Study On System Structure In Free Space Optics Communication

Free Space Optics (FSO) Communications, also called Optical Wireless, refers to the transmission of infrared laser beams modulated through an atmosphere. FSO is a line-of-sight technology that has the advantages both wireless communication and optical communication.This dissertation, consisting of four chapters, has analyzed the relevant content of FSO in details, from the optimization designs to the performance parameters.Chapter one at first gives an overview of FSO about the principles of operations, applications, advantages, and disadvantages as well as a brief analysis of its market prospect. This paper has introduced in detail the current situation of development of this technology in the leading developed countries and in China. Then, the systematic structure of FSO will be presented. At the end, a comparison between FSO and microwave communication technology will also be presented.Chapter two has discussed the atmosphere channel issue that will be met with when FSO is used on the earth's surface. In this paper, theoretical analysis is taken through attenuation channel and turbulence channel. Bit error rate, link range, wavelength selection, visibility and availability about FSO are also discussed. Based on the mathematical mode of channel, this paper analyzes the effects of atmospheric attenuation and detector sensitivity on the link range. The results show that higher power transmitted is unadvisable. The analysis in this paper is aimed to be helpful for system designs.In chapter three, at first, it has introduced the system of transmission. This paper analyzes the effects of beam divergence, and power transmission on the link range. The results show that it is effective to extend the range by narrower beam divergence and higher detector sensitivity. Then it has discussed system of acceptance, including coherent detection and non-coherent detection. Afterwards, it has discussed the functions, working processes of PAT(Pointing, Acquisition and Tracking) subsystem, the proposed system structure .ind design criterion suitable for the PAT. In latter part of the chapter, theoretical analysis and comparison is taken through three channel coding models: interleaving convolution coding, cascaded BCH coding and Turbo coding.In chapter four, concepts of wireless LAN based on the technology of FSO are presented. By analyzing the system's structure and key parts of LAN, the paper illustrates in detail MAC (Media-Access Control) protocols that are fundamental for the system's operations. Finally, the paper presents a crucial technology on WDM (Wavelength-division multiplexing).