# The Design of a Long-Distance Signal Transmission System in a Bistatic RCS Testing System
## 1 Introduction  
In the last two decades, modern wireless communication system technologies have undergone rapid growth in response to the corresponding demand [1]. The multipath effect, a prevalent phenomenon in wireless communication, occurs when radio signals arrive at the receiving antenna via multiple paths. This effect can arise from factors such as atmospheric ducting, ionospheric reflection, and reflections from surfaces like water bodies, mountains, and buildings. Interference and phase shifts can ensue when the same signal follows different paths to the receiver. Destructive interference resulting from this can cause signal fading, which may render a signal too weak to be reliably received in certain areas. This phenomenon is one of the most important causes of system fading. On the other hand, if multipath effects are utilized effectively, communication efficiency can also be improved. For example, using drones as relay stations can create intentional multipath effects, enabling transmission across obstacles in complex environments [2, 3] or ensuring reliable long-distance communication [4, 5]. Consequently, a comprehensive understanding of multipath fading channels is crucial for the efficient design of wireless communication systems [6]. In statistical channel models, distribution parameters are typically derived from extensive field measurement data [7], which are used to characterize typical scenarios. Several classical models have been established, such as the 3 GPP/ITU 2D/3D channel models and the 3D millimeter-wavechannel model [8; 9]. However, statistical models frequently fail to precisely capture channel performance at specific locations, leading to the widespread adoption of the ray-tracing method. As shown in Figure 1, the ray-tracing channel model simplifies the wavefront into multiple small ray tubes that propagate along their respective incident directions [10]. It is a deterministic model based on high-frequency electromagnetic simulation technology and uses [PERSON]'s equations and appropriate boundary conditions to develop a deterministic model that can ascertain the details of multipath propagation and derive the impulse response of multipath fading for specific environmental conditions [11; 12; 13]. Despite its advantages in providing accurate and detailed modeling of multipath propagation, ray tracing has several inherent drawbacks. One major limitation is its high computational complexity. Additionally, ray-tracing models are highly dependent on detailed and accurate environmental information. Constructing a precise geometric model of the propagation environment, including the shapes, positions, and material properties of all relevant obstacles, can be labor-intensive and time-consuming. Furthermore, while ray tracing can provide detailed insights into a signal's propagation paths, it does not inherently account for all wave phenomena, such as wave polarization and material anisotropy, which might be relevant in certain scenarios. Given these challenges, improving the accuracy of multipath channel modeling remains a significant concern in the wireless communication and radar community. Some researchers are attempting to improve the ray-tracing channel model. In [14], [PERSON] et al. explored combining ray-tracing techniques with terahertz communication to enhance multipath channel modeling. Their hybrid approach outperformed conventional models by better capturing the temporal-spatial characteristics of multipath channels and the role of ray tracing in accurate multipath propagation. However, it still faced challenges related to the high computational demands of ray tracing. In [15], [PERSON] et al. introduced Massive MIMO Communications, emphasizing the need to account for reflection, scattering, and diffraction to obtain more precise results. But they were unable to overcome the challenge of modeling multipath channels in complex environments. Some other researchers hope to improve the accuracy of multipath channel models by using more novel techniques. [PERSON] et al.