  Non-orthogonal multiple access (NOMA) has recently received considerable
attention as a promising candidate for 5G systems. A key feature of NOMA is
that users with better channel conditions have prior information about the
messages of the other users. This prior knowledge is fully exploited in this
paper, where a cooperative NOMA scheme is proposed. Outage probability and
diversity order achieved by this cooperative NOMA scheme are analyzed, and an
approach based on user pairing is also proposed to reduce system complexity in
practice.
