  Grant-free transmission is an important feature to be supported by future
wireless networks since it reduces the signalling overhead caused by
conventional grant-based schemes. However, for grant-free transmission, the
number of users admitted to the same channel is not caped, which can lead to a
failure of multi-user detection. This paper proposes non-orthogonal
multiple-access (NOMA) assisted semi-grant-free (SGF) transmission, which is a
compromise between grant-free and grant-based schemes. In particular, instead
of reserving channels either for grant-based users or grant-free users, we
focus on an SGF communication scenario, where users are admitted to the same
channel via a combination of grant-based and grant-free protocols. As a result,
a channel reserved by a grant-based user can be shared by grant-free users,
which improves both connectivity and spectral efficiency. Two NOMA assisted SGF
contention control mechanisms are developed to ensure that, with a small amount
of signalling overhead, the number of admitted grant-free users is carefully
controlled and the interference from the grant-free users to the grant-based
users is effectively suppressed. Analytical results are provided to demonstrate
that the two proposed SGF mechanisms employing different successive
interference cancelation decoding orders are applicable to different practical
network scenarios.
