Capacity Improvement with NOMA-CI Precoding for MU-MISO Downlink Systems

In the communication process of the Internet of Things (IoT), non-orthogonal multiple access (NOMA) technology is usually used to improve spectrum efficiency and system capacity. However, due to the wide application of multimedia in the IoT, the high amount of data generated puts a higher demand on the system capacity. Each targeted user in a NOMA group uses successive interference cancellation (SIC) to decode useful information where the signals from other users are usually taken as interference, which hinders the improvement of the system capacity. In addition, the transmit power distribution between groups does not consider edge users, which affects the fairness of users. To solve above problems, we propose to combine the constructive interference (CI) precoding with NOMA by making the interference between users constructive for each NOMA user. Specifically, the NOMA-CI precoding is dedicatedly designed to improve the system capacity and a proportional power distribution scheme between NOMA groups is deployed to improve the capacity for edge users. We conduct extensive simulation experiments by comparing with existing schemes under different modulation methods, which demonstrate our scheme can improve the system capacity than the NOMA scheme by about 60% and achieve significant increase of the edge user capacity, enhancing the system fairness.