## 4 Experiment
Figure 12: The performances of FSO-only, FSO with FEC, RF-only HARQ once or twice with FEC, and RF/FSO hybrid HARQ once or twice with FEC, \(\Psi=0.5\): (a) the BER performances versus the transmitted optical power of the FSO link; (b) the power gain of each HARQ strategy under different FEC overheads. retransmitted twice compared to the RF/FSO hybrid retransmission. The FSO signal acts as noise at low received power and influences the MRC process in cc-HARQ, resulting in poor performance of the system with RF/FSO hybrid retransmission twice compared to the system with RF-only retransmission twice. From the perspective of different relative symbol rates of the RF link over the FSO link \(\Psi\), Figure 13 shows the performances of FSO-only, FSO with FEC, RF-only HARQ once or twice with FEC, and RF/FSO hybrid HARQ once or twice with FEC under different \(\Psi\). We can see from Figure 13c that the first HARQ round achieved similar performance, no matter which HARQ strategy was taken. The second HARQ round showed a great difference in performance. When OH = 5%, \(\Psi=0.5\) compared with \(\Psi=0.25\), the power gains of 9.6 and 1.5 dB were obtained under RF retransmission twice and RF/FSO hybrid retransmission twice, and when OH = 15%, the power gains of 11.3 and 2.5 dB were obtained, respectively. Figure 13: The performances of FSO-only, FSO with FEC, RF-only HARQ once or twice, and RF/FSO hybrid HARQ once or twice under different \(\Psi\): (**a**) the BER performances with the RF-only HARQ strategy when FEC OH = 5%; (**b**) the BER performances with the RF-only HARQ strategy when FEC OH = 15%; (**c**) the BER performances with the RF/FSO hybrid HARQ strategy when FEC OH = 5%; (**d**) the BER performances with the RF/FSO hybrid HARQ strategy when FEC OH = 15%; (**e**) the power gain of each HARQ strategy under different FEC overheads. Next, we probed into the system performance utilizing the HARQ strategy under atmospheric turbulence. The implementation of various air-turbulence simulators makes it difficult to quantify the difference between weak, moderate, and strong turbulence. Since atmospheric turbulence essentially affects the amplitude of the signal being transmitted, we simulated the influence of turbulence on the system by generating the channel gain coefficient under turbulence of different intensities, just as is shown in Figure 4, and then applying the turbulence coefficient directly to the signal at the transmitting end. We chose the turbulence coefficients at 21 time slots appropriately, which contained the worst-case scenario, e.g., the smallest coefficient, of the three turbulence intensities. We examined the BER performances of all 21 time slots at strong, moderate, and weak turbulence intensities; three FEC overheads of 5%, 10%, and 15%; and two RF to FSO data rate ratios \(\Psi\) of 1:2 and 1:4. We first tested the system BER performance under strong atmospheric turbulence. Figure 13(a) is the strong turbulence coefficients at 21 different time slots, Figure 13(b) is the pre-FEC BER of FSO link without retransmission, Figure 13(c)-e is the BER performance at \(\Psi=0.25\) and different FEC overheads, and Figure 13(f)-h is the \(\Psi=0.5\) condition. This is noted because of the varied channel performances throughout our testing, Figure 13(b) shows the pre-FEC performance of continuous tests for 21 time slots, and the post-FEC performance differs between Figures 13(c)-e and 13(f)-h at the same FEC overhead. When the worst case of strong turbulence was encountered, the turbulence coefficient was 0.58 at the 11 th time slot, as is shown in Figure 13(a), indicating that the signal amplitude at the receiving end was 0.58 times that without turbulence. When FEC OH = 5%, no matter the RF to FSO data rate \(\Psi\) is, 1:2 or 1:4, the system can be transmitted to error-free by RF-only retransmission twice, as can be seen from Figure 13(c),f. As is discussed above, the RF-only retransmission strategy at \(\Psi=0.25\) had the worst system performance.