  This paper proposes a new communication protocol for a cooperative
non-orthogonal multiple access (NOMA) system. In this system, based on users'
channel conditions, each two NOMA users are paired to reduce system complexity.
In this concern, the user with a better channel condition decodes and then
forwards messages received from the source to the user with a worse channel
condition. In particular, the direct link between the paired users is assumed
to be unavailable due to the weak transmission conditions. To overcome this
issue, we propose a new cooperative NOMA protocol in which an
amplify-and-forward (AF) relay is employed to help the user-to-user
communications. To evaluate the proposed protocol, the exact closed-form
expressions of outage probability (OP) at the two paired users are derived.
Based on the analysis of the OP, we further examine the system throughput in a
delay-sensitive transmission mode. Finally, our analytical results verified by
Monte-Carlo simulation show that the proposed protocol is efficient in
enhancing the performance of NOMA system when the user-to-user communications
is limited.
