Performance Analysis In Cooperative Downlink NOMA System

As wireless communication technologies evolved,a large number of mobile terminals are connected to the network,and the frequency resources are increasingly limited.The traditional orthogonal multiple access(OMA)has been unable to meet the needs of people.Nonorthogonal multiple access(NOMA)allows one frequency channel to be allocated to multiple users in the same cell at the same time,improves spectral efficiency,and has become a key technology of the fifth generation(5G)communication system.In addition,due to the broadcast characteristics of wireless communication,information is easy to be eavesdropped by eavesdropper in the transmission process,so the physical layer security technology is an effective solution to secure the reliable transmission of NOMA users.This paper mainly studies the security performance of cooperative NOMA system,and the main work is summarized as follows:Consider a downlink NOMA system,including one base station,one eavesdropper and two users(U1 and U2).U2 has poor channel gain due to the fact that it is far away from the base station.Therefore,U1 is used as a relay to improve its reception reliability.Then,the closed expressions of the secrecy outage probability for two users are derived.Simulation results are provided to corroborate the correctness of the theoretical analysis.Consider a downlink NOMA system,including one base station,one trustable relay,one eavesdropper and two users.All nodes in the system model are equipped with a single antenna.A two-time slot cooperative jamming scheme is proposed to ensure communication security by using users to send jamming signals to the eavesdropper.Moreover,since the eavesdropper is only interested in the information of the weak user,this paper allocates smaller power to the weak user to so that the private signal is not easy to be eavesdropped,which is different from the traditional NOMA scheme.Then,the outage probability and secrecy outage probability of the two users are derived respectively.The simulation results prove that the scheme proposed in this paper is useful.Consider a downlink secure NOMA scenario,the communication between a base station and two users is assisted by a trustable relay.The relay equipped with multiple antennas,and all the other nodes are equipped with a single antenna.A two-slot jamming scheme is proposed to improve system security.In the first time slot,the user sends the jamming signal to the eavesdropper to ensure communication security.In the second time slot,the base station sends the jamming signal to deteriorate the eavesdropping.Moreover,the channel gain between the two users is increased by designing the beamforming vector at the relay to facilitate the execution of NOMA.The simulation results prove the correctness of the secrecy outage probability derived in this paper and the effectiveness of the proposed scheme.