The Research On Physical Layer Security Of FSO/CDMA System Based On Discrete Channel

Free Space Optical(FSO)communication combines the advantages of optical fiber communication and microwave communication and it has become one of the development directions of next-generation optical communication.FSO communication has attracted considerable attention thanks to its velocity,cost-effectiveness,quick deployment and flexibility.However,factors such as atmospheric turbulence,the radius and intensity of beam divergence,and the presence of external eavesdroppers can all affect the safety of FSO communications.All algorithm-based encryption methods have been proven to be creaked.Therefore,it is very important to study the physical layer security of FSO communications.At present,methods for increasing the physical layer security of the optical communication include optical quantum communication,optical chaotic communication,and optical code division multiple access(OCDMA)communication.Among them,the quantum secure communication technology can ensure the absolute security of communication,but is only for lower transmission rate;the security of chaotic optical communication system is limited to experiments under theoretical level and ideal conditions;OCDMA technology is a new communication method that combines code division multiple access communication technology with optical fiber communication technology,and is a preferred method for solving high-speed data transmission security problems.Therefore,this paper proposes free space optical code division multiple access(FSO/Code Division Multiple Access,CDMA)wiretap channel model based on discrete channel,and quantitatively analyzes the physical layer security of FSO/CDMA wiretap channel from the perspective of information theory.This project comes from the National Natural Science Foundation of China,“Research on Physical Layer Security and Optical Coding for Free Space Optical Communication”(NSFC61671306),and JCYJ “Research on Key Technologies of Optical Communication Based on Optical Encoding Physical Layer Security”(JCYJ20160328145357990).The main contents and research work of this paper are as follows:1)FSO/CDMA wiretap channel model based on On Off Keying(OOK)modulation is established in this paper.Legitimate channel and wiretap channel adopt binary symmetric channel model and taking into account the effects of atmospheric turbulence,atmospheric attenuation,shot noise,multiple access interference,thermal noise and background noise on the system.The intercept probability is used as a key metric in evaluating the physical layer security of FSO/CDMA wiretap channel model.Based on information theory security point of view,the closed expression of the interception probability and bit error rate(BER)of the wiretap channel model is derived,and the effects of transmission distance,code length,transmission power and a number of interfering users on the reliability,physical layer security and effectiveness of the wiretap channel model are quantitatively analyzed.The optimal parameters of the FSO/CDMA wiretap channel model based on OOK modulation are obtained when the wiretap channel model satisfies the reliability,security and effectiveness simultaneously.It is shown that reducing the transmission distance can reduce the interception probability of the system,and in order to meet the legitimate user's reliability and security simultaneously,the number of interfering users and code length must be in a certain interval.Furthermore,the maximum spectral efficiency can be obtained by selecting the transmission rate.In addition,the physical layer security of FSO/CDMA system is also verified by experiments.2)Using the pulse position modulation(PPM)technology.The FSO/CDMA wiretap channel model based on M-PPM is proposed in this paper.Legitimate channel and wiretap channel adopt strong symmetric discrete channel model and analyzes the physical layer security,reliability and effectiveness of the wiretap channel model simultaneously.The effects of code length,atmospheric turbulence and the number of interfering signals on safety,reliability and effectiveness are discussed.It is shown that,in order to meet the legitimate user's reliability and security simultaneously,the number of interfering users and code length must be in a certain interval.Given the Bob's BER,the intercept probability and the code word of the system,the optimal parameters of the wiretap channel can be found to achieve the maximum transmission rate.Then this paper also provides in-depth comparison and analysis with the performance of the OOK FSO/CDMA wiretap channel model.The results show that the FSO/CDMA wiretap model based on 4-PPM has better reliability and security by optimizing the parameters of the model.In addition,the paper also uses the optical simulation software---Optisystem to simulate the FSO/CDMA wiretap channel model.