Research And Optimization Of Wireless Optical Communication Based On Exponentiated Weibull Distribution

Wireless optical communication (WOC), also called free-space optical communication, springs up again as a promising kind of communication technology, with the development of the progress of high power semiconductor laser and high sensitivity optoelectronic detector. It has widely promising application prospects, due to its several advantages, such as large-capacity, wide-band, high-security, easy-to-deploy, effective anti-interference etc. However, the performance of WOC system can be severely reduced by the influences of atmospheric turbulence and pointing errors. Therefore, a proper understanding of how atmospheric turbulence affects the optical signal transmission is quite valuable for us to design effective, sensitive and inexpensive WOC system and network.In this thesis, the joint effects of turbulence-induced fading, also known as scintillation, beam wander and spread, atmospheric attenuation caused by light scatter and absorption, pointing errors induced by transmitter and receiver misalignment and non-Kolmogorov atmospheric turbulence, on WOC system performance are studied. As statistical regularity of atmospheric turbulence-induced scintillation is very important for the study of WOC, people have proposed several statistical models to describe scintillation. The most widely acceptable statistical models of scintillation are the log-normal (LN) and the Gamma-Gamma (GG) distribution models. Experimental studies have indicated that neither of them is adequate to model the irradiance data under all conditions of atmospheric turbulence and aperture averaging. Exponentiated Weibull (EW) distribution, proposed recently, has the best fit for data under all aperture averaging conditions in weak to strong turbulent regions.Many kinds of optimization techniques have been proposed to improve WOC system in performance, selecting an appropriate one is very useful to design a reliable WOC system. In this paper, the theoretical analysis of the performance improvement on WOC system with intensity modulation and direct detection by using block coding and selection diversity combining technique are done, considering in the atmosphere the characteristics of WOC transmission optical signal and a variety of factors on the actual system. The models of average channel capacity of WOC systems are derived based on LN distribution model and EW distribution model. The main results are listed as follows:1) In weak atmospheric turbulent channels, using LN distribution model, considering the joint effects of the spectral index of non-Kolmogorov turbulence, pointing errors, turbulence inner scale and outer scale, the average channel capacity model of WOC system, which is closer to the actual situation, is established.2) We model the average channel capacity of WOC system over weak to strong turbulent channels, using the EW distribution model. The joint effects of the spectral index of non-Kolmogorov turbulence, beam spread and wander, pointing errors, and atmospheric attenuation on system performance are included. ① The conditions and influences of large scale scintillation and small scale scintillation stepping into saturation regimes are different. ② In strong turbulent condition, two scale scintillation successively step into saturation regime, the plot of average capacity with the propagation distance is quite complex due to the effect of scintillation. ③ In weak turbulence region, atmospheric visibility is an important factor affecting WOC in performance. The quality of WOC system is better with higher visibility.④ The effect of the spectral index of non-Kolmogorov turbulence on WOC is stronger in strong turbulent regime, average capacity increases with the increase of the spectral index of non-Kolmogorov turbulence. ⑤Pointing errors has a significant effect on WOC systems under no matter weak or strong turbulent conditions. Using beam tracking technology can effectively improve the performance of WOC systems and eliminate the adverse effect caused by pointing errors. ⑥ In moderate turbulent region, the larger detection aperture is applied, the larger average channel capacity is. However, in weak or strong turbulence regions, the effect of detection aperture on WOC system can be ignored. It is shown that aperture averaging can efficiently improve the performance of WOC in moderate turbulence region.3) Based on EW distribution model, block error rate performance of WOC system in strong turbulent slant channel is modeled, using block coding technique, which at the receiver usually involves adding extra bits of information to correct errors that were made during transmission. The effects of the spectral index of non-Kolmogorov turbulence, beam spread and wander, and pointing errors on coding system performance are studied.4) Selection diversity combining techniques can maximize the quality of the combined signal and improve the immunity to turbulence-induced fading. Based on EW distribution model, outage probability, average BER and average capacity of WOC system using selection diversity combining technique over weak to strong fluctuation atmosphere fading channels are modeled. Results show that selection diversity combining technique leads to a significant improvement on the performance of WOC in moderate turbulent condition, as aperture averaging technique.