Wavefront distortion of data-carrying laser beam propagating through the atmosphere has been reported to have detrimental effects on the performance of Free-Space Optical (FSO) communication systems. Optical intensity fluctuation models generally assume clear air optical turbulence where atmospheric aerosols warming effects are neglected. This variation of the refractive index structure parameter (Cn2) of the atmosphere due to the aerosol induced warming and its influence on Bit Error Rate (BER) performance of FSO systems are studied in this paper using high-resolution radiosonde and multi-satellite observations of aerosols and atmospheric thermodynamics. Based on an approximate mathematical expression built on Gauss-Laguerre quadrature rule, and a radiative transfer model-based analysis, the BER of a Differential Phase Shift Keying (DPSK) FSO communication link through Exponentiated Weibull modelled turbulence with aperture averaging has been investigated. Our results show significant signal deterioration with the aerosol-induced turbulence taking a toll on the signal to noise ratio (SNR) over more than 15 dB. BER analysis under different receiver aperture dimensions is performed with the selected intensity fluctuation model. We show that aperture averaging does not have significant influence on the performance enhancement under aerosol perturbed atmospheric conditions.
Accurate characterization of atmospheric refractive index fluctuations (optical turbulence) is important in applications such as Free-Space Optical (FSO) communication and laser remote sensing, where atmospheric optical propagation is involved. In this paper, we present the statistics of near-surface optical turbulence derived from three-year sonic anemometer-thermometer observations at a semi-arid, flat terrain in peninsular India. Using concurrent and collocated measurements of meteorological fields and atmospheric aerosols (both scattering and absorbing type), the role of atmospheric boundary layer dynamics and aerosols in modulating the magnitude, evolution, and temporal variations (over diurnal and seasonal scales) of refractive index structure parameter (Cn2) are discussed. Absorption of solar radiation and the resulting atmospheric heating by aerosol particles will modify the land-atmosphere temperature gradient that regulates the near-surface Cn2. We discuss such a scenario using aerosol black carbon measurements close to the surface of the Earth and highlight the crucial role of ABL dynamics in controlling the influence of such aerosol radiative heating effects on Cn2. These results will be helpful in improving the weather model simulations of optical turbulence over semi-arid regions.
Free space optical (FSO) communication is a line of sight technology capable of carrying large volume of data using laser signals through the atmosphere. This unguided propagation of laser beams through the atmosphere confronts with turbulent fluctuations and suspended aerosol particles on its en route to the receiver. Random fluctuations in the atmospheric refractive index causes variations in the propagation constant and thereby affects the optical pulse propagation. We examine the local atmospheric warming effects of absorbing aerosols on the atmospheric refractive index fluctuation statistics and its influence on the group velocity dispersion (GVD) parameter. Black Carbon (BC) aerosols increase local temperature through solar absorption, which will be amplified when they reside in the upper atmosphere for longer duration, owing to the reduced atmospheric density prevailing at higher altitudes. To elucidate the implications of elevated BC layer heating on FSO links, vertical BC mass concentration was measured using an Aethalometer (Model AE-42, of Magee Scientific, USA) mounted on a hydrogen filled balloon. Long term analysis of multi-satellite observations along with in-situ measurements of aerosol parameters show dependence of GVD on aerosol induced local atmospheric warming. Effect of warming on outage probability of FSO systems employing chirped Gaussian pulses are also presented.
Performance of terrestrial and vertical Free-Space Optical (FSO) communication systems are strongly influenced by the atmospheric boundary layer (ABL) dynamics. In addition to the diurnal variations in the refractive index structure parameter (Cn2 ) caused by the wind speed and the temperature difference between the surface and the near-surface air mass, any other unprecedented temperature variations can lead to Cn2 variations. Even though the prevailing ‘simple’ Cn2 models capture the overall trend in vertical Cn2 variations, they fail to capture the effects of temperature inversions, especially in tropical regions. Influence of absorbing aerosols like Black Carbon (BC), which can improve the atmospheric stability by forming strong temperature inversion layers, are not considered in these models. BC can reduce the optical beam intensity by scattering and absorption and can also cause variations in refractive index by modifying the local temperature. The uncertainties in the implications of BC will be large, owing to their large spatio-temporal and vertical variations. Using high-resolution balloon measurements and multi-satellite observations coupled with a radiative transfer model, we substantiate the strong influence of absorbing aerosols on the vertical distribution of Cn2 . We report how vertical profiling of absorbing aerosols can be used to estimate altitudes with low refractive index fluctuations. The manifestations of high-altitude aerosol-induced atmospheric warming in FSO systems are also pointed out. We conclude by discussing how mass concentration of BC, a good tracer for ABL dynamics, is correlated with the near-surface refractive index fluctuations.
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