  Non-orthogonal multiple access (NOMA) represents a paradigm shift from
conventional orthogonal multiple access (MA) concepts, and has been recognized
as one of the key enabling technologies for 5G systems. In this paper, the
impact of user pairing on the performance of two NOMA systems, NOMA with fixed
power allocation (F-NOMA) and cognitive radio inspired NOMA (CR-NOMA), is
characterized. For FNOMA, both analytical and numerical results are provided to
demonstrate that F-NOMA can offer a larger sum rate than orthogonal MA, and the
performance gain of F-NOMA over conventional MA can be further enlarged by
selecting users whose channel conditions are more distinctive. For CR-NOMA, the
quality of service (QoS) for users with the poorer channel condition can be
guaranteed since the transmit power allocated to other users is constrained
following the concept of cognitive radio networks. Because of this constraint,
CR-NOMA has different behavior compared to F-NOMA. For example, for the user
with the best channel condition, CR-NOMA prefers to pair it with the user with
the second best channel condition, whereas the user with the worst channel
condition is preferred by F-NOMA.
