# A Hybrid RF/FSO Transmission System Based on a Shared Transmitter
## 2 Principle  
### Design of Hybrid RF/FSO Structure  
As shown in Figure 2, the RF signals and optical signals are combined and transmitted through a structure comprising a dielectric plate, a gradient-index (GRIN) lens, and a lens for RF focusing. The dielectric plate serves to reflect optical signals and can be fabricated from optical plastics that are transparent to the millimeter-wave band and capable of reflecting light. Polymethyl methacrylate (PMMA), a widely used optical plastic, is an example of such a material. It is a polymer commonly known as Plexiglas and exhibits high optical reflectivity and MMW transparency. Measurements indicate that, for incident optical power of 14 dBm, PMMA transmits 12 dBm and reflects 2 dBm. In this study, we take  
Figure 1: The hybrid RF/FSO system structure. (**a**) Discrete parallel structure; (**b**) discrete serial structure; (**c**) integrated shared transmitter structure. (Green: RF link; Yellow: FSO link.)  
advantage of its MMW high transmittance property by which RF signals can be transmitted without excessive loss, and its reflection property for optical signals by which FSO signals can propagate along the same channel, thus enabling a shared transmitter architecture. The GRIN lens, also referred to as a self-focusing lens, is employed for collimation and focusing of optical signals, producing a parallel beam after collimation. In this study, a Thorlabs F810 FC-1550 GRIN lens (Newton, NJ, USA) was used, which operates at 1550 nm and can effectively reduce beam divergence and improve the transmission stability of the system. For the RF focusing lens, plano-convex TPX (Polymethyl pentene) aspheric lenses from Thorlabs are adopted, as they exhibit broadband transparency in the MMW range and effectively perform collimation and a focusing of MMW signals. Experimental findings confirm that incorporating a lens after the photodetector can provide gain, which is dependent on the lens dimensions.  
After generating the optical signal at the transmitter, the signal is divided into two separate paths. In one path, a photodetector performs photoelectric conversion, and the resulting electrical signals are focused by a lens before passing through a dielectric plate and being emitted into free space. In the second path, the optical signals are transmitted through an optical fiber, with a GRIN lens used to create a parallel beam that is incident on a PMMA plate. The optical signals are then reflected by the dielectric plate, allowing them to travel along the same path as the electrical signals. The fusion signals are received by the respective RF and optical receivers. The optical receiver must be precisely aligned with the transmitter, while the electrical receiver can be positioned adjacent to the optical receiver. This structure enables the fusion transmission of RF and FSO signals through the shared transmitter.  
To verify the feasibility of this novel fusion transmission scheme, we chose to conduct experiments in a stable, short-range indoor laboratory environment. In this setting, the effects of atmospheric turbulence, pointing errors, and scintillation in the FSO channel are negligible. Additionally, to account for system nonlinearities, we strictly controlled the input signal power to remain within the linear region of the devices. Upon receiving the signal, we employed a series of digital signal processing (DSP) algorithms for carrier recovery to achieve optimal bit error performance.