Research On Throughput Optimization For Wireless Powered Communication Networks

With the development of wireless communication,wireless devices(WDs)are deployed throughout our lives.WDs are typically powered by their embedded batteries,the lifetimes of which are limited due to the small batteries' capacities.Wireless powered communication network(WPCN)is a new network paradigm,where WDs,especially the low-power consumption WDs,are powered by radio frequency(RF)based wireless power transfer(WPT)to eliminate the need for recharging or replacing the batteries manually and to prolong their lifetimes.This advance guarantees that the WPCN can be applied in the upcoming Internet of Things(IoT).However,the future application of WPCN still faces some challenges,such as the low energy transfer efficiency and low system throughput.In this dissertation,the throughput maximization problems of WPCNs are investigated,and the main contributions are summarized as follows.(1)In a WPCN,the distances between the users and the hybrid access point(HAP)are different,which results in unfair transmission rates among different users.NOMA,the basic principle of which is that the users can achieve multiple access by exploiting the power domain multiplexing,is applied in the WPCN to improve user fairness.However,the successive interference cancellation(SIC)constraints,the prerequisite of applying NOMA successfully,are not considered in most existing literatures.In this dissertation,the effect of SIC constraints on the throughput of the WPCN with NOMA is investigated,where the users harvest energy from RF signals radiated by the HAP,and then use the harvested energy to simultaneously transmit information to the HAP.First,the throughput maximization problem is formulated to find the optimal time allocation scheme for a general case and the numerical result is derived.To be specific,we derive the closed-form solution for a two-user case.Simulations on the effect of SIC constraints show the importance of the distinctness among users' channel power gains for the WPCN with NOMA.(2)A wireless powered relay network(WPRN)is considered,which consists of multiple users with stable energy sources and multiple energy-free relays.The relays harvest energy from the signals radiated by the HAP to forward the users' information to the HAP with a decode-and-forward(DF)approach.However,the harvested energy of the relays by the existing schemes is usually limited.Hence,to raise the harvested energy,the HAP with two antennas is considered,which works in the full duplex(FD)mode.The HAP simultaneously broadcasts energy signals to the relays and receives the users' information forwarded by the relays,which improves the relays' energy harvesting time significantly.Under the setup,the signal interference may be encountered at the HAP and relays,and the imperfect interference cancellation is considered.An optimization problem is formulated to allocate the time between the downlink energy transfer from the HAP to relays and the uplink information transmission from users to the HAP via relays for the sake of maximizing the system throughput,and an effective algorithm is proposed to obtain the optimal solution.Moreover,a mathematical proof is given to show that the DF mode leads to a higher system throughput than the amplify-and-forward(AF)mode.Simulation results demonstrate the advantage of the WPRN with an FD-HAP in terms of system throughout.(3)By exploiting the advantages of the harvest-then-transmit(HTT)mode and the backscatter communication(BackCom)mode,two schemes that integrating the HTT mode and the BackCom mode are proposed for the WPCN,i.e.,the hybrid user scheme and the hybrid mode scheme.For the hybrid user scheme,the WPCN consists of two types of users,which adopt the HTT mode and the BackCom mode,respectively.While,each user can work in either the HTT mode or the BackCom mode for the hybrid mode scheme.The system throughput maximization problems are formulated for the proposed schemes and the optimal solutions are derived,respectively.The optimal solution for the hybrid user scheme shows that only the user in the BackCom mode with the largest backscatter rate can be scheduled.Moreover,the optimal users' working mode permutation is studied for the hybrid mode scheme.Simulation results show the superiority of the proposed schemes over the single mode schemes in terms of system throughput.(4)A hybrid mode scheme is proposed for a cognitive WPCN(CWPCN)consisting of a primary communication system and a secondary communication system,following which each cognitive user(CU)can adopt the HTT mode,the ambient backscatter(AB)mode or the bistatic backscatter(BB)mode.The primary transmitter(PT)and the power beacon(PB)serve as the incident signal sources of the AB mode and the BB mode for CUs' information backscattering,respectively.When the primary channel is idle,CUs use the harvested energy from the PT and PB to transmit information to the information receiver(IR)following the HTT mode.The optimal time allocation between the three modes is investigated for the sake of maximizing the throughput of the secondary communication system,and the numerical result is derived.To be specific,the closed-form solution for a single CU case is derived and the optimal combination of the working modes is obtained.Numerical results demonstrate the advantage of the proposed scheme over the single mode schemes in terms of system throughput.(5)A relay cooperation scheme is proposed for a backscatter communication system(BCS),where the user backscatters incident signals from a carrier emitter(CE)to an IR and a relay simultaneously,and then the relay forwards the user's information to the IR for throughput improvement.Two cases are considered that the relay is with an embedded energy source and the relay is without an embedded energy source.If the relay does not have an embedded energy source,it first harvests energy from the CE and then uses its harvested energy for information forwarding.For both cases,the time allocation problems on the user's information backscattering,the user's information forwarding,or the relay's energy harvesting are formulated to maximize the system throughput,and then closed-form solutions are derived.Simulation results demonstrate the advantage of the proposed relay cooperation scheme with the optimal time allocation in terms of system throughput.