Research On Throughput Optimization For Wireless Powered Communication Networks With Non-Orthogonal Multiple Access

With the vigorous development of the Internet of Things and the deepening of the concept of green communication,scholars focus on improving the utilization of wireless communication resources and using renewable energy as the main energy supply of wireless communication networks.For low-power devices such as sensors deployed in the Internet of Things,manual battery replacement or wired charging is usually difficult to achieve due to the large number of devices and the possible distribution in harsh ecological environment.In order to ensure that mobile terminals can work continuously for a long time,energy harvesting(EH)technology based on radio frequency(RF)signals has become a hot research topic in recent years.Based on these premises,Wireless powered communication network(WPCN)is widely used in the field of Internet of Things.In WPCN,wireless devices supply power to themselves remotely through energy harvesting technology,which prolong the service life of energy-constrained devices.However,the current practical application of WPCN still faces many shortcomings,such as low energy transmission efficiency,low user fairness and low system throughput.On the other hand,Non-orthogonal multiple access(NOMA)technology,as a key technology for 5G,can provide higher spectrum efficiency,which provides great convenience for resource conservation.For this reason,some scholars have introduced NOMA technology into WPCN,and use NOMA technology to simultaneously transmit information in uplink information transmission,which greatly increases user fairness and system throughput.For NOMA-based WPCN,it is necessary to consider the size of the collected energy,the limitation of the transmission power,and the transmission time of the information.Therefore,resource allocation problems such as time and power become complicated.For the actual demand of massive information transmission in future communication,how to optimize the transmission scheme of NOMA-based WPCN to improve system throughput is of great significance.This thesis is aimed at this background.The main contributions and innovations of this thesis are summarized as follows.(1)The SIC constraints in NOMA-WPCN have an impact on system throughput and user fairness,which is more pronounced when the number of users increases.For this reason,this thesis proposes a new scenario for NOMA-WPCN: clustering users in the network.All users first collect energy from the power beacon,and then the two users in a cluster simultaneously transmit information to the base station based on the harvested energy via NOMA,while time division multiple access(TDMA)is used among clusters.Since the energy collected during the energy transmission phase affects the transmission power of the users,this thesis studies the joint optimization of energy transmission time,information transmission time and user's transmission power to maximize system throughput.The author constructs an optimization problem by considering the causality of energy collection and the SIC constraints,and proves the optimization problem is convex.By optimizing power and time allocation,an optimal time allocation scheme is proposed.The simulation results show that,compared with the NOMA-WPCN without user clustering,the time optimization algorithm proposed in this thesis improves the system throughput and can significantly improve the network performance.Simulation results also reveal the effect of threshold power in SIC constraints on the system throughput of NOMA-WPCN.(2)In order to further expand the network coverage,this thesis studies the relay network.In order to improve the system performance and improve the system throughput of users,this thesis studies the wireless powered relay network based on NOMA.The system throughput of this scheme is limited by the causality of relay acquisition energy and transmission power.In the process of energy and information transmission,the relay first collects radio frequency energy signal broadcasted by the base station,and then the user transmits information to the relay through TDMA.After relays decoding and forwarding,all relays send information to the base station simultaneously through NOMA.In this scenario,the optimization model of the system throughput is constructed.In order to maximize the system throughput,the methods are used to optimize the energy transmission time and information transmission time,and the sub-gradient algorithm is used to obtain the optimal solution of the problem.The simulation results verify the performance superiority of the wireless power relay network using NOMA.Compared with the wireless power relay network using equal time allocation,the proposed scheme can achieve higher system throughput.