Digital beamsteering (DBS) is an appealing technique to overcome the severe millimeter-wave path loss, with less complexity and less channel feedback. In multiuser systems, DBS generates a beam toward each user. However, in congested cells, such a system suffers from high inter-user interference causing by the neighboring beams. In this paper, a new approach of space division multiple access (SDMA) with power-domain non-orthogonal multiple access (PD-NOMA) is considered for millimeter-wave channel to significantly reduce the inter-user interference and thus enhance the overall throughput. The proposed scheme is designed by selecting the users located at the same direction to share one beam, in which PD-NOMA multiplexes the users in power domain. The proposed user clustering method built upon an interference metric, and followed by a fixed power allocation method based on the received power, makes possible the design of a low-feedback NOMA-DBS scheme. Simulation results, based on the New York university mm-wave simulator, show that the multiuser MIMO system can achieve higher sum-rate with our proposed joint SDMA and PD-NOMA scheme, with respect to only SDMA (e.g., up to 20 bps/Hz rate gain for 45 users in the cell). Index Terms-Beamforming, multiuser MIMO, millimeterwave, antenna array, power-domain NOMA.

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Thanks to the high directionality of millimeter-wave (mmWave) channels, angle-domain beamforming is an appealing technique for multi-user multiple-input multiple-output (MU-MIMO) in terms of sum-throughput performance and limited feedback. By utilizing only the angular information of users at the transmitter, we propose an angle-domain non-orthogonal multiple access (NOMA) scheme to enhance the sum-throughput of the mmWave MU-MIMO system, especially in congested cells. We first derive a set of angular-based performance metrics, such as the inter-user spatial interference, the user channel quality, and the sum-throughput, by exploiting the specific features of the mmWave propagation. Then, a multi-user clustering algorithm is developed based on the spatial interference metric, and a new user ordering strategy is proposed using the angular-based channel quality metric. Additionally, we design a power allocation method that maximizes the angular-based sum-throughput. Extensive numerical results show that the proposed scheme significantly improves the performance of the mmWave MU-MIMO system by achieving up to 39% increase in the spectral efficiency when the number of users is closed to the number of antennas. Moreover, we find that the proposed user ordering strategy outperforms other limited feedback strategies, and the angular-based power allocation allows for efficient successive interference cancellation.INDEX TERMS Millimeter-wave networks, multiple-input multiple-output, beamforming, non-orthogonal multiple access.