  Semi-grant-free (SGF) transmission has recently received significant
attention due to its capability to accommodate massive connectivity and reduce
access delay by admitting grant-free users to channels which would otherwise be
solely occupied by grant-based users. In this paper, a new SGF transmission
scheme that exploits the flexibility in choosing the decoding order in
non-orthogonal multiple access (NOMA) is proposed. Compared to existing SGF
schemes, this new scheme can ensure that admitting the grant-free users is
completely transparent to the grant-based users, i.e., the grant-based users'
quality-of-service experience is guaranteed to be the same as for orthogonal
multiple access. In addition, compared to existing SGF schemes, the proposed
SGF scheme can significantly improve the robustness of the grant-free users'
transmissions and effectively avoid outage probability error floors. To
facilitate the performance evaluation of the proposed SGF transmission scheme,
an exact expression for the outage probability is obtained and an asymptotic
analysis is conducted to show that the achievable multi-user diversity gain is
proportional to the number of participating grant-free users. Computer
simulation results demonstrate the performance of the proposed SGF transmission
scheme and verify the accuracy of the developed analytical results.
