# A Novel System of Mixed RF/FSO UAV Communication Based on MRR and RIS by Adopting Hybrid Modulation
Meanwhile, in order to improve the performance of the communication system by enhancing the signal, the reconfigurable intelligent surface (RIS) is applied in RF communication [17]. The RIS is an emerging technology to improve the performance of wireless communication and will have promising applications in the internet of things (IoT) as well as in 6G [18]. More specifically, the RIS is an artificial surface composed of electromagnetic materials. It adjusts the reflection of incident electromagnetic waves through low-cost and simple passive reflection elements to realize the intelligent reconstruction of the signal propagation environment, thereby enhancing the signal and improving the performance of the wireless communication system [19]. At present, RIS is mainly used in two transmission scenarios. One is the RIS-assisted scenario, where the RIS is deployed on the building between the signal source and the receiver. The other is the RIS-equipped scenario, where the RIS is placed together with the RF signal generator as a RF source. The outage probability, average BER, and average channel capacity of the RIS-assisted dual-hop RF UAV communication system were analyzed in [20], and it was found that RIS did improve the performance of the system. Nonetheless, in the RIS-assisted case, it was observed that the analytic and the simulated results matched well only when the number of reflective elements of the RIS was large [21]. By contrast, in [22], the RIS-equipped scenario reflected better diversity gain and coding gain. Moreover, the theoretical and simulated results also matched well when the number of reflective elements was low. Additionally, the modulation technique has a great impact on the BER performance of the communication system [23]. There have been many studies using modulation techniques to improve communication performance. Compared to traditional modulation schemes, the hybrid modulation is prominent. In [24], the authors applied M-ary quadrature amplitude modulation multi-pulse pulse-position modulation (QAM-MPPM) to the FSO system and found that the average BER performance was significantly better than that of the conventional scheme. Additionally, the BER of the FSO communication system by using hybrid pulse position modulation-binary phase shift keying-subcarrier intensity modulation (PPM-BPSK-SIM) was investigated in [25; 26; 27]. Moreover, the hybrid \(L\)-ary pulse position modulation-minimum shift keying-subcarrier intensity modulation (\(L\)-PPM-MSK-SIM) was adopted in deep space wireless optical communication system to improve the BER performance [28]. Inspired by the above, we apply MRR and RIS to the mixed RF/FSO UAV communication system, where MRR solves the limitation of UAV relay and RIS achieves signal enhancement. Moreover, the RIS-equipped scheme has better diversity gain and codinggain. In practical application scenarios, the user side needs to take into account factors such as size, which requires that the RIS does not have too many reflective elements, and the RIS-equipped scheme simulates well with a smaller number of reflective elements, which is not the case with the RIS-assisted scheme. The use of UAV as mobile relay allows for increased flexibility in communication, can be better applied to unexpected and occasional events, and the special operating principle of MRR can largely address the inherent limitations of UAV. As far as we know, there are rather few studies on mixed RF/FSO systems based on MRR, and even fewer studies that consider both atmospheric turbulence and pointing error. More importantly, the theoretical research combining MRR and RIS in the existing literature is still in a vacuum. Besides, we raise the BER performance of the system by the hybrid \(L\)-PPM-BPSK-SIM scheme. In the existing research on MRR FSO systems, only the downlink is mostly investigated, in our work, the end-to-end uplink and end-to-end downlink are comprehensively studied and the differences between them are analyzed.