  Hybrid non-orthogonal multiple access (NOMA) has recently received
significant research interest due to its ability to efficiently use resources
from different domains and also its compatibility with various orthogonal
multiple access (OMA) based legacy networks. Unlike existing studies on hybrid
NOMA that focus on combining NOMA with time-division multiple access (TDMA),
this work considers hybrid NOMA assisted orthogonal frequency-division multiple
access (OFDMA). In particular, the impact of a unique feature of hybrid NOMA
assisted OFDMA, i.e., the availability of users' dynamic channel state
information, on the system performance is analyzed from the following two
perspectives. From the optimization perspective, analytical results are
developed which show that with hybrid NOMA assisted OFDMA, the pure OMA mode is
rarely adopted by the users, and the pure NOMA mode could be optimal for
minimizing the users' energy consumption, which differs from the hybrid TDMA
case. From the statistical perspective, two new performance metrics, namely the
power outage probability and the power diversity gain, are developed to
quantitatively measure the performance gain of hybrid NOMA over OMA. The
developed analytical results also demonstrate the ability of hybrid NOMA to
meet the users' diverse energy profiles.
