Research On Coded Multilevel Modulation Technology Based On LDPC Codes For Free Space Optical Communication System

Free space optical(FSO)communication,a kind of optical communication technology,refers to the use of modulated laser beam through free space transmission of optical wireless communication technology,also known as wireless optical communication.The advantages of FSO system are large communication capacity,high bandwidth,high-speed transmission,convenient and fast deployment,high security,low power consumption,low cost and so on.Nowadays,with the scarcity of spectrum resources,the communication of optical frequency band has great potential.As an effective complement to the wireless communication network,the FSO system can be used to solve the "last mile" problem of Internet,which is an important part of the next generation ultra high speed wireless communication network.FSO communication mainly uses the atmosphere as transmission medium to transmit information,the performance of the FSO system is mainly affected by atmospheric turbulence.How to reduce the effect of atmospheric turbulence on the FSO system and improve the performance of the system is one of the key research targets in the field of FSO.In recent years,researchers have proposed a hybrid modulation method,which can improve the performance of the system to a great extent.The combination of traditional coding technology and hybrid modulation technology can effectively improve the bandwidth efficiency and BER performance of FSO system.The application of relay cooperative transmission technology in FSO system can further suppress the adverse effects of atmospheric turbulence and improve the performance of FSO system.In this paper,we propose a PPM-BPSK-LDPC coded multilevel modulation scheme based on low density parity check(LDPC),pulse position modulation(PPM)and binary phase shift keying(BPSK)modulation technology.According to the proposed PPM-BPSK-LDPC coded multilevel modulation,the transmitter and receiver models of FSO communication system are designed;Under the lognormal distribution atmospheric turbulence channel,we derive the code-decode formula and bandwidth utilization expression of the PPM-BPSK-LDPC coded multilevel modulation;Theoretical analysis and simulations verified the bandwidth utilization and the BER of coded multilevel modulation.The results show that compared with traditional LDPC coding and PPM modulation,PPM-BPSK-LDPC coded multilevel modulation has higher bandwidth utilization and lower BER.Therefore,the PPM-BPSK-LDPC coded multilevel modulation method has a certain application space in the FSO system.Due to the influence of atmospheric turbulence,the single hop FSO link can not meet the BER requirements of FSO system when the transmitter and receiver are far away from each other.In order to further improve the performance of the system,this paper proposes a relay cooperative PPM-BPSK-LDPC coded multilevel modulation scheme.This method is based on the PPM-BPSK-LDPC coded multilevel modulation,using the relay nodes for cooperative communication.Through the model of three point relay cooperative system,the BER of FSO system is simulated and analyzed in Gamma-Gamma atmospheric turbulence channel.The simulation results show that the relay cooperative PPM-BPSK-LDPC coded multilevel modulation scheme has better BER performance than the PPM-BPSK-LDPC coded multilevel modulation scheme without relay cooperation.This method combines the advantages of the relay cooperation and the PPM-BPSK-LDPC coded multilevel modulation,which can be applied to the FSO communication system with far distance between transmitter and receiver.The relay cooperative PPM-BPSK-LDPC coded multilevel modulation scheme can improve the performance of the system effectively.