Free Space Optical Link Availability Research Articles

Availability of Free-Space Laser Communication Link with the Presence of Clouds in Tropical Regions

Free-space laser communication (lasercom), a great application of using free-space optics (FSO) for satellite communication, has been gaining significant attraction. However, despite of great potential of lasercom, its performance is limited by the adverse effects of atmospheric turbulence and cloud attenuation, which directly affect the quality and availability of lasercom links. The paper, therefore, concentrates on evaluating the cloud attenuation in the FSO downlinks between satellite and ground stations in tropical regions. The meteorological ERA-Interim database provided by the European Center for Medium-Range Weather Forecast (ECMWF) from 2015 to 2020 is used to get the cloud database in several areas in tropical regions. This study proposed a novel probability density function of cloud attenuation, which is validated by using a well-known curve-fitting method. Moreover, we derive a closed-form of satellite-based FSO link availability by applying the site diversity technique to improve the system performance. Numerical results, which demonstrate the urgency of the paper, reveal that the impact of clouds on tropical regions is more severe than in temperate regions.

A survey on performance enhancement in free space optical communication system through channel models and modulation techniques

Free space optical (FSO) communication system has obtained significant importance in communication field due to its unique features: unlimited spectrum, larger bandwidth and high data rate, low mass and less power requirements, quick and easy deployability. FSO system better suits in disaster recovery, defense and last mile problems in networks, remote sensing and so on. However, FSO system has greater advantage, its performance is mainly degraded by adverse effects like beam wandering and spreading, scattering and mainly a major degradation factor is atmospheric turbulence and pointing errors which leads to severe degradation in Bit error rate (BER) and Signal to noise ratio (SNR) of the FSO link and makes the communication link infeasible. This paper gives a detailed survey on performance enhancement of FSO communication system and also discusses various channel distribution models and modulation techniques to have high reliability and FSO link availability. In this paper, the various atmospheric effects like turbulence, fog, absorption and scintillation and so on are discussed. The first part of the paper analysis the channel models and the latter part of the paper summarizes the different modulation techniques, diversity techniques and also the comparative study of the (SNR) and (BER) under various atmospheric factors of the FSO system. This survey provides the comprehensive details in order to provide low cost and high capacity FSO link design.

Availability modelling of terrestrial hybrid FSO/RF based on weather statistics from tropical region

Free-space optical (FSO) communication links are point-to-point wireless technologies. It provides high data rates up to a gigabit per second. FSO link availability is easily affected by different weather conditions. Radio-frequency (RF) can be a complimentary link to the FSO link to increase availability. Hybrid FSO/RF is a feasible technology for providing high availability of point-to-point wireless links. In tropical regions, normally RF link is relatively sensitive to rain. Availability prediction of hybrid FSO/RF with the selection of suitable RF is indispensable, especially in tropical regions. In this study, empirical models are proposed to predict the link availability of RF, and hybrid FSO/RF over link ranges up to 5 km. The availability prediction models are based on the long-term statistics of atmospheric attenuations from the tropical climate zone. The RF link availability prediction model is developed for frequency ranges from 10 to 100 GHz using the International Telecommunications Union-Radio (ITU-R) rain attenuation model. It is link distance, fade margin, and radio frequency dependent. The proposed availability models can predict carrier as well as enterprise class availability. The availabilities predicted by proposed models are found to be in good agreement with those estimated by the ITU-R.

Resource Allocation for Hybrid RF/FSO Multi-Channel Multi-Radio Wireless Mesh Networks

The overwhelming data rates in next generation wireless networks impose a burden on the high-capacity network planning. One promising strategy to meet the demand for high-capacity communications is to augment radio frequency (RF) based multi-channel multi-radio (MCMR) wireless mesh network (WMN) by free-space optics (FSO). In this paper, we construct a hybrid RF/FSO MCMR WMN topology and address its resource allocation (RA) problem in terms of interface assignment, channel allocation, routing, FSO link allocation, and topology control. Considering the weather effects on FSO link availability and the fading nature of RF links, the RA problem is formulated as a two-stage optimization problem with the objective of maximizing the network throughput. In our optimization model, we formulate each stage as a mixed integer linear program, and the bottleneck RF links are gradually upgraded by FSO links. To avoid the computational complexity of the second stage optimization, an improved iterated local search algorithm is proposed. Simulation results show that our RA scheme is efficient and the throughput can be enhanced dramatically by proper FSO link augmentation.

Availability Prediction Methods for Terrestrial Free-Space-Optical Link Under Tropical Climate

<p>Free-space optical (FSO) links provide high speed point-to-point wireless communication, but its availability can easily be affected by weather conditions. In heavy rainfall regions, FSO links are relatively sensitive to rain. Availability prediction of FSO is indispensable part, especially in tropical areas. In this paper, methods to predict FSO link availability are presented. The overall performance of FSO link is described in terms of the availability that can be achieved over anticipated link distance. The availability prediction methods demonstrated for an FSO link ranges up to 5 km. The availability prediction methods are based on long-term statistics of atmospheric attenuations and FSO link budget under tropical climate condition. In tropical regions, for a terrestrial FSO link, carrier class availability can be achieved over a few hundred meters only; whereas enterprise class availability can be achieved over a few kilometers link distances. </p>

Wireless optical signal availability and link range analyses over strong fluctuating meteorological conditions with case study in Senegal

