# Performance of Cooperative Relay NOMA with Large Antenna Transmitters
## 1 Introduction
### Review of Existing Works and Motivation
#### 1.1.2 Performance of CR-NOMA with CBSM Channel Model  
The review of performance of CR-NOMA using AF/DF coding schemes is presented in this section. As previously stated, the performance of CR-NOMA has been extensively studied utilizing various network coding schemes and channel characteristics [23; 24; 25; 26; 27]. The use of a relay in NOMA system is to improve system performance and extend the network coverage [28]. The effect of various network coding schemes on the performance of CR-NOMA has been studied in [21; 29]. In order to improve reliability, enhance the spectrum, power effectiveness and network connectivity, certain network terminals interact and aid one another by using the broadcast aspect of the wireless connections [30]. In CR-NOMA, users or devices with poor channel conditions, who often suffer from power losses at the cell edge, can be assisted by other users or devices with strong channel conditions by serving as transmission relays. This can greatly improve the reliability of the weak user's reception. There are two well-known network coding schemes: amplify-and-forward (AF) and decode-and-forward (DF) coding schemes [31; 32]. The AF coding scheme relay simply amplifies and forwards a scaled version of its observation to the destination [33; 34; 35; 23; 36]. A CSI-assisted AF relay exploits instantaneous CSI of source-to-relay link to adjust its gain which has superior performance over the fixed gain AF relay has been studied in [37; 38; 39; 40; 41; 42]. DF coding scheme relay, on the other hand, decodes the received signal first before re-transmitting the decoded signal to the destination [23]. Others such as compress-and-forward (CF) and computer-and-forward(CpF) have been studied in [43; 44; 45; 46; 47; 48; 49; 15] respectively. With the CF approach, the signal is compressed and sent to the target location(s), whereas with the CpF however does not requires CSI and exploits signal couplings produced by the channel to remove noise noise and mitigate interference at the relays thereby reducing the number of relays required. Resource allocation optimization has been analyzed using the above coding schemes in [45; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60] in order to improve CR-NOMA performance. Majority of the CR-NOMA performance evaluations (OP, achievable rate, BER, etc.) stated above have been limited to CBSM channel models. Table 1, presents a summary of CR-NOMA performance using CBSMs with AF/DF coding schemes. Figure 2 presents the CR-NOMA model that used and studied in the above studies, where the channel between the transmitter, relay and the users is modeled as CBSM. To improve system spectral efficiency and reduce challenges in channel state information (CSI) acquisition, CR-NOMA with MIMO and mMIMO have been studied in [47; 61]. In [47], the authors considered the Saleh-Valenzuela millimeter-wave channel model for beamforming analysis, and no network coding schemes were considered. Again the analysis focused on user relay and not dedicated relay. The authors in [61], studied the application of relay to mMIMO NOMA. However, the analysis was based on CBSM. They proposed a general framework for a multiple-relay aided massive NOMA system. At the TX, the authors considered circular array for the analysis which is not a typical antenna array typologies for massive MIMO systems according to [17].