  Non-orthogonal multiple access (NOMA), as the newest member of the multiple
access family, is envisioned to be an essential component of 5G mobile
networks. The combination of NOMA and multi-antenna multi-input multi-output
(MIMO) technologies exhibits a significant potential in improving spectral
efficiency and providing better wireless services to more users. In this
article, we introduce the basic concepts of MIMO-NOMA and summarize the key
technical problems in MIMO-NOMA systems. Then, we explore the problem
formulation, beamforming, user clustering, and power allocation of
single/multi-cluster MIMO-NOMA in the literature along with their limitations.
Furthermore, we point out an important issue of the stability of successive
interference cancellation (SIC) that arises using achievable rates as
performance metrics in practical NOMA/MIMO-NOMA systems. Finally, we discuss
incorporating NOMA with massive/millimeter wave MIMO, and identify the main
challenges and possible future research directions in this area.
