results demonstrate its function to improve the performance of FSO. When APDAGC is over 1 km FSO, the variance of intensity fluctuation is lowered from 0.046 to 0.009, with the bit error rate reduced from 4.82E-6 to 1E-12. Comparatively, as APDAGC is over 6.5 km FSO, the variance of intensity fluctuation is decreased from 1.767 to 0.376, with the bit error rate varying from 4.6E-2 to 2.4E-4. Moreover, it was used for the first FSO transmission of 5G floating base station signals on an airship platform.

Reliable FSO system performance matching multi-level customer needs in Alexandria City, Egypt, climate: sandstorm impact with pointing error

Free space optical (FSO) communication is one of the recent promising new access technologies. FSO provides data transmission with high security and unlimited bandwidth with low cost of installation. Optical signal absorption and scattering are the two main factors of attenuation due to the terrestrial channel. Pointing error is a technical factor that causes attenuation of the signal which results from the misalignment of the transmitter and the receiver. This paper investigates the FSO performance for an atmospheric channel in Alexandria city, Egypt. The study covers the operating wavelength, modulation technique, receiver aperture diameter, and type of photodetector. The study considers the practical average of meteorological measurements for Alexandria city obtained from Egyptian Meteorological Unit. The system performance is evaluated by its bit error rate, signal-to-noise ratio, and the received power at different system parameters. The system is studied by simulation and experimentally using Optisystem ver. 15.2. It is found that the FSO system is so sensitive to the transmitter–receiver misalignment. However, the effect of thermal noise and background noise could not be neglected compared to the resulting pointing error due to the Mediterranean climate which is mainly characterized by high temperature and humidity. After this integrated parametric study for the system performance, we deduced the practical application availability for the system starting from low to higher data rates with respect to climate and system parameters. Therefore, one can ensure that the system can be used on a wider scale of customers.

Investigation and Analysis of Data Rate for Free Space Optical Communications System Under Dust Conditions

In this study, the performance of free-space optics (FSO) communication link under dust conditions was analysed. The data were modulated by the non-return-to-zero technique. The optical link was investigated with varying data rates under the effect of dust conditions. The performance of the FSO system was tested in terms of the bit error rate (BER), Q-factor and received power at the maximum link. Simulation results were obtained to compare between two transmit data rates (0.3 and 0.7 Gbps). The system has a high BER at a high data rate. In the meantime, it has a high Q-factor at a low data rate.

Comparative analysis of MIMO-based FSO and MIMO-based MGDM communications

The ever-growing need for high data rate, bandwidth efficiency, reliability, less complexity, and less power consumption in the communication systems is on the increase. Modern techniques have to be developed and put in place to meet these requirements. In the last few years, free space optical (FSO) communication systems have attracted considerable research efforts mainly due to their inherent potential transmission capacity, much higher than that offered by radio transmission technologies. However, despite of great potential of FSO communication, its performance is limited by the adverse effects (viz., absorption, scattering, and turbulence) of the atmospheric channel. Different studies on weather conditions and techniques employed to mitigate their effect are studied. On other hand, a MMF network may constitute the backbone network that feeds fixed-wired services (e.g. voice telephony) as well as wireless services (e.g. IEEE 802.x) throughout the building. The large bandwidth makes MMF a very attractive medium for low-cost broadband in building networks, in which several independent sets of services are integrated. In addition, research has shown that compared to conventional single-input single-output (SISO) systems, multiple-input multiple-output (MIMO) can actually increase the data rate of a communication system, without actually requiring more transmit power or bandwidth. In this paper, we develop a comparison between MIMO over multimode fiber (MMF) using MGDM and MIMO based FSO links. The achievable performance improvements, including received power levels, bit error rate (BER), and quality factor (Q-factor) are presented. Numerical results are obtained for single-input single-output (SISO) as well as for multiple-input multiple-output (MIMO) configurations. Simulation results show that MMF outperforms FSO links in terms of max Q-factor, minimum BER, and eye diagram. Seen that the dependence of the FSO system performance on the weather conditions is one of the most significant FSO limitations, we analyze the performance of FSO under clear, haze, and fog conditions using Q-factor, bit error rate (BER), etc., at varied distances. From the results, it can be concluded that as we move from clear weather conditions to haze and fog weather conditions, Q-factor of the received signal decreases.

Performance of geometrical effect in wavelength filtrate detection using 10gbps data rate for free space optical communication system

&lt;span&gt;In this paper, the analysis of comparison between proposed detection called as Wavelength Filtrate Detection with existing detection have been analyzed to determine the best system performance in term of power received and bit error rate (BER). The analysis focus on the geometrical effect of aperture receiver and beam divergence. The performance of Free Space Optics is validated through simulation analysis using parameter 10Gbps data rate and in 14dB/km atmospheric attenuation. This analysis shows that Wavelength Filtrate Detection increase the power received of 6dBm and improve the absolute value of error rate 10-9 in acceptable threshold required by (ITR-U) as much as 50% compare to others existing detection&lt;/span&gt;

Performance of multiple-input multiple-output free space optics using hybrid subcarrier intensity modulation with avalanche photodiode receiver

In order to improve the performance of a free space optical (FSO) communication system, hybrid pulse position modulation-binary phase shift keying-subcarrier intensity modulation (PPM-BPSK-SIM) technique is proposed. The system performance is further improved by using multiple-input multiple-output (MIMO) configuration and avalanche photodiode (APD) receiver. The use of multiple lasers and photo detectors in MIMO FSO system reduces the scintillation effect in the atmosphere. In this study, the impact of different parameters like irradiance, link distance, and turbulence level have been considered for the bit error rate (BER) analysis. The relevant mathematical model is also presented. The simulation results show that the hybrid PPM-BPSK-SIM scheme performs better compared to PPM and BPSK-SIM schemes. Also, APD receiver shows a noticeable BER improvement than p-type intrinsic n-type (PIN) receiver in our proposed scheme.