  A novel semantics-empowered two-user uplink non-orthogonal multiple access
(NOMA) framework is proposed for resource efficiency enhancement. More
particularly, a secondary far user (F-user) employs the semantic communication
(SemCom) while a primary near user (N-user) employs the conventional bit-based
communication (BitCom). The fundamental performance limit, namely
semantic-versus-bit (SvB) rate region, of the proposed semantics-empowered NOMA
framework is characterized. The equivalent SvB rate region achieved by the
conventional BitCom-based NOMA is provided as the baseline scheme. It unveils
that, compared to BitCom, SemCom can significantly improve the F-user's
performance when its permitted transmit power is strictly capped, but may
perform worse when its permitted transmit power is high. Guided by this result,
the proposed semantics-empowered NOMA framework is investigated over fading
channels. An opportunistic SemCom and BitCom scheme is proposed, which enables
the secondary F-user to participate in NOMA via the most suitable communication
method at each fading state, thus striking a good tradeoff between its own
achieved performance and the interference imposed to the primary N-user. Two
scenarios are considered for employing the opportunistic scheme, namely on-off
resource management and continuous resource management. For each scenario, the
optimal communication policy over fading channels is derived for maximizing the
ergodic semantic rate achieved at the secondary F-user, subject to the minimum
ergodic bit rate constraint of the primary N-user. Numerical results show that
the proposed opportunistic scheme in both scenarios can achieve higher
communication performance for NOMA than the baseline schemes merely employing
SemCom or BitCom.
