## 4 Experiment  
The experimental setup is shown in Figure 9. In the transmission digital signal processing (Tx DSP) block, the data stream to be transmitted was first CRC encoded, then FEC encoded, and then modulated into QPSK signals, as is introduced in Figure 2. The output modulated symbol stream is denoted as \(S_{FSO}\); it is then downsampled N times to obtain bit stream \(S_{RF,N}\). In the following experiment, two times and four times downsampling were adopted according to the different transmission rate ratios, that is, the number n in the bit sample block is 2 and 4, respectively. Depending on the retransmission times of the RF link, down-sampling sequences of different positions are generated. Sequence \(S_{RF,1}\) and \(S_{RF,2}\) in the figure represent the transmitting data stream when RF link is retransmitted once and twice, respectively. After the symbol sample, the signal stream of the FSO link \(S_{FSO}\) and the signal stream of the RF link \(S_{RF,N}\) were then sent into a 60-GSa/s sampling rate arbitrary waveform generator (AWG). The output electrical signals of the \(S_{FSO}\) and \(S_{RF,N}\) modulated sequences were used to drive the Machzender modulator (MZM) of the FSO link and RF link, respectively. In the FSO link, Laser 1 operated at 1550 nm and was used as the light source of the FSO link. The output signal light passed through an erbium-doped fiber amplifier (EDFA) and a variable optical attenuator (VOA) before transmission. The VOA was used to adjust the transmitted optical  
Figure 8: The outage probability and throughput versus the receiving side SNR of the FSO link under different FEC overheads with twice RF-only retransmission: (**a**) outage probability; (**b**) throughput. signal power. A pair of fiber collimators served as antennas in the FSO link. The collimator in the transmission side was used to collimate free space light propagating from the tip of the fiber. After 3.8 m of line-of-sight (LoS) free space transmission, another collimator was used to focus the received FSO light into the fiber at the receiving end. The received FSO signals were then detected by a 10 GHz photodetector (PD) and sent into a digital storage oscilloscope (DSO) with a sampling rate of 100 GHz for subsequent joint off-line digital signal processing (DSP). In the RF link, the 80 GHz MMW signal was generated by the optical beat method. Laser 2 operated at 193.1 THz and was used as the modulated signal source for RF links. Laser 3 operated at 193.18 THz and was used as the local oscillator light of the RF link with an output optical power of 7 dBm and a frequency difference of 80 GHz from Laser 2. It is noted that the power and the polarization of the beat light and the modulated signal light should be consistent so that the generated MMW signal can obtain the optimal heterodyne gain. A 3 dB polarization maintaining optical coupler (PM-OC) was utilized to combine the optical signals and the LO light. The combined optical signal was sent into a high-speed PD with 100-GHz bandwidth and was up-converted to MMW. A pair of Cassegrain antennas with 45-dBi gains was used to transmit the MMW signal. After 4 m LoS transmission, the MMW signal was received by a low noise amplifier (LNA), and then it was converted down to an intermediate frequency (IF) signal by an envelope detector (ED) and amplified by a power amplifier (PA). It was then stored by the DSO for subsequent joint off-line digital signal processing. The offline digital signal processing block is shown in Figure 2, where the signals transmitted and received by MMW and FSO links were synchronized and resampled, respectively, and the channel estimation was also carried out. Then, the two synchronized signals were combined by the ADCT based on the MRC algorithm with the best binding ratio \(\alpha\) determined by the SNR of the two channels. If the CRC check failed, then the HARQ controller would implement RF or RF/FSO hybrid retransmission.