Investigation On Physical Layer Security In Cognitive Two-Way Relaying Systems

With the rapid development of the information society,it has brought higher requirements to wireless communication technologies.Thus,how to provide high speed and secure data transmission has been becoming a hot research topic in wireless communications.Cognitive relaying combining cognitive radio and cooperative relaying technologies together is now becoming an effective solution to address the scarcity of radio spectrum resources.However,due to the open nature of wireless channels,eavesdropping threats are faced in cognitive relay systems.Physical layer security is one of the most important wireless communication security technologies which takes the advantages of randomness nature of wireless channels to achieve communication confidentiality based on information security theory.This dissertation focuses mainly on the security cooperation between primary users and secondary users in cognitive two-way relaying systems.The main work and contributions are given as follows.(1)The intercept probability and the secrecy-sum-rate of the two-way relaying wire-tap channel model based on the amplify and forward,decode and forward and denoise and forward protocols are analyzed and compared.The simulation results show that the amplify and forward,and the denoise and forward protocols can significantly improve system secrecy-sum-rate,and reduce intercept probability compared to the decode and forward protocol.As the transmit power increases,the security performance of the amplify and forward protocol and the denoise and forward protocol increases significantly.Compared to the amplify and forward protocol,the denoise and forward protocol can result in better security performance when the eavesdropping channel is good,the main channel is bad or the transmit power is low.(2)Security cooperation technologies between primary and secondary users in the cognitive two-way relaying system are studied.First,a relay selection scheme maximizing the secrecy capacity is investigated.For the case where the eavesdropping channel state information is unknown,another relay selection scheme is proposed to maximize the system sum-rate.It can be proved that the above two relay selection schemes are equivalent.Based on the framework of the proposed relay selection schemes,a security cooperation protocol between primary and secondary users is developed and analyzed in terms of the achievement of the secondary users with respective to the number of secondary users attending the cooperation.The simulation results verify the correctness of the theoretical analysis,and show that the proposed relay selection schemes can improve the security performance of the primary users significantly.With increasing the number of secondary users to be selected,the security performance of the primary users increases as well,and the achievement of the secondary users also increases.However,the gain of the primary uses' security performance per secondary user decreases.Taking the costs of the secondary users into consideration,the achievement of secondary users attending the cooperation increases first and then decreases with increasing the number of secondary users.The optimal number of secondary users is related to the weight of the unselected secondary users.(3)Under an assumption that secondary users are all equipped with buffers and could act as relay nodes,security cooperation technologies between primary and secondary users is further studied.With the objective of maximizing the secrecy rate of primary users,an adaptive link selection strategy is proposed.The simulation results verify the correctness of the theoretical analysis,and show that the proposed adaptive link selection strategy can significantly improve the security performance of the primary users compared to the conventional relaying system without buffers.When the eavesdropping channel becomes better,the proposed strategy performs better.When increasing the buffer size,the security performance of the primary users increases as well,and meanwhile the secondary users can be rewarded more achievements.When the buffer size reaches a certain size,the security performance cannot increase further.(4)Considering an untrusted secondary user acting as a two-way relay,physical layer security technologies of the cognitive two-way relaying system are investigated.First,secure cooperation regions where the primary users' messages can be kept secret from the secondary users are analyzed.Then,four power allocation schemes are: 1)power allocation maximizing the system secrecy-sum-rate subject to a peak-power constraint;2)power allocation maximizing the system secrecy-sum-rate subject to a sum-power constraint;3)power allocation maximizing the minimal secrecy-rate subject to a peak-power constraint;4)power allocation maximizing the minimal secrecy rate subject to sum-power constraint.The simulation results show that the proposed power allocation schemes can effectively improve the security performance of the primary users and always keep the primary users' messages secret from the untusted secondary users.The secrecy sum-rate maximization power allocation schemes can always provide the maximum secrecy sum-rate,while the secrecy rate of the primary user with a weak channel is suppressed when the channel is asymmetric.The minimal secrecy-rate maximization power allocation schemes can release the negative influence on the secrecy rate of the primary user arising from the above case.