  This paper proposes a tractable solution for integrating non-orthogonal
multiple access (NOMA) into massive machine-type communications (mMTC) to
increase the uplink connectivity. Multiple transmit power levels are provided
at the user end to enable open-loop power control, which is absent from the
traditional uplink NOMA with the fixed transmit power. The basics of this
solution are firstly presented to analytically show the inherent performance
gain in terms of the average arrival rate (AAR). Then, a practical framework
based on a novel power map is proposed to associate a set of well-designed
transmit power levels with each geographical region for handling the no
instantaneous channel state information problem. Based on this framework, the
semi-grant-free (semi-GF) transmission with two practical protocols is
introduced to enhance the connectivity, which has higher AAR than both the
conventional grand-based and GF transmissions. When the number of active GF
devices in mMTC far exceeds the available resource blocks, the corresponding
AAR tends to zero. To solve this problem, user barring techniques are employed
into the semi-GF transmission to stable the traffic flow and thus increase the
AAR. Lastly, promising research directions are discussed for improving the
proposed networks.
