  In this paper, we take a critical and fresh look at the downlink
multi-antenna NOMA literature. Instead of contrasting NOMA with OMA, we
contrast NOMA with two other baselines. The first is conventional Multi-User
Linear Precoding (MULP). The second is Rate-Splitting Multiple Access (RSMA)
based on multi-antenna Rate-Splitting (RS) and SIC. We show that there is some
confusion about the benefits of NOMA, and we dispel the associated
misconceptions. First, we highlight why NOMA is inefficient in multi-antenna
settings based on basic multiplexing gain analysis. We stress that the issue
lies in how the NOMA literature has been hastily applied to multi-antenna
setups, resulting in a misuse of spatial dimensions and therefore loss in
multiplexing gains and rate. Second, we show that NOMA incurs a severe
multiplexing gain loss despite an increased receiver complexity due to an
inefficient use of SIC receivers. Third, we emphasize that much of the merits
of NOMA are due to the constant comparison to OMA instead of comparing it to
MULP and RS baselines. We then expose the pivotal design constraint that
multi-antenna NOMA requires one user to fully decode the messages of the other
users. This design constraint is responsible for the multiplexing gain erosion,
rate loss, and inefficient use of SIC receivers in multi-antenna settings. Our
results confirm that NOMA should not be applied blindly to multi-antenna
settings, highlight the scenarios where MULP outperforms NOMA and vice versa,
and demonstrate the inefficiency, performance loss and complexity disadvantages
of NOMA compared to RS. The first takeaway message is that, while NOMA is not
beneficial in most multi-antenna deployments. The second takeaway message is
that other non-orthogonal transmission frameworks, such as RS, exist which
fully exploit the multiplexing gain and the benefits of SIC to boost the rate
in multi-antenna settings.
