## 4 Results  
The simulation results are presented in Figures 2-5 for three MIMO configurations: \(4\times 4\), \(6\times 4\), \(8\times 4\) with two resolvable multipath components. Effective Signal to Noise Ratio (SNR) was set to \(0\) dB. The output measured metric is the average BER of all active MSs in the network, while the OMA transmission scheme was evaluated as well. Processing gain was set to \(32\)[23], in order to simulate increased inter-user interference scenarios. In the first set of simulation results (i.e., Figure 2), NGs can be either \(2\), \(4\), \(8\) or \(10\) while MSs may vary from \(20\) to \(30\) with a step of \(2\). As can be observed, for the increased number of MSs in the network, the average BER deterioration compared to the OMA case is  
Figure 1: Successive interference cancellation (SIC) for a pair of mobile stations (MSs).  
practically negligible. For 8 NGs and 30 MSs, BER in the OMA/NOMA case is \(7.76\times 10^{-4}\)/\(7.93\times 10^{-4}\), respectively. Hence, in this case, a CAG of almost 25% can be achieved, since 22 codes are now assigned compared to the 30 codes in the OMA scheme. For 30 active MSs in the network and 10 NGs, corresponding average BER values are now \(7.76\times 10^{-4}\)/\(8.11\times 10^{-4}\), respectively. Hence, in this case, BER deterioration is limited to 4.5%, with an equivalent CAG that reaches 33% (20 codes in the NOMA case).  
Figure 3: BER values for various NGs—\(6\times 4\) MIMO configuration.  
Figure 2: Bit error rate (BER) values for various NOMA groups (NGs)—\(4\times 4\) multiple-input multiple-output (MIMO) configuration.  
Figure 4: BER values for various NGs—\(8\times 4\) MIMO configuration.  
Figure 5: BER values for 10 NGs—Comparison with the non-SIC approach. (\(4\times 4\), \(6\times 4\),\(8\times 4\) MIMO configurations).  
In Figures 3 and 4, a \(6\times 4\) and a \(8\times 4\) MIMO configuration were considered, respectively. As it can be observed, the average BER deterioration is improved when considering 30 active users and 10 NGs, since the diversity order (i.e., \(M_{t}\times M_{r}\)) of the system is increased: The corresponding values are \(1.23\times 10^{-4}\)/\(1.28\times 10^{-4}\) in the \(6\times 4\) case and \(2.48\times 10^{-5}\)/\(2.56\times 10^{-5}\) in the \(8\times 4\) case. Hence, average BER deterioration is limited to 4%/3%, respectively.  
In Figure 5, considering 10 NGs, SIC has been omitted in order to evaluate the potential degradation of our proposed approach compared to the OMA case. The average BER value in the NOMA scheme is now \(8.8\times 10^{-4}\) considering the \(4\times 4\) MIMO configuration. Therefore, although the BER degradation compared to OMA transmission is now almost 13%, CAG values of 33% are still achieved. Hence, depending on the tolerable BER per requested service, a trade-off can be made between average BER and transceiver complexity reduction. In the \(8\times 4\) configuration, the corresponding BER values are \(2.81\times 10^{-5}\)/\(2.53\times 10^{-5}\) for the OMA/NOMA case, respectively. Therefore, the average BER degradation is further reduced to 11%.  
Unavoidably, SIC introduces a decoding delay. Hence, BER degradation can be alternately mitigated via channel encoding techniques, where the trade-off dipole is now the average BER/net bit rate [26; 27]. Moreover, it should be noted that BER in the NOMA case can significantly degrade for a reduced number of MSs in the network and an increased number of NGs. This is rather expected, since in this case multiuser diversity is reduced, and the derived performance metric of Equation (7) will unavoidably select pairs with increased interuser interference. Finally, performance gain values come in agreement with the results presented in [14], where an Orthogonal Frequency Division Multiple Access (OFDMA) NOMA scheme was considered.