The Design And Research On Modulation System In Fso Communication

The Free Space Optical Communication (FSO) is a kind of newly arisen wireless communication system. In this system, information was modulated on laser pulse transmitted in THz spectrum. It has many advantages such as great capacity, cheap cost, high transmitting rate and safe confidentiality. PPM (Pulse Position Modulation), as one of key technologies in Free Space Optical Communication, arouses more and more attention by the virtue of high power efficiency and strong resistance to interference. Based on studying several kinds of laser signal modulation principle, according to its signal characters, and discussed some related problems, it designs a PPM modulation system.This thesis studies the following aspects:1. Described the development of Free Space Optical Communication system and the composition of the system, discussed the development prospects of FSO.2. Base on introducing various optical signal modulation methods, discussed the application prospects of interior and exterior modulation, especially, the reasons why in wireless laser communications, PPM modulation will be widely used.3. According to PPM signal characters, the mathematics model of PPM signal is established and the signal is analyzed in detail by MATLAB emulator. Compared with OOK, the power efficiency, bandwidth efficiency, transmission capacity of PPM signal is discussed.4. Based on analyzing the working principle and working flow of PPM system, a model of PPM modulation system is put forward. Based on DSP technology, TMS320LF2407A is selected as DSP processor which is one product of TI Co., and its software language uses assembler and C to realize digital signals'PPM modulation and serial communication between DSP processor and main computer.It emphasizes on the modulation principle of PPM, transformation principle of PWM to PPM, generation methods of PWM and PPM signals in this thesis. The results of experiments indicate that the performance of this system is stabile greatly and the information can be modulated correctly using the PPM system for Free Space Optical communication.