Precoder for Proportional Fair Resource Allocation in Downlink NOMA-MIMO Systems

Non-Orthogonal Multiple Access (NOMA) allows the base station (BS) to simultaneously transmit data to such a number of users that exceeds the number of transmit antennas. For that, the users are grouped according to their channel conditions and the user with a stronger channel uses a method called Successive Interference Cancellation (SIC) to reduce interference from the signals for other users in a group with a weaker channel. In downlink NOMA-MIMO (Multiple-Input Multiple-Output) systems the BS uses specific precoders to serve multiple users. Typically, such precoders form beams focused on users with higher channel gains of each group. Such an approach leads to a high interference at users with lower channel gain and, as a result, their throughput degrades and fairness in the network decreases. To solve this problem, the paper proposes a novel semi-orthogonal precoder that forms non-orthogonal beams for users within each group while maintaining orthogonality between the groups. The obtained simulation results show that the proposed precoder significantly improves geometric mean user throughput and outperforms the state-of-the-art approach in the area of moderate signal-to-interference-plus-noise ratio and can be used jointly with existing precoders for downlink NOMA-MIMO systems.