# A Hybrid RF/FSO Transmission System Based on a Shared Transmitter
## 3 Experimental Setup  
Figure 3 illustrates the experimental setup of the RF and FSO fusion system. On the transmitter side, a baseband signal is generated by a 120 GSa/s arbitrary waveform  
generator (AWG), model M8100 series from Keysight (Santa Rosa, CA, USA). The signal is then amplified by electric amplifiers (EAs) and modulated by an I/Q modulator onto an optical carrier generated by ECL1 (at 1550.73 nm). After passing through a polarization-maintaining Erbium-doped fiber amplifier (PM-EDFA) from Amonics (Hong Kong, China), the modulated signal is combined with the optical signal from ECL2 (at 1550.00 nm), resulting in a signal at 91.25 GHz, in the millimeter-wave band. The signal is split into two paths: in the electrical path, it first passes through an attenuator (ATT) to adjust the power input to the photodiode (PD), model Finisar XPDV3120R (Sunnyvale, CA, USA). The PD converts the optical signal into an electrical millimeter-wave signal, which then passes through a low-noise amplifier (LNA), model AT-LNA-75110-3504 HP from AT Microwave (Shanghai, China) with 35 dB gain, and is transmitted via a horn antenna (HA) with 25 dBi gain into free space. The signal passes through a dielectric plate made of PMMA, transparent to MMW radiation. In the optical path, a self-focusing lens (GRIN lens), model F810 FC-1550 from Thorlabs, generates parallel light incident on the PMMA plate, which is 5 mm thick. The plate reflects the optical signal, and by adjusting the angle of the plate, the optical and electrical signals remain on the same path. The wireless transmission distance in this experiment is 1 m, eliminating the need for lens-based focusing due to the short distance. The physical diagram of the key components in the shared transmitter is shown in Figure 4.  
At the receiving end, electrical and optical signals are simultaneously received by separate receivers. The optical receiver must be precisely aligned with the transmitter, while the electrical receiver can be positioned within a 0.2 m range from the optical receiver. For the FSO link, the optical signal received by the FSO receiver (model M1505009-02B,  
Figure 4: Key components of the shared transmitter physical diagram. Figure 3: The experimental setup of the RF/FSO fusion system. (ECL: external cavity laser; DSP: digital signal processing; AWG: arbitrary waveform grating; I/Q Mod: I/Q modulator; PM-EDFA: polarization-maintaining Erbium-doped optical fiber amplifier; PM-OC: polarization maintaining optical coupler; OC: optical coupler; ATT: attenuator; PD: photodiode; LNA: low noise amplifier; HA: horn antenna; PMMA: Polymethyl methacrylate; FSO: free space optical; LO: local oscillator; EA: electrical amplifier; EDFA: Erbium doped fiber amplifier; ICR: integrated coherent receiver; OSC: oscilloscope.) 34 th Research Institute of China Electronics Technology Group Cooperation, CETC, Guilin, China) is pre-amplified by an Erbium-doped fiber amplifier (EDFA). Measurements indicate that, when the optical power input to the GRIN lens is 14 dBm, the power received by the fiber connected to the FSO receiver is \(-\)19 dBm, which is subsequently amplified by the EDFA. The signal is then processed by the integrated coherent receiver (ICR), with the local oscillator (LO) employed for homodyne detection. Finally, an oscilloscope (model DSA-X 96204Q, Agilent Technologies, Santa Clara, CA, USA) with a 160 GSa/s sampling rate is used to capture the signals for offline digital signal processing (DSP). As shown in Figure 5, the offline DSP of the FSO signal includes the constant modulus algorithm (CMA), frequency offset estimation (FOE), common phase error (CPE) correction, and decision-directed least mean square (DD-LMS) adaptation. For the RF receiver, the signal is initially received by a horn antenna (HA) with a gain of 25 dBi. A low-noise amplifier (LNA) with a gain of 20 dB is used to boost the signal before down-conversion. The down-conversion is achieved using a mixer driven by a 75 GHz local oscillator, resulting in an intermediate frequency (IF) of 16.25 GHz.