## 3 DT Model of the Electromagnetic Coupling Process of the BTS  
### Structural Design of DT Model of Electromagnetic Coupling Process of the BTS  
The equipment components of the BTS and their operating principles were discussed in Section 2.1. The factors influencing the electromagnetic coupling process of the BTS were discussed qualitatively in Section 2.2. On the basis of these two subsections, this subsection discusses the DT model structure of the electromagnetic coupling process of the BTS. Then the design scheme of the system-level model is proposed, and the modeling objectives and tasks of each sub-model are specified. The mapping relationship between the real-world BTS and the DT model of the system's electromagnetic coupling process is shown in Figure 3.  
The downlink excitation circuit of the BTM sends the RF power signal to the balise through the onboard transmitting antenna, and the balise receiving antenna receives the downlink magnetic flux. The transmission process of this RF power signal requires a downlink transmission model for describing the effect on the magnetic flux received by the balise receiving antenna due to the RF power signal current strength, the antenna shape and size, and the relative position change of the antennas during the train movement.  
After the balise receiving antenna receives the RF power signal, the balise is activated from the dormant state. The internal conversion circuit of the balise processes the RF power signal to provide the transmitting current for the uplink signal to the passive balise transmitting antenna. This process requires an equivalent current model of the passive balise that describes the mapping of the magnetic flux strength received by the balise receiving antenna to the uplink signal current strength of the balise transmitting antenna.  
The onboard receiving antenna receives the uplink signal from the balise transmitting antenna through the air gap interface. The magnetic flux received by the onboard receiving antenna is affected by the antenna shape and size, the current strength on the balise transmitting antenna, and the relative position between the antennas. This process requires an uplink transmission model.  
The matching circuit of the onboard receiving antenna can improve the performance of the receiving antenna and reduce the loss of transmission performance caused by impedance mismatching and resonance frequency offset. Therefore, a matching circuit model of the onboard receiving antenna is needed to predict the relationship between the magnetic flux of the uplink signal received by the onboard receiving antenna and the uplink signal power on the input port of the BTM.  
Since the relative positions of the OAU and balise are constantly changing during train movement, both uplink and downlink transmission models need to fully reflect the changes in signal transmission characteristics caused by such dynamic factors. The uplink and downlink transmission models have similarities and are both electromagnetic  
Figure 3: Schematic diagram of mutual inductance model between balise antenna and onboard antenna.  
induction models. Therefore, for both models, the fundamental task was to establish an accurate model of mutual induction between the antennas.  
By combining the above sub-models, a DT model of the electromagnetic coupling process of BTS can be successfully established in theory. In order to realize the quantitative analysis of the electromagnetic coupling characteristics of BTS, it is a challenging task to accurately build each of the above sub-models and realize the lossless connection of the sub-models.