## 6 Discussion and Conclusions  
In this study, a DT model to quantitative analyze the electromagnetic coupling characteristics of BTS was established. The sensitive factors of the electromagnetic coupling characteristics were analyzed by using fishbone diagram method, and the suitable models for three important links in the electromagnetic coupling process were established according to the analysis results. By fusing these sub-models, a DT model of the electromagnetic coupling process of BTS was obtained. The designed two experiments proved that the proposed model had sufficient accuracy and could be used for the quantitative analysis of the main sensitive parameters involved in the electromagnetic coupling process.  
The contribution of this article was twofold. First, the DT model established in this paper could accurately predict the electromagnetic coupling characteristics of the BTS. Traditionally, the electromagnetic coupling characteristics of the BTS are not clear until the uplink field conformity and the tele-powering field conformity measurements have finished. In the process of research and development of onboard equipment, the impact of the size structure of the OAU and the components of the matching circuit on the test results are more dependent on the experience of the engineers. This leads to a high cost of trial-and-error. According to the DT model proposed in this paper, how the selection of some key parameters and the matching of these parameters affect the electromagnetic coupling characteristics of BTS can be calculated directly, which helps to reduce the time and capital cost of new equipment development. Second, the influence of some parameters on the electromagnetic coupling process was quantitatively analyzed. This is not only an important basis for quantitative analysis of the information transmission reliability of the BTS, but also helps manufacturers and operators adjust their strategies to obtain better information transmission quality.  
However, the entire BTS model has not yet been established. The DT model developed in this paper cannot predict whether the telegram signal can be received properly when the BTS is subjected to electromagnetic disturbance. If we can attain the digital twin model of the entire BTS, it would be possible to solve the problems on electromagnetic susceptibility and reliability of the BTS, which are very important but difficult to be analyzed by conventional methods. The authors' next work is to build such a system level digital twin model.  
In addition, the study in this paper ignored the situation where the balise surface was covered by foreign objects such as rain, snow, hail, and debris, which may occur under some severe weather conditions in reality. When these conditions occur, the uplink signal transmission performance is bound to be lost, which cannot be evaluated by the DT model established in this paper. These issues still need further study.  
**Author Contributions:** Conceptualization, Q.G., Y.W. and D.Z.; methodology, Q.G., Y.W. and D.Z.; validation, J.X. and Y.Z.; investigation, J.X. and L.Z.; writing--original draft preparation, Q.G.; writing--review and editing, Y.W. and D.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded in part by the National Natural Science Foundation of China (No. U1734203), in part by Basic Research Business Expenses of Beijing Jiaotong University--Special Project of Frontier Science Center of Smart High Speed Railway System (No. 2020)BZD004), and in part by the Enterprise Project (AIPEC/BJIT/20 BW1219).  
**Infrared Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** The authors wish to thank the reviewers for their valuable comments and suggestions concerning this manuscript. In addition, the authors wish to thank [PERSON], a senior engineer from Beijing Jiaoda Signal Technology Co., Ltd., for his insightful suggestions and help.  
**Conflicts of Interest:** The authors declare no conflict of interest.