# A Hybrid RF/FSO Transmission System Based on a Shared Transmitter
## 1 Introduction
Meanwhile, the RF/FSO fusion transmission system is able to determine remote and efficient data transmission and sensing by combining FSO with radio frequency identification (RFID) sensing technology, which is particularly suitable for Internet of Things (IoT) and Industrial Internet of Things (IIoT) applications in complex environments [16; 17; 18; 19]. Such systems not only optimize communication and sensing resources to improve reliability and coverage, but can also incorporate deep learning technologies to enhance signal processing capabilities, thus playing an important role in areas such as smart monitoring and structural health monitoring (SHM) [16; 18]. For the transmission architecture, the proposed hybrid FSO/RF transmission models have been divided into two main categories. One of these is the serial transmission structure, where the FSO link is responsible for high-speed data transmission from the core network to remote base stations or relay nodes, leveraging its high capacity and immunity to spectrum interference. Meanwhile, in the access layer or complex environments, RF communication handles coverage from base stations to end-user devices, using its excellent multipath adaptability and wide-area flexibility to meet the demands of multiple users [20; 21; 22; 23]. Another approach is to adopt a parallel architecture for FSO and RF transmission, where the system dynamically allocates resources for the RF and FSO links by monitoring real-time link quality (such as signal-to-noise ratio). Most parallel architectures are based on link selection and switching techniques, the FSO link carries primary traffic under high-quality conditions, while the RF link takes over or offloads data in low-quality scenarios, ensuring the continuity of service quality (QoS). This architecture is particularly well suited for high-speed backbone link deployment from the core network to remote base stations [12; 24; 25; 26; 27; 28; 29]. [PERSON] et al. developed an RF/FSO hybrid fusion system that includes two FSO links and two RF links, achieving the transmission of an of a 1.196 Tbps signal over 800 m [30]. Moreover, a demonstration of hybrid fiber/FSO/millimeter wave transmission for 6G robust backhaul is also presented [31]. As shown in Figure 1, existing studies mainly use independent transmitters to realize RF and FSO converged systems, and the system requires two transmitters or two RF and FSO links, which increases the complexity and weight of the system. In addition, there are also studies from an algorithmic point of view, such as adaptive soft switching schemes based on machine learning [32] and neural network optical energy transmission optimization schemes [33]. From an antenna design perspective, [PERSON]. [34] describes an FSO/RF composite antenna developed by [PERSON] et al., which achieves a common aperture for transmitting and receiving optical and RF signals. However, this design is bulky, structurally complex, and limited to data rates of 6.7 Mbps for FSO and 100 kbps for RF links. Subsequently, Ref. [35] presents an optimized FSO/RF antenna with a reduced size, but only tests 7 Mbps BPSK FSO signals. Furthermore, the integrated antenna structure does not support further amplification of fused signals post-generation. In this paper, a hybrid RF/FSO integrated transmission architecture is proposed for the first time, which enables the system to transmit both RF and FSO signals on the same link by sharing the transmitter. Firstly, we discuss the hybrid RF/FSO transmission model, in which the attenuation suffered by each of the two links due to environmental influences is taken into account, and a proof-of-principle is then carried out to demonstrate the feasibility of such a shared transmitter architecture. The results show that the system supports the converged transmission of 25 GBaud RF/FSO signals, so such a shared transmitter and common link transmission model is feasible.