# A Novel System of Mixed RF/FSO UAV Communication Based on MRR and RIS by Adopting Hybrid Modulation
## 4 Numerical Results and Discussion  
In this section, we utilize the mathematical expressions derived in Section 3 to evaluate the performance of the mixed RF/FSO UAV system based on MRR and RIS. Here, we do not consider factors such as the jitter of the UAV. In our analysis, the wavelength of the laser is set to 1550 nm, the aperture of both the MRR at the relay and the receiver at the destination is fixed at 10 cm, and the beamwidth at distance \(d\) is \(0.03 mrad*d\). In addition, the distance of the RF link is set to 1.0 km, and the FSO communication distance is generally 5.0 km without special instructions. The height of the optical receiver at the destination from the ground is 50 m, the height difference between the UAV and the optical receiver is 100 m, and the angle between the FSO link and the horizontal direction is 1.2 degrees. The number of iterations of the simulation is \(10^{6}\). The atmospheric refractive index structure parameters are set to \(C_{n}^{2}=5.2\times 10^{-16}m^{-2/3}\) (\(\alpha=11.8\), \(\beta=10.2\)), \(C_{n}^{2}=6.5\times 10^{-15}m^{-2/3}\) (\(\alpha=4.0\), \(\beta=1.6\)), and \(C_{n}^{2}=3.0\times 10^{-14}m^{-2/3}\) (\(\alpha=6.0\), \(\beta=1.1\)) corresponding to weak, moderate, and strong turbulence, respectively. Because of the use of MRR, the beam is immediately reflected from the UAV relay to the destination, we set \(\alpha_{1}=\alpha_{2}=\alpha_{3}=\alpha\) and \(\beta_{1}=\beta_{2}=\beta_{3}=\beta\). For the pointing error, we let \(\zeta=10\) as small pointing error and \(\zeta=1\) as large pointing error. The outage probability of the mixed RF/FSO UAV dual-hop system are shown in Figures 3-5. In Figure 3, under moderate turbulence and small pointing error, we set \(N=5,10,15,20\) to analyze the impact of RIS on the outage probability of end-to-end downlink and end-to-end uplink by varying the number of reflective elements and compare the performance with that of the scheme without RIS. As expected, the outage probability with RIS is several orders of magnitude lower than the scheme without RIS. The simulation results show that the performance of the end-to-end downlink is better than that of the end-to-end uplink, which is caused by the fact that the FSO uplink experiences twice the atmospheric channel distance than the FSO downlink. In addition, both have their own performance thresholds, because we fixed the SNR of the FSO link, when the SNR is low, the performance of the whole system is greatly influenced by the RF link. Furthermore, when the SNR gradually increases, the outage probability of RF link will be very small and the performance of the whole system will be dominated by the FSO link. Specifically, when \(\overline{\gamma}_{1}=5\) dB and \(N=10\), the end-to-end uplink and end-to-end downlink outage probabilities are \(3.3\times 10^{-2}\) and \(3.3\times 10^{-3}\), respectively. Moreover, the outage probability decreases more significantly as \(N\) increases. Therefore, the use of RIS can improve system performance to a large extent by reducing outage probability, especially in low SNR regions. In Figure 4a,b, the outage probability of end-to-end uplink and end-to-end downlink are analyzed under different atmospheric turbulence and pointing error. Here, we set \(\overline{\gamma}_{1}\!=\!\overline{\gamma}_{2}\,=\,\overline{\gamma}\). As clearly seen from the result, the outage probability of the uplink and downlink increases with the growth of atmospheric turbulence and pointing error. Furthermore, the downlink outperforms the uplink in the same weather conditions. For instance, under weak turbulence and small pointing error, the outage probability of uplink and downlink at \(\overline{\gamma}\!=\!20\) dB are \(8.1\times 10^{-3}\) and \(8.6\times 10^{-6}\), respectively. The reason for this phenomenon is that the atmospheric channel distance experienced by the FSO uplink is twice that of the FSO downlink.