Resource Allocation In Wireless Powered Communication Networks With Hybrid Multiple Access

Using RF-based wireless energy harvesting techniques,users in the Wireless Powered Communication Networks(WPCNs)can harvest energy from wireless signal,which addresses the problem of energy supply constraints of the conventional wireless networks.Current researches on WPCNs mainly focus on Time Division Multiple Access(TDMA)and Non-Orthogonal Multiple Access(NOMA).However,due to the dependence of the users on the harvested energy,WPCNs with TDMA will result in more serious system unfairness than the traditional wireless networks,and WPCNs with NOMA will lead to unacceptable high decoding complexity at the receiver.Therefore,it's necessary to combine TDMA and NOMA,fully utilize the high system fairness and throughput enhancement brought by NOMA,as well as the simply non-interference transmission of TDMA,further improve the network performance.In this context,we focus on resource allocation in WPCNs with hybrid multiple access.We consider the so called harvest-then-transmit WPCNs,whose frame is divided into energy harvesting phase and information transmission phase.During the energy harvesting phase,the BS transmits energy to the users,and d uring the information transmission phase,users are divided into multiple user groups,where NOMA is implemented within each group and different groups are allocated with orthogonal time resources.Firstly,with the optimization objective of maximizing system throughput,optimal allocation of the user pairing algorithm and the durations of the energy harvesting and information transmission phases,the optimization problems of this paper are listed.Since the formulated optimization problem is a mixed-integer nonlinear programming problem,a heuristic step-by-step solution is proposed in this paper to solve the formulated optimization problem.Secondly,based on the dependence of the users on the harvested energy,an energy harvesting user pairing algorithm is proposed.According to users' harvested energy,energy harvesting user pairing algorithm divides the users into strong users and weak users,and gives corresponding user pairing schemes for the three situations with different relative numbers of strong users and weak users.Then we derived the optimal value of the durations of the energy harvesting and the multiple user groups' information transmission.Since the user pairing is completed,it is proved that the system throughput is a concave function of the durations of the energy harvesting and information transmission of each user group.And then we show that the formulated optimization problems can be optimally and efficiently solved by dual function method,thus the closed-form expression of the optimal time allocation is obtained.Finally,the simulation results show that the resource allocation algorithm proposed in this paper of WPCNs with hybrid multiple accesses achieves higher performance in terms of system throughput,fairness and energy efficiency.The energy harvesting user pairing algorithm achieves high system throughput among the three different scenarios: users are uniformly distributed in the cell,users are concentrated in the cell edge,and users are centrally distributed in the cell center.