  Being able to accommodate multiple simultaneous transmissions on a single
channel, non-orthogonal multiple access (NOMA) appears as an attractive
solution to support massive machine type communication (mMTC) that faces a
massive number of devices competing to access the limited number of shared
radio resources. In this paper, we first analytically study the throughput
performance of NOMA-based random access (RA), namely NOMA-RA. We show that
while increasing the number of power levels in NOMA-RA leads to a further gain
in maximum throughput, the growth of throughput gain is slower than linear.
This is due to the higher-power dominance characteristic in power-domain NOMA
known in the literature. We explicitly quantify the throughput gain for the
very first time in this paper. With our analytical model, we verify the
performance advantage of the proposed NOMA-RA scheme by comparing with the
baseline multi-channel slotted ALOHA (MS-ALOHA), with and without capture
effect. Despite the higher-power dominance effect, the maximum throughput of
NOMA-RA with four power levels achieves over three times that of the MS-ALOHA.
However, our analytical results also reveal the sensitivity of load on the
throughput of NOMA-RA. To cope with the potential bursty traffic in mMTC
scenarios, we propose adaptive load regulation through a practical user barring
algorithm. By estimating the current load based on the observable channel
feedback, the algorithm adaptively controls user access to maintain the optimal
loading of channels to achieve maximum throughput. When the proposed user
barring algorithm is applied, simulations demonstrate that the instantaneous
throughput of NOMA-RA always remains close to the maximum throughput confirming
the effectiveness of our load regulation.
