Design And Implementation Of LDPC Codes In Atmosphere Laser Communication System

Atmosphere Laser Communication refers to the two-way transmission technology of voice, data and image through laser beam in atmosphere. It has good application foreground in solve of"The Last Kilometer Problem"and emergence communication.In the atmosphere laser communication system, laser can be transimitted via atmosphere, because of many factors such as fog, rain, snow and atmosphere onflow, laser signal pulse will be widen, the Signal-to-Noise Ratio(SNR)will be fall and the error codes increases. So, it is very important to investigate the error-correcting codes technology with high speed, which has the character of highly anti-interference.Low-Density Parity-Check (LDPC) codes are linear block codes based on sparse matrix, originally invented in the early 1960's by Gallager. Its rates extremely near to the Shannon limit. It can match the requirement of high speed data transmission and high performance in the future.According to the character of atmosphere channel and high performance of LDPC, this paper select LDPC codes as channel code. Firstly, the basic principle of the FSO (Free Space Optical Communication, FSO) system and the development of LDPC codes are introduced, then it introduce the error-correcting codes, studies the encoding and decoding arithmetic; Secondly, studies the quasi-randomπ-rotation LDPC code, which are suited for hardware implementations. We use Simulink emulation tool to analyze performance under different typical weather conditions in the atmosphere channel based on PPM modulation, the results show that using theπ-rotation LDPC error-correcting codes in atmospheric laser communication systems can get the high performance, the SNR increases averagely 5dB at BER is 10-6. Thirdly, this paper introduce the output model of the avalanche photodiode demodulator. It discusses the performance of LDPC codes in PPM channel of FSO system usig the information of APD, the results show that theπ-rotation LDPC error-correcting codes can improve the system performance remarkably, the number of signal photons can decrease averagely 30 at BER is 10-6. Lastly, this paper investigates how to hardware implementation theπ-rotation LDPC by using Verilog-HDL on FPGA(Filed Programmable Gate Array), The optimization designs of LDPC codes implemented on FPGA with pipeline technology are presented which can improve the speed of the encoding mostly, then all functional moduls are discussed in detail, and simulation is finished with ModelSim-SE.