Power Allocation Schemes For Downlink Non-orthogonal Multiple Access Systems

In order to meet the requirements of the fifth generation(5G)mobile communication for low latency,high spectrum efficiency,and large-scale connections,a number of key technologies such as massive multiple input multiple output(MIMO),millimeter wave,and non-orthogonal multiple access(NOMA)have been proposed.For power domain NOMA,signals from multiple users with different powers allocated by base station are superimposed on the same time-frequency resource and successive interference cancellation(SIC)technology is used to detect the desired signals.Power domain NOMA uses power's difference to distinguish users,significantly improve spectral efficiency and reduce system latency.Power allocation affects the performance of the NOMA system,such as sum rate,fairness,and energy efficiency,etc.Therefore,power allocation in the NOMA system has received much attention in recent years.This paper mainly studies the power allocation schemes of the single cell downlink NOMA system.For the problems of existing schemes,the corresponding improvement methods are proposed.The main works of this paper are as follows:(1)The minimum rate requirements of users are not taken into consideration in the existing power allocation schemes for maximum fairness of NOMA systems.For this problem,the power allocation scheme for fairly improving user rate in NOMA systems is proposed.Firstly,the minimum power required for each cluster is calculated based on channel conditions and rate requirement for each user.Secondly,in the case of meeting the minimum rate requirements of all users,the power allocation optimization problem of improving each user's rate fairly is established,with the constraints of minimum power required by each cluster and the total power required by all clusters.Finally,the power allocation scheme,which satisfies the minimum rate demand of the user and fairly increases the user's rate,is obtained by adjusting the power of the part clusters.The simulation results show that when the minimum rate requirements of users are different,the increased minimum rates of users and the outage probabilities of the proposed scheme both outperform these of the existing schemes in the same scenario.(2)For a NOMA system with multiple clusters and each cluster consists arbitrarily users,firstly,an optimization problem of weighted sum rate maximization(WSRM)is formulated by optimizing power allocation among users,under the constraints of user rate requirement.Then,the optimization problem is decomposed into a series of sub-problems to maximize the weighted sum rate for a single cluster.Finally,the power allocation optimization problem among users is transformed into that between clusters,which is solved by water-filling algorithm.Simulation results show that the weighted sum rate and the outage probability of the proposed scheme is better than that of the existing schemes.(3)For a multi clusters NOMA system with arbitrary users in each cluster,an optimization problem of maximizing tradeoff between sum rate and energy efficiency is formulated by optimizing power allocation among users,under the constraints of user rate requirements.Then,the optimization problem is decomposed into a group of subproblems with the aim of maximizing sum rate and energy efficiency tradeoff for each cluster,which is solved by using bisection method and monotonicity of function.Finally,the power allocation optimization problem among users is transformed into that between clusters,and a two steps inter-cluster power allocation algorithm is developed to solve this problem.Simulation results show that the tradeoff between sum rate and energy efficiency of the proposed scheme is better than that of the existing schemes.