Effects of aerosols and atmospheric boundary layer dynamics on refractive index fluctuations: implications for Free-Space Optical communication

Abstract

Laser communication through atmospheric channels is an emerging wireless technology, commonly known as Free-Space Optical (FSO) communication. It facilitates unprecedented channel capacity and very large bandwidth, favouring huge-volume data transfer across spatially separated locations. The performance of FSO links depend largely on the atmospheric channel state. There are two main processes leading to signal degradation in FSO links: attenuation due to scattering and absorption, and intensity fluctuations due to scintillation and beam wander (represented using the refractive index structure parameter C_n^2). While the attenuation effects can be quantified and compensated, it is difficult to quantify C_n^2, owing to the inherent randomness of atmospheric turbulence. Due to its large spatio-temporal and vertical variations, and wide range of physical, optical, and chemical properties, aerosols, the tiny solid/liquid particles suspended in the air, can introduce considerable inhomogeneity in the atmosphere. Even though the effects of aerosol scattering and absorption on laser propagation and FSO communication have been reported previously, studies on the radiative effects of aerosols and the resulting temperature (and refractive index) fluctuations are nonexistent. This thesis, reports for the first time, the heating effects of aerosols such as Black Carbon (BC) on C_n^2 and their implications for FSO communication, using dedicated in-situ observations, multi-satellite data analysis, balloon observations, and radiative transfer modelling. Absorption of solar radiation by aerosols will heat the atmosphere and induce perturbations in C_n^2, the strength of which depend on the vertical distribution and residence time of aerosols, besides their absorption properties. Heating of higher levels of the atmosphere by absorption of solar radiation by elevated BC (EBC) layers suppresses C_n^2. This acts as a positive feedback and sustains the reduced C_n^2 at EBC layer. Though the higher BC concentration within the EBC layer leads to higher signal attenuation due to absorption, this effect is alleviated by the large suppression of C_n^2 and the consequent reduction in beam wander. This makes EBC layers conducive for implementing high-performance, cost-effective aerial FSO communication links. Hence, BC aerosols, usually looked upon as a bane in FSO communication, can be a boon in aerial FSO communication. Atmospheric Boundary Layer (ABL) dynamics regulates the temporal variations in C_n^2. A comprehensive characterization of ABL, simultaneous with aerosol measurements, has been carried out at a semi-arid region in peninsular India, to find out the role of ABL dynamics (and the resulting impacts on aerosols) on FSO communication. The daytime temporal variations in mixing layer height and the impact of those on the concentration and vertical distribution of aerosols are modelled using location and season specific data. During daytime, temperature fluctuations controls C_n^2, whereas during nighttime, it is the wind shear induced fluctuations that control C_n^2. Inverse relationship between BC and C_n^2 has been observed, which is strongest close to the local noon and subdued under high wind speeds. The immediate response of C_n^2 to BC aerosols depends on the BC concentration, ABL dynamics, and time of the day. The merits of reduced C_n^2 during nighttime are vitiated by the high aerosol extinction, due to which the nocturnal signal-to-noise ratio will not be as high as expected. The fractional contribution of aerosols could be as high as 25% during the convectively stable atmospheric conditions and can impart more than 2 dB attenuation to FSO links, with distinct diurnal variations imparted by the ABL dynamics. Finally, the effects of aerosols and ABL dynamics on the performance of vertical FSO communication links are characterized in terms of data rate, coherence length, and scintillation index. Scintillation index for up and down FSO links exhibit strong seasonality, with higher values during pre-monsoon summer when higher surface layer optical turbulence and stronger convective activities persist. The temporal variations in the data rate of vertical FSO links, thus estimated, show reduction as high as 7 Gb s-1 due to aerosol-induced perturbations. The results from this thesis highlight the immediate requirement of incorporating the effects of aerosols in C_n^2 models, and while estimating the FSO communication link budget.