# A Novel System of Mixed RF/FSO UAV Communication Based on MRR and RIS by Adopting Hybrid Modulation
## 4 Numerical Results and Discussion
Figure 5 analyzes the outage probability with different FSO communication distance under moderate turbulence and small pointing error, where \(d\) is set to 3.5 km, 5.0 km and 6.5 km. As expected, the result shows that the outage probability gradually drops as the FSO communication distance decreases. Likewise, the performance of the downlink still outperforms the uplink. Moreover, when \(\overline{\gamma}\!=\!30\) dB, the outage probability of the uplink  
Figure 4: Outage probability under different atmospheric turbulence and pointing error when \(\gamma_{th}=0\) dB, \(\xi=10,N=10,d=5.0\) km. Figure 5: Outage probability under different FSO communication distance when \(\gamma_{th}=0\) dB, \(\xi=10\), \(N=10\). are \(5.8\times 10^{-2}\), \(1.0\times 10^{-1}\) and \(1.3\times 10^{-1}\) at 3.5 km, 5.0 km and 6.5 km, while they are \(3.3\times 10^{-3}\), \(1.1\times 10^{-2}\) and \(1.9\times 10^{-2}\) in the downlink. Figure 6 illustrates the average BER performance of the end-to-end downlink using \(L\)-PPM-BPSK-SIM under conditions of moderate turbulence and small pointing error. Since many communication systems use BPSK modulation scheme with low BER to improve the communication performance, Figure 6 compares the \(L\)-PPM-BPSK-SIM and BPSK schemes. We set \(L=4,8,16,32,64\), the data reveals that the average BER with \(L\)-PPM-BPSK-SIM less than that with the BPSK when the value of \(L\) is greater than eight, and the average BER decreases continuously as the value of \(L\) increases. For example, when \(\overline{\gamma}=30\) dB, the average BER with the BPSK scheme is \(2.7\times 10^{-3}\), the average BER of the \(L\)-PPM-BPSK-SIM scheme reaches \(1.6\times 10^{-3}\), \(7.7\times 10^{-4}\) and \(3.9\times 10^{-4}\) for \(L\) of 16, 32 and 64, respectively. Figure 7 depicts the average BER of end-to-end downlink and end-to-end uplink with different number of RIS reflective elements under moderate turbulence and small pointing error. Expectedly, the average BER performance of the downlink is significantly better than that of the uplink, and the average BER decreases to the threshold as the SNR increases, for reasons explained in Figure 3. Moreover, the larger the value of \(N\), the faster the average BER decreases. Specifically, when \(\overline{\gamma}_{1}\)\(=-\) 10 dB and \(N=10\), the end-to-end uplink and downlink average BER is \(3.1\times 10^{-3}\) and \(1.3\times 10^{-4}\). Obviously, the average BER performance with RIS has a great improvement over that the scheme without RIS. Figure 8a,b demonstrate the average BER of the end-to-end uplink and end-to-end downlink under different atmospheric turbulence and pointing error. As expected, like the performance of outage probability, the average BER performance of the downlink is still better than that of the uplink. From Figure 8b, the average BER of weak turbulence is significantly smaller than that of moderate and strong turbulence. Moreover, when \(\overline{\gamma}\)\(=20\) dB, the average BER of uplink and downlink under weak turbulence in Figure 8a are \(2.2\times 10^{-2}\) and \(1.5\times 10^{-2}\), while in Figure 8b, the average BER of uplink and downlink are \(1.7\times 10^{-5}\) and \(4.6\times 10^{-8}\). Figure 6: Average BER of end-to-end downlink under different \(L\) when \(\alpha=4.0\), \(\beta=1.6\), \(\xi=10\), \(d=5.0\) km, \(N=10\). In Figure 9, the average BER of different FSO communication distance is analyzed under moderate turbulence and small pointing error. Based on this, we set the FSO communication distance as 3.5 km, 5.0 km, and 6.5 km. Expectedly, the average BER of both the uplink and downlink reduces as \(d\) decreases. For instance, when \(\overline{\gamma}=30\) dB, the average BER of the uplink are \(3.4\times 10^{-3}\), \(1.2\times 10^{-2}\) and \(2.4\times 10^{-2}\) at 3.5 km, 5.0 km and 6.5 km, while they are \(1.6\times 10^{-4}\), \(7.7\times 10^{-4}\) and \(1.8\times 10^{-3}\) in the downlink. Figure 8: Average BER under different pointing error and atmospheric turbulence when \(d=5.0\) km, \(L=32\), \(N=10\).