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 downlink NOMA system with imperfect successive interference cancellation (SIC), we derive bounds on power allocation factors and formulate a minimum signal-to-interference-plus-noise ratio (SINR) difference criterion for NOMA pair formation. Through extensive simulations, we show the effect of imperfect SIC on the rate performance of a NOMA pair and the trade-off with its OMA (orthogonal multiple access) counterparts. We propose an adaptive user pairing (A-UP) algorithm for achieving better user rates. Further, the proposed A-UP algorithm results in better performance than state-of-the-art NOMA pairing algorithms in the presence of SIC imperfections. © 2012 IEEE.