  The aim of this paper is to reveal the importance of hybrid successive
interference cancellation (SIC) and power adaptation (PA) for improving
transmission robustness of uplink non-orthogonal multiple access (NOMA).
Particularly, a cognitive radio inspired uplink NOMA communication scenario is
considered, where one primary user is allocated one dedicated resource block,
while M secondary users compete with each other to be opportunistically served
by using the same resource block of the primary user. Two novel schemes are
proposed for the considered scenario, namely hybrid SIC with PA (HSIC-PA)
scheme and fixed SIC with PA (FSIC-PA) scheme. Both schemes can ensure that the
secondary users are served without degrading the transmission reliability of
the primary user compared to conventional orthogonal multiple access (OMA)
based schemes. Rigorous analytical results are presented to evaluate the
performance of the proposed two schemes. It is shown that both schemes can
avoid outage probability error floors without any constraints on users' target
rates in the high SNR regime. Furthermore, it is shown that the diversity gain
achieved by the HSIC-PA scheme is M, while that of the FISC-PA scheme is only
1. Numerical results are provided to verify the developed analytical results
and also demonstrate the superior performance achieved by the proposed schemes
by comparing with the existing HSIC without PA (HSIC-NPA) scheme. The presented
simulation results also show that HSIC-PA scheme performs the best among the
three schemes, which indicates the importance of the combination of HSIC and PA
for improving transmission robustness.
