## 4 Simulation Analysis and Measurement Verification
### Analysis of Balise I/O Characteristics  
To characterize the relationship between the uplink FSK signal transmission current intensity and the magnetic flux intensity of the received downlink telepowering signal in the BTS, the electrical model of the balise I/O characteristics is verified according to the current specification [24]. Figure 12 illustrates the test system used for this verification, comprising a core control unit, an excitation unit, and an acquisition unit. In Figure 12, the excitation unit comprises a signal generator (NI-PXIe 5451), a power amplifier (E&I 325 LA), two attenuators (Attenuator A: 6 dB 20 W, Attenuator B: 3 dB 100W), and custom-made components (balun and transmitting antenna). The acquisition unit consists of a vector signal analyzer (NI-PXI 5661), a power meter (RS-NRVD), a low-pass filter (Mini Circuits, BLP-10.7), and custom-made components (balun and receiving antenna). The custom-made balun, transmitting antenna, and receiving antenna collectively form the test antenna. The two channels of the vector signal analyzer and power meter are used to measure the voltage of the 4.234 MHz uplink FSK signal and the power of the  
Figure 11: S21 parameter variation curve versus horizontal displacement; (**a**) X-axis (h = 220 mm); (**b**) Y-axis (h = 220 mm); (**c**) X-axis (h = 340 mm); (**d**) Y-axis (h = 340 mm); (**e**) X-axis (h = 460 mm); (**f**) Y-axis (h = 460 mm). 27.095 MHz downlink telepowering signal. The core control unit (NI-PXIE 8135) is used for instrument control and data analysis. All modules of the test system are designed based on the ETCS SUBSET-085 specification. When a train provides an appropriate telepowering signal, the loop current in the balise is determined by the received magnetic flux and the mutual inductance of the BTS. Table 1 shows the relevant parameter requirements for the balise as specified in ETCS SUBSET-036, which defines the upper and lower limits of the balise I/O characteristics. The measurements start with the lowest flux, and with increasing flux, will reach \(\varphi_{44}\); then, the flux is decreased again to the lowest flux. In this experiment, we configure the NI-PXIE 5451 to 27.095 MHz in CW mode. The test antenna is positioned at a height of 220 mm relative to the balise. We adjust the amplitude of NI-PXIE 5451 to achieve the desired power level and record the value. We then measure and record the 4.2 MHz power level. Using these measurements, we calculate the flux and balise loop current. This process is repeated for all flux levels. The balise is measured in accordance with the specifications. Figure 13 shows the measurement layout. The test program and procedures adhered to the specification requirements. Figure 14 illustrates the experimental results of these characteristics in both free space and water debris, revealing the corresponding relationship between the excitation flux and loop current. The x-axis shows the magnetic flux of the telepowering signal received by the balise, while the y-axis depicts the current on its transmitting antenna. This current results from the received magnetic flux processed by internal conversion circuits. \begin{table}
\begin{tabular}{c c} \hline
**Telepowering Magnetic Flux (nVs)** & **Loop Current (mA)** \\ \hline \(\varphi_{41}=4.9\) & \(I_{u1}=37\) \\ \(\varphi_{42}=7.7\) & \(I_{u2}=59\) \\ \(\varphi_{43}=5.8\) & \(I_{u3}=186\) \\ \(\varphi_{44}=130\) & \(I_{u4}=186\) \\ \hline \end{tabular}
\end{table} 
Table 1: I/O transfer characteristics for a balise. Figure 12: Schematic diagram of the connection relationship; (**a**) Instruments for I/O characteristics measurements; (**b**) Test set-up for I/O characteristics measurements. The experiments are distributed and included the results of the balise in free space and with underwater debris. Initially, the balise I/O characteristics are measured in free space. The saturated loop current of a balise in free space is approximately 95.2 mA, representing its baseline performance.