Performance Analysis And Transmission Scheme Design Of Wireless Powered Communication Networks

How to prolong the operation lifetime of energy constrained wireless devices has become a critical issue,especially with the explosive growth of sensor nodes due to the fast development of internet of things.As a practically viable solution to address this challenge,a new paradigm has emerged where the wireless devices harvest energy from the radio-frequency(RF)signals.Since the RF signals can be fully controlled,it is more reliable and efficient.The combination of wireless power transfer(WPT)and information transfer has resulted in a new topic,generally referred to as simultaneous wireless information and power transfer(SWIPT),where one signal ean transmit information and energy at the same time,and wireless powered communication network(WPCN),where energy constrained wireless devices are first powered by WPT and then perform information transmission using the harvested RP energy.Although SWIPT and WPCN enjoy the great flexibility enabled by WPT,its application is limited to relative short distance communication scenarios due to the high attenuation of microwave energy over distance and its performance is critically depended on the energy harvested at the nodes.To address this problem,in this paper,multi input multi output(MIMO)and relay assisted SWIPT and WPCN are proposed to improve the energy and information transmission efficiency.Specifically,we firstly consider a source-relay-destination dual-hop system where the relay is powered via RF energy harvesting,and we assume that the antennas at the relay node are spa?tially correlated.Two separate cases are studied,depending on the availability of CSI at the relay,namely,instantaneous CSI and statistical CSI.For both cases,exact analytical expressions for the outage probability are obtained.The findings suggest that,with instantaneous CSI,full diversity order can be achieved,which implies that the antenna correlation does not affect the achievable diversity order.In addition,the antenna correlation degrades system performance in the moder?ate or high SNR regime and increases the system performance in the low SNR regime.However,with statistical CSI,only unit diversity order can be achieved,and the antenna correlation becomes always beneficial.Next,we investigate a three-node wireless powered communication system where a hybrid access point(H-AP)communicates with a energy constrained user with the assistance of a energy constrained relay.The optimal energy beamforming vector and time split maximizing the achiev?able throughput is firstly studied.To further reduce the computational complexity,we propose a simple upper bound for the achievable throughput,and then obtain closed-form expressions for the optimal energy beamforming and time split maximizing the throughput upper bound.The out-come of the paper indicates that implementing multiple antennas at the H-AP can significantly improve the system performance,and the near optimal performance can be attained by employing the simple suboptimal design.Then,a power beacon(PB)assisted two-way relaying network is proposed,where two single antenna energy constrained users first harvest energy from a multi-antenna PB and then commu-nicate with each other with the assistance of a relay.Specifically,two different design objectives are investigated,namely,max-min rate design and sum rate maximization design.Due to the non-convex nature of the optimization problems,the global optimal solutions are difficult to obtain.In-stead,we propose an alternating optimization design framework where near optimal performance can be achieved through iterative optimization.In addition,to further reduce the computation com-plexity,closed-form suboptimal designs are provided.Simulation results are presented to validate the effectiveness of the proposed alternating optimization design and suboptimal design.The out-comes of the paper suggest that adopting the proposed energy beamforming vector at the PB can substantially boost the system performance.Also,the topology of the network has a significant impact on the achievable performance.Finally,we consider a wireless powered two-way relaying system consisting of two energy constrained single antenna sources and one multi-antenna relay with constant power supply.The time division protocol is adopted,where the relay first acts as the energy source and employs energy beamforming to charge the two sources,and then switches its role as a relay to help forward the information to the sources.To maintain user fairness,we aim to maximize the minimum rate of two sources,by jointly optimizing the energy beamforming vector,time splitting factor and relay transformation matrix.To further reduce the complexity of the optimal algorithm,we propose an alternating optimization method,where closed-form expressions for the energy beamforming and time splitting factor are obtained.Numerical results show that the performance of the proposed low-complexity alternating optimization method approaches the optimal algorithm over the entire SNR range,but has a significantly low complexity.