Research On Physical Layer Security Of Cooperative NOMA System

Non-orthogonal multiple access(NOMA)technology offers significant advantages in terms of spectral efficiency and user connectivity by enabling the use of non-orthogonal resources.Cooperative NOMA systems can further improve the reliability of information transmission through relay.However,due to the broadcast nature of the wireless channel,information can be exposed to illegal users,and the higher network complexity of cooperative NOMA systems makes them less secure.Physical layer security(PLS)technology does not depend on the upper layer protocols and keys of the wireless network,and can provide secure wireless transmission by exploiting the randomness of wireless communication.Therefore,the physical layer security of cooperative NOMA system has important research significance.In this thesis,we focus on the physical layer security of cooperative NOMA systems with different architectures.First,the physical layer security of cooperative NOMA systems is studied in the scenario where the user acts as a relay and there is an eavesdropper.Due to the high energy consumption of the node acting as a relay,simultaneous wireless information and power transfer(SWIPT)technology is introduced for this node.In addition,in order to improve the physical layer security of the cooperative NOMA system,full duplex technology and cooperative jamming technology are introduced to the node with poor channel conditions,so that the node emits cooperative jamming signals during information transmission to reduce the signal-to-noise ratio of eavesdroppers and improve the system security.In this system,there is a cooperative relationship between the users with better and worse channel conditions,so the interference information is considered to be shared between them.The expression of the user's secrecy interruption probability is derived under the rayleigh fading channel,and the correctness of the theoretical derivation and the superiority of the proposed scheme are verified by simulation.Secondly,the physical layer security of the untrusted relay cooperative NOMA system is studied under rayleigh fading channel.The untrusted relay will attempt to decode information for eavesdropping while forwarding information for the user.In this thesis,a cooperative jamming transmission scheme is proposed for this system to improve the system security by sending a cooperative jamming signal from a user with poor channel conditions,but the signal may reduce the signal-to-noise ratio of the relay eavesdropping while also reducing the signal-to-noise ratio of the user with good channel conditions.Therefore,the interference information between two users is divided into two cases of shared and unshared,and the lower bound expressions for the traversal secrecy rate of users are derived and their asymptotic characteristics are analyzed in these two cases respectively,and finally,it is verified by simulation that the scheme can effectively improve the security of information transmission.