## 1 Introduction
As such, the RIS-UAV has been a popular research topic in recent years [15; 16; 17] to serve as a mobile relay node to forward the incident signal toward the destinations, especially when the direct links between the source and destination are weak or blocked by obstacles. However, the existing RIS-UAVs mostly comprise passive elements only, in which the RIS-UAV reflects the signals without signal regeneration or amplification. Hence, the passive RIS-UAV suffers from the double path-loss problem, especially when the passive RIS-UAV is placed distant from the BS and UE [6; 17]. Even though active and hybrid terrestrial RIS are demonstrated to obtain a better performance than the passive terrestrial RIS in [18] and [8], respectively, the application of active and hybrid RIS-UAVs in assisting ground communication is still an open issue. To date, there have been several comparative studies between terrestrial RIS and active relays, but their comparison in UAV-assisted communication is still limited. The comparison of the energy efficiency between a passive RIS-UAV and the full-duplex DF UAV relay is examined in [17]; however, the half-duplex UAV relaying mode is not taken into consideration. Despite the fact that the full-duplex relaying scheme has twice the spectral efficiency of the half-duplex relaying scheme, the half-duplex relaying scheme is free of interference, while the full-duplex relaying scheme experiences self-interference. Furthermore, Refs. [19; 20] investigate the performance between a passive RIS-UAV and the conventional AF UAV relay. The simulated results reveal that the passive RIS-UAV yields a higher energy efficiency than the AF UAV relay. This paper proposes the use of active and hybrid RIS-UAVs in assisting ground communications and investigates the energy efficiencies of the active, hybrid, and passive RIS-UAVs, as well as the conventional UAV relays. The main contributions of this paper are summarized as follows:  
* We propose the use of active and hybrid RIS-UAVs to assist ground communication with ground-to-air (G-A) and air-to-ground (A-G) channel modeling. * The energy efficiencies of active and hybrid RIS-UAVs are analyzed and compared to that of existing passive RIS-UAVs and the conventional DF and AF UAV relays, considering the total power budget. A practical power consumption model is applied, which captures not only the transmit power at the BS and RIS or relay but also includes the hardware power consumption, namely the power consumption of the control circuit and phase shift switch of the RIS that scales with the number of total reflecting elements, the power consumption of the direct current (DC) biasing at each active RIS element, as well as the hardware-dissipated power at the BS, UE, and relay. * The comprehensive comparison of the energy efficiency of all the RIS-UAVs and UAV relaying schemes is presented with varying system parameters, including the placement and altitude of the UAV, the number of total reflecting elements, the number of active elements in the hybrid RIS-UAV, and the total power budget. * The comprehensive comparison between the RIS-UAVs and UAV relaying schemes reveals that the active and hybrid RIS-UAVs, which exploit the benefit of signal amplification using active elements, outperform the passive RIS-UAV and conventional UAV relays. Even with a single active element, the hybrid RIS-UAV performs better than the passive RIS-UAV, in which the performance gain can be up to 14 times if compared to the passive RIS-UAV. The rest of this paper is organized as follows. In Section 2, the channel model of the UAV and the SNR of each of the RIS-UAVs and the UAV relaying scheme are discussed. In Section 3, the simulation results are presented to evaluate the performance of each of the RIS-UAVs and the UAV relaying scheme. Finally, discussions are made and concluded in Section 4.