Joint Clustering and Precoding for a Downlink Non-Orthogonal Multiple Access System with Multiple Antennas

In this paper, we first consider a downlink multiple-input multiple-output non-orthogonal multiple access (MIMO-NOMA) system with a base station and four users, where the users are equally divided into two clusters and inter-cluster interference is induced in transmission. Then we propose a joint clustering and precoding algorithm for the system such that not only the inter-cluster interference can be effectively eliminated but also the sum capacity can be enhanced. Specifically, a set of precoder pairs (one pair having two precoders for the two clusters) are constructed based on the eigenspaces of channel matrices, and one of them is selected to maximize the sum capacity. The proposed joint algorithm involves a high-complexity issue due to the need of exhaustive search for the best precoder pair. To reduce the search complexity, we explore the channel gain of each user and construct a reduced-size precoding set for selection. Combined with orthogonal frequency division multiplexing, the proposed four-user scheme is further extended to the general multiuser case, where every four users share a subcarrier for MIMO-NOMA transmission. Simulation results show that the proposed MIMO-NOMA scheme can provide more system capacity than a MIMO orthogonal multiple access approach.