# A Hybrid RF/FSO Transmission System Based on a Shared Transmitter
## 4 Results and Discussion  
To evaluate the performance of the hybrid RF/FSO system, experiments were conducted over a 1 m wireless transmission distance. For the RF link, Figure 6 illustrates the bit error rate (BER) curves for QPSK signals at baud rates of 10 Gbaud, 15 Gbaud, 20 Gbaud, and 25 Gbaud, with input power to the photodiode (PD) ranging from \(-\)4 to 2 dBm. The measured BER values satisfied the 20% soft-decision forward error correction (SD-FEC) threshold of 2.4 \(\times\) 10\({}^{-2}\). The BER initially decreased with increasing input power but began to rise beyond approximately 0 dBm, attributed to signal saturation over the short transmission distance, which degraded signal quality. For the FSO link, no errors were observed at the tested baud rates and input optical powers. Consequently, the hybrid  
Figure 5: The digital signal processing (DSP) flowchart of the system. RF/FSO system successfully supported 25 GBaud QPSK fusion transmission over the 1 m wireless link. To further evaluate the system's performance, the baud rate in the FSO link was increased. As the BER and error vector magnitude (EVM) shown in Figures 7 and 8, the FSO link supports 32 GBaud QPSK transmission when the optical power input to the GRIN lens exceeds 7 dBm, meeting the 7% hard-decision forward error correction (HD-FEC) threshold of 3.8 \(\times\) 10\({}^{-3}\). Additionally, at input optical powers above 12 dBm, the system achieves 100 GBaud QPSK transmission while satisfying the 7% HD-FEC threshold of 3.8 \(\times\) 10\({}^{-3}\). Furthermore, as illustrated in Figure 9, the system supports 32 GBaud 16 QAM transmission at input powers exceeding 13 dBm, meeting the 20% SD-FEC threshold of 2.4 \(\times\) 10\({}^{-2}\). Figure 6: BER of the received RF signals versus the power into the PD. Figure 7: BER and EVM of the received 32 GBaud QPSK FSO signals. The experimental results demonstrate that the fusion system can support 25 GBaud hybrid RF/FSO transmission, and the FSO link supports a higher baud rate transmission compared to the RF link, as shown in Table 1. This is attributed to the narrower bandwidth of 35 GHz in the W-band electrical signal used for the RF link, whereas the FSO link leverages a spectral range spanning tens to hundreds of RF, offering a significantly broader bandwidth. Consequently, FSO supports higher data rates. Furthermore, as the experiment was conducted indoors over short distances, the optical signal avoided degradation from atmospheric absorption, scattering, and turbulence, maintaining its strength and quality. However, achieving precise alignment for the FSO link is challenging. Misalignment in components such as the collimating lens, dielectric plate, or FSO receiver can cause  
Figure 8: BER and EVM of the received 100 GBaud QPSK FSO signals. Figure 9: BER and EVM of the received 32 GBaud 16 QAM FSO signals. signal losses. Although RF transmission also requires alignment, it is relatively easy and stable. Outdoors, FSO performance is further affected by ambient light and atmospheric turbulence, while RF transmission is less affected by these factors. Based on the gamma-gamma turbulence and Nakagami-m fading models, theory suggests that the BER of the pure FSO link will exceed the threshold when the fog fading is greater than 250 dB/km or the lateral offset is greater than 1.5 mrad, whereas the hybrid system maintains the reliability by automatically switching to the RF link. This suggests that channel conditions and alignment errors can significantly degrade pure FSO performance, while the hybrid architecture circumvents this risk through dynamic link selection. While the current experiments focus on proof-of-principle in ideal environments, the theoretical model provides a basis for practical analysis, and subsequent work will focus on outdoor field tests to quantify the effects of turbulence, haze, rain, and snow on the hybrid link BER.