Secrecy Outage Probability of a NOMA Scheme and Impact Imperfect Channel State Information in Underlay Cooperative Cognitive Networks Security performance and the impact of imperfect channel state information (CSI) in underlay cooperative cognitive networks (UCCN) is investigated in this paper. In the proposed scheme, relay R uses nonorthogonal multiple access (NOMA) technology to transfer messages     e 1    ,     e 2     from the source node S to User 1 (U     1    ) and User 2 (U     2    ), respectively. An eavesdropper (E) is also proposed to wiretap the messages of     U 1     and U     2    . The transmissionxe2x80x99s security performance in the proposed system was analyzed and performed over Rayleigh fading channels. Through numerical analysis, the results showed that the proposed systemxe2x80x99s secrecy performance became more efficient when the eavesdropper node E was farther away from the source node S and the intermediate cooperative relay R. The secrecy performance of U     1     was also compared to the secrecy performance of U     2    . Finally, the simulation results matched the Monte Carlo simulations well.

Performance Analysis of Cooperative NOMA Networks with Imperfect CSI over Nakagamim Fading Channels In this paper, we investigate a downlink cooperative nonorthogonal multiple access (NOMA) network with decodeandforward relaying, where two scenarios of user relaying with direct link and user relaying without direct link are discussed in detail. More particularly, the performance of cooperative NOMA system under the assumption of imperfect channel state information (ipCSI) is studied over Nakagamim fading channels. To evaluate the outage performance of the above discussed two scenarios, the closedform expressions of outage probability for a pair of users are derived carefully. The diversity orders of users are achieved in the high signaltonoise region. An error floor appears in the outage probability owing to the existence of channel estimation errors under ipCSI conditions. Simulation results verify the validity of our analysis and show that: (1) NOMA is superior to conventional orthogonal multiple access; (2) The best user relaying location for cooperative NOMA networks should be near to the base station; and (3) The outage performance of distant user with direct link significantly outperforms distant user without direct link by comparing the two scenarios.