## 4 Experiment
We generated a series of quantifiable time-varying turbulence coefficients based on channel and turbulence models. As mentioned earlier, due to the much shorter transmission time of each signal frame compared to the coherence time of turbulence, channel gain was assumed to remain constant during the transmission of each FSO data stream. Therefore, the impact of turbulence on the overall data flow was consistent and the turbulence-induced optical power fluctuations in received power were able to be reflected by applying the  
Figure 9: Experimental setup of the hybrid MMW and FSO links system with HARQ and adaptive combining techniques (PA: power amplifier; MZM: Mach–Zehnder modulator; EDFA: erbium-doped fiber amplifier; PD: photodetector; ED: envelop detector; DSO: digital storage oscilloscope; LNA: low noise amplifier). same attenuation coefficient to the signal at the receiving end, and this was equivalent to applying the same attenuation coefficient at the transmitting end. Thus, the influence of atmospheric turbulence on the FSO link was simulated by applying different turbulence coefficients on the data stream of the transmit side, as is shown in Figure 9.  
Figure 10 shows the experimental setup of our hybrid RF/FSO transmission system, and Table 2 lists the basic experimental quantities' values. On the left side is the transmitter, where the RF link signal was converted into millimeter-wave signals through laser beating and then passed through a PD before being transmitted simultaneously with the FSO link. The combined optical signal before the PD was 4.43 dBm. At the receiver, the RF link signal was received by an antenna and then went through LNA with the gain of 38 dB, ED, and PA with the gain of 23 dB, before being input into a digital storage oscilloscope. The FSO link, on the other hand, was collimated by a collimator, where the aperture of the FSO antenna used was 11.2 cm. It was then input directly into a PD for photoelectric conversion before being sent to a digital storage oscilloscope, where it awaited further joint digital signal processing. retransmits once, twice, and three times; and RF and FSO retransmit once and twice to obtain power gains of 1.6, 1.3, 1.3, 2.1, 1.9, and 1.5 dB. It can be seen that increasing the FEC overhead improved the system performance. However, it should be noted that the increase in the overhead reduced the throughput, and the FSO link was able to continue the transmission of the next data frame with RF-only retransmission when the received SNR of the FSO link was high enough. Consequently, the throughput was much higher than that of the RF/FSO hybrid retransmission, which can also be seen in the previous numerical simulation. As for the \(\Psi=0.5\) condition in Figure 12, when FEC OH = 5%, RF-only retransmission once and twice obtained 1.0 and 11.0 dB power gain, respectively, while the RF/FSO hybrid retransmission once and twice obtained 2.0 and 5.0 dB power gain, respectively. When FEC OH = 15%, RF-only retransmission once and twice obtained 0.6 and 12.0 dB power gain, while RF/FSO hybrid retransmission once and twice obtained 2.1 and 5.5 dB power gain, respectively. Compared with the FEC OH = 5% condition, single FSO transmits without HARQ, RF retransmits once or twice, and RF and FSO retransmit once or twice to obtain power gains of 2.0, 1.6, 3.0, 2.1, and 2.5 dB. RF-only retransmission twice can achieve error-free transmission, even when the FSO transmitted power is \(-10\) dBm and the received power is \(-27.8\) dBm, thanks to the higher transmission rate of the RF link. The RF link transmits half of the information of the FSO link, and thus it achieves larger gain when  
Figure 11: The performances of FSO-only, FSO with FEC, RF-only HARQ once or twice, and RF/FSO hybrid HARQ once or twice, \(\Psi=0.25\): (a) the BER performances versus the transmitted optical power of the FSO link; (b) the power gain of each HARQ strategy of different FEC overheads.