Amplify-and-forward relay selection in cooperative non-orthogonal multiple access

Cooperative non-orthogonal multiple access (NOMA) assisted by a dedicated relay is an effective way to improve the spectral efficiency and reliability of a cognitive radio network. Two-stage relay selection is useful in cognitive radio scenario that consists of primary user and secondary user. However, the performance of existing decode-and-forward (DF) relay selection is limited by the decoding requirements of DF relay. This paper proposes an amplify-and-forward (AF) relay selection for cooperative NOMA in cognitive radio network. A relay is selected from a group of AF relays to forward messages to a primary user and a secondary user. The spatial randomness of relays are captured in this paper by applying stochastic geometry tools. In addition, the impact of power allocation coefficient is also investigated in this paper. Simulation results demonstrate the superior performance gains of the proposed scheme over the existing scheme assisted by DF relays in terms of outage probability and ergodic rate.