# On the Performance of LDPC-Coded Massive MIMO Schemes with Power-Ordered NOMA Techniques
## 4 Simulation Results and Analysis
\begin{table}
\begin{tabular}{c c c c c} \hline Baseline 1 & Conventional NOMA & MIMO \(8\times 64\) & w/out LDPC & 2 users [0.5 1] \\ \hline Baseline 2 & Conventional NOMA & MIMO \(8\times 64\) & LDPC & 2 users [0.5 1] \\ \hline Baseline 3 & Conventional NOMA & MIMO \(8\times 64\) & w/out LDPC & 3 users [1 0.5 2] \\ \hline Baseline 4 & Cooperative NOMA & MIMO \(8\times 64\) & w/out LDPC & 3 users [1 0.5 2] \\ \hline Baseline 5 & Conventional NOMA & MIMO \(8\times 64\) & LDPC & 3 users [1 0.5 2] \\ \hline Baseline 6 & Cooperative NOMA & MIMO \(8\times 64\) & LDPC & 3 users [1 0.5 2] \\ \hline Baseline 7 & Conventional NOMA & MIMO \(8\times 64\) & LDPC & 4 users [1 0.5 2 4] \\ \hline Baseline 8 & Cooperative NOMA & MIMO \(8\times 64\) & LDPC & 4 users [1 0.5 2 4] \\ \hline Baseline 9 & Conventional NOMA & MIMO \(8\times 256\) & LDPC & 3 users [1 0.5 2] \\ \hline Baseline 10 & Cooperative NOMA & MIMO \(8\times 256\) & LDPC & 3 users [1 0.5 2] \\ \hline \end{tabular}
\end{table} 
Table 1: List of Baselines utilized in simulations. Figure 5 shows the performance of conventional NOMA (designated in figures as \"NOMA\") and cooperative NOMA (designated in figures as \"COOP NOMA\") with 8 \(\times\) 64 MIMO with and without LDPC (baselines 3-6) with both ZF and MRC receivers. Three NOMA users with relative powers [1, 0.5, 2] were considered, sharing the spectrum: the reference with the power 1, an interfering user with power 0.5 (i.e., with 3 dB less power than the reference user), and another interfering user with power 2 (i.e., with 3 dB more power than the reference user). It is known that the sharing of spectrum with regular power-ordered NOMA tends to be limited in terms of performance [16; 17; 18] while improving the capacity. This occurs because higher power users are unable to cancel the interference generated by lower power users, representing residual interference. Cooperative NOMA aims to mitigate this limitation by making lower power users relay the signals of higher power users over the air without interference. Weak users are those that tend to be further from the base station (or can also be due to, e.g., fading); thus, the propagation losses are higher. With NOMA, this is mitigated by employing higher transmit power. Therefore, it is assumed that the interfering user with transmit power 0.5 tends to be closer to the base station than the reference user with a transmit power of 1. As can be seen, due to the high level of NOMA interference, the results obtained with conventional NOMA are limited. Cooperative NOMA overcomes such limitations and originates a high-performance improvement with or without LDPC for both receiver types. Cooperative NOMA comprises the cancellation of the interfering signals associated with all users and exploits diversity. Cooperative NOMA considers that the lower power users retransmit the symbols detected by the SIC of higher power users (typically using decode-and-forward in time division multiplexing). These additional signals are utilized by higher power users to exploit diversity as the same signals are also received directly from the base station (assuming downlink). These signals are then combined to improve performance and exploit diversity. Figure 5 shows that a combination of signals performed with cooperative NOMA results in good performance improvement  
Figure 4: Results for 2 NOMA users with powers [0.5, 1], with 8 \(\times\) 64 MIMO (with and without LDPC codes). when compared with conventional NOMA, using both the MRC and ZF receivers, whose performances are close to that of the MFB. It is also viewed that, in this NOMA scenario and for conventional NOMA, the ZF tends to achieve better performance than the MRC for lower levels of noise (higher levels of \(E_{b}/N_{0}\)). With a high level of interference associated with the non-cancelled interfering NOMA user, the symbol estimates performed in each iteration of the MRC receiver are poor; therefore, it is not able to perform well.