  This letter studies the impact of relay selection (RS) on the performance of
cooperative non-orthogonal multiple access (NOMA). In particular, a two-stage
RS strategy is proposed, and analytical results are developed to demonstrate
that this two-stage strategy can achieve the minimal outage probability among
all possible RS schemes, and realize the maximal diversity gain. The provided
simulation results show that cooperative NOMA with this two-stage RS scheme
outperforms that with the conventional max-min approach, and can also yield a
significant performance gain over orthogonal multiple access.
