Investigation of single beam near-infrared free space optical communication under different weather anomalies

The Free space optics (FSO) is a wireless optical communication system that connects directly to the atmosphere, where the connection is established between transmitter and receiver within in the line of sight. The FSO poses a high-speed broadband, which is the last mile wireless optical communication, deployed relatively fast. However, there are some weather factors may affect the performance of FSO transmission. In this paper, we analyzed the performance of Non-Return to Zero (NRZ) modulation schemes, which is used in FSO communication under extreme weather conditions over a range of 2Km. The performance has been analyzed under 980nm wavelength, Bit Error Rate (BER), and Q-factor using Opt system. The largest attenuation measured is 340dB/Km, correlate to the visibility of 50m. In addition the visibility exceeding about 50m, The Kruse formula provides a good measurement of optical attenuation over long distances under the clear weather and haze conditions respectively

Relay-assisted WDM-FSO System: A Better Solution for Communication under Rain and Haze Weather Conditions, Journal of Telecommunications and Information Technology, 2017, nr 4

Among various conventional wireless communication systems, Free Space Optics (FSO) may be distinguished as well, which provides a good level security, high data rates, an enormous bandwidth and negligible susceptibility to electromagnetic interference. The main problem arises with unfavorable weather conditions, which affect the system’s parameters and require an aversion to other paths. The weather conditions discussed are rain and haze. Elimination of their adverse effects is essential for establishing a better communication system. The routing path is a major issue, as its decreased parameters lead to the diversion of the transmitted signal or to its total loss. In this article, increased system path length and proficiency levels are demonstrated by using relays, which are assisted by Wavelength Division Multiplexing (WDM)

Performance evaluation of FSO communication systems over weak atmospheric turbulence channel for eastern coast of South Africa.

Masters Degree. University of KwaZulu-Natal, Durban.Free space optical (FSO) communication, otherwise known as optical wireless communication (OWC), is an established line-of-sight telecommunication technique which utilises an optical signal carrier to propagate modulated signals in the form of a light wave (visible or infrared) over the atmospheric medium. It has numerous advantages, including ease of deployment, large bandwidth, cost effective, full duplex high data rate throughput, protocol independence, highly secured data rate transmission, unregulated frequency spectrum, limited electromagnetic interference, and minimum amount of power consumption. With all the inherent advantages in FSO systems, the technology is impaired by atmospheric turbulence. Atmospheric turbulence occurs due to the persistent random changes of the refractive index as a result of variations in atmospheric temperature and pressure. This results in fluctuations in the irradiance of the laser (simply referred to as scintillation), which may lead to attenuation of optical signals in the FSO communication system. Thus, atmospheric attenuation and turbulent conditions have negative effects on the performance and ease of deployment of FSO communication systems. In this dissertation, we examine the performance of FSO systems over weak atmospheric turbulence channel for the eastern coast of South Africa. We evaluate the feasibility of the FSO link and how to improve the reliability by estimating the link margin, probability of attenuation exceedance, power scintillation index, overall power loss due to attenuation and turbulence, link budget estimate for different link lengths and wavelengths. The FSO system availability estimated for the eastern coast of South Africa is above 99% for link distances ranging from 1 km-4 km at 850 nm, 950 nm and 1550 nm. It is also observed that the FSO link availability increases with corresponding increase in wavelengths. Adopting the Kim model to estimate the atmospheric attenuation at 850 nm wavelength, the attenuation due to scattering contributes 9.47% to the absolute atmospheric losses while the atmospheric turbulence loss contributes 90.53% to the overall power loss at a link range of 4 km. Using the Ferdinandov model for a link range of 4 km at 950 nm wavelength, the attenuation due to scattering contributes 8.81% to the total power loss while the atmospheric turbulence loss contributes 91.19% to the overall power loss. It is observed that the attainable link distance increases with increase in atmospheric visibility status. The FSO system availability reduces with increase in the propagation link distance. Furthermore, it is found that the fading loss from scintillation effects strongly depends on the power scintillation index. An increase in the power scintillation index, causes an increase in the fading loss. Thus, the power scintillation index also increases per unit increase in transmission link length and refractive index. The compensation margin for such atmospheric fading loss increases with decrease in accessible FSO system bound probability. Therefore, for a highly reliable FSO system link, extra margin must be incorporated to compensate for fading loss caused by scintillation

Analog free-space optical links.

