Research On Key Technologies In Buffer-Aided Non-Orthogonal Multiple Access Networks

The emergence of various emerging services and the rapid growth of mobile data traffic have brought new demands and higher challenges for the sixth-generation(6G)wireless communication system.It is very important to fully explore and utilize the available network resources under limited spectrum resources to design wireless transmission optimization schemes to improve the network spectrum efficiency and energy efficiency.Non-orthogonal multiple access(NOMA)as a key technology for future wireless networks,which can achieve high spectrum efficiency,support massive connectivity and reduce energy consumption,has been widely used in the transmission design of various systems.In addition,taking full advantage of the time freedom brought by buffer for transmission scheme design and resource optimization can further improve the long-term network performance.Accordingly,focusing on the demand for high spectrum efficiency and energy efficiency in future wireless networks,this thesis studies how to use NOMA technology and buffer resources to design efficient transmission and resource optimization schemes for various network scenarios.The main contributions are summarized below:Firstly,this thesis considers the use of data and energy buffering in secure relay networks with simultaneous wireless information and power transfer(SWIPT),with the aim to increase the secrecy throughput.Specifically,it is assumed that the source node transmits information to the destination node by using half-duplex randomize-and-forward relaying with SWIPT,under the existence of a passive eavesdropper.For the case of a non-buffered relay,an optimal baseline policy is designed.Focusing on buffer-aided relaying,we maximize the secrecy throughput under the stability requirement for data and energy queues.Accordingly,we propose an adaptive secure transmission policy,which dynamically allocates the available resources,i.e.,time and power,with respect to the channel state information,as well as the states of the buffers.In addition,our analysis reveals the existence of a trade-off between the secrecy throughput and the average queue delay.Secondly,to increase the feasibility of wireless powered communication networks(WPCNs),the synergy of efficient multiple access protocols and diverse types of resources,e.g.,multiple antennas,memories,and energy storage devices is paramount.This thesis studies a bufferaided WPCN with NOMA,in which a multi-antenna hybrid access point(H-AP)can both transmit energy and receive information.Each buffered source sends information to the H-AP in a NOMA manner.The proposed scheme aims at maximizing the α-utility function,which is determined by the achieved average data rate and fairness among sources.Accordingly,we propose an online fairness scheduling scheme,which allocates the available resources according to the real-time buffer states and the channel state information.The derived analytical solutions reveal the impact of data buffers’ length on the decoding order of sources’ messages.It is shown that the proposed scheme achieves superior average data rate and fairness compared to the considered baseline schemes with non-buffered sources and buffered sources with orthogonal multiple access.Thirdly,a novel jointly adaptive transmission strategy for buffer-aided multiple-relay NOMA networks is proposed.Specifically,by leveraging time diversity brought by the data storage buffer,and multiple relays’ beamforming gains,the source node broadcasts data in the first hop and all relays perform distributed beamforming to transmit information to users with NOMA in the second hop.Our focus is on maximizing the average network throughput(ANT)by jointly optimizing the mode selection,power,and rate allocation at the source node,as well as the rate allocation and distributed beamforming coefficients at the relays along the time.As a solution to this complex problem,we first adopt the Lyapunov optimization methodology to convert the ANT maximization problem into the drift-plus-penalty function minimization problem at each time slot.Focusing on the non-convexity of the transformed problem at each time slot,a novel two-loop algorithm is proposed as an effective solution.Extensive performance evaluation results have revealed the superiority of the proposed scheme as compared to other benchmark access schemes.Finally,we study the application of reconfigurable intelligent surface(RIS)to cooperative non-orthogonal multiple access networks with SWIPT.We aim for maximizing the rate of the strong user with guaranteed weak user’s quality of service by jointly optimizing power splitting factors,beamforming coefficients,and RIS reflection coefficients in two transmission phases.The formulated problem is difficult to solve due to its complex and nonconvex constraints.To tackle this challenging problem,we first use the alternating optimization framework to transform it into three subproblems and then apply the penalty-based arithmetic-geometric-mean approximation algorithm and the successive convex approximation-based method to solve them.Numerical results verify the superiority of the proposed algorithm over the baseline schemes.