## 3 Performance Evaluation
### Average Number of Transmission Times (\(T_{avg}\))
However, when \(\alpha\) is very high, the time allocated for the data transmission phase is reduced, which also leads to low channel capacity and high \(\Phi_{i}\). Therefore, \(\Phi_{i}\) reaches its lowest value at a medium value of \(\alpha\). For example, with \(x_{\text{SB}}=0.3\), \(\Phi_{1}\) and \(\Phi_{2}\) obtain their minimum values at \(\alpha=0.5\). In addition, the position of SB significantly impacts on \(\Phi_{1}\) and \(\Phi_{2}\). Indeed, as shown in Figure 3, the value of \(\Phi_{1}(\Phi_{2})\) as \(x_{\text{SB}}=0.3\) is lowest (highest) because SS\({}_{1}\)(SS\({}_{2}\)) is nearest (farthest) to SB. When \(x_{\text{SB}}=0.65\), \(\Phi_{2}\) is lower than \(\Phi_{1}\) because SS\({}_{2}\) is closer to SB than SS\({}_{1}\). \begin{table}
\begin{tabular}{c c c} \hline
**Notations** & **Meaning** & **Value** \\ \hline PL & Path-loss exponential & 3 \\ \(\sigma_{0}^{2}\) & Variance of Gaussian noise & 1 \\ \(H_{\text{min}}\) & Minimum number of FC packets required for data recovery & 5 \\ \(N_{\text{max}}\) & Maximum number of FC packets exchanged between two sources & Change \\ \(C_{\text{th}}\) & Target rate & 1 \\ \(\eta\) & Conversion efficiency & 0.5 \\ \(\Delta\) & \(\Delta=P_{\text{SB}}/\sigma_{0}^{2}\): Transmit SNR & Change \\ \(x_{\text{SB}}\) & x-coordinate of the SB node & \(0<x_{\text{SB}}<1\) \\ \(\alpha\) & Fraction of time allocated for EH & \(0<\alpha<1\) \\ \(\Phi_{i}\) & Probability that one FC packet \(p_{i}\) is unsuccessfully relayed & \(0<\Phi_{i}<1\) \\ \(\text{SS}_{i}\) to SS\({}_{j}\), where \(i,j=1,2,\ i\
eq j\) & \(0<\Phi_{i}<1\) \\ \hline \end{tabular}
\end{table} 
Table 1: Mathematical notations and their values. In Figure 4, we present OP at two sources in the Cov-Scm scheme as a function of \(\Delta\) (dB) when \(x_{\text{SB}}=0.35\), \(\alpha=0.5\) and \(N_{\text{max}}=6\). With \(x_{\text{SB}}=0.35\), the distance from SS\({}_{1}\) to SB is shorter than that from SS\({}_{2}\) to SB, resulting in \(\Phi_{2}<\Phi_{1}\), and OP at SS\({}_{2}\) is lower than that at SS\({}_{1}\). Hence, we observe from Figure 4 that the OP performance of SS\({}_{2}\) is better than that of SS\({}_{1}\) for all values of \(\Delta\) and \(K\). It is also shown that OP of both sources decreases when the values of \(\Delta\) and \(K\) increase. Furthermore, the OP gap between the two sources also increases as \(\Delta\) increases. It is worth noting that the results obtained in this figure can be used to design/optimize the OP performance. For example, with \(K=4\), the OP of both sources in the Cov-Scm is lower than 0.01 when the value of \(\Delta\) is from 11 dB to 20 dB. In other words, the SB station can use a minimum transmit power of 11 dB to ensure that OP of both sources remains below 0.01. Figure 5 shows OP at two sources in the Mod-Scm scheme as a function of \(\Delta\) (dB) when \(x_{\text{SB}}=0.35\), \(\alpha=0.5\), \(K=3\), and \(N_{\text{max}}=6\). Furthermore, we consider two scenarios: (i) SS\({}_{1}\) transmits first (named Mod-Scm-1); (ii) SS\({}_{2}\) transmits first (named Mod-Scm-2). We observe that in Mod-Scm-1, OP of SS\({}_{2}\) is lower than that of SS\({}_{1}\) at low and medium \(\Delta\)  
values, and at high \(\Delta\) values, OP of SS\({}_{1}\) is higher. In Mod-Scm-2, OP of SS\({}_{2}\) is always better than that of SS\({}_{1}\) for all values of \(\Delta\). Moreover, the OP gap between the two sources in Mod-Scm-2 is much higher than that in Mod-Scm-1, and OP at SS\({}_{2}\) and SS\({}_{1}\) in Mod-Scm-2 are lowest and highest, respectively. Therefore, Figure 5 shows that Mod-Scm-1 achieves greater performance fairness for two sources. To determine whether Mod-Scm-1 or Mod-Scm-2 performs better, Figure 6 compares SOP of all the considered schemes. In this figure, the parameters are set to \(x_{\text{SB}}=0.35\), \(\alpha=0.5\), \(K=5\), and \(N_{\text{max}}=6\). As shown, Mod-Scm-1 obtains the best SOP performance, while that of Cov-Scm is worst. We also observe that SOP of Mod-Scm-1 is much lower than those of Mod-Scm-2 and Cov-Scm.