# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 4 Performance Evaluation
In order to provide performance comparisons among the different schemes, two scenarios are considered. The first scenario focuses on a network within a circular region of interest with radius \(R=500\) m, consisting of a total number of users \(N=32\) uniformly distributed, having an elevation angle equal to \(10^{\circ}\), with 50% of users experiencing WC conditions, 37.5% SC and 12.5% MC and different values towards the available terrestrial bandwidth \(B_{d}\). Then, the second scenario consists of a network within a circular region of interest with \(R=500\) m, a total number of users \(N=32\) uniformly distributed in the region of interest, having an elevation angle equal to \(10^{\circ}\), with 50% of users experiencing WC conditions and 50% SC, for different values for the available terrestrial bandwidth \(B_{d}\). In Figure 4, the spectral efficiency performance for scenario 1 is presented for different schemes. It can be observed that in the range of 0-26 dB regarding the satellite transmit SNR value, the proposed technique for \(B_{d}=0.4\) MHz achieves better results than any other value of \(B_{d}\), and compared to the conventional OMA and NOMA. For SNR values greater than 26 dB, NOMA and SANOCO-D2D with \(B_{d}=0.1\) MHz achieve slightly better results with almost identical performance. Specifically, for low to medium SNR values, SANOCO-D2D with \(B_{d}=0.4\) MHz outperforms NOMA. Therefore, the value of 0.4 MHz for the terrestrial D2D link is near the optimal value in order to achieve high spectral efficiency. For terrestrial bandwidth values less than 0.4 MHz, the spectral efficiency of SANOCO-D2D is close to that of NOMA. On the other hand, for \(B_{d}>0.4\) MHz, because there is no need for extra bandwidth, spectral efficiency starts to decrease, and this claim can be verified by (14) which states that the rate \begin{table}
\begin{tabular}{l l} \hline Simulated frames & 100,000 \\ \hline Number of UEs \(N\) & 32 \\ \hline Region of interest circle radius \(R\) & 500 m \\ \hline Satellite downlink frequency \(f_{s}\) & 1.625 GHz \\ \hline D2D operating frequency \(f_{d}\) & 2 GHz \\ \hline Satellite Tx antenna gain \(G_{t}^{s}\) & 24 dBi \\ \hline Satellite transmit SNR & 0–30 dB \\ \hline UE satellite Rx antenna gain \(G_{r}^{s}\) & 2.7 dBi \\ \hline System receiver noise temperature \(T_{s}\) & 25.7 dBK \\ \hline UE transmit SNR and \(P_{d}\) & 23 dBm and 0.1995 Watt \\ \hline System receiver noise temperature \(T_{d}\) & 24.6 dBK \\ \hline D2D Tx/Rx antenna gain \(G_{g}^{d}\) with \(g=\{t,r\}\) & 0 dBi \\ \hline D2D pathloss model & \(127+30\log_{10}{(d)}\) \\ \hline D2D log-normal shadowing & 8 dB \\ \hline Satellite Bandwidth \(B_{s}\) & 5 MHz \\ \hline UE speed & 3 km/hr \\ \hline Maximum Doppler Shift & 40 kHz \\ \hline Terrestrial Environment & Urban \\ \hline \end{tabular}
\end{table} 
Table 3: Simulation parameters. of the weak user is always limited by the decoding rate of its signal at the strong user. In addition, SANOCO-D2D with \(B_{d}=0.8\) MHz and \(B_{d}=1.2\) MHz behave similarly to the case of \(B_{d}=0.1\) MHz. When the terrestrial bandwidth value is below \(B_{d}=0.4\) MHz, spectral efficiency degradation is observed, since the weak user chooses to be served directly from the satellite, as the small bandwidth decreases the rate of the D2D channel; i.e., \(R_{i}^{D2D(i)}\leq R_{i}^{DECODED(i)}\). In the case where the terrestrial bandwidth value is slightly above \(B_{d}=0.4\) MHz, the use of extra bandwidth reduces the spectral efficiency performance, and in particular for high SNR, NOMA exhibits superior performance. In Figure 5 the sum rate performance for scenario 1 is presented. It can be easily observed that independently of the terrestrial bandwidth value, SANOCO-D2D outperforms OMA and NOMA. In greater detail, for \(B_{d}=0.1\) MHz, SANOCO-D2D behaves better for low to medium SNR values, while for high SNR, its sum rate is slightly better than that of NOMA.