  This paper considers a cognitive radio inspired uplink communication
scenario, where one primary user is allocated with one dedicated resource
block, while $M$ secondary users compete with each other to opportunistically
access the primary user's channel. Two new designs of NOMA schemes, namely
hybrid successive interference cancellation with power adaptation (HSIC-PA) and
fixed successive interference cancellation with power adaptation (FSIC-PA), are
proposed. The significant advantages of the proposed schemes are two folds.
First, the proposed two schemes can ensure that the secondary users are
opportunistically served without degrading the transmission reliability of the
primary user. Besides, the transmission robustness of the served secondary
users can be guaranteed. Specifically, the outage probability error floors can
be avoided for the secondary users, which is proved by asymptotic analysis in
the paper. Extensive simulation results are also provided to demonstrate the
superior performance of the proposed schemes.
