Semi-orthogonal Precoder for Improving Throughput and Fairness in Downlink NOMA-MIMO Systems

In Non-Orthogonal Multiple Access (NOMA) systems, the base station can simultaneously transmit data from an antenna to a group of users. In this group, a user with higher channel gains performs Successive Interference Cancellation (SIC) to mitigate interference from the signal destined for the user with a weak channel. Typically, downlink NOMA-MIMO (Multiple-Input Multiple-Output) systems use precoders that form beams focused on users with higher channel gains of each group. This approach induces interference on the users with low gain, degrading throughputs and fairness in the network. This paper proposes a novel semi-orthogonal precoder that forms non-orthogonal beams for users within each group, while maintaining orthogonality between the groups. Simulations show the efficiency of this approach in improving the geometric mean user throughput.