*Terahertz (THz) communication can provide high-speed, low-latency and high-capacity data transmission, making it a potential for 6G communication; therefore, it is necessary to measure and model the THz channel. This article discusses the use of the NI PXIe-5841 vector signal transceiver to measure the reflection of vertically polarized waves from wood, plaster, tempered glass and mirrored glass at multiple incident angles ranging from 230 to 330 GHz in indoor environments. The relationship between the reflection coefficient and the material, incident angle and frequency is analyzed. Experimental results show that the trend of the reflection coefficient with incident angle is consistent with theory and increases with the incident angle. The relationship between the reflection coefficient and frequency satisfies the Rayleigh roughness coefficient theory. In addition, relative permittivity is estimated and analyzed using the minimum mean square error method. The results show that the method fits well for non-metallic materials but exhibits a large deviation for metals, which may be related to their special properties. Finally, prospects for the study of THz channel reflection characteristics are discussed.*

The need for high-speed and low-latency mobile communication services is increasing with societal growth and developments in artificial intelligence, IoT and smart manufacturing placing higher demands on mobile communication networks. 5G technology is no longer able to meet that need.^{1}

6G is the next generation of mobile communication technology, with faster speeds, wider bandwidths, stronger connectivity, lower latency and higher security.^{2} 6G can support new applications that require higher communication speeds and lower latency, such as virtual and augmented reality, high-definition video transmission and intelligent networks.^{3}

THz communication technology can provide high-speed, low-latency and high-capacity data transmission, which is in line with 6G communication goals. It can also provide new spectrum resources to meet the growing demand for bandwidth. Therefore, THz communication is an essential part of 6G with significant potential.^{4-6}

The physical environment has a profound impact on electromagnetic wave propagation at THz frequencies and reflection is a significant contributor. For THz communication systems, the study of channel reflection characteristics is essential to:^{7,8}

- Determine the transmission path. By measuring the reflection of THz waves, the transmission path is determined, providing a reference for system design.
- Evaluate signal quality and stability. Reflection measurements are used to evaluate signal integrity, including signal-to-noise ratio and power distribution. This ensures normal operation of the communication system and is necessary for improving system performance and efficiency.
- Identify interfering sources. Reflections of THz waves can identify the nature and location of interferers, enabling mitigation measures.

In the study of reflection characteristics, many different aspects have been investigated. Sato et al.^{9} proposed a layered model for estimating the refractive index and used a stepped frequency radar system based on a network analyzer to measure the reflectivity of various building materials. Piesiewicz et al.^{10} introduced the Rayleigh roughness factor calculated from the measured surface height distribution of samples and derived a modified Fresnel equation based on Kirchhoff scattering theory. Reflectivity was derived from the material parameters. Surface measurements of reflectivity in the 100 to 1000 GHz frequency range were in close agreement. Ma et al.^{11} demonstrated a THz wireless link using 1 Gbit/s data streams at 100, 200, 300 and 400 GHz frequencies in indoor and outdoor environments, confirming the feasibility of using THz carriers for data transmission. Pan et al.^{12} measured the THz reflection channel in the 240 to 310 GHz band and observed that the reflectivity of common building materials is related to the incident angle of the electromagnetic wave. They proposed a statistical model of reflectivity dependent upon the incident angle, giving the model parameters. Yang et al.^{13} measured and simulated the propagation characteristics of THz waves in an indoor desktop scene, analyzed the characteristics of line-of-sight and reflection path environments and simulated the reflection path environment. Statistical parameters of line-of-sight and reflection paths were also calculated. The plasma development process in line-of-sight and reflection path environments and the plasma development process of different materials in the reflection path environment were analyzed. Wang et al.^{14} conducted reflection measurements of five materials under parallel-polarized waves at multiple incident angles in D-Band (110 to 170 GHz) and proposed a dual-parameter reflection model based on the minimum mean square error (MMSE) criterion.

This article describes the use of a PXI vector signal transceiver platform to measure and analyze the reflection coefficients of four common materials in the 230 to 330 GHz range. The relationship between the reflection coefficient, material, incident angle and frequency is analyzed. In addition, the relative dielectric constant of the materials is estimated using the MMSE method.

**REFLECTION MEASUREMENT PRINCIPLE AND THEORETICAL MODEL**

**Reflection Coefficient Measurement Principle**

Line-of-sight received power in free space is determined by the Friis equation:

Where *P*_{t} is the power of the transmitting antenna and *Gt* and *Gr* represent the gains of the transmitting and receiving antennas, λ represents the wavelength, and d_{LoS} represents the line-of-sight propagation distance.^{15,16}

When electromagnetic waves are incident on the surface of an object at a certain angle, reflection usually occurs, forming a reflected wave. he ratio of the amplitude of the reflected wave to the amplitude of the incident wave is defined as the reflection coefficient, represented by |Γ|.

Assuming the size of the reflecting boundary is much larger than the distances *d _{1}* and

*d*, and the surface area of the reflecting boundary is much larger than the illuminated portion, the received power after reflection can be regarded as the received power of the line-of-sight path with a path length of (

_{2}*d*+

_{1}*d*) multiplied by | Γ |

_{2}^{2}, as shown in Equation (2):

^{17}

Where *d _{1}* and

*d*are the distances from the transmitting and receiving antennas to the reflecting surface, respectively. Dividing Equation (2) by Equation (1) yields:

_{2}**Reflection Theoretical Model**

For non-magnetic materials, the Fresnel reflection coefficient of a vertically polarized wave can be obtained from the Fresnel equations. The proportion of incident light power reflected by the interface is called the reflectivity R. When the incident radiation is vertically polarized (s-polarization), the reflectivity is:

where *θi* is the incident angle, *θt* is the refracted angle and *n*_{1} and *n*_{2} represent the refractive indices of the incident medium and the reflective medium, respectively. The last step in Equation (4) uses Snell's law to derive *θt* as a function of *θi*:

For non-magnetic materials, neglecting complex refractive index, we have:

Where ε and μ are the relative permittivity and relative permeability of the material, and for non-magnetic materials, μ=1.