## 1 Introduction  
The fifth generation (5G) wireless networks is deployed for actualizing the full potential of the Internet of Things (IoT). The key idea of IoT is a large number of devices which connect to the internet, collect and share data together. As mentioned in [1], the big challenge in IoT implementation is to obtain reliable communication under the condition of low cost and limited spectrum. Owing to superior spectral efficiency of the non-orthogonal multiple access (NOMA), it is a promising multiple access technique for future wireless systems, especially the NOMA is considered to be applied to 5G mobile networks [2; 3; 4]. Being different to another orthogonal multiple access (OMA) method, such as code division multiple access (CDMA), frequency division multiple access (FDMA) and so on, the NOMA method uses the power domain for multiple access technique at transmitters, and it applies successive interference cancellation (SIC) method for detecting the desired signals at users site [5]. The SIC separates superimposed symbols and mitigates the inter-user interference while operating at users. In the downlink NOMA MU system, the users that has better channel conditions are provided by less transmit power, whereas the users that has worse channel conditions are allocated more transmit power [6; 7]. This strategy can keep the balance between the performance of MU systems and the fairness of every user [8]. In the other hand, the NOMA MU system allocates transmit power based on priority of users, the user with higher priority is provided by more transmit power, whereas the user with lower priority is allocated less transmit power [9; 10]. The previous researches on NOMA downlink MU systems are summarized as follows. In [11], [PERSON] et al. investigated the simple NOMA downlink MU system, in which all usersare located randomly, and the approximated expressions of outage probability (OP) and ergodic capacity (EC) of the system are derived. In [12], [PERSON] et al. researched on downlink and uplink of MU NOMA systems. The authors proposed the dynamic power allocation based on QoS of different users was proposed. According to the obtained results, it is indicated that more whippiness to fairness performance of all users is provided based on the proposed dynamic power allocation method. The OP and EC were discussed under the condition of fairness of quality for the users in [13; 14]. The authors in [15; 16] analyzed the NOMA MU--MIMO systems, and shown that the performance of NOMA MU--MIMO systems is improved when MU are gathered into a cluster. Throughout the previous works, the throughput and the bandwidth efficiency of NOMA systems are improved, furthermore, many billions electronic devices are provided an ultrahigh connectivity by the non-orthogonal property of NOMA technology. Moreover, comparing with another multiple access method such as: sparse-code multiple access, MU shared access, pattern-division multiple access, the NOMA MU systems has a low-complexity [3]. Therefore, the NOMA technology is suitable for downlink MU systems. On the other hand, the precoding in NOMA multiple input single output (MISO) systems was proposed and analyzed [17]. However, every cluster was assumed to consist of only two users, we are going to extend this work in order to propose a system that can serve more end-users. We have proposed and analyzed the dual hops MU system with precoding at the first hop and NOMA at the second hop in the short paper [18], and the combination of NOMA and beamforming for downlink MU systems in [19; 20]. However, these systems were analyzed under assumption of perfect SIC, perfect channel state information (CSI), perfect beamforming, and completely decoding superposition signals. It is almost impossible in practical applications, therefore we are going to analyze the proposed system with combination of precoding and NOMA under the condition of imperfect SIC.