## 3 System Model
### Cooperated Clustered PD-NOMA  
In wireless communication, transmitting more than one copy of a signal is an encouraging technique to reduce the effects of fading. Among the range of existing schemes, the cooperative network is considered to be a better scheme. The cooperative network helps in achieving improved reliability by transmitting multiple signals through relaying. Moreover, for users with bad channel conditions located at farther distances, where a direct link between BS and a user could be established, relaying proves to be the solution. In general, relaying is preferred because it is relatively closer to the BS and therefore requires less power at BS to transmit the signal at relay. It determines that relay networks have more benefits in terms of reliability, extensive coverage area, and power saving [19; 33]. As with the PD-NOMA in the cooperative network, the receiver's SNR can be improved significantly over a restricted distance. NOMA enhances with relaying the system's throughput. The spectral efficiency of PD-NOMA improves by relaying signals and backing the cell-edge users. Therefore, it is necessary to consider the cooperation in PD-NOMA-based communication systems. Two types of cooperation in the downlink scenario are possible, namely cooperation between the PD-NOMA users/BSs and cooperation by using fixed relays [19; 34; 35].  
In this article, we considered cooperation within the PD-NOMA clusters among users, as shown in Figure 4a. The basic design and working of cooperative PD-NOMA is similar to the function of SIC. According to the working principle of SIC, the users with strong channel conditions perform the decoding, and thus the messages of the weak user will be decoded first by the users with the better channel condition. Therefore, the users with the good channel condition exploit this information by working as relays for other users with weak channel states and improving the reliability of users with weak channel states. We considered a multi-user downlink cooperative PD-NOMA with a single SBS and two or more end users. For two users, the downlink transmission will be performed in two time slots as shown in Figure 4b, i.e., the direct phase and the cooperative phase. In the direct phase, a superposed message of both users U1 and U2 will be broadcasted by the SBS.  
Consider a scenario where the channel state of U1 is supposed to be strong, so SIC will be implemented at U1. Considering the working principal of SIC, U1 decodes the message of U2 before decoding its own message, while in the second time slot, U1 will function as a relay and forwards the former decoded information of U2. With this cooperation scheme, two copies of signals will be received at U2 via different paths. This cooperation is important and much needed for weak users, since the throughput of weak users would suffer due to the interference from the strong user.  
Therefore, the signal reception of the farther user will be improved by having two copies of a message. For larger distances, this cooperation would require additional spectral resources. However, it can be recompensed by working with shorter distances, which is achievable with the formed PD-NOMA clusters. The other method to implement cooperative PD-NOMA is with dedicated relays, which can be considered in future research.