Beamforming Technology For AN Assisted Relay SWIPT Communication System

In wireless communication systems,some communication terminals are used in environments where battery replacement or recharging is inconvenient.Obtaining energy from RF signals can help these terminals extend their operating time.The emerging energy harvesting(EH)technique enables relays equipped with EH modules to collect energy from RF signals and convert electromagnetic radiation energy into electrical energy through rectification.However,in addition to energy issues,wireless communication networks also need to address information security issues caused by the broadcasting characteristics of wireless transmission media.Adopting EH relays to assist communication enjoys the advantages of cooperative communication and avoids energy waste,but also suffers from higher risk of information leakage.Physical layer security(PLS)technology is widely used to combat eavesdropping in wireless communication systems,and beamforming and artificial noise(AN)are the two most commonly used techniques.This thesis focuses on the EH relay assisted Simultaneous Wireless Information and Power Transfer(SWIPT)systems.From the perspective of security performance,the main research content is as follows.Firstly,for a decode-and-forward relay SWIPT system where the relay splits its received signal with power splitting(PS)method,the full duplex(FD)technology is introduced to enable multiple legitimate destinations to transmit AN signals while receiving signals form the relay.The forwarded signal and the AN signal are both beamformed to combat eavesdropping.Aiming at the maximization of the secrecy rate,this thesis provides beamformers for the forwarded signal and the AN signal through solving a two-variable-coupled optimization problem.Due to the coupling of the two beamformers,alternative optimization is adopted to solve the problem.Simulation results show that the proposed secure communication scheme provides better security performance compared with traditional half duplex(HD)scheme,AN transmitted by relay scheme and non-EH scheme.Secondly,for an amplify-and-forward relay SWIPT system with FD destination transmitting AN,a general hybrid power time splitting(HPTS)method which covers the traditional PS method and time switching(TS)method is studied and the confidential message is forwarded by the relay in a beamforming manner.The beamforming vector is jointly designed with the power-time splitting ratios and the AN power to improve secrecy rate.The corresponding maximization problem is also multi-variable coupled,thus it is solved by utilizing AO method,along with mathematical tools including Successive Convex Approximation(SCA).Simulation results show that compared to traditional PS and TS methods,the proposed scheme also has significant performance advantages.Finally,for a relay SWIPT system operating in PS mode with multiple eavesdroppers,considering the estimation error of the eavesdropping channels,a robust relay beamforming scheme is designed along with the PS ratio and the AN power to enhance system security.An AO algorithm is proposed to solve the optimization problem where the Charnes Cooper transformation is adopted to transform the first order fractional problem into a second order linear programming problem when solving the sub-problem of the relay beamforming matrix.Simulation results show that the proposed robust beamforming scheme can provide better security performance.