  In this paper, we propose a cognitive radio-inspired rate-splitting multiple
access (CR-RSMA) scheme to assist semi-grant-free (SGF) transmissions in which
a grant-based user (GBU) and multiple grant-free users (GFUs) access the
base-station (BS) by sharing the same resource block. Using the cognitive radio
principle, the GBU and admitted GFU are treated as the primary and secondary
users, respectively, and rate-splitting is applied at the admitted GFU to
realize SGF transmissions. The admitted GFU's transmit power allocation, target
rate allocation, and successive interference cancellation decoding order at the
BS are jointly optimized to attain the maximum achievable rate for the admitted
GFU without deteriorating the GBU's outage performance compared to orthogonal
multiple access. Due to the extended non-outage zone, CR-RSMA-assised SGF
(CR-RSMA-SGF) transmissions achieve a lower outage probability than SGF
transmissions assisted by cognitive radio-inspired non-orthogonal multiple
access. Exact expressions and asymptotic analysis for the admitted GFU's outage
probability are derived to evaluate the system performance achieved by
CR-RSMA-SGF transmissions. The superior outage performance and full multiuser
diversity gain achieved by CR-RSMA-SGF transmissions are verified by the
analytical and simulation results.
