Research On Multi-user Cooperative NOMA Physical Layer Security

With the rapid development of wireless communication technology,the future mobile network will carry a large number of user devices,and the proliferation of user data brings a severe test on the limited spectrum resources.To effectively solve this problem,NonOrthogonal Multiple Access(NOMA)technology has emerged,which can effectively improve the system spectrum efficiency and solve the spectrum scarcity problem while ensuring the number of user accesses.To further increase system capacity,Cooperative Transmission(CT)technology can be applied to NOMA systems to optimise the communication experience of edge users and thus meet the rising demand for data exchange.In addition to spectral efficiency and system capacity,future mobile networks will also need to effectively address the problem of information leakage caused by the large number of users accessing the system,to prevent malicious users in the system from taking advantage of the broadcast nature of the wireless channel to eavesdrop on information.As the proliferation of information transmission data will lead to the inability of traditional encryption methods to effectively guarantee the confidentiality of the system,researchers have focused their attention on Physical Layer Security(PLS)technology,which is an underlying encryption technology that can use the inherent properties of the physical channel to protect user communications.Based on the above research background,this thesis will conduct a study on physical layer security in collaborative NOMA systems.Most of the existing collaborative NOMA systems use proprietary relays to assist users in transmission.In fact,in NOMA systems,the near user needs to decode the far user’s information before decoding its own information,and this feature can be exploited by selecting the near user to assist in the transmission of the far user’s information.This idea can improve transmission reliability while reducing system overhead.The NOMA system allows multiple users to share the same spectrum resources,which can improve the spectrum efficiency and system capacity in multi-user systems.Based on this,this paper investigates a downlink NOMA system with near-user collaborative far-user transmission.Considering the heterogeneity of user Qo S requirements,a two-stage user scheduling scheme is designed to ensure the confidentiality of the system by scheduling the near users to assist the far users in transmission while taking into account the different Qo S requirements of the far and near users.At the same time,the optimal scheduling scheme is designed with the objective of minimising the probability of confidentiality interruption in order to ensure the optimal confidentiality performance of the system.Finally,the closed-form solutions for the above two schemes are derived,and the accuracy of the derived results is verified by simulation experiments,and the results show that both schemes can achieve the optimal secrecy interruption performance.On the other hand,the eavesdropping user in the current study usually performs only singlestage eavesdropping,while in this paper the case of global eavesdropping by the eavesdropper is considered.Collaborative transmission studies usually consider eavesdroppers with low eavesdropping capabilities,i.e.low eavesdropper signal-to-noise ratios,while the confidentiality performance of the systems studied above is significantly reduced for the case of high eavesdropper signal-to-noise ratios.In order to guarantee the secrecy of the system in the case of high eavesdropping SNR,collaborative jamming techniques are introduced in this paper to improve the performance of the system.Commonly used collaborative jamming techniques usually use beamforming or multiple antenna techniques to send jamming noise and usually only have a single phase of collaborative jamming.In this paper,a joint collaborative user-interferer scheduling algorithm is designed to achieve the effect of global jamming by jointly scheduling the nearuser node and the transmitter node to generate jamming noise,which aims to differentiate the channel capacity between the legitimate link and the eavesdropping link as much as possible without affecting the transmission performance of the legitimate user,thus ensuring the secrecy of the system.In order to evaluate the performance of the proposed scheme,a closed-form solution for the secrecy interruption probability of the above scheduling scheme is derived,and the accuracy of the study is verified by simulation analysis.The performance of the system with and without interference noise is also compared,and the results show that a reasonable addition of interference noise can improve the secrecy performance of the system.