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Non-orthogonal multiple access (NOMA) and millimeter wave (mmWave) are twokey enabling technologies for the fifth-generation (5G) mobile networks andbeyond. In this paper, we consider uplink communications with a hybridbeamforming structure and focus on improving the spectral efficiency (SE) andenergy efficiency (EE) of mmWave multiple-input multiple-output (MIMO)-NOMAsystems with enhanced user grouping and power allocation. Exploiting thedirectionality feature of mmWave channels, we first propose a novel initialagglomerative nesting (AGNES) based user grouping algorithm by taking advantageof the channel correlations. It is noted that the optimization of the SE/EE isa challenging task due to the non-linear programming nature of thecorresponding problem involving user grouping, beam selection, and powerallocation. Our idea is to decompose the overall optimization problem into amixed integer problem comprising of user grouping and beam selection only,followed by a continuous problem involving power allocation and digitalbeamforming design. To avoid the prohibitively high complexity of thebrute-force search approach, we propose two suboptimal low-complexity usergrouping and beam selection schemes, the direct AGNES (DIR-AGNES) scheme andthe successive AGNES (SUC-AGNES) scheme. We also introduce the quadratictransform (QT) to recast the non-convex power allocation optimization probleminto a convex one subject to a minimum required data rate of each user. Thecontinuous problem is solved by iteratively optimizing the power and thedigital beamforming. Extensive simulation results have shown that our proposedmmWave-NOMA design outperforms the conventional orthogonal multiple access(OMA) scenario and the state-of-art NOMA schemes.

Enhanced User Grouping and Power Allocation for Hybrid mmWave MIMO-NOMA systems