Research On Physical Layer Secure Transmission In Cognitive Radio System

With the sudden increase in the number of handheld mobile devices,the problem of shortage of spectrum resources has become increasingly prominent.Cognitive radio technology is considered to be one of the important methods to effectively solve the current low utilization of spectrum resources.At the same time,the broadcast characteristics of the cognitive radio networks wireless channel and the coexistence of the primary user and the secondary user in the licensed frequency band pose a more severe challenge to the secure communication.The traditional secure transmission of wireless communication mainly relies on high-level encryption algorithms.However,in a distributed networks,since the management and distribution of keys are not easy to implement,and wireless devices with limited computing capabilities are not suitable for using complex encryption algorithms and other shortcomings,the application of traditional encryption technologies in wireless networks has encountered difficulties.As a method of perfect security in the sense of information theory,that physical layer security brings new ideas for the security of cognitive radio networks.The basic idea of physical layer security is to use natural differences between legitimate channels and eavesdropping channels to escort for secure communications.Therefore,it is of great significance to study the physical layer security transmission technology in cognitive radio networks.In cognitive radio networks,spectrum sensing is the primary issue that needs to be addressed.Unlike other wireless communication systems,secondary users in the cognitive radio networks naturally provide relay and interference roles for the primary user's transmission.Since the secondary user uses the licensed band of the primary user,the secondary user is willing to provide cooperation for the secure communication of the primary user as a reward.Therefore,the main research focus of this paper is to use the space resources brought by secondary users to provide secure transmission strategies for the primary users.In general,the main contributions of this paper can be summarized as follows:(1)From the perspective of secure transmission,the primary user randomly changes the state of the information transmission in order to prevent eavesdropping by illegitimate users.When the status of the primary user changes several times during the whole of secondary user's frame time,even the primary user's quality of service,the sensing performance and the throughput of the secondary user's will be detrimental effected too.Aiming at soloving the problems mentioned above,a multivariate perception hypothesis model based on multiple changes of the primary user's state was established,and the influence of the multi-state change of the primary user on secondary user sensing performance was analyzed.Taking the secondary user's quality of service as the optimization goal,an optimization objective function problem is established to maximize the throughput of secondary users.At the same time,in order to guarantee the service quality of the primary user,the interference temperature is introduced as a constraint,and the total transmit power of secondary users is also constrained.The simulation results show that compared with the system model without considering the activity of the primary user,the secondary user multi-power allocation scheme proposed for the multi-status change of the primary user enables the secondary user's throughput to be effectively improved.(2)For cooperative cognitive radio networks systems with multiple secondary users,the issue of enhancing the performance of the primary user's security system by exploiting multi-user gain is studied.Two kinds of multi-user selection strategies are proposed:one is called the minimum interference based selection strategy,and the other is called the maximum interference rate based selection strategy.In the minimum interference secondary user selection strategy,the closed expression of the achievable ergodic secrecy rate lower bound for the primary user is deduced,the relationship between the secrecy rate and the interference temperature is revealed,the impact of the peak power of the secondary user on the primary user security performance is analyzed.The theoretical results show that in the high SNR region,if the interference temperature threshold is fixed,the multiuser security gain increases with the number of secondary users.In the maximum interference rate secondary user selection strategy,the closed expression of the achievable ergodic security rate for the primary user is deduced,and the scaling law of the multi-user security gain is revealed.Simulation results show that the two multi-user selection strategies proposed in this thesis can improve the security performance of the primary user significantly due to multi-user gain.Relatively speaking,from the perspective of secure communication of the primary user,the maximum interference rate selection strategy is a better security transmission strategy.(3)To utilize cooperative relay and cooperative interference to enhance the security of the primary user is further studied in cooperative cognitive radio networks.The secondary user's selfishness characteristics are considered in the system model.In order to encourage the secondary user to provide cooperative jamming and cooperative relay for the secure transmission of the primary user,the energy transmission technology is used to provide the energy reward for the cooperative jamming node,and the licensed band usage rights is provided as incentive for nodes to cooperate relay.The secondary user's service quality is regarded as the optimization goal.The primary user's confidential communication is used as the constraint condition,and the secondary user's maximize transmission rate is achieved through the optimal allocation of transmission time.The experimental results show that under the premise of guaranteeing the secure communication of the primary user,all users can benefit from the system or not lose due to collaboration at least,and the throughput of secondary users is also improved.(4)In order to make full use of the space resources brought by secondary users to protect the secure communication of the primary user,a cognitive radio networks system adopting orthogonal frequency division multiple access is considered.According to the different subcarrier channel quality which could help the primary user can conduct secure communication,the goal of maximizing the security rate of the primary user can be achieved by optimizing the allocation of subcarrier resources and power resources.For both the full channel state information and part of the channel state information,the secondary user transmission rate and the secrecy outage probability are respectively introduced as constraints to maximize the primary user security rate.The optimization problem of joint subcarriers and power allocation is established,and the specific expression of power allocation is obtained.The simulation results confirm that the joint subcarrier and power allocation transmission strategies proposed in this thesis can effectively improve the secrecy rate of the primary user and ensure the transmission rate of secondary users.