Research On Physical Layer Security Techniques For Wireless-Powered Cooperative Relay Systems

With the explosive growth of mobile data traffic,the problems of information secu-rity and user privacy are facing great challenges due to the diversified and differential user demands of communication services.On the one hand,the future wireless communication network architecture is evolving with the characteristics of ultra-dense and heterogeneous.The coexistence of multiple access networks makes the network more open and the in-formation leakage more serious caused by the broadcast nature of wireless medium.On the other hand,with the arrival of the big data era,the computing and learning capability of communication nodes have been continuously enhanced.Traditional encryption-based security mechanisms on the upper layer have been greatly challenged.Besides,the ex-change and management of encryption keys among users can hardly be conducted since the ultra-dense deployment of wireless networks.To this end,physical layer security,which can improve information security by exploiting the inherent randomness of wire-less channel,has been proposed recently and drawn intensive attention.A particularly interesting physical layer security technology is cooperative relaying,since it can not on-ly extend network coverage,but also reduce information leakage.However,such kind of technologies faces several problems.Firstly,the half-duplex operation of relays causes considerable spectral efficiency loss.Though full duplex techniques can improve spectral efficiency,the loopback interference(LI)introduced will degrade the system security per-formance.Secondly,the battery-powered mobile users may unwilling to participate in the cooperative communication because of their starving energy storage.Last but not least,persons who hold mobile terminals have different kinds of social behaviors.Whether the social relationships among persons can be used to guide the design of cooperative relay mechanism and further improve information security is still an open problem.Therefore,how to design an efficient wireless-powered secure transmission mechanism to alleviate the limited energy storage of cooperative relays and reduce information leakage thus to achieve high-secure transmission is the key research issue of this work.Besides,the social relationship-aware cooperative relay systems will also be explored.Towards the future trend of wireless communications and the main contradiction between the rapid growth of communication demands and information transmission secu-rity,this paper aims to reduce the information leakage and maximize system secrecy rate on the basis of full-duplex(FD)technology,wireless power transfer and social network analysis.Taking cooperative relay technology as a core method,this paper mainly focuses on the fundamental theories and key technologies of physical layer security for wireless-powered cooperative relay systems.The main research contents and contributions are summarized as follows.1)The secure transmission of wireless-powered FD relay systems is investigated.A novel secure transmission mechanism is first investigated,in which FD relay is wireless energy harvesting-enabled,adopting both transmit and receive antennas to harvest ener-gy in a time switching(TS)mode.To maximize the system secrecy rate through jointly designing the energy beamforming vector,the information beamforming vector and the TS coefficient,an optimization problem is formulated.To solve this non-convex prob-lem,an iterative algorithm is proposed to convert it into three convex subproblems,based on which the closed form solutions for the beamforming vectors are derived and the TS coefficient is obtained.Convergence property of the iterative method is analyzed.Simu-lation results reveal that the security performance of the proposed method approaches the analytical upper bound of achievable secrecy rate.2)The secure transmission scheme of wireless-powered relay systems with imper-fect eavesdropper channel state information(ECSI)is investigated.On the basis of the above work,a novel two-phase protocol is proposed.Compared with those existing pro-tocols,the proposed design possesses two main advantages:1)it fully exploits the avail-able hardware resource(antenna element)of relay and can offer higher secrecy rate;2)it enables self-energy recycling(S-ER)at relay,in which the loopback interference gen-erated by FD operation is harvested and reused for information relaying.To maximize the worst-case secrecy rate(WCSR)through jointly designing the source and relay beam-formers coupled with the power allocation ratio,an optimization problem is formulated.This formulated problem is proved to be non-convex and the challenge to solve it is how to concurrently solve out the beamformers and the power allocation ratio.To cope with this difficulty,an alternative approach is proposed by converting the original problem into three subproblems.By solving these subproblems iteratively,the closed form solutions of robust beamformers and power allocation ratio for the original problem are achieved.Simulation results demonstrate that the proposed S-ER based secure transmission scheme outperforms the traditional time-switching based relaying(TSR)scheme at a maximum WCSR gain of 80%.3)The social-aware cooperative secure transmission method is investigated.By defining the social trust degree between relays and communication pairs,the social trust mechanism is proposed,and the analytical expression of system secrecy rate is derived.On the basis of social relationships between users,a novel cooperative jamming scheme is further investigated and opportunistic energy harvesting approach is designed.Taking into account the relay initial energy storage,the analytical representation of the oppor-tunistic TS scheme is obtained theoretically,and an optimal relay selection scheme is proposed.Simulation results demonstrate that the system secrecy performance is sub-stantially boosted.