Free-space optics (FSO) communications is a technology that uses modulated infrared optical beams to transmit information line-of-sight through the atmosphere. There has been a substantial increase in the use of FSO technology over the last few years, mainly for "last mile" applications, because FSO links provide the transmission capacity to overcome bandwidth bottlenecks between backbone optical fiber links and metropolitan concentrations of end users. Optical fiber has been traditionally deployed for the transmission of both digital and analog signals. While transmission techniques for analog radio frequency (RF) intensity-modulated signals over optical fibers is well-established, prior to the investigations presented in this dissertation, there is no report of research on the efficiency of FSO for transmission of analog signals in the technical literature. This dissertation research investigated the effectiveness of FSO to transport modulated RF analog signals and compares key performance measures against those of fiber optic links. In addition, a new method to setup temporary IS-95 CDMA microcells or permanent IS-95 CDMA macrocells using FSO was proposed and its viability investigated. Finally, a new transmission technique for transmitting multiple RF signals (channels) over a single FSO link using wavelength division multiplexing (WDM) technology for potential CATV applications was demonstrated

Mitigation of Scintillation Effects in WDM FSO System using Multibeam Technique, Journal of Telecommunications and Information Technology, 2017, nr 2

Free Space Optical communication (FSO) has engrossed a large section of researchers in recent times due to its wide bandwidth, effortless deployment and immune links making it appropriate for communication purposes. This wireless optical technique requires clear and non-turbulent atmospheric conditions for efficient transmission. In this paper, authors aim at reducing the effect of turbulent atmospheric conditions like scintillation effect on FSO. Multibeam technique, which uses spatially diverse transmitters for transmission, has been used for increasing the achievable link distance of the FSO system. Parameters like quality factor and bit error rate have been used to check the received signal quality

The impact of visibility range and atmospheric turbulence on free space optical link performance in South Africa.

Doctoral Degree. University of KwaZulu-Natal, Durban.In the recent years, the development of 5G and Massive Internet of Things (MIoT) technologies are fast increasing regularly. The high demand for a back-up and complimentary link to the existing conventional transmission systems (such as RF technology) especially for the “last-mile” phenomenon has increased significantly. Therefore, this has brought about a persistent requirement for a better and free spectrum availability with a higher data transfer rate and larger bandwidth, such as Free Space Optics (FSO) technology using very high frequency (194 −545 ) transmission system. There is currently unavailable comprehensive information that would enable the design of FSO networks for various regions of South Africa based on the impact of certain weather parameters such as visibility range (mainly in terms of fog and haze) and atmospheric turbulence (in terms of Refractive Index Structure Parameter (RISP)) on FSO link performance. The components of the first part of this work include Visibility Range Distribution (VRD) modeling using suitable probability density function (PDF) models, and prediction of the expected optical attenuation due to scattering and its cumulative distribution and modeling. The VRD modelling performed in this work, proposed various location-based PDF models, and it was suggested that the Generalized Pareto distribution model best suited the distributions of visibility in all the cities. The result of this work showed that the optical attenuation due to scattering within the coastal and near-coastal areas could reach as high as 169 / or more, while in the non-coastal areas it varies between 34 / and 169 /, which suggests significant atmospheric effects on the FSO link, mostly during the winter period. The BER performance analysis was performed and suitable mitigating techniques (such as 4 × 4 MIMO with BPSK and L-PPM schemes) were suggested in this work. The general two-term exponential distribution model provided a good fit to the cumulative distribution of the atmospheric attenuation due to scattering for all the locations. In order to ascertain how atmospheric variables contribute or affect the visibility range, which in turn determines the level of attenuation due to scattering, a time series prediction of visibility using Artificial Neural Network (ANN) technique was investigated, where an average reliability of about 83 % was achieved for all the stations considered. This suggests that climatic parameters highly correlate to visibility when they are all combined together, and this gave significant predictions which will enable FSO officials to develop and maintain a strategic plan for the future years. The modules of the second part of this work encompass the determination of the Atmospheric Turbulence Level (ATL) for each of the locations in terms of RISP (2) and its equivalent scintillation index, and then the estimation of the optical attenuation due to scintillation. The cumulative distributions of the optical attenuation due to scintillation and its modeling were also carried out. This research work has been able to achieve the prediction of the ground turbulence strength (through the US-Army Research Laboratory (US-ARL) Model) in terms of RISP using climatic data. In an attempt to provide a more reliable study into the atmospheric turbulence strength within South Africa, this work explores the characteristic behavior of several meteorological variables and other thermodynamic properties such as inner and outer characteristic scales, Monin-Obhukov length, potential temperature gradient, bulk wind shear and so on. According to the predicted RISP from meteorological variables (such as temperature, relative humidity, pressure, wind speed, water vapour, and altitude), location-based and general attenuation due to scintillation models were developed for South Africa to estimate the optical attenuation. The attenuation due to scintillation results show that the summer and autumn seasons have higher ATL, where January, February and December have the highest mean RISP across all the locations under study. Also, the comparison of the monthly averages of the estimated attenuations revealed that at 850 nm more atmospheric turbulence with specific attenuations between 21.04 / and 24.45 / were observed in the coastal and near-coastal areas than in the non-coastal areas. The study proposes the two-term Sum of Sine distribution model for the cumulative distribution of the optical attenuation based on scintillation, which should be adopted for South Africa. The obtained results in this work for the contributions of scattering and turbulence to the optical link, and the design of the link budget will serve as the major criteria parameters to further compare the outcomes of these results with that of the available terrestrial FSO systems and other conventional transmission systems like RF systems