Performance Analysis And Experimental Investigation Of Physical-Layer Security In OCDMA-Based Hybrid FSO/fiber Wiretap Channel

Free-space optical(FSO)communications have attracted considerable attention due to the flexibility,cost-effectiveness and high-capacity.They have been considered as an attractive alternative to provide high-speed communication services where fiber infrastructure deployment is impractical or deficient.Moreover,the future optical networks will be hybrid,composed of different fiber and FSO links,in order to support a wide range of services over different network types at a high speed.Therefore,hybrid FSO/fiber networks have been proposed and widely studied recently.However,there are some security risks in hybrid FSO/fiber networks.FSO communications suffer from eavesdropping because of the openness of wireless channels.In addition,the eavesdroppers can get access to a small number of optical signals by bending the optical fiber,which is not easy to be detected by legitimate users.Encryption on the data layer cannot guarantee absolute security.Quantum communication technology can provide absolute security,but only for low rate transmission.The physical-layer encryption is proposed,in order to ensure secure communication.Optical code division multiple access(OCDMA)technology is useful to prevent eavesdropping,thus improving the physical-layer security.In this paper,OCDMA technology is applied to hybrid FSO/fiber network,and the physical-layer security of the system is evaluated theoretically and experimentally.This project was supported by NSFC"Research on physical layer security and optical coding for free space optical communication(61671306)",and JCYJ"Research on key technologies of optical communication based on optical encoding physical layer security(20160328145357990)".The main work of this thesis is as follows:1)OCDMA-based hybrid FSO/fiber wiretap channel is proposed,and the physical-layer security is analyzed by using the conditional secrecy outage probability as the performance metric.When the eavesdropper intercepts information on the FSO link,according to the closed expression of the conditional secrecy outage probability,the effects of secrecy rate,transmission distance,eavesdropping ratio,eavesdropping location and the number of interfering users are analyzed.The results show that as the secrecy rate,transmission distance and eavesdropping ratio increase,the physical-layer security of the system decreases.And the farther Eve is from the transmitter of the FSO link,the more secure the system is.When Eve uses unmatched decoder,the physical-layer security of the system is better,and increasing the number of interfering users can improve the physical-layer security of the system.Assuming the eavesdropper intercepts information on the fiber link and the minimum conditional secrecy outage probability is zero,the safe transmission distance and interception distance of the system are defined and studied.When the transmission distance of the FSO link is fixed,if the transmission distance of the fiber link is less than the safe transmission distance,the system can guarantee absolute security wherever the eavesdropper is located.On the contrary,a monitoring device should be set up within the interception distance,in order to prevent Eve from eavesdropping,thus ensuring the absolute security of the system2)A 10Gb/s experimental system of OCDMA-based hybrid FSO/fiber wiretap channel is built for the first time.A two-dimensional reconfigurable optical encoder and decoder based on Wavelength Selection Switch(WSS)and fiber delay line is used in the experimental system.The reliability of the system is analyzed by measuring the bit error rate(BER)of legitimate user at different receiving power.Then,under the condition of guaranteeing the reliable transmission,the interception distance of the system is obtained by measuring the BER of the eavesdropper at different eavesdropping locations and ratios.The results show that legitimate users can achieve reliable transmission under certain receiving power.At the same time,with the increase of eavesdropping ratio,the interception distance of the system increases.For example,when the eavesdropping ratio is r =0.01,the interception distance is Le =0km.When the eavesdropping ratio is re= 0.02,the interception distance is Le = 5km.