| As the number of devices connected to the network increases,the shortage of wireless spectrum resources meets more challenges.It also leads to a significantly complex communication environment,in which security problems become more and more important because of the openness of wireless communications.Compared with the conventional orthogonal multiple access(OMA)technology,non-orthogonal multiple access(NOMA)has the ability to provide services for multiple users within the same physical time and/or frequency resource block.In the other words,compared with OMA,NOMA has outstanding capabilities in enhancing spectral efficiency and user connectivity.However,an appropriate power difference between the users is vital to unleashing NOMA’s full potential.Furthermore,the communication security based on key encryption technology is adversely affected by the improvement of the computing capabilities of the device.Thanks to the physical layer security(PLS)technology,which has the ability to improve the decoding ability of legitimate users but reduce that of the eavesdropper by taking advantage of the inherent characteristics of wireless channels.The recently proposed reconfigurable intelligent surface(RIS)has the ability to manage the reflection/refraction properties of the radio waves,which results in appropriate channel differences between NOMA users,and unleashes the potential of NOMA.Moreover,it also introduces a new degree of freedom for the design of PLS,which may improve the performance of PLS.Therefore,the RIS assisted NOMA networks can exhibit higher reliability and better performance of secure data transmission.This dissertation addressed the limited release of the potential of NOMA and PLS,integrated RIS and NOMA techniques,and researched the PLS for RIS assisted NOMA systems.First,for small-scale RIS,a legitimate user signal enhancement method is proposed to improve the performance of PLS.Second,for large-scale RIS,in order to solve the problem that the security performance of RIS assisted NOMA system will be damaged with the increase of the number of RIS elements after reaching the upper bound of PLS in legitimate user signal enhancement method,the eavesdropper signal elimination method is proposed.Finally,for the simultaneously transmitting and reflecting(STAR)RIS assisted NOMA secure communication system,the impact of the eavesdropper located in different half-spaces of STAR-RIS on the performance of PLS for the STAR-RIS assisted NOMA system is studied.The main innovative contribution of this dissertation can be concluded as follows:1.Aiming at the challenge to performance analysis of PLS,which is caused by the inaccurate distribution of RIS concatenated channel,the average channel gain of the concatenated link is given for the first time,when received signals are co-phased,by utilizing the relationship between the Laplace transforms and moments.The secrecy outage probability(SOP)and the average secrecy capacity(ASC)of users in the RIS assisted NOMA network are analysed.In order to gain further insights,the asymptotic behaviour is analysed in the high signal-to-noise-ratio(SNR)regime.The secrecy diversity order of the cell-centre user is 1.By contrast,the secrecy diversity order of the cell-edge user is related to the number of RIS elements and the Nakagami-m fading parameters.The high SNR slope of the cell-centre user and cell-edge user are 1 and 0,respectively.In other words,for the cell-centre user,the ASC will increases as the transmit SNR increases.However,there is an upper bound on the cell-edge user’s ASC.2.To solve the problem that the increase of RIS elements harms the secrecy performance of RIS assisted NOMA networks,a secure RIS aided NOMA wireless communication system is established,in which there are direct links between the BS and multiple users as well as the eavesdropper.Then,a novel design of the RIS is proposed for eliminating the risk of the information being eavesdropped on,when the channel state informations of users and the eavesdropper are known.Furthermore,the performance analysis of the RIS assisted NOMA network based on the order statistics is proposed.When the number of RIS elements meets the requirement,there are no influences on the system’s performance by increasing the number of RIS elements.The diversity orders of NOMA users are decided by the index number of them.Under the novel design of the RIS,the problem that increasing the number of RIS elements harms the secrecy performance of RIS assisted NOMA systems is solved.Besides,it also can avoid the need for wiretap codes,and the system is able to use conventional channel codes to achieve secrecy.3.Aiming at the problem that the whole space may be eavesdropped on due to the full space coverage,a STAR-RIS assisted NOMA secure communication system is established.Then we analyse the secrecy performance based on two scenarios.More specifically,in scenario 1,the Eve is on the same side with the BS regarding the STARRIS.While in scenario 2,the Eve is on the other side of the BS regarding the STAR-RIS.The equivalent concatenated channel gains are given,by utilizing the Triangular distribution and the relationship between Laplace transforms and moments.According to which half-spaces the eavesdropper is located,the individual users have different security performance,which is caused by the amplitude reflection and refraction coefficient of the STAR-RIS and the power allocation factors.However,the secrecy diversity orders of the individual users are equal for the two cases.Moreover,for the two cases,when the distance between the STAR-RIS and the eavesdropper tends to infinity,the secrecy performance of the individual users tends to be equal. |
