On Sum Rate Maximization For Intelligent Reflecting Surface Assisted NOMA System

With the rapid development of wireless communications,the future wireless communication systems will support more number of devices and services,which will inevitably require the higher transmission rates.Therefore,there is an urgent need to develop some innovative technologies to achieve sustainable growth of future mobile network traffic with low cost,complexity and energy consumption.Recently,intelligent reflecting surface(IRS)has been proposed to provide new approaches and ideas to solve the above problems.IRS consists of a large number of low-cost reflecting elements,and by adjusting the phase shifts of all reflecting elements,the propagation of reflected signals can be intelligently configured.In this case,the wireless propagation environment is changed from passive adaptation to active controllability,thus building an intelligent wireless environment.Besides,the non-orthogonal multiple access technology,as a key technology beyond5 G,can greatly improve the communication efficiency and spectrum utilization of the communication system.Therefore,combining the two effective techniques can further improve the transmission performance of communication systems.In this thsis,the investigation on the performance of IRS-assisted NOMA is carried out in order to improve the effectiveness and reliability of the communication system.The main research works are given as follows:(1)For the IRS-assisted downlink NOMA information transmission system,an optimization problem on decoding order,power allocation,and phase shift is established in order to maximize the sum rate of the system.Firstly,a low-complexity user ordering scheme is proposed based on the sum of combined channel gains,in which the effect of phase shift is also considered,and then the optimal user ordering scheme is obtained using the Rayleigh quotient and eigenvalue decomposition method.Secondly,to optimize the rate performance and ensure user fairness,an alternating optimization algorithm is used to design the power allocation coefficient of the base station and the phase shift coefficient of the IRS.Specifically,when solving the power allocation coefficient,the optimal solution is obtained by using the equal signal-to-noise ratio of each user;when solving the subproblem of the IRS phase shift coefficients,a discrete phase shift design model is used by considering the realization of the practical system,and a sequential rotation algorithm is developed to determine the optimal value of each phase shift.The simulation results show that the optimization scheme proposed in the paper can significantly improve the system sum rate compared with a variety of existing schemes.(2)For the IRS-assisted downlink NOMA secure communication system,an optimization problem on the transmit power of the base station,the power allocation coefficient and the reflecting phase shift is established in order to maximize the secure rate of the system.Firstly,due to the Qo S requirements,a minimum transmit power that satisfies the Qo S requirements of all users must exist,so the minimum transmit power of the system is obtained by sequentially solving for the power of each user.Secondly,to further improve the transmission rate of legitimate users,the power allocation coefficient of the base station and the phase shift coefficient of the IRS are jointly optimized.Specifically,when solving the power allocation coefficient,the problem is transformed into searching a common solution for each maximization subproblem by introducing the auxiliary variables,and the closed-form solution of the power allocation coefficient of the base station is obtained by solving the KKT condition;when solving the subproblem of IRS phase shift coefficients,the optimal solution of the problem is found by using a block coordinate descent algorithm.The simulation results show that the optimization scheme can effectively improve the secure rate of legitimate users.