Objectives: Free-Space Optics (FSO) can provide high data rate, up to 10 Gbps. The performances of FSO systems are generally affected by weather conditions. In that paper we establish percentages of availabilities, the power margins and optimal link ranges of some FSO systems, for different strong meteorological conditions in Senegal. Methods/Statistical Analysis: Received signal and availability of FSO links are subjected to link distance and meteorological conditions like, clear atmosphere, rain, and sandstorm or in presence of heavy dust. In this paper we investigate the effects of weather conditions on the performance of FSO links, taking the climate of Senegal as a case study. However, the effect of turbulence induced irradiance fluctuation, varying beam divergence angle and misalignment conditions, and their impacts on the performances of the FSO systems were analyzed and estimated. Based on visibilities data recorded from 2003 to 2014, particular FSO link optimization’s simulation cases were analyzed, in terms of power margin, availability and maximum distance. The results were performed by a lognormal distribution with good precision of the tests of correlations. Findings: Range and availability of optical link for different weather conditions are established easily and with high precision. As there is no known study on the effects of weather conditions in this country, this paper offers an attempt to analyze and identify the challenges related to the deployment and optimization of FSO systems under Senegalese’s weather. Application/Improvements: For more flexibility, we forecast using an application programming interface to make easy use of the modeling approach by professional and academic actors. We plan to extend the study in all the cities in Senegal and for the Sahel region.

Availability Assessment of Free-Space-Optics Links With Rain Data From Tropical Climates

Rainfall in tropical environments acts as a dominant parameter for estimating the availability of free-space optics (FSO). Long fades are caused by precipitation in particular heavy rains, and this precipitation increases atmospheric attenuation due to rain and reduces the availability of optical power from FSO links. In this paper, the link availability of an FSO system is estimated under the impact of rain in a tropical climate. The influence of rain on the propagation of FSO signal is investigated based on experimental and statistical analyses of rain-rate measurements under tropical conditions. Rain-rate data are measured in Malaysia for three years and used to estimate the availability of FSO links. Models developed in Malaysia are used to predict atmospheric attenuation due to rain. Long-term statistical measurements of rain data can enable good estimation of link availability. Based on the prediction model and measured rain-rate data, the FSO link budget is analyzed, and the link availability as a function of distance is predicted. Carrier class availability limits the FSO link to a few hundred meters only, whereas enterprise class availability can exceed FSO links over a few kilometers long.

Availability analysis of free-space-optical links based on rain rate and visibility statistics from tropical a climate

Availability is one of the major parameters to measure the performance of Free Space Optics (FSO) communication link. In tropical climate, rainfall acts as a dominant factor that affects the FSO link availability; whereas haze has no significant effect compare with rain. But for longer FSO link distances, the effect of haze on the overall FSO link availability becomes serious and need to be considered. In this paper, availability of an FSO link is estimated under the impact of rain, haze and the combination of the two phenomena. The availability estimations based on predicted attenuation due to rain and haze using long-term statistical measurements of rain rate (mm/hr) and visibility (km) data measured in Malaysia. The effect of individual weather phenomena was demonstrated over FSO link ranges from 1 to 5km. The combine effect of both rain and haze also has been analyzed using the cumulative distribution function (CDF) of predicted atmospheric attenuations. In tropical regions, for short FSO link distance, the influence of haze on the availability of FSO signal is negligible compare with rain; whereas for longer FSO distances, haze attenuation will reduce the overall link availability. This effect of haze needs careful consideration for the deployment of FSO systems especially in Malaysian tropical climates.

Proposed parameters of specific rain attenuation prediction for Free Space Optics link operating in tropical region

As the demand for higher and unlimited bandwidth for communication channel is increased, Free Space Optics (FSO) is a good alternative solution. As it is protocol transparent, easy to install, cost effective and have capabilities like fiber optics, its demand rises very fast. Weather condition, however is the limiting factor for FSO link. In the temperate region the major blockage for FSO link feasibility is fog. In the tropical region high rainfall rate is expected to be the major drawback of FSO link availability. Rain attenuation is the most significant to influence FSO link availability in tropical region. As for now the available k and α values are developed using data from temperate regions. Therefore, the objective of this paper is to propose new parameters for specific rain attenuation prediction model that represents tropical weather condition. The proposed values are derived from data measured in Malaysia and using methods recommended by ITU-R.

Availability Evaluation of Ground-to-Air Hybrid FSO/RF Links

Recently, a hybrid architecture that utilizes the complementary nature of free-space optics (FSO) and radio frequency (RF) links with respect to their individual weather sensitivities was proposed to significantly increase availability for terrestrial broadband links. Based on this architecture, we developed a channel model integrating both the RF and FSO channels. Using the model and cloud distribution data obtained from the International Satellite Cloud Climatology Project, availability of an airborne hybrid FSO/RF link is evaluated. From the results, we conclude that if the FSO link alone is used, availability is greatly hampered by clouds due to attenuation and temporal dispersion. Contrarily, the RF signals are relatively immune to cloud influence, thus improving the hybrid link availability significantly. Furthermore, because of the significant temporal dispersion caused by multiple scattering of cloud particles, availability of FSO links can be improved by using frequency division schemes, though far from compensating for losses incurred by clouds.

Free-space optics wavelength selection: 10 µm versus shorter wavelengths [Invited

Here I address the many inquiries about 10-µm free-space optics (FSO) performance compared with that of shorter wavelengths. The growing interest in better understanding FSO weather effects is the result of carrier requests as well as recent progress in analyzing fog effects on FSO signal propagation. Extensive studies in modeling fog and simulating FSO attenuations revealed the complexity behind estimating FSO link availability in a given geographical location. There are many different types of fog that are inhomogeneous along the propagation path. Each type is characterized by water-droplet size and concentration; these parameters are used in Mie scattering theory to compute FSO signal attenuation. As a result, some vendors are augmenting their FSO links with a microwave backup link or simply investigating other wavelengths claimed to be more resistant to fog such as 10 µm. Here I analyze ways to improve FSO link availability, 10-µm improvement compared with shorter wavelengths, and challenges behind successful microwave backup installation.