Research On Hybrid Precoding-Based Massive MIMO-NOMA Systems

Non-Orthogonal Multiple Access(NOMA)technology and millimeter-wave massive multiple-input multiple-output(MIMO)technology as the key technologies for 5G communications,the combination of them i.e.,the millimeter-wave massive MIMO-NOMA,can significantly improve the spectrum efficiency and increase the number of users,further increasing the system capacity.However,there are two serious technical obstacles in the massive MIMO-NOMA communication system.The first problem is that the number of users is much larger than the number of radio-frequency(RF)chains,which can lead to severe interbeam and intra-beam interference,i.e.,inter-user interference problem.The other problem is that the number of users in the 5G communication system has increased dramatically,making the energy consumption greatly increased,i.e.,power allocation problem.Therefore,inter-user interference and power allocation are the two essential challenges that need to be solved in 5G communication systems.Based on the above mentioned,the main research work of this thesis is as follows.(1)We study the user grouping and the hybrid precoding scheme in the downlink millimeter-wave massive MIMO-NOMA system.Considering the scenario of physically clustering users,we utilize the directionality of millimeter-wave channels based on the channel correlation between users to achieve user grouping,which makes the channel correlation of users in different beams is high and the channel correlation of users in the same beam is low to suppress the inter-user interference.Then we design a hybrid precoding scheme to eliminate the inter-user interference,and we design the analog precoder by aligning the channels to maximize the channel array gain,and then use the zero-forcing algorithm to further eliminate the inter-user interference and maximize the SINR of the users.(2)After performing the user grouping and hybrid precoding,it is very difficult to solve the power allocation for each user in the downlink millimeter-wave massive MIMO-NOMA system,so the thesis converts the non-convex sum rate efficiency maximization problem into a convex intercluster power allocation problem so that the closed solution of the power allocation problem can be found quickly and effectively according to the KKT condition.The simulation results show that the proposed scheme can effectively improve the spectral efficiency of the system.(3)Energy efficiency optimization is also a critical issue in millimeter-wave communication systems.In this thesis,we constructed the energy-efficiency optimization problem in the downlink millimeter-wave massive MIMO-NOMA system under the minimum rate constraint of users,and we propose a fast iterative algorithm based on the Dinkelbach method to obtain the closed-form solution of the power allocation by Lagrange dual and sub-gradient algorithms.The simulation results show that the proposed power allocation algorithm can achieve superior energy efficiency performance.