## 3 Results
On one hand, the hybrid RIS-UAV performs better than the half-duplex UAV relays because the hybrid RIS-UAV can receive and transmit the signals simultaneously. On the other hand, the hybrid RIS-UAV is more energy efficient than the full-duplex UAV relays because the full-duplex UAV relays suffer from residual self-interference. In addition, as the passive RIS-UAV works in the full-duplex mode, it achieves a higher energy efficiency than the half-duplex UAV relay when the altitude of the UAV increases. Moreover, it can be noticed that the DF UAV relay outperforms the AF UAV relay because the latter forwards amplified noise to the UE during the signal forwarding. Figure 4 investigates the effect of increasing the antennas or elements on the energy efficiency performance of different relaying schemes. The coordinate of the UAV is fixed at the coordinate (90, 0, 20), i.e., the UAV is placed near to the UE, to enable a comparison between the RIS-UAVs and UAV relays. Specifically, Figure 4 is simulated to investigate the scaling effect of increasing the number of antennas or elements, without considering whether the dimensions of the RIS or relay are feasible to be mounted on the UAV. From Figure 4, the proposed active RIS-UAV outperforms the other relaying schemes when the number of elements is small, thanks to its amplification capability to amplify the attenuated signal while reflecting it to the UE. However, the energy efficiency of the active RIS-UAV degrades to zero when the number of elements becomes very large because the power budget is insufficient to supply its hardware components, thus failing to transmit signals. On the other hand, because the hardware power consumption of passive RIS-UAV is much smaller, and its SNR scales with \(N^{2}\), thus making it more energy efficient when the number of elements is very large. The observation is in line with the result reported in [26] for terrestrial RIS. For the proposed hybrid RIS-UAV, its energy efficiency has steadily declined as the number of elements grows because the SNR of the hybrid RIS-UAV is significantly affected by its amplification factor, and this factor remains nearly constant, even increasing the number of elements. This is because of the weak channel gain between the BS and RIS-UAV; hence, the same amount of the amplification factor is required to amplify and reflect the signals. As a result, the steady decline in the energy efficiency performance of the hybrid RIS-UAV is only due to the decrease in the available transmit power. Moreover, it can be observed that the energy efficiencies of the conventional half-duplex and full-duplex UAV relays drop significantly when the number of antennas increases because the power budget is inadequate to supply the UAV relays' hardware components and circuits. In other words, the UAV relays can only be equipped with a small number of antennas because the hardware-dissipated power at each antenna is substantially larger than the RIS elements as they are equipped with the radio-frequency chains. Generally, a higher number of antennas will increase the dimensions of the RIS and relays because an adequate distance is required between the antennas or RIS elements in order to ensure the channels are uncorrelated. In practice, there is a trade-off between the number of antennas or elements and the size of the relay or RIS, such that the higher number of elements can provide a higher energy efficiency to the system, but its dimensions might grow larger until a point where it might not be feasible to be mounted on the UAV. If a smaller size of the RIS or relay is used, the energy efficiency of the relaying schemes will be lower. Hence, the number of antenna elements has to be chosen carefully in order to be mounted on the UAV feasibly while providing a good performance to the system. In short, an active RIS-UAV is preferred because it can achieve a higher energy efficiency, even if the number of elements is small.