## 1 Introduction  
The deployment of fifth-generation (5G) mobile cellular networks is inextricably connected with the provision of high data rates to mobile stations (MSs) in order support bandwidth demand and zero latency applications [1; 2; 3]. To this end, various novel technologies have been introduced over the last few years: mmWave transmission [4; 5], massive multiple-input multiple-output (MIMO) systems [6; 7], as well as non-orthogonal multiple access (NOMA) transmission schemes [8; 9]. In the latter case, available network resources (i.e., frequency blocks or codewords) can be reused by certain groups of users, thus improving overall network capacity. Therefore, two main NOMA categories can be identified in literature: power domain NOMA [10; 11] and code-domain NOMA [12]. In the first approach, the transmitted signals share the same frequency and time resources but vary in power. The second approach includes, among others, multiple access solutions relying on low-density spreading (LDS) and sparse code multiple access (SCMA). In power domain NOMA, major scientific challenges include, among others, appropriate power allocation to MSs, and advanced transceiver techniques to mitigate intracode interference. In this context, MSs with improved channel conditions employ successive interference cancellation (SIC) to remove the messages intended for other MSs before decoding their own. In [13], the impact of user pairing on the performance of two NOMA systems, NOMA with fixed power allocation (F-NOMA) and cognitive radio inspired NOMA (CR-NOMA), was studied. According to the presented results, the performance gain of F-NOMA over conventional orthogonal multiple access (OMA) assignment schemes can be further improved by selecting users whose channel conditions are more distinctive. In [14], system-level evaluations were performed, according to which downlink NOMA with SIC can improve both the capacity and cell-edge user throughput performance based on wideband channel quality indicator (CQI) without relying on the availability of the frequency-selective CQI at the base station (BS) transmitter side. In this context, multi-user transmit power allocation and multi-user scheduling are performed according to sub-optimum algorithms. In [15], the design and performance evaluation of precoding and detection matrices for MIMO-NOMA is described. In this framework, user pairing was considered. However, users were randomly divided into multiple groups. The work in [16] focused on leveraging the capabilities provided by low-complexity single-antenna cooperative relays with buffers and NOMA transmission. In the same context, in [17], a novel relay selection policy is presented, namely flex-NOMA, that facilitates simultaneous transmissions from multiple sources to multiple relays, exploiting channel state information at the reception, and dynamic decoding ordering by the relays performing SIC. The work in [18] presented a sub-optimal hierarchical power allocation scheme along with a vertical user-pairing approach to deal with a large number of users in the NOMA system. An optimization problem is formulated to maximize the sum rate with total power and minimum rate constraints. In [19], the authors studied user pairing and power allocation in a downlink 2-user NOMA network. In addition, the globally-optimal closed-form solution in a general NOMA system was obtained. In [20], the paper focused on the user pairing and power allocation in the 2-user NOMA system and proposed the user pair power allocation (UPPA) scheme based on the optimal closed-form solution with the proportional fairness objective. Simulation results verified that the proposed UPPA scheme improves the performance gains of NOMA over orthogonal frequency division multiple access (OFDMA) in terms of both efficiency and user fairness. It becomes apparent from the above that a major scientific challenge in power-domain NOMA schemes is the deployment of computationally efficient approaches for user pairing during transmission.