# Enhanced Integrated Satellite-Terrestrial NOMA with Cooperative Device-to-Device Communication
## 4 Performance Evaluation  
In this section, simulation results are presented in terms of sum rate and spectral efficiency for different system parameters and transmission techniques. The whole transmission takes place in an urban terrestrial environment. Towards this end, a custom made simulator was implemented in C++. Furthermore, the maximum weighted perfect matching problem was solved with the use of the LEMON graph library [26]. The average Loo model channel parameters \((M,\Sigma,MP)\) were selected based on the Table XIII of [27] and are presented in Table 2 for an urban area, operating in the L-Band and handheld antennas. Regarding the satellite channel conditions with terrestrial users, it should be noted that in urban areas, users being a few meters away may experience different channel conditions because of trees, roads, moving vehicles and buildings with different heights acting as scatters and affecting LoS connectivity [27,28]. All users in our scenario have the same elevation angle and range from and to the satellite but experience different channel conditions (LoS), including intermediate and deep shadowing as presented in Table 2, thereby representing the complex nature of an urban environment. On the contrary, in suburban or rural areas the channel conditions of the users would not significantly differ. Moreover, Table 3 includes the different simulation parameters used for comparison purposes and the following scenarios. SANOCO-D2D is compared against standalone NOMA with optimal user pairing [6] and OMA scheme in the time domain without D2D communication capabilities between the UEs. In the NOMA satellite transmission, perfect SIC at the strong user is assumed. Furthermore, different operating frequencies for satellite and D2D links are considered. The strong user transmits the decoded signal of the weak user through the entire duration of the time-slot and the time for decoding and re-encoding of the weak user's signal is considered to be negligible. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{\(\mathbf{E}^{\text{o}}\)} & \multicolumn{3}{c}{**Line-of-Sight**} & \multicolumn{3}{c}{**Intermediate Shadow**} & \multicolumn{3}{c}{**Deep Shadow**} & \multirow{2}{*}{\(\mathbf{D}\) **(km)**} \\ \cline{2-2} \cline{5-10}  & \(\mathbf{M}\) (dB) & \(\mathbf{\Sigma}\) (dB) & \(\mathbf{MP}\) (dB) & \(\mathbf{M}\) (dB) & \(\mathbf{\Sigma}\) (dB) & \(\mathbf{MP}\) (dB) & \(\mathbf{M}\) (dB) & \(\mathbf{\Sigma}\) (dB) & \(\mathbf{MP}\) (dB) & \\ \hline \(10^{\circ}\) & \(-0.7\) & \(1.9\) & \(-38.3\) & \(-18.4\) & \(8.6\) & \(-14.7\) & \(-24.4\) & \(9.4\) & \(-23.9\) & \(2300\) \\ \hline \(20^{\circ}\) & \(0.7\) & \(2.1\) & \(-25.5\) & \(-10.0\) & \(4.9\) & \(-23.3\) & \(-25.3\) & \(7.0\) & \(-26.5\) & \(1700\) \\ \hline \(40^{\circ}\) & \(-0.2\) & \(1.0\) & \(-32.9\) & \(-8.6\) & \(3.8\) & \(-16.1\) & \(-15.1\) & \(2.6\) & \(-16\) & \(1000\) \\ \hline \(60^{\circ}\) & \(0.1\) & \(1.9\) & \(-27.2\) & \(-6.9\) & \(2.2\) & \(-18.6\) & \(-13.1\) & \(4.2\) & \(-19.7\) & \(900\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Loo’s model’s channel parameters and average range \(D\) between the LEO satellite and UE for different elevation angles and channel states. Figure 3: Optimal user pairing policy \(\mathbf{u}_{\mathbf{i}\mathbf{j}}^{\ast}\) that gives the maximum sum rate \(R_{\text{max}}\) for \(N=4\) UEs and the studied toy network. We define the cases of weak channel (WC), medium channel (MC) and strong channel (SC) users regarding the satellite link. In WC, Loo fading is generated using the parameters presented in Table 2 for the deep shadow state based on the value of the elevation angle. Accordingly, for the SC and MC, UE fading is generated using the parameters of the LoS and intermediate shadow state, respectively and for different elevation angle values. Additionally, as presented in Table 2, based on the elevation angle value and independently of the UE channel state, an average range \(D\) in km between the satellite and each UE is considered.