# Machine Learning-Based Methods for Enhancement of UAV-NOMA and D2D Cooperative Networks
## 5 Performance Evaluation
Moreover, spectral efficiency degradation is observed when the terrestrial D2D bandwidth \(B_{d}\) is greater than 1.2 MHz. In this case, the weak users can not efficiently exploit the capabilities offered by the wireless D2D channel link, as the rate received through the terrestrial cooperation is restricted by the decoding rates achieved by the strong users of each pair. This observation is derived as a result of the constraints imposed by (17)-(19), as well as from the explanation of cases 1 and 2 in Section 2.4. As an illustrative case for this phenomenon, the baseline standalone OMA scheme behaves better than the NOMA-D2D scheme with \(B_{d}=3.0\) MHz in terms of spectral efficiency. Therefore, in the case of \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Values** \\ \hline Simulated frames & 100,000 \\ Number of GMTs \(N\) & 20 \\ Region of interest circle radius \(R\) & 500 m \\ UFBS downlink frequency \(F_{u}\) & 1.8 GHz \\ D2D operating frequency \(F_{d}\) & 2 GHz \\ UFBS transmit power \(P_{u}\) & 0–24 dBm \\ GMT transmit power \(P_{d}\) & 24 dBm \\ UFBS Tx antenna gain \(G_{t}^{u}\) & 0 dBi \\ GMT Rx antenna gain \(G_{t}^{g}\) with \(g=\{u,d\}\) & 0 dBi \\ GMT Tx antenna gain \(G_{t}^{g}\) & 0 dBi \\ Terrestrial environment \(t\) & Urban \\ Urban environment parameters & \(a=9.61\), \(b=0.16\), \(\eta_{LoS}=1\), \(\eta_{NLoS}=20\) \\ UFBS bandwidth \(B_{u}\) & 5 MHz \\ Receiver noise temperature \(T_{g}\) with \(g=\{u,d\}\) & 24.6 dBK \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Simulation parameters. cooperative NOMA schemes such as the proposed one, the value of the terrestrial D2D bandwidth \(B_{d}\) should be carefully chosen to avoid wasting spectrum resources. Also, in the NOMA-D2D cooperative network, for UFBS transmit power in the range of 0 to 12 dBm, it can be observed that the spectral efficiency is approximately the same for \(B_{d}\) values equal to 0.1 and 0.2 MHz. However, for UFBS transmit power higher than 12 dBm, the proposed method with \(B_{d}\) = 0.2 MHz achieves higher spectral efficiency than the others. In other words, \(B_{d}\) = 0.2 MHz is a near-optimal D2D bandwidth value for the considered communication system. In Figure 8, the sum rate performance of the proposed ANN-based PSS is examined for the different network schemes. It can be easily observed that the employment of the suggested PSS technique in the NOMA-D2D cooperative network readily outperforms OMA and NOMA schemes for all UFBS transmit power values and regardless of the D2D bandwidths value allocations. Moreover, for the NOMA-D2D cooperative network, we observe that the sum rate is approximately the same for any value of \(B_{d}\) > 0.1 MHz. This can be supported by (21), which demonstrates that there is no need to devote more bandwidth to D2D transmission. Also, for UFBS transmit power in the range of 0 to 12 dBm, it can be observed that the sum rate is approximately the same for all \(B_{d}\) values. Hence, large \(B_{d}\) values for low-to-medium UFBS transmit powers are thus seen as a waste of resources. Therefore, for that UFBS transmit power range, there is a maximum value \(B_{d}\), which should not be exceeded to avoid wasting resources. Nevertheless, the findings from Figures 7 and 8 demonstrate that dynamic bandwidth allocation is required for D2D out-band communication to improve both the sum rate and spectral efficiency performance. Figures 9 and 10 show the effects caused by the different placement methods on the spectral efficiency and the system sum rate, respectively. More specifically, Figure 9 illustrates the spectral efficiency performance of the different communication schemes, NOMA-D2D with \(B_{d}\) = 0.2 MHz, NOMA, and OMA, utilizing the different placement procedures. As can be observed, the ANN-based PSS applied to the NOMA-D2D cooperative network scheme achieves significant spectral efficiency gains compared to MEA-DP and MED-DP for all UFBS power transmission values.