Relay Selection And Secrecy Performance Analysis In Cooperative NOMA Systems

With the development of wireless mobile communication,user's demands have gradually changed from voice call to multiple information exchange.In the future,mobile communication will certainly cover everything.Applying Cooperative Transmission(CT)technology in non-orthogonal multiple access(NOMA)systems can effectively enhance system capacity and improve the service quality of edge users.At the same time,Cooperative-NOMA system can meet the increasing information exchange demand.Massive data also bring challenges to information security,the traditional encryption may be difficult to meet the encryption demands of all users.Moreover,if the computing power of illegal eavesdroppers is strong enough,the traditional top-level encryption could be brute-force attack.Therefore,we need to find new encryption technology to meet user's requirements for information security.Physical layer security(PLS)use the physical characteristics of the wireless channel to achieve the secure transmission of information,so it without additional costs but more simple.Therefore,PLS has received a lot of attention and became the primary target of research.Considering information security,this thesis aims to study how to make full use of the inherent characteristics of communication nodes to ensure the reliable transmission of user information.The specific methods include relay selection and cooperative jamming.There are two common defects in the existing studies.First,most of the studies only consider eavesdroppers can overhear message from relay nodes but do not overhear message from source node.Secondly,most studies mainly use cooperative jamming of relay nodes or source nodes to improve system performance.But this kind of cooperative jamming need to share jamming information in advance,or need multiple relays and multiple antennas to realize beamforming which will waste the system resources.Based on the limitations of the existing studies,this thesis fully considered the ability of eavesdroppers,and proposed new transmission scheme to improve the security performance of cooperative communication system.The specific research contents are summarized as follows:1.The first part studies the physical layer security of multiple relays NOMA system when eavesdroppers can overhear message from both source node and relay nodes.This part mainly improves the security performance by proposing the relay selection strategy.The proposed Two-Step Relay Selection strategy and Optimal Relay Selection strategy consider both legal link and eavesdropping link to ensure that all legitimate information can be securely transmit and the multi-relay NOMA system can obtain the optimal security performance.This thesis analyzes closed-form expression of secrecy outage probability(SOP)and asymptotic SOP value to evaluate system security performance under proposed relay selection strategies.Both theoretical and simulation results show that the two relay selection strategies have the same SOP,and the security performance is better than the Random Relay Selection scheme.And the system performance getting better while the relays number rising.2.The second part on the basis of the first part,in order to further enhance the system security performance,I introduced artificial noise to disturb eavesdropper.Different from previous research,in the first time slot,I choose a legal user to send the artificial noise.In the second time slot,the AF relay forward the message to legal users.Based this,the artificial noise can disturb both two time slot eavesdropper decoding secret message.Using destination node cooperative jamming and AF relay to amplify and forward signals can avoid to share the jamming information between source and destination nodes and avoid the waste of relay resources.However,the correlation of the signal-to-noise ratio(SNR)becomes higher when AF relay forward the message and interference.It brings great difficulties to the analysis,so I mainly study the security performance under high SNR.Through theoretical and simulation analysis,the proposed relay selection strategies have the same SOP,and the security performance is better than the Random Relay Selection strategy.At the last,by comparing the results of the first part and the second part under the same conditions,it can be concluded that introducing artificial noise can effectively improve the system security performance.