## 2 Design Concept of Smart Transfer Planer
### Beamforming Network  
To meet the usage scenario, the reception of outdoor signals and the transmission of indoor signals were simplified into three directions: forward, positive lateral angle, and negative lateral angle. Accordingly, we planned beamforming circuits with three directions and selected a Rotman lens [6; 7; 8; 9; 10], for its high design flexibility. The principle behind the Rotman lens involves feeding any point on the circular focal arc, passing through the parallel plate cavity formed by the upper and lower metals, reaching the inner contour C1, and then through the delay lines to the outer contour C2 [6]. This ensures that the path lengths through the lens are equal, achieving scanning functionality.  
The basic structure and derivation of the outer curve of the Rotman lens are presented in the references, along with several important design parameters, including the issue of different propagation constants between the Rotman lens cavity and transmission lines. We designed a Rotman lens with three input ports and eight output ports, where the geometrical shapes of the lens on the left and right are identical to achieve symmetrical offset of the radiation pattern. To reduce losses and interference, the design employs substrate-integrated waveguide (SIW) technology [11; 12; 13; 14], Figure 4.  
Figure 4: Geometry of the 3T/8R Rotman lens.  
Figure 3: (**a**) Geometry of the propose antenna array. (**b**) Simulation result of \(8\times 8\) antenna array with beamforming. (**c**) S-parameter of \(8\times 8\) antenna array.  
Initially, the calculated lens profile places the input and output ports along the focal positions, with the cavity structure of the Rotman lens in-between. To prevent multiple reflections of electromagnetic waves within the cavity from affecting the output and phase, dummy ports are designed on both sides of the cavity, and the optimal positions are determined by observing the angle of the dummy ports' impact on the multiple reflections of electromagnetic waves [8]. Finally, by adjusting the feeding angles of the three input ports, individual phase differences are achieved in the output ports. By simulating and observing the reflection coefficients and phase differences, and using an 8 \(\times\) 8 antenna array to observe the simulated results of the radiation pattern, the antenna radiation pattern offset can be determined in this step. The input ports are denoted as A1-A3; the dummy ports are located on both sides of the cavity; and the output ports are denoted as B1-B8. The reflection coefficients of each input port are observed through a simulation, as shown in Figure 5a, where the reflection coefficient of forward input A1 is below \(-10\) dB between 11.2 GHz and 13.2 GHz, and that of lateral inputs A2 and A3 is below \(-10\) dB between 11.7 GHz and 14.72 GHz, meeting the requirements for STP applications. Figure 5b shows the phase of the forward input A1 to each output port B1-B8, with phase differences between each output approaching \(0^{\circ}\). Figure 5c,d show the phase of the lateral inputs A2 and A3 to each output port B1-B8, respectively, with phase differences between each output approaching \(-45^{\circ}\) and \(45^{\circ}\), respectively.