  Enabled by hybrid grant-based (GB) and grant-free (GF) transmission
techniques, GF users of internet of things (IoT) devices and massive
machine-type communications (mMTC) meet opportunities to share wireless
resources with GB users. In this paper, we propose a rate splitting multiple
access (RSMA) strategy for an emerging semi-grant-free (SGF) transmission
system to increase connectivity and reliability. In the proposed RSMA assisted
SGF (RSMA-SGF) scheme, the GF users apply the rate splitting principle to
realize distributed contentions and utilize transmit power most effectively for
robust transmissions, meanwhile keeping themselves transparent to the GB user.
Compared to existing non-orthogonal multiple access (NOMA) assisted SGF
schemes, the RSMA-SGF scheme significantly decreases outage probability and
achieves full multiuser diversity gain without restricting the GB and GF users'
target rates to a limited value region. Exact expressions and asymptotic
analysis for the outage probability are provided to facilitate the system
performance evaluation of the proposed RSMA-SGF scheme. Computer simulation
results clarify the superior outage performance of the RSMA-SGF scheme and
verify the accuracy of the developed analytical results.
