QoS Guarantee And Energy Conservation For Wireless Powered Communication Networks

With the popularization of wireless device,e.g.,smart phones and tablet computers,and the maturity of Internet of Things,more and more wireless communication devices have been emerg?ing in people's daily lives.However,due to the limited battery power of these devices,it is difficult to provide long-term and stable guarantee to the quality of user experience and quality of service(QoS)of wireless communication networks,and networks may even be paralyzed when battery replacement is impossible.To address the above issue,academics and industries have proposed the concept of wireless powered communication networks(WPCN),in which wireless communication devices can obtain far-field electric energy supplementation through radio frequency-based wire-less energy transfer(WET).Although WPCNs get rid of the limitation of battery power,there are still several technical challenges:1)Imbalance of energy supplies and requirement,causing that some users lack energy thus resulting in poor QoS,while some others have redundant energy thus leading to wastage;2)High energy loss,containing those from double propagation attenuations of energy transfer and data transfer and from converting loss while users are harvesting wireless energy.3)Difference between energy transfer and data transfer in terms of time,frequency and power resources and contradiction between QoS guarantee and energy conservation.This thesis aims to explore the technologies of QoS guarantee and energy conservation in some main scenarios of WPCNs.For WPCNs supporting delay-sensitive services,a multihop transmission-based cross-layer control policy and user scheduling scheme are studied.To handel the high waiting delay caused by energy imbalance,a protocol termed adaptive harvest-then-cooperate is proposed:every user first harvests wireless energy from a hybrid access point(HAP)that is a joint energy provider and data transceiver,and then transmits data to the HAP either directly or relayed by other users;according to the distribution of existing energy and traffic load over the network,the HAP makes a deci-sion consisting of transmit power in the physical layer and routing topology in the network layer.To avoid data queue overflow when relay users are receiving others' data packets,a scheduling scheme matched with the proposed protocol is proposed.To reduce average waiting time,a delay-rminimized decision problem is formulated through Markov decision process(MDP),and a sample-path-based low-complexity solving algorithm is proposed.Simulation verifies the effectiveness of the proposed protocol and algorithm in terms of balancing energy supply and requirement and the reduction of average waiting time.For backscatter communication-enabled WPCNs,techniques for multi-user QoS guarantee and energy conservation are investigated.Backscatter-based transfer is introduced into WPCNs to deal with the high energy loss:by mismatching antenna resistance,passive nodes(PN)modulate their data over the signal received from an active node(AN)and then scatter the modulated signal to a receiver(Rx).An protocol called harvest-while-scatter(HWS)is proposed:the AN contin-uously transmits a single-carrier wave,while PNs first havest wireless energy from the AN,then successively modulate their data on that wave and scatter it to the Rx.To reduce energy dedicatedly'consumed for wireless energy transfer,each PN utilizes the time while others are scattering to har-vest energy.Since backscatter-enabled PNs are unable to actively send pilots or feed back channel state information(CSI),a channel estimating method matched with the HWS protocol is proposed,in which AN,PN and Rx cooperate together to obtain statistical CSI.To reduce overall energy con-sumption while guaranteeing QoS,a robust algorithm is proposed to jointly control AN's transmit power and allocate time among PNs with only above statistic CSI.Simulation validates the use-fulness of proposed protocol and algorithm in QoS guarantee and energy conservation,and reveals the influence of nodes,relative locations on power control and time allocation.For hybrid-powered communication networks involving source-powered and wireless-power users,an energy saving approach based on resource allocation optimization and adaptive transmis-sion is studied.According to different time-frequency resource requirements of energy transfer and data transfer,a frequency-division transfer mode is proposed,where energy signals and data signals are transferred in low-frequency narrowband and high-frequency wideband,respectively.To address the imbalance issue,an energy beamforming-based method termed beam switch is pro-posed:HAP controls the energy beam to point at different users in turn,and meanwhile jointly controls energy transfer power and charging durations so as to deliver energy on users,demands.To minimize energy consumption while provisioning QoS,an optimal decision problem is for-mulated,where data transfer power,energy transfer power and charging durations are adaptively controlled.First,a decision model is built up,which involves instantaneous CSI,energy distribu-tion and traffic distribution;Second,by fixing uplink-downlink time allocation ratio,the decision problem is decomposed into two subproblems with respect to data transfer and energy transfer,respectively;Third,through discretizing channel state and MDP formulation,the two subproblems are transformed into tractable linear programming problems;Finally,a solving algorithm is pro-posed.Simulation and numerical results verified the effectiveness of beam switch and the proposed algorihtm and,it is revealed that the adaptive transmission decision utilizes queue dynamics and time-dimensional channel diversity to realize energy conservation,i.e.”prestoring energy under good channel condition so as to reduce average overall energy consumption.For large-scale heterogeneous networks consisting of several WPCNs within the same area,a crowdsourcing-based energy-service trading platform with high energy efficiency is investigated.In the platform,a user can provide energy as reward to employ other users to help complete its task,so as to reduce overall energy consumption.An energy bank is proposed,which uses energy as the trading media,maintains accounts for all users and supervises the above trading.Users can with-draw their energy savings from the bank through nearby power beacons to get energy supply,or transfer their savings to other accounts in a loss-free bookkeeping manner.During the trading,the energy bank freezes the employer's payment,then after worker users complete the task unfreezes and transfers the payment to workers' accounts.The employer's energy expense-minimized deci-sion and workers' energy profit-maximized decisions are analyzed,which composes a Stackelberg equilibrium.To demonstrate the application of the proposed platform and quantify the performance of the proposed mechanism,they are applied to a relay-based wireless sensor network in which a source node employs relay nodes to help forward data with a minimum data rate requirement.A heuristic algorithm is proposed to minimize the employer's energy expense in this scenario.Sim-ulation result validates that the proposed platform makes win-win situations for both the employer and workers and meanwhile improves energy efficiency of the whole network,and the result also reveals the preference of the source node on relays' locations.