## 4 Conclusions  
The integration of RIS and UAVs has been envisaged as a new promising technology for beyond-5G networks. Therefore, this paper has investigated the performance of various relaying schemes and demonstrated that the proposed active and hybrid RIS-UAVs are better options than a passive RIS-UAV to assist ground communication. Based on the results simulated, active and hybrid RIS-UAVs, which amplify the signal while reflecting it to the UE, can overcome the double path-loss problem suffered by the passive RIS-UAV. The results numerically show that the proposed active and hybrid RIS-UAV schemes can provide up to 26 times and 14 times improvement in the energy efficiency as compared to the passive RIS-UAV, respectively, when the UAV is placed in the middle between the BS and UE. Meanwhile, when the UAV is placed close to the BS or UE, the performances of the proposed active and hybrid RIS-UAVs are about 187% and 18.9% higher than the passive RIS-UAV, respectively. Even though the energy efficiency of the existing passive RIS-UAV increases as the altitude of the UAV increases, the proposed active and hybrid RIS-UAVs still outperform it. In addition, the proposed active and hybrid RIS-UAVs can provide a high energy efficiency to the system even with a small number of total elements. By increasing the active elements of the hybrid RIS-UAV, the proposed hybrid RIS-UAV can double or triple the energy efficiency of the passive RIS-UAV. On the other hand, when compared to the conventional half-duplex UAV relays that require two phases to transmit signals, and the full-duplex UAV relays that are significantly affected by residual self-interference, the proposed active and hybrid RIS-UAVs show their superiority.  
Even though an active RIS-UAV provides a higher energy efficiency than a hybrid RIS-UAV in most cases, it requires a larger power budget in order to supply the hardware components and circuits. Considering the limitation of the power budget of the UAV, the hybrid RIS-UAV is hence preferred as it requires lesser power to operate, such that the hybrid RIS-UAV can save up to 5 dBm while achieving an acceptable energy efficiency. Additionally, the hybrid RIS-UAV can be modified to become a passive RIS-UAV or an active RIS-UAV by dynamically adjusting its number of active elements based on the available power budget and the system requirements. Therefore, the application of a hybrid RIS-UAV in assisting the communication between the source and destination is a potential future work. In addition, the optimal altitude of the UAV and the number of total reflecting elements are worth to study for improving the performance of the hybrid RIS-UAV scheme.  
Conceptualization, C.Y.G. and C.Y.L.; methodology, C.Y.G. and C.Y.L.; software, C.Y.G. and C.Y.L.; validation, C.Y.G. and C.Y.L.; formal analysis, C.Y.G., C.Y.L. and R.N.; investigation, C.Y.G., C.Y.L. and R.N.; resources, C.Y.G. and C.Y.L.; data curation, C.Y.G. and C.Y.L.; writing--original draft preparation, C.Y.G.; writing--review and editing, C.Y.G., C.Y.L. and R.N.; visualization, C.Y.G. and C.Y.L.; supervision, C.Y.L.; project administration, C.Y.L.; funding acquisition, C.Y.L. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the Ministry of Higher Education Malaysia under the Fundamental Research Grant Scheme FRGS/1/2020/TK0/UTM/02/68, in part by H2020-MSCA-RISE-2020 under Grant 101008085, and in part by Universiti Teknologi Malaysia under Grant 08G83, Grant 22H33, and Grant 05E07.  
Not applicable.  
The authors declare no conflict of interest.