## 4 Experiment
Therefore, if the goal was to maintain high throughput in this system at the minimum RF link power, then the error-free transmission would be achieved through two RF retransmissions at strong atmospheric turbulence. When using the RF/FSO hybrid retransmission strategy at FEC OH = 5%, the error-free transmission was achieved through one RF/FSO hybrid retransmissions. We also recorded the system BER performance when FEC OH = 10%, wherein the error-free transmission was achieved through one RF retransmission, as is shown in Figure 13(d),g. When FEC OH = 15%, the error-free transmission was achieved right after the FEC decoding, as is shown in Figure 13(e),h. Thus, we can draw the conclusion that when the retransmission round was limited, throughput or system consumed power can be sacrificed for the similar BER performance with more retransmission rounds by either utilizing the FSO link to retransmit together, increasing the FEC overhead, or improving the RF data rates. We then examined the system performances under moderate and weak atmospheric turbulence. Figure 14(a) is the moderate turbulence coefficients at 21 different time slots, Figure 14(b) is the pre-FEC BER of the FSO link without retransmission, Figure 14(c),d is the BER performance at \(\Psi=0.25\) and different FEC overheads, and Figure 14(e),f is the \(\Psi=0.5\) condition. As mentioned above, Figure 14(b) shows the pre-FEC performance for 21 time slots in one of our selected sets of continuous tests, and the post-FEC performance differed between Figure 14(c),d and Figure 14(e),f at the same FEC overhead. As is illustrated in Figure 14(c)-f, the system can achieve error-free transmission by RF-only retransmission once at 5% FEC OH and right after FEC decoding at 10% FEC OH, regardless of the RF to FSO data rate \(\Psi\). Figure 15g shows the weak turbulence coefficients at 21 different time slots, and Figure 15f shows the pre-FEC BER of the FSO link without retransmission under the weak turbulence condition. After testing, the system can achieve error-free transmission right after FEC decoding at 5% FEC OH in the weak turbulence condition. From the above, we can see that the HARQ strategy can significantly improve the performance of the FSO/RF hybrid transmission system under the influence of atmospheric turbulence. In practice, we need to dynamically select the retransmission strategy and FEC overhead according to the planned transmitting power of the FSO and RF link, the size of turbulence, and other factors. By substituting the actual parameters into the numerical simulation in Section 3, we summarized the output probability and throughput at different circumstances, which corresponded to the aforementioned BER performances. The SNRs at the receiving end were obtained by averaging the SNR of 21 groups of data tested under different turbulences and rate ratios in the actual experiment. These SNRs were calculated by error vector magnitude (EVM), which is called the calculated SNR. Table 3 shows the output probability and throughput under strong turbulence intensity, with different RF to FSO data rate ratios \(\Psi\), FEC overheads, and HARQ strategies. It can also shed light on the great performance improvement brought by the hybrid FSO/RF communication system with the HARQ technique. Figure 14: The BER performance under strong atmospheric turbulence: (**a**) strong turbulence coefficients at 21 different time slots; (**b**) the pre-FEC BER of the FSO link without retransmission; (**c**) 5% OH with HARQ-aided BER performances at \(\Psi=0.25\); (**d**) 10% OH with HARQ-aided BER performances at \(\Psi=0.25\); (**e**) 15% OH with HARQ-aided BER performances at \(\Psi=0.25\); (**f**) 5% OH with HARQ-aided BER performances at \(\Psi=0.5\); (**g**) 10% OH with HARQ-aided BER performances at \(\Psi=0.5\); (**h**) 15% OH with HARQ-aided BER performances at \(\Psi=0.5\).