## 3 Results
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Default Value** \\ \hline \((x_{BS},y_{BS},z_{BS})\) & \((0,0,0)\) \\ \((x_{UAV},y_{UAV},z_{UAV})\) & \((90,0,20)\) \\ \((x_{UE},y_{UE},z_{UE})\) & \((100,0,0)\) \\ \(\alpha_{c}\); \(\alpha_{o}\) & \(1.5\); \(2\) \\ \(a\); \(b\) & \(11.95\); \(0.136\) \\ \(K_{min}\); \(K_{max}\) & \(5\) dB; \(15\) dB \\ \(N\) & \(256\) \\ \(L\) & \(1\) \\ \(f_{c}\) & \(2.3\) GHz \\ \(B\) & \(10\) MHz \\ \(\sigma_{r}^{2}\sigma^{2}\) & \(-90\) dBm \\ \(R_{SI}\) & \(10\) dB \\ \(P_{sw}\) & \(-10\) dBm \\ \(P_{dc}\) & \(-5\) dBm \\ \(P_{s}\); \(P_{d}\); \(P_{relay}\) & \(10\) dBm \\ \(P_{tot}\) & \(30\) dBm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation Parameters. Compared to the conventional half-duplex and full-duplex UAV relays, the proposed hybrid and active RIS-UAVs are superior. This is because the performances of the full-duplex UAV relays are limited by the 10 dB of residual self-interference, and it is well-known that the full-duplex relays cannot fully cancel their self-interference even with the recent technologies. On the other hand, the energy efficiency of the half-duplex UAV relays is lower than other relaying schemes because they transmit the signals using two time slots due to the half-duplex constraints. However, when the UAV is placed near the UE, the half-duplex UAV relays can even outperform the full-duplex UAV relays as the full-duplex UAV relays experience a weak channel gain between the BS and UAV relays and also its residual self-interference. Furthermore, the figure shows that the DF UAV relay achieves a higher energy efficiency than the AF UAV relays because the noise is amplified by the AF UAV together with the attenuated signals. In addition, Figure 3 studies the performance of the relaying schemes by varying the altitude of the UAV. In order to study the impact of the UAV's altitude and elevation angle on the energy efficiency performance of different relaying schemes, the UAV is placed in the middle between the BS and UE. Figure 3: Energy efficiency versus altitude of the UAV. Figure 2: Energy efficiency versus the horizontal distance of UAV from the BS. As shown in Figure 3, the energy efficiencies of all the schemes increase with the altitude of the UAV. This is due to the increase in the elevation angle between the UAV and the ground nodes, which provides a better clearance from the ground obstructions, such as buildings and other man-made structures. As a result, the probability of the LoS and Rician factor increase, as the Rician factor is defined as the ratio of the power in the LoS component to the power in the NLoS components. The higher the probability of the LoS, the higher the Rician factor. The high Rician factor strengthens the channel gain between the UAV and the ground nodes, thus improving the SNR of the relaying schemes. In short, the energy efficiencies of all the relaying schemes increase as the altitude of the UAV increases, as validated with the Al-Hourani model [23]. However, it can be observed that the energy efficiencies of all the schemes decrease after the UAV's altitude increases to above 65 m in height. The increase in the distance between the UAV and the ground nodes has caused a higher path loss to be experienced by the A-G channel, thus degrading the system performance. Furthermore, Figure 3 demonstrates that the proposed active RIS-UAV is superior to other relaying schemes in all the UAV deployments. Similar to Figure 2, when the UAV is placed in the middle between the BS and UE, the energy efficiency of the proposed hybrid RIS-UAV is higher than the passive RIS-UAV, even if the altitude of the UAV increases. This is because the passive RIS-UAV experiences severe double path-loss problems when the UAV is placed in the middle between the BS and UE. Additionally, the proposed hybrid RIS-UAV also outperforms the half-duplex and full-duplex UAV relays.