| With the rapid development of mobile communications and Internet of Things(Io T),numerous intelligent devices are connected through wireless communication networks.However,due to the broadcasting and open charactersitcs of wireless medium,information transmitted to legitimate users is easily eavesdropped by illegal users.Thus,solutions to achieve secure transmsission is an urgent need.Different from that the traditional key encryption method has the risk of being cracked,the physical layer security exploiting artificial noise(AN)and wireless channel random fading is an emerging solution to achieve secure transmsission.Recently,intelligent reflecting surface(IRS),composed of many reflecting elements,is considered as a promising technology to achieve and enhance the physical layer security.By adjusting the phase of reflecting elements,the reflecting beamforming can be constructed to beam confidential information to the legitimate users and suppress the signal power received by the illegal users.It is worth nothing that the IRS may be deployed in places without stable energy supply,e.g.,ceilings and mountains.In this scenario,the tranitional way of cable power supply is infeasible.In addition,the signal reflected by the IRS usually suffers from the severe path-loss,which leads to a limited improvement to system security performance.To address the above issues,this thesis focuses on applying new types of RIS to design secure transmission enhancement schemes for multi-input single-output(MISO)systems.The main contributions are summarized as follows:To solve the issue of stable energy supply,the self-sustainable IRS empowered scheme is proposed.To guarantee that the IRS has a perpetual lifespan,the time-switching(TS)based energy harvesting scheme is proposed.Thus,the transmission time slot is divided into energy collecting stage and information reflecting stage.In particular,the IRS first harvests energy from the base station in the energy collecting stage and then uses the harvested energy to assist the information transmission of the base station in the information reflecting stage.Under the constraints of the base station power and the unit phase shift modulus of the IRS,a joint optimization of beamforming vectors at the base station,time allocation and the IRS reflection phases is considered to maximize the system secrecy transmission rate.Since the two stages of the TS scheme are independent of each other,the optimization problem can be transformed into two sub-problems.To optimize the beamforming vectors at the base station and the IRS reflection phases in the information reflecting stage,the optimal solution of the sub-problem is obtained through the alternating optimization algorithm and bisection search algorithm.To optimize the beamforming vectors of the base station and the time allocation in the energy collecting stage,the semidefinite relaxation(SDR)and successive convex approximation(SCA)techniques are used to transform the problem into a convex problem,so that the sub-optimal solution of which can be obtained.Simulation results show that the proposed scheme can effectively improve the system secrecy transmission rate.To enhance the secure transmission performance,the active IRS and AN aided scheme is proposed.The active IRS is used to amplify the base station signals to improve the system secrecy transmission rate.In particular,the active IRS is deployed between the base station and the legitimate user,which can enhance the confidential information transmission from the base station to the legitimate user and suppress the information received by the eavesdropper.Meanwhile,the beamforming vectors and the AN at the base station are designed to further enhance the system performance.To maximize the system secrecy transmission rate,a joint optimization problem of the base station beamforming vectors,AN,active IRS reflection phase and active IRS amplification factor is formulated.Since the formulated problem is non-convex,an efficient algorithm based on the alternating optimization,SCA techniques and SDR is proposed to obtain a suboptimal solution.By applying the alternating optimization framework,the formulated problem is transformed into two sub-problems.Specifically,with the active IRS reflecting phase and active IRS amplification factor being fixed,the SCA and SDR are used to obtain the sub-optimal solution of the beamforming vectors and AN at the base station.Then,with the fixed beamforming vectors and AN at the base station,the sub-optimal solution of the subproblem can be obtained by optimizing the active IRS reflection phase and active IRS amplification factor.The simulation results show that compared with the benchmark schemes,the proposed scheme can achieve a higher secrecy transmission rate. |
PDF Full Text Request