Transmission Policies And Performance Analysis For Wireless Powered Communication Networks With Non-linear EH Models

Radio frequency wireless energy transfer(RF WET)can be regarded as one of the promising technologies to address the energy shortage problem for future communications networks,i.e.,Internet of Things(Io T)networks.The key idea of RF WET is to harvest energy from ambient RF signals to power the energy-constrained nodes due to the fact that RF signals can carry both information and energy.Besides,in order to satisfy the everincreasing requirements for high spectrum efficiency and high energy efficiency(EE),various high efficient wireless communications networks have emerged,i.e.,multiple antennas communications networks,Device-to-Device(D2D)communications networks,two-way relay networks,etc.The combination of RF WET and the high efficient wireless communications networks forms high efficient wireless powered communications networks which can not only prolong the lifetime of the networks,but also improve the system performance,bringing a lot of attentions.However,most of existing works on high efficient wireless powered communications networks are based on a conventional linear energy harvesting(EH)model,while the energy conversion efficiency of the practical EH circuit is non-linear.Motivated by this,this paper focuses on the wireless powered multiple antennas communications network,the wireless powered D2 D network and the wireless powered two-way relay network,and studies the design of the transmission policies and performance analysis for the three networks under the non-linear EH model systemically.The main researches and contributions of this paper are summarized as follows.1.Transmission policies designed for wireless powered multiple antennas communications networks with the non-linear EH model considered.Firstly,based on both the linear and the non-linear EH models,we formulate two optimization problems by optimizing the transmission power of each antenna at the transmitter to maximize the EE of point-to-point wireless powered multiple antennas communications networks with multiple constraints,i.e.,the minimum harvested energy at the receiver and the maximum total transmission power at the transmitter and propose a power allocation iterative algorithm to obtain optimal solutions.Simulation results verify the superiority of the proposed power allocation scheme under the non-linear EH model and also show that the use of the linear EH model will lead to a resource mismatch,reducing the system performance.Then considering that the storage efficiency of a practical battery is also non-linear,combining the existing non-linear EH model,we further study the transmission policy designed for the point-to-multiple-points wireless powered multiple antennas communications networks.Specifically,by jointly optimizing the transmission power of each antenna at the transmitter,the EH time and the used energy from the battery at each receiver,we formulate an optimization problem to maximize the system EE while satisfying multiple constraints,i.e.,the minimum transmission rate at each receiver,the limited battery capacity and the maximum total transmission power at the transmitter.Since the consideration of the nonlinear EH model makes the formulated optimization problem non-convex and difficult to solve,we propose a novel non-linear EH model-piecewise linear EH model.Based on such a model,we further propose two iterative algorithms which can be combined to obtain the optimal resource allocation scheme.Simulation results illustrate the advantage of the proposed scheme by comparing with other existing schemes.2.Transmission policies designed for wireless powered D2 D communications networks with the non-linear EH model considered.Firstly,by means of stochastic geometry,we provide a tractable analytical framework to model the cellular networks with underlaid ambient backscatter communications(AmBack Coms),where each ultra-low-power device harvests energy from the ambient cellular signals via the time-switching(TS)/power-splitting(PS)scheme for operating circuits,reflects and modulates the cellular signals to realize the direct communication between two ultra-low-power devices.On this basis,considering the nonlinearity of the practical energy harvester,by optimizing the TS and PS ratios,we formulate two optimization problems to maximize the outage capacity of Am Back Coms under the TS and PS schemes,respectively.Then based on the statistical channel state information(CSI),we propose the TS and PS schemes with low complexity to obtain the near-optimal outage performance.In order to characterize the outage performance of the system and the effectiveness of the proposed schemes,considering the interference between cellular transmissions and backscattering and the interference among different ambient backscatter(AB)transmitters,we further derive the successful transmission probabilities for AB transmitters and cellular downlink transmissions with a given TS/PS ratio under the non-linear EH model.Simulation results show that compared with the optimal results obtained by the exhaustive search method,the proposed TS and PS schemes can achieve the near-optimal outage performance while enjoying lower computational complexity.Besides,by comparing with results obtained by Monte Carlo simulations,the correctness of our derived expressions can also be verified and the use of the linear EH model leads to over-estimated system performance.3.Transmission policies designed for wireless powered two-way relay networks with the non-linear EH model considered.As for wireless powered two-way relay networks,the TS or PS scheme is considered to realize simultaneous wireless information and power transfer(SWIPT)between the sources and the relay.In particular,for a TS SWIPT based three-step two-way decode-and-forward(DF)relay system,a simpler non-linear EH model-piecewise linear EH model with 2 segments is adopted.In order to know how the relay combines the received signals from two sources,we propose an optimal combining scheme to minimize the system outage probability and derive the optimal combining ratio in closed form.On this basis,we also derive the expressions of the system outage probability and capacity under the proposed scheme.Simulation results verify the correctness of the derived results,the superiority of the proposed scheme and the necessity of considering the non-linear EH model.For a PS SWIPT based three-step two-way DF relay network,a more practical non-linear EH model-piecewise linear EH model with N + 1 segments is considered.On this basis,we successively study the maximization of the total outage capacity and the EE for the considered network.Specifically,we propose a dynamic heterogenous PS scheme to maximize the total outage capacity of the system and derive the end-to-end outage probability and capacity.Simulation results demonstrate the correctness of the derived results and the advantage of the proposed PS scheme.Then based on the non-linear EH model,we formulate an optimization problem to maximize the system EE by jointly optimizing the transmission power of two sources,the PS ratios at the relay and the time used for the source-relay transmission while satisfying multiple constraints,i.e.,the minimum transmission rate of each end-to-end link and the maximum transmission power,and propose two iterative algorithms which can be combined to obtain the optimal resource allocation scheme.Simulation results show the superiority of the resource allocation scheme designed based on the non-linear EH model by comparing with other existing schemes.