Error Performance of NOMABased Cognitive Radio Networks With Partial Relay Selection and Interference Power Constraints Nonorthogonal multiple access (NOMA)based cognitive radio (CR) networks have recently emerged as a promising solution to enhance the spectral efficiency and massive connectivity problems. In this paper, we investigate the error rate performance of relayassisted NOMA with partial relay selection in an underlay cognitive radio network. In this setup,    K    relays are used to assist in transmission between secondary NOMA users and a secondary base station (SBS), where the relay (R) with the strongest link with the SBS is selected to amplifyandforward (AF) its received signals to the secondary receivers. We derive an accurate approximation for the pairwise error probability (PEP) of the secondary users with imperfect successive interference cancellation (SIC). Subsequently, the derived PEP expression is utilized to deduce a union bound, which is considered as an upper bound on the bit error rate (BER). We further formulate an optimization problem to calculate the optimum power coefficients that minimize the derived union bound. Numerical and Monte Carlo simulation results are presented to corroborate the derived analytical expressions and give some useful insights into the error rate performance of each user.

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.