The Research On Physical-layer Security Of OCDAM System Based On Information Theory

Optical fiber communication system has been widely applied in commercial and military communication fields due to its unique advantages such as wide frequency bandwidth,large transmission capacity,low loss and strong anti-interference ability.With the rapid development of science and technology,there are serious security risks in the optical fiber transmission system,e.g.,an eavesdropper can obtain access to a small number of optical signals by bending the optical fiber,which is not easily detectable by legitimate users.As a result,three types of security schemes are developed which are algorithmic cryptography,quantum key distribution,and physical-layer cryptography.Algorithmic cryptography is implemented in Layer 3 or the upper layers.However,its security is not entirely free from constant threats,and all algorithm-based encryption methods have been proved to be decipherable.Due to the limitations of the physical mechanism,the maximum key generation rate at present is around 1 Mb/s.Therefore,Physical layer security encryption can be a moderate scheme to achieve long distance and large capacity transmission,and its security originates from the information theory security with provable security.Optical code division multiple access(OCDMA)is considered as a good candidate to provide physical-layer security.Therefore,based on information theory,the physical layer security performance of the OCDMA system is studied in this thesis.The main work of this thesis is as follows:1)Based on information theory security point of view,the physical layer security performance of the multiuser coherent OCDMA system is analyzed.By establishing the wiretap channel model of a coherent OCDMA system,security capacity,security leakage factor and safe receiving distance are employed to evaluate the physical-layer security.In the case of satisfying the BER of the legitimate user and the security leakage factor of the eavesdropper,the influences of the extraction location,the extraction ratio,the number of active users,and the length of the code on the physical-layer security are investigated.For example,when extraction ratio of the eavesdropper is 0.5%,taking into account the security leakage factor 0.1% and BER ? 10-9,the number of active users must be in the interval of 2 to 4 in order to meet the requirements of security and reliability.2)We propose two schemes to enhance the physical layer security performance of the OCDMA system: One is to improve the OCDMA security performance by power control of interference signals.By establishing a single-user wiretap channel model of coherent OCDMA system with power control,and the influence of power control on the OCDMA Wiretap Channel is studied.For example,when the extraction ratio is 1% and security leakage factor is 0.01% and 0.05% respectively,the safe receiving distances are 20.5km and 48.5km respectively for 1?(28).While for?(28)1.4,safe receiving distances will be increased to 23.9km and 100 km respectively.The other is to construct a quasi-synchronous coherent time-spreading OCDMA wiretap channel based on LA codes,and compare it with Gold codes.The influences of the extraction location,the extraction ratio,the number of active users on the physical-layer security are explored.The numerical results prove that the physical-layer security performance of LA code is always better than that of Gold code.For example,when the extraction ratio of the eavesdropper is 1%,the extraction distance is 5 km and the number of users is 3,the security leakage factor of the LA(156,8,16)code is 0.0083%,while the security leakage factor of 127 Gold code is 0.375%.