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Non-orthogonal multiple access (NOMA) system can meet the demands of ultra-high data rate, ultra-low latency, ultra-high reliability and massive connectivity of user devices (UE). However, the performance of the NOMA system may be deteriorated by the hardware impairments. In this paper, the joint effects of in-phase and quadrature-phase imbalance (IQI) and imperfect successive interference cancellation (ipSIC) on the performance of two-way relay cooperative NOMA (TWR C-NOMA) networks over the Rician fading channels are studied, where two users exchange information via a decode-and-forward (DF) relay. In order to evaluate the performance of the considered network, analytical expressions for the outage probability of the two users, as well as the overall system throughput are derived. To obtain more insights, the asymptotic outage performance in the high signal-to-noise ratio (SNR) region and the diversity order are analysed and discussed. Throughout the paper, Monte Carlo simulations are provided to verify the accuracy of our analysis. The results show that IQI and ipSIC have significant deleterious effects on the outage performance. It is also demonstrated that the outage behaviours of the conventional OMA approach are worse than those of NOMA. In addition, it is found that residual interference signals (IS) can result in error floors for the outage probability and zero diversity orders. Finally, the system throughput can be limited by IQI and ipSIC, and the system throughput converges to a fixed constant in the high SNR region.Electronics 2020, 9, 249 2 of 16 signals of other users with lower channel gains before decoding their own signals based on the SIC technique [7]. Hence, NOMA can support multiple users with limited resources and improve spectral efficiency, which has attracted considerable attention from academia and industry. For instance, in [8,9], the authors discussed power allocation under various criteria for a downlink NOMA system. In order to improve the system throughput, a network NOMA technique was proposed and analysed for the uplink coordinated multi-point transmission (CoMP) in [10].Cooperative communications is another solution to mitigate the fading effects of wireless environments [11]. NOMA in cooperative communication systems is commonly referred to as cooperative-NOMA (C-NOMA), which is able to enhance the system reliability [12]. The authors analysed the effect of the power allocation coefficient on the error performance under a one-way relay C-NOMA system in [13]. The authors in [14] presented a new system detection scheme for a one-way relay C-NOMA system, which used the maximal ratio combining (MRC) and SIC to decode signals. Sheng Luo et al. in [15] proposed an adaptive transmission scheme based on a one-way relay C-NOMA network with a dedicated relay, which could temporarily store the received information. Recently, two-way relaying (TWR) has attracted a great deal of research attention because of its ability to achieve higher spectral efficiency in comparison to one-way relayin...

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