  Non-orthogonal multiple access (NOMA) has been recognized as a key driving
technology for the fifth generation (5G) and beyond 5G cellular networks. For a
practical dowlink NOMA system with imperfect successive interference
cancellation (SIC), we derive bounds on channel coefficients and power
allocation factors between NOMA users to achieve higher rates than an
equivalent orthogonal multiple access (OMA) system. We propose an adaptive user
pairing (A-UP) algorithm for NOMA systems. Through extensive simulations, we
show that NOMA with imperfect SIC is not always superior to OMA. Further, the
proposed A-UP algorithm results in better performance than state-of-the-art
NOMA pairing algorithms in presence of SIC imperfections.
