## 4 Simulation and Discussion
### Performance Analysis for Multiple Links  
First, we focus on multiple PLs and SLs. In each snapshot of the Monte Carlo simulation, as shown in Figure 2, we only evaluate the system performance within the first transmission frame.  
In Figure 3, we depict the overall WSP versus the number of SLs. The optimal scheme achieves the minimum overall WSP, while the random scheme performs the worst. It can be observed that the curves of the proposed scheme and the optimal scheme are overlapped. This phenomenon indicates that the proposed one is also optimal. The performance of the DRL-based scheme is very close to the optimal (proposed) scheme, with an average of 2-3 percentage points WSP higher than that of the latter two. In addition, the optimal WSEE maximization scheme performs between the optimal (proposed) and the random schemes. However, the average overall WSP of the optimal WSEE maximization scheme is far higher than that of the optimal (proposed) scheme, more than 400 times that of the latter. This result confirms that WSEE maximization may exacerbate energy consumption. Thus, the EE or WSEE are not very good optimization goals for energy-limited systems.  
Figure 4 shows the SE of the PLs and SLs versus the number of SLs. Similarly, we can observe that the optimal and the proposed schemes have the same performance. This result further demonstrates that our proposed scheme is optimal. Although it is difficult  
Figure 3: Overall WSP vs. the number of SLs.  
Figure 2: A snapshot of the Monte Carlo simulation when \(M=10\) and \(N=5\).  
to distinguish between the curve of the DRL-based scheme and the curve of the optimal (proposed) scheme, there are still subtle differences. The SE of the DRL-based scheme is approximately 1% higher than that of the latter two. It can be noticed that the link SEs of all the considered schemes linearly increase before \(N=10\) and remain largely the same when \(N>10\). The reason for this phenomenon is easy to understand: in our system model, the spectrum sharing among the PLs and SLs is one-to-one correspondence; thus, when the number of the SLs exceeds the number of the PLs, the link SLs almost no longer increase. Additionally, although the random scheme has the best link SEs in this configuration, it cannot ensure the QoS requirements of all the links. In the following subsection, we will prove it.  
In Figure 5, we show the overall WSEE versus the number of SLs. The optimal WSEE maximization scheme achieved the best performance. We can estimate from the figure that the optimal (proposed or DRL-based) scheme can achieve an average of 80% WSEE of the former. The difference in WSEE between the two is not significant in the region where \(N\) is small. For example, when \(N=7\), the WSEE of the optimal WSEE maximization scheme is about \(1.903\times 10^{9}\) bit/J and the WSEE of the optimal (proposed or DRL-based) scheme is about \(1.684\times 10^{9}\) bit/J.  
From Figures 3-5, we can verify that our scheme is optimal in terms of WSP while maintaining the performance gap with the optimal WSEE maximization scheme in terms of WSEE. Furthermore, although the above results are based on the configuration in Table 3, a similar trend will be observed when the configuration changes.  
Figure 4: Link SE vs. the number of SLs.  
Figure 5: Overall WSEE vs. the number of SLs.