# Power-Efficient Resource Allocation for Active STAR-RIS-Aided SWIPT Communication Systems
## 4 Simulation Analysis
As observed in Figure 4, the AP transmission power assisted by active STAR-RIS is the lowest, while the AP transmission power assisted by passive RIS is the highest. This is because there is loss in the signal propagation process, while passive RIS can only reflect the signal and can only meet the minimum power requirements received by the energy receiver by increasing the AP transmission power. Active STAR-RIS can reflect accepted signals as well as transmit and amplify it, making full use of spectrum resources while reducing path loss, resulting in lower AP transmission power. From Figure 4, it can also be seen that when the passive beamforming variables \(q_{t}\) and \(q_{r}\) are taken as a random value, and only the active beamforming variable \(\{w_{i},\
u_{j}\}\) is optimized, the AP transmission power is greater than that of the active STAR-RIS. This is because when the passive beamforming variable is fixed, its coefficient matrix cannot be adjusted, and there is a lack of iterative optimization process. Additionally, as the quantity of transmitting antennas \(N\) increases, AP transmission power of the five models gradually decreases. This is because more transmitting antennas mean that signals have more transmission paths; therefore, the AP transmission power gradually decreases. Figure 5 shows the relationship between AP transmission power and the quantity of active STAR-RIS units \(M\), where \(N=2\), and \(\overline{\gamma}_{0}=10\,\mathrm{dB}\). It can be observed that within a certain range, as the quantity of active STAR-RIS units increases, AP transmission power of five models gradually decreases from Figure 5. This is because, in one respect, when the quantity of active STAR-RIS units increases, active STAR-RIS has more degrees of freedom to achieve more reflection or transmission channels related to active STAR-RIS, achieving the effect of improving beamforming gain. In another respect, the more elements an active STAR-RIS has, the more paths and power it has to reflect or transmit \begin{table}
\begin{tabular}{c c} \hline \hline
**Simulation Variables** & **Value** \\ \hline Quantity of information receivers & 2 \\ Quantity of energy receivers & 2 \\ Noise power & \(-120\,\mathrm{dBm}\) \\ Active STAR-RIS distance from AP & \(10\,\mathrm{m}\) \\ Distance between IRs and active STAR-RIS & \(20\,\mathrm{m}\) \\ Distance between ERs and active STAR-RIS & \(2\,\mathrm{m}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: System Simulation Settings. Figure 3: Algorithm Convergence. AP transmission signals, thus increasing the efficiency of the energy. However, as the quantity of active STAR-RIS units increases, the decreasing trend of AP transmission power gradually decreases. This indicates that within a certain range, increasing the quantity of active STAR-RIS elements is useful for the performance improvement of systems. When exceeding this range, the quantity of active STAR-RIS elements is no longer the key factor influencing AP transmission power. From Figure 5, it can also be seen that when the passive beamforming variables \(q_{t}\) and \(q_{r}\) are fixed, and only the active beamforming variables \(\{w_{i},v_{j}\}\) are optimized, the AP transmission power is highest compared to the other four models. This is because when the passive beamforming variables are fixed, the performance of RIS is not fully utilized. However, the other four models can be continuously optimized through iteration to achieve optimal performance. Therefore, the AP transmission power is maximized by fixing the passive beamforming variables and only optimizing the active beamforming variables. Figure 6 presents how the power received by the minimum ERs changes with variations in the AP's transmission power, where \(N=2\), and \(\overline{\gamma}_{0}=10\,\mathrm{dB}\). From Figure 6, it can be seen that in all four models, the AP transmission power gradually grows with the growth of the minimum power accepted by ERs.