Study On Mechanism And Mitigation Technology Of Atmospheric Effects In Optical Wireless Communications

With the rapid development of the information technology, conventional radio frequency (RF) systems can not meet the increasing demand for the transmission rate. Recently, the optical wirelesss communication (OWC) becomes a research focus in the field of wirless communication. However, the atmosphere has a series of effects on the propagaton of a laser signal and can degrade the performance of OWC systems. The atmospheric effects have become an obstacle that limits the wide application of OWC technology.Focusing on the OWC application, this dissertation investigated the mechanism of atmospheric effects on the laser propagation and its mitigation technology. The dissertation consists of four important parts. The main contents are as follows:Firstly, the mechanism of atmospheric effects on the laser propagation in OWC was theoretically studied. One, based on the theory of atmospheric absorption and scattering, the problem of power attenuation caused by atmospheric extinction was researched, and the attenuation characteristics of the atmospheric channel with various visibility distances were caculated and analyzed. Two, based on the atmospheric multi-scattering model, the formula of the diffuse intensity and the reduced intensity at the receiver side were derived using radiative transfer theory. Three, the time-domain responses of a laser pulse through atmospheric channels with various optical thicknesses were computed using Monte Carlo ray tracing method, and then the transmission bandwidth model was proposed. Four, the turbulence-induced effects on the the laser propagation were systematically researched, and five kinds of turbulence effects and their mathematical models were discussed. Then, simulation results of turbulence effects under various conditions were analyzed.Secondly, a field experiment system of laser propagation was designed, and the experiments over three different paths were conducted. Based on the data collected during the experiments, the intensity fluctuation, the angle-of-arrival fluctuation and the spot size on the focus plane were analyzed. The results show that the large aperture receiving and the multi-beam transmission can reduce the depth, probability and frequency of the received intensity degradation.Thirdly, the power model of the laser propagation through the atmosphere was established. And, the fiber coupling efficiency under various conditions was computed. This model can give a support for the power budget of OWC systems. In addition, the detection models of avalanche photodiodes (APD) with different approximation conditions were given, which layed a good foundation for analyzing the performance of OWC systems.Fourthly, the solutions to the three kinds of problems in OWC, caused by atmospheric effects on a detected signal, were studied in detail. One, focusing on the intersymbol interference caused by the time-domain extending of a laser pulse, the equalization filering and the wavelet-modualtion-based transimission-rate diversity were researched and the system performance was evaluated and analyzed by simulation. Two, the adaptive optimal thresholding method was investigated to overcome the turbulence-induced fluctuations of the detected signal; the system architecheture and the adaptive prediction process were described in detail; a comparision of the adaptive thresholding performance between the least-mean-square method and Kalman method was presented based on the simulation results. Three, the performance analysis of a link with a partially coherent beam was presented, and then a novel technology, called the optimal initial coherence technology of a laser source, for reducing the turbulence-induced effects was proposed; the optimization criterion and the system architecheture were also discussed in detail. Four, considering the receiving diversity scheme, the formula of the equivalent aperture averaging factor for a multi-subaperture receiver was derived; it shows by analysis that with the same receiving area, the aperture averaging performance of a multi-subaperture receiver is better than a single-large-aperture receiver; then, the bit-error-rate models for both the maximum-likelihood combining and the equal-gain combining were suggested, and a performance comparision between the two combining scheme was also presented.The results obtained in this dissertation are very benefical and valuable for the design of atmospheric OWC systems.