# A Hybrid RF/FSO Transmission System Based on a Shared Transmitter
## 1 Introduction  
Free-space optical (FSO) communication serves as a critical enabler for applications demanding ultra-high-speed, secure, and reliable wide-bandwidth data transmission, including satellite-ground links, inter-satellite networks, and next-generation wireless access systems. This technology offers distinct advantages: access to unlicensed spectrum, exceptional spectral efficiency, highly directional beam propagation, and minimal signal attenuation under clear atmospheric conditions. The narrow beam divergence inherent in point-to-point FSO links inherently mitigates multipath interference, thereby simplifying carrier recovery algorithms in receiver-side digital signal processing (DSP). However, FSO performance is fundamentally constrained by atmospheric turbulence and meteorological impairments such as fog, haze, and snow, which induce severe range limitations and signal degradation [1, 2, 3]. Experimental measurements reveal attenuation exceeding 400 dB/km in dense fog conditions with visibility below 50 m [4]. Furthermore, FSO systems exhibit acute sensitivity to pointing errors, necessitating sub-millimeter-level alignment precision between transceivers to maintain line-of-sight propagation. Even minor mechanical vibrations or thermal drifts can disrupt optical path integrity, compromising link stability. Although state-of-the-art implementations have demonstrated 10 Tb/s over 220 m and100 Gb/s over 1 km in controlled environments, persistent challenges from atmospheric disturbances continue to hinder the scalability and reliability of FSO technology [5]. As a robust complement to FSO systems, radio frequency (RF) links, particularly millimeter-wave (MMW) bands, demonstrate superior stability under adverse meteorological conditions. Field trials reveal that 40 GHz RF augmentation elevates system availability from 0.51% to 100% through hybrid signal diversity [6]. Unlike FSO's stringent line-of-sight requirements, mmWave signals exhibit multipath resilience through reflection and diffraction mechanisms, enabling limited non-line-of-sight (NLOS) communication critical for urban environments with dense obstructions. While atmospheric turbulence and pointing errors constrain FSO to sub-kilometer operational ranges, mmWave systems achieve extended reach exceeding 30 km through adaptive beamforming and atmospheric penetration capabilities [7; 8; 9]. Therefore, integrating RF signals with FSO signals for transmission effectively combines the strengths of both communication methods, addressing the limitations inherent to single-mode communication in diverse scenarios while enhancing overall system bandwidth. Hybrid RF/FSO systems are recognized as promising solutions for high-speed, point-to-point terrestrial communication in last-mile access networks, representing a pivotal approach for next-generation wireless networks beyond 5G [10; 11; 12]. This architecture offers a variety of potential application scenarios. For instance, in low Earth orbit (LEO) inter-satellite links (ISLs), Free Space Optical (FSO) communication provides high-speed data transmission, while Radio Frequency (RF) links serve as backup during satellite attitude adjustments or beam obstruction events. Additionally, the architecture can be applied to high-altitude platform stations (HAPS) and unmanned aerial vehicle (UAV) communication systems, using FSO for high-capacity backhaul links while relying on RF to maintain stable connectivity under adverse weather conditions that degrade optical signals [13; 14]. For example, in extreme fog fading scenarios, RF links can maintain a base rate of 25 Gbps to ensure 100% availability, while FSO unleashes a high bandwidth potential of 425 Gbps in low-loss environments [15].