Research On Low Complexity Decoding Algorithm For LDPC Codes In Free Space Optical Communication

Free space optical(FSO)communication is a new type of wireless communication.As the last mile solution,it has become a hot area of research in recent years.In this way,information is modulated on laser(carrier)to transmit.FSO communication has a wide range of applications.In particular,FSO communication can be adopted in deep space,in middle orbit,among ground stations which are difficult to realized in conventional methods caused by landform,and in some special situations,between satellites and ground stations.Generally,the laser communication between ground stations is called as “atmospheric laser communication”.FSO system attracts more and more scholars,mainly because of following advantages.FSO technology has a very high transmission rate which is comparable to the optical fiber.In some special conditions,its throughput can reach 56Gb/s.Besides,FSO technology is not limited by the bandwidth.Thus,it takes a great advantage in terms of spectrum resources compared with the RF wireless communication.Although the FSO communication technology has many advantages,there are bottlenecks in the development.In laser communication,atmospheric attenuation and fluctuation of light intensity are the main reasons limiting communication quality and distance.In order to suppress the effect of atmosphere,scholars conducted researches in different aspects.The major technologies are light source technology,adaptive correction,encoding and modulation,etc.This paper is focus on channel coding and decoding.Low density parity check(LDPC)code is a good approximation to the performance of the Shannon limit.It has very low error level.Because of its excellent performance,LDPC code is widely used in the field of wireless communication,and becomes one of the standards in 4G mobile communication.However,in the field of FSO communication,there is a lot of research space.Theoretical knowledge of atmospheric laser communication is introduced in this paper.After introducing several basic channel models and analyzing different types of noise,the atmospheric channel model is established.Synthesizing the interference caused by various atmospheric turbulence channel noise to the communication link,the Gamma-Gamma distribution which is consistent with the measured data is used as the channel model.In the third chapter,we introduce the structure of LDPC code and the construction of parity check matrix.Then,in the fourth chapter,several iterative decoding algorithms of LDPC code are introduced,including the probability domain BP algorithm,LLR BP algorithm,Min Sum algorithm,normalized BP algorithm and iterative APP algorithm etc.Afterwards,an improved algorithm based on normalized BP algorithm is proposed.Complexity and performance of different decoding algorithms are compared and simulated in various turbulence channels.After comparing the simulation results of these decoding algorithms on the Gamma-Gamma channel model,we could analysis the improvement of the proposed algorithm to Min Sum algorithm and LLR BP algorithm.Finally,we discussed how the effects of the number of iterations and the length of code exerted on the decoding performance.