Impact of Atmospheric Aerosols on Free-Space Optical Communication Systems
Free–space optical (FSO) communication is a wireless technology with an unguided laser signal carrying a large volume of data through the atmosphere. Numerous advantages such as higher bandwidth, high security, easy deployment, compatibility with fibre communication systems, etc. make it an attractive solution for high data demanding communication networks. Despite possessing these advantages, atmospheric channel effects such as molecular and particle, absorption, and scattering as well as atmospheric turbulence, are potential impediments to its usefulness. Random fluctuations in the optical signal's intensity, better called as scintillation, due to the stochastic variations in the atmosphere's refractive index, degrade the error performance and set limits on the reliable information rate, and accelerates outage of FSO communication systems. In addition to this, atmospheric aerosols induced regional warming can alter the atmosphere's refractive index distribution, whereby influence the optical signal propagation. This thesis investigates the performance analysis of FSO systems under the influence of atmospheric aerosols induced warming and optical pulse broadening due to enhanced scintillation caused aerosol radiative effects. Bit error rate (BER), channel capacity, and outage probability of FSO systems have been studied under realistic atmospheric conditions while beam misalignment between transmitter and receiver taking into consideration. Closed-form expressions were derived for the analytical part and a discrete ordinate radiative transfer model employed for aerosol radiative forcing calculations. Multi-satellite, multi-wavelength observations, along with GPS radiosonde and ground-based dedicated instrument-based measurements, have been used for the study at various geographic locations in the Indian subcontinent. Closed-form analytic expression for the BER of a Binary Phase Shift Keying (BPSK) FSO communication system under the influence of enhanced optical scintillation due to the aerosol induced warming and beam misalignment are studied. Results show that improved beam accuracy would not result in a corresponding improvement in such systems' error performance while the perturbed aerosol conditions are incorporated into the analysis. In addition to this, the seasonal variations observed under natural turbulence conditions disappear during aerosol conditions. Thus, this study suggests that system design for optimized parameters must consider local refractive index fluctuations due to atmospheric aerosols. Group velocity dispersion is known to produce pulse broadening due to the spectral dependence of velocity fluctuations. Atmospheric propagation of optical pulses in the femtosecond regimes undergoes additional broadening (greater than 10 for pulses < 60 fs wide) due to the enhanced optical turbulence induced by aerosol induced local atmospheric warming. These results show that the reliably achievable data rate in FSO communication systems is restricted further by the atmospheric aerosols. Moreover, spectral redistribution of energy in the optical pulse due to the aerosol radiative effect shows that the abundance of absorbing aerosols in the atmosphere is characterized by lower values of single scattering albedo (SSA) a considerable reduction in the energy of the received pulses. This variation is vital for urban locations where absorbing aerosols like soot, concentrations are large. These results show that random variations in the atmospheric refractive index, enhanced by aerosol-induced local warming, manifest as a dispersive channel and disproportionately affect the transmitted pulse's different spectral components, leading to pulse broadening and deterioration. Considerable uncertainty in the spatial and temporal distribution of atmospheric aerosols influences the channel capacity of DPSK systems in urban areas. Direct absorption of light due to black carbon (BC) aerosols in the atmosphere and its influence on the channel capacity of Differential Phase Shift Keying (DPSK) systems have been investigated further in an urban location, Chennai on the east coast of southern India. Aerosol absorption drastically reduces the channel capacity, and improved performance due to better beam accuracy could be disrupted by the absorbing aerosols such as BC in the atmosphere. Moreover, atmospheric aerosols' radiative properties can reduce the channel capacity by introducing perturbations in the laser signal's intensity fluctuations. Since the direct radiative forcing due to aerosol BC crucially depends on BC's vertical distribution, the outage probability of FSO communication under perturbed aerosol conditions has been estimated further for vertical and high-altitude horizontal links. Results show significant variation when aerosol induced warming is taken into consideration for vertical links. Simultaneously, elevated BC layers frame a conducive environment for elevated horizontal links, which is attributed to the increased atmospheric stability due to large warming produced by a strong BC layer. Thus, high data rate systems such as atmospheric profiling using aerial vehicles and drones could be improved by deploying them into existing elevated aerosol layers.