## 2 System Principle
### Slot Structure and Radiation Characteristics of LCX Cable  
The LCX cable consists of four parts arranged from the inside out: the inner conductor, insulation layer, outer conductor, and the outer jacket. Electromagnetic waves propagate axially along the inner conductor and radiate outward through periodically arranged  
Figure 3: Signal propagation profile diagram inside the tunnel. Figure 2: Signal propagation diagram inside the tunnel. slots on the outer conductor. Functionally, the outer conductor acts like dipole antennas, providing uniform wireless signal coverage in enclosed or narrow environments. Based on the different types of radiation slots on the outer conductor, the LCX cable can be classified into three types: T-type LCX cable, V-type LCX cable, and inclined LCX cable. The structure of the T-type LCX cable is shown in Figure 4a, where the radiation slots on the outer conductor are periodically arranged in a vertical pattern. The electromagnetic wave signals transmitted within the cable radiate outward in a direction perpendicular to the cable's axis, as illustrated in Figure 4b. The slotted structure of the V-type LCX cable is shown in Figure 5a, where the radiation slots on the outer conductor are arranged in a V-shaped pattern at equal intervals. The signal transmission paths are illustrated in Figure 5b. Signal A is driven from the A port of the leaky cable and radiates outward uniformly at a radiation angle \(\alpha\) within the cable. Similarly, signal B is driven from the B port of the leaky cable and radiates outward uniformly at a radiation angle \(\beta\). The slotted structure of the inclined-type LCX cable is shown in Figure 6a, where the radiation slots on the outer conductor are arranged at equal intervals in an inclined pattern. The signal transmission path is illustrated in Figure 6b. Signal A is excited from port A of the leaky cable, and signal B is excited from port B. Both signals radiate uniformly outward from the cable at a radiation angle \(\alpha\). Figure 4: (a) Slotted structure of the T-type LCX cable; (b) signal propagation of the T-type LCX cable. Figure 5: (a) Slotted structure of V-type LCX cable; (b) signal propagation of V-type LCX cable. Among the three types of LCX cables mentioned above, the T-type LCX cable radiates electromagnetic waves outward at a 90-degree radiation angle. For the other two types of LCX cables, the radiation angle is closely related to the slot structure on the outer conductor and the signal frequency. The radiation angle of the leaky cable can be calculated using Equation (1). \[\theta_{\rm m}=\arcsin\biggl{(}\sqrt{\epsilon_{\rm r}}+\frac{m\lambda}{\rm P} \biggr{)} \tag{1}\]  
In Equation (1), m is the harmonic order. Typically, the value of m is set to \(-1\). P is the slot period in the outer conductor, \(\epsilon_{\rm r}\) is the relative permittivity of insulation layer, and \(\lambda\) is the wavelength of the transmitted signal. Currently, the insulator in LCX cables is typically made of foaming polyethylene, with a relative permittivity ranging from 2.1 to 2.3. Under conditions where the relative permittivity is 2.3, the LCX slotting period is 0.2 m, and the harmonic order is set to \(-1\); the operating radiation angle of the GNSS signals used in this study falls within the range of 0\({}^{\circ}\) to 90\({}^{\circ}\). References [29; 30] conduct relevant studies on the coupling losses of LCX cables. Reference [29] suggests that coupling loss can be reduced, and LCX cables coverage enhanced, by printing periodic metallic stripes on the LCX cables jacket. In Reference [30], it is proposed that increasing the signal radiation angle of LCX cables, reducing the spacing between radiation slots, and raising the transmission frequency can all effectively lower LCX cables' coupling losses. Foaming technology can reduce the dielectric constant of the insulation layer.