Study On Codeword Interception Probability And Security Performance Enhancement In FSO-CDMA System

FSO-CDMA system combines the widely used CDMA technology in mobile communication and optical communication,which can not only take advantage of CDMA technology anti-interference,anti-fading and simple protocol,but also utilize the rich bandwidth resources of optical communication.The application prospect has become one of the research hotspots in the field of space optical communication.Before the universal application of free-space optical communication technology,we can not ignore the fact that lasers are easily intercepted and exposed to information leakage during bare atmospheric channel transmission.Many studies have shown that freespace optical communication has the possibility of eavesdropping.The optical signal can be separated by a lens to realize eavesdropping within the beam range;outside the beam range,information leakage may also occur due to atmospheric scattering.OCDMA technology itself has certain security,but in the face of some highly targeted and powerful eavesdropping methods such as energy detection,brute force search,codeword interception,and differential detection,these basic security guarantees are far away not enough.This paper focuses on the analysis of FSO-CDMA system security and studies its security performance enhancement methods.This project comes from: National Natural Science Foundation of China,“Research on Physical Layer Security and Optical Coding Method of Free Space Optical Communication”(NSFC 61671306),Shenzhen Discipline Layout Project “Research on Key Technologies of Optical Communication Based on Optical Encoding Physical Layer Security”(JCYJ20160328145357990),the main research work is as follows:Firstly,theoretically deducing the eavesdropper's probability and expected time formula for FSO-CDMA system succession through codeword interception and eavesdropping,and using it as an indicator to measure the security performance of the system,quantitative analysis of the impact of eavesdropping signal-to-noise ratio,codeword,system user number,asynchronous/synchronous,modulation mode and channel state on the security performance of FSO-CDMA communication system.The eavesdropper is far away from the transmitting end,the extraction ratio is small,the codeword is complex,and the number of users is large.The PPM modulation is adopted,and when the turbulence intensity is large,the system has good security performance.Secondly,the FSO-CDMA system experimental platform is built to simulate the process of the eavesdropper receiving the signal cracking information.By observing the optical signal encoded waveform and comparing the legal user matching decoding,the eavesdropper non-matching decoding signal eye diagram and error code.The rate is found to be very fragile in the OOK-based FSO-CDMA system used in practice.Especially in the case of a single user,the codeword is easily cracked,and when the two-dimensional address code is used,the system security performance is better than the case of using the one-dimensional address code.Then,for the FSO-CDMA security,when the eavesdropper is relatively close to the eavesdropping signal and noise,the system has a large security risk and the security rate is limited.A cross-layer security design based on OCDMA and data encryption is proposed.The solution further enhances the security of the communication system to meet the high security and high rate business requirements.And based on the previous analysis method,a safety assessment is made for the cross-layer security system.Finally,two problems brought about by data encryption: key leakage and overhead increase,and propose corresponding solutions.Channel openness may cause the key to leak in the transmission,and the reciprocity of the atmospheric channel combined with the physical layer transmission technology can be used to implement a secure communication scheme without key sharing.The Optisystem simulation software is used to verify the scheme.The sender and the receiver do not need to transmit the key.They only need to transmit the same pilot sequence.After the channel is transmitted,both parties can receive the feature quantity and the information can be obtained.The same key,and the eavesdropper is inconsistent with the main channel because the eavesdropping channel state is inconsistent,and it is impossible to restore the same key.In addition,an adaptive security strategy is adopted to adjust the parameters of the transmitting end in real time according to the channel state,so as to ensure that each location of the link conforms to the security model to ensure the dynamic security of the entire system.