Effective Transmission Schemes For Simultaneous Wireless Information And Power Transfer Systems

Simultaneous Wireless Information and Power Transfer(SWIPT)is a frontier research field,which is a combination of wireless communication technology and wireless energy transfer.The technology of S WIPT utilizes the radio frequency signal as the vehicle to transport information and energy concurrently,which is an exciting emerging technology to solve the energy-constrained problem of wireless networks and achieve green communication,and has drawn considerable research attentions from academics and industry.How-ever,SWIPT faces challenges of fading and secure communications.Therefore,the study on how to realize efficient energy transfer and secure communication in SWIPT system has important practical significance.In traditional wireless networks,multi-antenna precoding and physical layer security technologies can effec-tively reduce the impact of channel fading and ensure the security of communications,respectively Conse-quently,by combining these two technologies,in this thesis,we mainly focus on the topic of beamforming design based on energy harvesting proportional fairness in SWIPT system,secure energy-efficiency precoder design in MIMO SWIPT system,secure beamforming for SWIPT in multiuser MISO broadcast channel with confidential messages,secure beamforming design for SWIPT in MISO broadcast channel with confiden-tial messages and external eavesdroppers,cooperative precoding for coexisting wireless energy transmission and cognitive radio systems,and multi-objective precoder design for coexisting wireless energy transfer and information transmission systems.The detailed contents and main contributions of this thesis are listed as follows:1.We propose a beamforming design based on energy harvesting proportional fairness to overcome the un-balance of energy harvesting in SWIPT system.We aim at achieving the energy harvesting proportional fairness while guaranteeing the signal to interference plus noise ratio constraints at the information re-ceivers and total power constraint at the transmitter by optimizing the beamforming vectors.This opti?mization problem is non-convex and hence difficult to solve.In order to solve it,in this paper,we first equivalently transform it into its epigraph form,and then use the semi-definite relaxation technique as a tool to turn the corresponding non-convex optimization problem into a convex optimization problem.Be-sides,based on worst-case method,we also extend our result to a robust case where the transmitter only knows a part of the channel state information.Simulation results show that our proposed beamforming designs can achieve the energy harvesting proportional fairness.2.We propose a secure energy-efficiency(SEE)precoder design for a three-node MIMO SWIPT system.Using SEE as the design criteria in SWIPT systems is still an open problem.Similar to the definition of energy efficiency in traditional wireless networks,here we define the SEE as the ratio of the maximum achievable secrecy rate and the total consumed energy.We aim to maximize the SEE of the system subject to the secret rate,energy harvesting and transmit power constraints,which is a nonconvex optimization and hard to tackle.To solve it,we first transform it into an equivalent parameterized polynomial subtrac-tive form and then approximate the nonconvex equivalent problem into a convex optimization problem by means of Taylor series expansion.Based on that,an iterative algorithm with proved convergence is proposed.Numerical results show that the proposed algorithm can always achieve the maximum SEE,and it also can reach the maximum secrecy rate in the smaller transmit power region.3.We consider SWIPT in multiuser MISO broadcast channel with confidential messages,where each receiv-er acts as a potential eavesdropper for the confidential messages sent to the others.Under this scenario,the secure transmission design remains an open problem.Therefore,we propose a secure beamforming de-sign,and our goal is to minimize the total transmit power while guaranteeing the secrecy rate and harvested energy constraints at each receiver by jointly optimizing the transmit beamforming vectors,artificial noise vector,and PS ratios.We propose a two-stage approach to address the original non-convex problem as follows:we first fix the signal-to-interference ratio(SINR)requirement of each receiver and obtain the globally optimal solution of the corresponding problem by utilizing the technique of semidefinite relax-ation;then,the optimal solution of the original problem can be found by K-dimensional(K-D)exhaustive search over the SINR requirement of all receivers.Finally,since the K-D exhaustive search has high computational complexity,we further propose a low-complexity suboptimal solution based on the genetic algorithm.4.In addition to the other receivers,the confidential messages in multiuser MISO broadcast channel may be also wiretapped by external eavesdroppers.Therefore,we further investigate the secure transmission design for a MISO broadcast channel with confidential messages and external eavesdroppers(BCCE).Our objective is to minimize the total transmit power while guaranteeing the secrecy rate and energy harvest-ing constraints at each receiver.Similarly,we first propose a two-stage optimization approach to solve the original non-convex problem global optimality,and also provide a low-complexity suboptimal solution based on the particle swarm optimization algorithm.Furthermore,we slao extend the above described se-cure beamforming design to the case of colluding eavesdroppers whereby all external and potential eaves-droppers are seen as one virtual multiple-antenna eavesdropper.The corresponding optimization problem is more challenging than the above non-colluding eavesdroppers case due to the existing of determinant function in constraints.Under the framework of the two-stage optimization approach,the corresponding problem is solved by employing some basic matrix results as well as the semidefinite relaxation technique.Finally,we extend our work to the imperfect CSI case,and furhter propose a robust secure beamforming design.5.We propose a cooperative precoding design for a coexisting wireless energy transfer(WET)and cognitive radio(CR)system.Different from the conventional wireless communication networks where interference is treated as a detrimental phenomenon,under this new paradigm,interference may be a useful rather than harmful resource for both the WET and the CR systems.Specifically,the interference generated by the secondary transmitter can be regarded as an energy source for the energy receiver in WET system.Meanwhile,the interference generated by the energy transmitter can be seen as an artificial noise,which can be exploited to achieve secure communication between the secondary transmitter and the secondary receiver.We propose an iterative algorithm to deal with such a challenging problem by applying the first-order Taylor series expansion as well as the Lagrange dual decomposition.Besides,we prove that the proposed algorithm monotonically converges to a Karush-Kuhn-Tucker(KKT)point of the original problem.Moreover,the complexity of the proposed algorithm is also analyzed.Simulation results show that the proposed cooperative precoding design can concurrently reduce the power consumption of both the energy transmitter and the secondary transmitter.6.Based on the multi-objective optimization framwork,we formulate the transmit covariance design as a multi-objective optimization problem(MOOP)for a coexisting WET and wireless information transmis-sion(WIT)systems,so as to jointly maximize the total harvested energy in WET system and maximize the secrecy rate in WIT system while guaranteeing the transmit power constraints.To solve such a non-convex MOOP,we first find its Utopia point.Then,we transform the MOOP into two different single-objective optimization problems(SOOPs)via the weighted sum and the weighted Tchebycheff approaches,respec-tively.The two SOOPs are non-convex and non-smooth,and thus they are difficult to solve.By exploiting the two-stage optimization method and the first-order Taylor series expansion,we develop two iterative algorithms with provable convergence to solve them,respectively.Moreover,we propose a distributed precoding design for the coexisting WET and WIT systems based on the non-cooperative game approach.The existence and uniqueness of the Nash equilibria for the formulated game is also analyzed.