## 3 Performance Evaluation
### Average Number of Transmission Times (\(T_{avg}\))
Therefore, in this simulation, the source SS\({}_{1}\) in Mod-Scm should be prioritized to transmit its data first. Figure 7 presents both OP and SOP of the considered schemes as a function of \(\alpha\) when \(\Delta=\)8.5 dB, \(x_{\text{SB}}=0.35\), \(K=3\), and \(N_{\text{max}}=7\). It is noted that because SB is located at  
\((0.65,0.5)\), we have \(\Phi_{2}<\Phi_{1}\); hence, in Cov-Scm, OP of SS\({}_{1}\) is lower than that of SS\({}_{2}\). As emphasized in Section 3, we can confirm from Figure 4 that OP of SS\({}_{1}\) in Cov-Scm is equal to that of SS\({}_{1}\) in Mod-Scm-2, and OP of SS\({}_{2}\) in Cov-Scm is equal to that of SS\({}_{2}\) in Mod-Scm-1. This figure also shows that there are optimal values of \(\alpha\) at which OP of each user is lowest. For example, in Mod-Scm-2, the OP performance at SS\({}_{1}\) and SS\({}_{2}\) is best when \(\alpha=0.35\) and \(\alpha=0.4\), respectively. For the SOP performance, we see that Mod-Scm-2 obtains the best performance, while Mod-Scm-1 still outperforms Cov-Scm. Similarly, there exists optimal values of \(\alpha\) which provides the best SOP performance for the considered schemes. Based on the results obtained from Figures 6 and 7, we can conclude that in the Mod-Scm scheme, if \(\Phi_{j}<\Phi_{i}\) (then SOP of Mod-Scm-\(j\) is lower than that of Mod-Scm-\(i\)), hence, the source SS\({}_{j}\) should be prioritized to transmit data first. Now, let us consider examples of designing the proposed schemes. If the required quality of service (QoS) dictates that the SOP performance must be below 0.01, then, as shown in Figure 7, only the Mod-Scm-2 scheme can satisfy this requirement. Moreover, the value of \(\alpha\) must be designed within the range of 0.325 to 0.4. For another example, to determine the optimal value of \(\alpha\) in the Mod-Scm-2 scheme, we follow the following steps:  
* Step 1: As shown in Figure 7, the simulation and theoretical results of the SOP performance over a wide range of \(\alpha\) were used to confirm the existence of an optimal value of \(\alpha\). * Step 2: Identifying the interval that contains the optimal value of \(\alpha\). For example, in Figure 7, the interval of \(\alpha\) is (0.325, 0.4). * Step 3: Using the derived expression of SOP (i.e., Equation (27)) to search the optimal value of \(\alpha\) within the interval determined in Step 2. Figure 8 studies the impact of the positions of the power station (SB) on the OP and SOP performance when \(\Delta=\)11 dB, \(\alpha=0.375\), \(K=4\), and \(N_{\max}=7\). Due to the symmetry, we can see that in Cov-Scm, OP of SS\({}_{1}\) at \(x_{\rm SB}=a\) equals to that of SS\({}_{2}\) at \(x_{\rm SB}=1-a\), where \(0<a<1\). Similarly, OP of SS\({}_{1}\) in Mod-Scm-1 at \(x_{\rm SB}=a\) equals to that of SS\({}_{2}\) in Mod-Scm-2 at \(x_{\rm SB}=1-a\). For the SOP performance, we see that SOP of Cov-Scm is symmetrical about \(x_{\rm SB}=0.5\), while SOP of Mod-Scm-1 at \(x_{\rm SB}=a\) equals to that of Mod-Scm-2 at \(x_{\rm SB}=1-a\). Therefore, as \(x_{\rm SB}<0.5\) (\(x_{\rm SB}>0.5\)), the source SS\({}_{1}\) (SS\({}_{2}\)) should be selected to transmit data  
first. Finally, it is worth noting from Figure 8 that the SOP of Cov-Scm, Mod-Scm-1, and Mod-Scm-2 is lowest when \(x_{\text{SB}}=0.5\), \(x_{\text{SB}}=0.3\), and \(x_{\text{SB}}=0.7\), respectively. In Figure 9, we present the average number of transmissions of FC packets for the successful data exchange between two sources in the Mod-Scm scheme as a function of \(\Delta\) (dB) when \(x_{\text{SB}}=0.4\), \(\alpha=0.2\), and \(N_{\text{max}}=7\). It is worth noting that the number of transmissions in Cov-Scm is always \(2N_{\text{max}}\). Figure 9 presents that the average number of transmissions in Mod-Scm-1 and Mod-Scm-2 decreases with the increasing of \(\Delta\). When the \(\Delta\) values are high enough, the average number of transmissions in Mod-Scm-1 and Mod-Scm-2 will reach \(2H_{\text{min}}\).