# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 4 Performance Evaluation
Again, SANOCO-D2D with \(B_{d}=0.4\) MHz achieves better results than any other value of \(B_{d}\), compared to OMA and NOMA, for an SNR range of 0-26 dB. On the contrary, for SNR values above 26 dB, NOMA and SANOCO-D2D with \(B_{d}=0.1\) MHz, achieve slightly better results. In general, we can see similar behavior to that of scenario 1, and more specifically, the spectral efficiency performance of SANOCO-D2D for \(B_{d}>0.4\) MHz (i.e., \(B_{d}=0.6\) MHz or \(B_{d}=0.8\) MHz) is slightly improved, since more SC users exist, leading to higher rates from the satellite, thereby leading to an improved usage of the terrestrial bandwidth. This applies to the whole range of transmit SNR but can be perceived at medium and high SNR values. The same is true for the case of \(B_{d}=0.4\) MHz for the whole range of different transmit SNR values. Furthermore, for \(B_{d}<0.4\) MHz, e.g., \(B_{d}=0.1\) MHz, results in an almost identical spectral efficiency to NOMA. This is attributed to the increased number of SC users who achieve higher rates and are able to better assist the weak users. However, usually, a \(B_{d}=0.1\) limits the rate of the weak users while cooperating with the strong users. Next, Figure 9 shows the sum rate performance for scenario 2. As it was the case in scenario 1, SANOCO-D2D outperforms OMA and NOMA for any \(B_{d}\) value. Moreover, the sum rate in scenario 2 is higher than that of scenario 1, due to the number of SC users, being equal to that of WC users, thereby improving the performance of NOMA in the satellite. Regarding OMA, as the number of SC users is increased, its total sum rate of the system increases. Furthermore, SANOCO-D2D with \(B_{d}\geq 0.4\) MHz offers better results than NOMA. For \(B_{d}<0.4\) MHz, as it is the case of \(B_{d}=0.1\) MHz this increase is significantly smaller, because the allocated D2D bandwidth limits the transmission rate of the strong user. It should be noted that compared to scenario 1, the sum rate in scenario 2 using SANOCO-D2D with \(B_{d}>0.4\) MHz starts to increase at approximately 10 dB, faster than scenario 1. As illustrated in Figure 9, a significant increase in the value of \(B_{d}\) cannot offer much higher sum rate, and causes a significant decrease in the spectral efficiency, as presented in Figure 8, because the strong user decoding rate is the upper bound for the weak user rate, of the D2D link's achievable rate. Thus, dynamic bandwidth allocation is needed, in order to achieve a balance between sum rate and spectral efficiency performance. Figure 7: Sum rate for scenario 1, \(B_{d}=0.4\) MHz and different elevation angle values. The final comparisons for scenario 2 are depicted in Figures 10 and 11. In greater detail, the spectral efficiency and sum rate performance, with \(B_{d}=0.4\) MHz and different values of elevation angle is evaluated. Regarding the spectral efficiency, similarly to scenario 1, for low and medium transmit SNR values and any elevation angle, SANOCO-D2D exhibits better performance than NOMA. Then, for higher SNR and equal elevation angles, SANOCO-D2D behaves slightly worse than NOMA. Furthermore, at low SNR and \(E=40^{\circ}\) or \(E=60^{\circ}\), SANOCO-D2D offers improved spectral efficiency, compared to NOMA. For medium and high SNR, differences for the two schemes are quite small. Next, the sum rate comparison outlines that for any SNR and elevation angle value, SANOCO-D2D performs better than NOMA. Moreover, the sum rate of SANOCO-D2D with \(E=40^{\circ}\) is higher than that of NOMA with \(E=60^{\circ}\) for SNR values approximately up to 8 dB, due to the increased number of SC users. Finally, for high SNR values with \(E=40^{\circ}\) or \(E=60^{\circ}\), the sum rate results of SANOCO-D2D and NOMA are almost identical. Figure 8: Spectral efficiency for scenario 2, \(E=10^{\circ}\) and different terrestrial D2D bandwidth values. Figure 9: Sum rate for scenario 2, \(E=10^{\circ}\) and different terrestrial D2D bandwidth values.