Research On Physical Layer Security In Swipt-enabled Untrusted Relay Networks

In the era of Internet of Everything,higher data transmission rate and spectrum efficiency are required with the explosive growth of data traffic and communication devices.Cooperative relaying,as an effective approach to expand coverage and improve system throughput,plays an important role in the Internet of Things,where the relay usually amplifies or decodes the received signals before forwarding them to improve the communication quality.However,the relay is often untrusted and does not have the rights to access the information,due to its low security level in some practical applications with high requirements for information security,such as financial,military and government intelligence networks.The untrusted relay is a collaborator during the signal transmission while may be an eavesdropper for the date,since it might intentionally overhear the information signal when forwarding and threaten the security of the system.In addition,the ourdoor relay nodes are usually powered by batteries.Charging or replacing batteries frequently may not only damage the user experience,but also not be easy to implement in harsh environments.In view of the information security and energy-constrained issues for untrusted relay networks,two secure transmission strategies,for two-way relay network and cooperative downlink non-orthogonal multiple access(NOMA)system,are proposed based on physical layer security and simultaneous wireless information and power transfer(SWIPT)techniques.The main research contents are as follows:(1)The basic theory of physical layer security and traditional relay cooperative network are first studied.Several physical layer security strategies for untrusted relay networks are summarized,and then the signal transmission process of two-way relay networks and downlink cooperative NOMA systems are elaborated.Finally,typical architectures of SWIPT receivers and SWIPT-enabled cooperative relaying theory are described.(2)For the two-way untrusted relay networks,a SWIPT-based and artificial noiseaided secure transmission scheme is proposed.The power splitting(PS)strategy and amplify and forward(AF)protocol are adopted by the untrusted relay to realize the confidential communication,where a full-duplex jammer is assigned to send the artificial noise while harvesting energy,to ensure the system security.The closed-form lowerbound expression of the ergodic secrecy sum rate(ESSR)is derived under the statistical channel state information(CSI).To maximize the secrecy sum rate(SSR),the PS ratio is optimized,and the closed-form expression of optimal PS ratio is also derived in the high signal-to-noise ratio regime.Besides,the impacts of channel estimation error on the system SSR are analyzed for the imperfect CSI.Simulation results validate the correctness of the theoretical derivation and demonstrate that the proposed transmission scheme outperforms the benchmark ones based on time switching(TS)strategy or destination-aided jamming.(3)For the cooperative downlink NOMA systems with an untrusted relay,a physical-layer security transmission scheme based on SWIPT and destination-aided jamming is proposed.The PS and AF are adopted by the untrusted relay to assist NOMA users at the edge of the cell for legitimate communication,while the users send the artificial noise simultaneously to ensure the system security.Through mathematical tools,such as Gauss-Chebyshev,Gauss-Laguerre and exponential integrals,the closed-form lower-bound expression of ESSR is derived in the high signal-to-noise ratio regime.The PS ratio is also optimized to maximize the SSR and the closed-form expression of optimal PS ratio is derived under the instantaneous CSI.In particular,the impacts of residual interference signal on the ESSR are analyzed,when the successive interference cancellation is imperfect.Simulation results validate the correctness of the theoretical derivation and demonstrate that the proposed SWIPT-NOMA scheme achieves higher secure transmission rate than that based on orthogonal multiple access.