  This letter proposes a modified non-orthogonal multiple-access (NOMA) scheme
for systems with a multi-antenna base station (BS) and two single-antenna
users, where NOMA transmissions are conducted only when the absolute
correlation coefficient (CC) between the user channels exceeds a threshold and
the BS uses matched-filter (MF) precoding along the user with the stronger
average channel gain. We derive the average minimal transmit power to guarantee
the signal-to-interference-plus-noise-ratio (SINR) levels of both users. Our
results show that the average minimal power grows logarithmically in the
reciprocal of the CC threshold and a non-zero threshold is necessary for the
modified NOMA scheme to have finite average minimal transmit power. Further,
for the massive MIMO scenario, we derive the scaling laws of the average
transmit power and outage probability with respect to the antenna numbers, as
well as their tradeoff law. Simulation results are shown to validate our
theoretical results.
