# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 4 Performance Evaluation
This can be attributed to the small terrestrial bandwidth at high SNR values, leading the weak user to prefer the satellite channel for receiving its signal. At low and medium SNR values, the sum rate of SANOCO-D2D is approximately the same for any value of \(B_{d}>0.4\) MHz. This is justified by considering (14) which highlights the fact that there is no need to allocate additional bandwidth for D2D cooperation. For SNR values above 14 dB, SANOCO-D2D with \(B_{d}=0.4\) MHz achieves a slightly smaller sum rate than the higher values of \(B_{d}\), as the decoding rate at the strong user is higher than the rate of the D2D link, and so, a small extra bandwidth is needed for optimal sum rate performance. For example, when \(B_{d}=0.6\) MHz, at high SNR, the sum rate surpasses that of \(B_{d}=0.4\) MHz. Thus, it is recommended to use a D2D bandwidth value within \(0.4<B_{d}<0.6\), as the adoption of \(B_{d}\) values above 0.6 MHz significantly wastes spectral resources. Hence, from Figures 4 and 5 it is concluded that dynamic bandwidth allocation for the D2D out-band communication is necessary, in order to optimize both the sum rate and the spectral efficiency performance. In Figure 6 the spectral efficiency for scenario 1, with \(B_{d}=0.4\) MHz and different elevation angle values is illustrated for SANOCO-D2D and NOMA. It can be easily observed that for low and medium SNR values and for any value of the elevation angle, the spectral efficiency of SANOCO-D2D outperforms that of NOMA. Specifically, at low and medium SNR values and for \(E=10^{\circ}\) and \(E=20^{\circ}\), SANOCO-D2D is superior to NOMA. At high SNR, SANOCO-D2D behaves marginally worse, compared to NOMA in terms of spectral efficiency. Similarly, at low SNR and \(E=40^{\circ}\) and \(E=60^{\circ}\), SANOCO-D2D behaves better than NOMA. For medium and high SNR values, the difference in performance becomes negligible. An interesting point in this comparison is that for fairly low SNR  
Figure 4: Spectral efficiency for scenario 1, \(E=10^{\circ}\) and different terrestrial D2D bandwidth values. values, the spectral efficiency of SANOCO-D2D with \(E=40^{\circ}\) is slightly better than that of NOMA technique with \(E=60^{\circ}\). In Figure 7, the sum rate performance for scenario 1, \(B_{d}=0.4\) MHz and different elevation angle values is presented for SANOCO-D2D and NOMA. It can be seen that throughout the SNR range and independently of the elevation angle value, the sum rate of SANOCO-D2D is better than that of NOMA. In greater detail, for \(E=10^{\circ}\) and \(E=20^{\circ}\), SANOCO-D2D outperforms NOMA, and in particular, at low SNR the sum rate of SANOCO-D2D with \(E=10^{\circ}\) is closer to the sum rate of NOMA with \(E=20^{\circ}\). The interesting point here is that the sum rate of SANOCO-D2D with \(E=40^{\circ}\) is better than that of NOMA with \(E=60^{\circ}\) for SNR values until approximately 7 dB. For high SNR values and \(E=40^{\circ}\) or \(E=60^{\circ}\), the sum rate performance of SANOCO-D2D and NOMA is almost identical. Hence, from Figures 6 and 7, it can be concluded that SANOCO-D2D with appropriately selected terrestrial bandwidth surpasses the performance of NOMA, especially under unfavorable channel conditions, encountered in low and medium transmit SNR values and elevation angles. Figure 5: Sum rate for scenario 1, \(E=10^{\circ}\) and different terrestrial D2D bandwidth values. Figure 6: Spectral efficiency for scenario 1, \(B_{d}=0.4\) MHz and different elevation angle values. In Figure 8, the spectral efficiency performance for scenario 2 is depicted for different schemes. Firstly, it can be observed that the spectral efficiency of scenario 2 is higher that that of scenario 1. This is justified by considering the lack of users in intermediate channel conditions and the equal percentages of SC and WC users. Thus, depending on the terrestrial channel conditions, each SC user will be paired with a WC user, significantly benefiting the performance of NOMA in the satellite link.