Research On Antenna Diversity Technique In MIMO-NOMA Systems

The fifth generation(5G)wireless communication networks require higher spectral efficiency,lower latency,and massive connectivity.As a very promising multiple access technology,nonorthogonal multiple access(NOMA)can effectively solve these problems,so NOMA has been one of the key technologies in 5G.In order to further improve the spectral efficiency,NOMA researches have been extended to multiple-input multiple-output(MIMO)systems,i.e.,the data transmission rates in NOMA systems are increased further by using spatial multiplexing techniques.On the other hand,the space resources provided by MIMO can be also utilized for antenna diversity or space-time coding to improve the outage performance and the reliability of data transmission in NOMA systems,but the research in this area is just beginning and there are relatively few results in this area.Thus,for downlink NOMA systems with non-perfect channel state information(CSI)in open-loop scenarios,antenna diversity techniques are utilized to improve the systems' outage performance.The main contributions and innovations of this paper are summarized as follows:1.Under the condition that the transmitter only obtains partial CSI,i.e.,the base station(BS)only knows each user's distance and channel direction information,this paper designs the precoding and detection vectors based on antenna diversity,which makes the proposed MIMO-NOMA system degenerate to a single-input single-output(SISO)-NOMA system with statistical CSI.The outage analysis indicates that after implementing the proposed antenna diversity scheme,NOMA system can achieve the maximum diversity order,which improves the system's outage performance.2.Under the condition that the transmitter only obtains statistical CSI,i.e.,the BS only knows the statistical characteristic of each user's channel matrix,the precoding and detection vectors based on antenna diversity are also designed,which also makes the proposed MIMO-NOMA system degenerate to a SISO-NOMA system with statistical CSI.However,the outage analysis shows that after implementing the proposed antenna diversity scheme,NOMA system only achieves receive diversity and the acquisition of transmit diversity needs to be further combined with space-time coding.3.Based on SISO-NOMA systems with statistical CSI,a dynamic power allocation scheme is proposed in this paper.This power allocation scheme guarantees the quality of service of ergodic rates for the weaker users(except the cluster-head)and maximizes the ergodic rate of cluster-head in each NOMA group.The outage analysis shows that the users' diversity orders are linear withtheir successive interference cancellation(SIC)decoding orders,and the user has a larger diversity order when its message has higher priority in SIC decoding.4.User clustering also plays an important role in NOMA systems,so under the proposed power allocation scheme,by comparing the corresponding orthogonal multiple access system,this paper analyzes the impact of user clustering on the ergodic sum rate in the considered NOMA group,and then designs a low complexity but very effective user clustering algorithm.