PDF Estimation and Liquid Water Content Based Attenuation Modeling for Fog in Terrestrial FSO Links

Terrestrial Free-space optical communication (FSO) links have yet to achieve a mass market success due to the ever elusive 99.999% availability requirement. The terrestrial FSO links are heavily affected by atmospheric fog. To design systems which can achieve high availability and reliability in the presence of fog, accurate and better models of fog attenuation need to be developed. The current article puts forth appropriate probability density function estimates for received signal strength (hereafter RSS) under fog conditions, where variations in the RSS during foggy events have been statistically characterized. Moreover, from the surface observations of fog density, liquid water content (hereafter LWC) of fog is estimated. The actual measured optical attenuations are then compared with the optical attenuations estimated from LWC. The results presented suggest that fog density measurements carried out are accurate representation of the fog intensity and the attenuation predictions obtained by the LWC estimate match the actual measured optical attenuations. This suggests that the LWC is a useful parameter besides visibility range to predict optical attenuations in the presence of hydrometeors

Experimental study of bit error rate of free space optics communications in laboratory controlled turbulence

This paper reports experimental results for the performance of an free space optical (FSO) communication link employing different modulation schemes under the influence of the atmospheric scintillation. A dedicated experimental atmospheric simulation chamber has been developed where weak and medium turbulence can be generated and its effect on the FSO link is investigated. The experimental data obtained is compared to the theoretical prediction. The paper also shows that the effect on the data transmission performance depends on the position of turbulence source positioned within the chamber

Bit error rate measurement of free space optical communication links under laboratory controlled conditions

This paper outlines the experimental investigation of the fog effect on the bit error rate (BER) performance of the free space optical (FSO) communication link under a controlled laboratory environment. The link transmittance and the received signal Q-factor are measured against different levels of fog densities. The link visibility derived from fog attenuation measurement is used to characterize the fog within the chamber. Moreover, the effect of using different average transmitted optical communication power (Popt) on the transmittance and received Q-factor of the link is also studied for light and dense fog densities

Probabilistic Model for Free-Space Optical Links Under Continental Fog Conditions

The error characteristics of a free-space optical (FSO) channel are significantly different from the fiber based optical links and thus require a deep physical understanding of the propagation channel. In particular different fog conditions greatly influence the optical transmissions and thus a channel model is required to estimate the detrimental fog effects. In this paper we shall present the probabilistic model for radiation fog from the measured data over a 80 m FSO link installed at Graz, Austria. The fog events are classified into thick fog, moderate fog, light fog and general fog based on the international code of visibility range. We applied some probability distribution functions (PDFs) such as Kumaraswamy, Johnson SB and Logistic distribution, to the actual measured optical attenuations. The performance of each distribution is evaluated by Q-Q and P-P plots. It is found that Kumaraswamy distribution is the best fit for general fog, while Logistic distribution is the optimum choice for thick fog. On the other hand, Johnson SB distribution best fits the moderate and light fog related measured attenuation data. The difference in these probabilistic models and the resultant variation in the received signal strength under different fog types needs to be considered in designing an efficient FSO system

Experimental Characterisation and Modelling of Atmospheric Fog and Turbulence in FSO

Free space optical (FSO) communication uses visible or infrared (IR) wavelengths to broadcast high-speed data wirelessly through the atmospheric channel. The performance of FSO communications is mainly dependent on the unpredictable atmospheric channel such as fog, smoke and temperature dependent turbulence. However, as the real outdoor atmosphere (ROA) is time varying and heterogeneous in nature as well as depending on the magnitude and intensity of different weather conditions, carrying out a proper link assessment under specific weather conditions becomes a challenging task. Investigation and modelling the ROA under diverse atmospheric conditions is still a great challenge in FSO communications. Hence a dedicated indoor atmospheric chamber is designed and built to produce controlled atmosphere as necessary to mimic the ROA as closely as possible. The experimental results indicate that the fog attenuation is wavelength dependent for all visibility V ranges, which contradicts the Kim model for V < 0.5 km. The obtained result validates that Kim model needs to be revised for V < 0.5 km in order to correctly predict the wavelength dependent fog attenuation. Also, there are no experimental data and empirical model available for FSO links in diverse smoke conditions, which are common in urban areas. Therefore, a new empirical model is proposed to evaluate the wavelength dependent fog and smoke attenuation by reconsidering the q value as a function of wavelength rather than visibility. The BER performance of an FSO system is theoretically and experimentally evaluated for OOK- NRZ, OOK-RZ and 4-PPM formats for Ethernet line data-rates from light to dense fog conditions. A BER of 10-6 (Q-factor ≈ 4.7) is achieved at dense fog (transmittance, T = 0.33) condition using 4-PPM than OOK-NRZ and OOK-RZ modulation schemes due to its high peak-to-average power ratio albeit at the expense of doubling the bandwidth. The effects of fog on OOK-NRZ, 4-PAM and BPSK are also experimentally investigated. In comparison to 4-PAM and OOK-NRZ signals, the BPSK modulation signalling format is more robust against the effects of fog. Moreover, the effects of using different average transmitted optical communication powers Popton the T and the received Q-factor using the OOK-NRZ modulation scheme are also investigated for light and dense fog conditions. The results show that for an FSO system operating at a Q-factor of 4.7 (for BER = 10-6), the required Q-factor is achieved at T of 48% under the thick fog condition by increasing Popt to 1.07 dBm, whereas the values of T are 55% and ~70% for the transmit power of 0.56 dBm and -0.7 dBm, respectively. The experimental characterisation and investigation of the atmospheric turbulence effect on the Ethernet and Fast-Ethernet FSO link is reported using different modulation schemes. The experiment is carried out in a controlled laboratory environment where turbulence is generated in a dedicated indoor atmospheric chamber. The atmospheric chamber is calibrated to mimic an outdoor turbulence conditions and the measured data are verified against the theoretical predictions. The experiment also demonstrates methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. The results show that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric turbulence. The results also show that the BPSK and OOK-NRZ modulation signalling formats are more robust against the weak atmospheric turbulence conditions than PAM signal

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

Analysis of Link Availability in FSO-OFDM System under Various Climatic Conditions

Free Space Optics (FSO) is an emerging solution for the last mile broad band connectivity where deployment of fiber is expensive. This wireless technology has attracted the researchers due to the ease of erecting along the windows and terrace and the abundant bandwidth available in the unlicensed band. Combining OFDM with FSO gives a hybrid technique -&nbsp;OFDM based FSO or FSO-OFDM in which OFDM symbol modulates the light that passes through free space. Various predictable and unpredictable phenomena cause variation in the light that causes the fading effect of the received signal. Besides the atmospheric effect, the signal power varies within the symbol. The link availability has been calculated by setting threshold power at the receiver. The simulation of the OFDM based FSO system under three climatic conditions namely summer, winter and rainy has been observed that shows the link availability is 99.3% for summer, 98.32% for winter and 98.99% for rainy condition. Under worst climatic condition link availability is 98.17%. Thus FSO- OFDM is able to provide better connectivity even under any atmospheric condition