## 1 Introduction
### Prior Works  
Driven by 5G, NOMA has been rigorously researched and studied in academia and industries. To date, the research on NOMA technology is still actively ongoing. Recently, research works on the analysis of rate region, studies of bit error rate (BER) performance, and investigations of various resource allocation schemes for NOMA systems have been reported in [4; 5; 6] to validate the viability and effectiveness of NOMA in handling more users and managing the scarce radio resources. Particularly, explorations of numerous resource management techniques employing NOMA have been performed, among which some of the techniques are reviewed and recommended for 5G. In [7], the performance of the downlink NOMA scheme has been scrutinized with a random distribution of users in which the proposed NOMA scheme is capable of achieving a superior ergodic sum-rate and better outage performance than conventional multiple access methods. However, judicious selection of the users' targeted data rates and allocated power is crucial as the user's outage probability will always be one if the two parameters are not properly chosen. Next, the unicast-multicast transmission with fixed power allocation and dynamic power allocation schemes for the NOMA system has been developed in [8] by proposing the best user selection scheme that can fully utilize the diversity order offered by the multicast users. The drawback of this approach is that the heterogeneity among the multicast users is not taken into account and thus it is unable to best serve all the users concurrently. A fair NOMA approach pairing a near-BS user and a cell-edge user has been proposed in [9] to prove that the ergodic capacity limit can be enhanced for every user by utilizing a fraction of the total transmit power as compared to the capacity per user achieved by OMA. However, this power allocation strategy requires the BS to acquire perfect instantaneous channel state information which is impossible to be obtained in practical wireless systems. Meanwhile, the performance of the cooperative NOMA system has been inspected in [10] where the closed-form expressions of outage probability for near user helping the far user was derived to overcome the zero-diversity order for the far user. To implement such a system, the cancellation of both self-interference and inter-user interference is necessitated. Over the last few years, research has focused on optimizing the user clustering (UC) for NOMA systems to improve NOMA performance. Unlike OMA, NOMA allows users to share the radio resources, therefore the cluster formation for NOMA plays a vital role in determining the ultimate bandwidth performance. The impact of user pairing on the performance of a fixed power allocation NOMA and cognitive radio assisted NOMA was studied for a downlink 5G transmission system in [11] to demonstrate how the NOMA system can offer a larger sum-rate than its OMA counterpart. In a fixed power allocation NOMA, users have been paired based on the significant difference in their channel gains. On the other hand, in a cognitive radio-assisted NOMA, a user that does not experience a significant difference in channel gains was opportunistically paired with another user under the condition that the former's interference does not adversely affect the QoS requirement of the latter. Conclusively, it has been found that cognitive the radio-assisted NOMA prefers to pair the first strongest user with the second strongest user, whereas the fixed power allocation NOMA favors pairing the strongest user with the weakest user. However, both techniques have certain limitations. The fixed power allocation NOMA might be unable to satisfy the user's predefined QoS if the target data rate for the weak user is large. On the other hand, in the cognitive radio-assisted NOMA, the performance of the strong user might be sacrificed as this user will only be served after the QoS of the weak user is fulfilled.