# Reflection Characteristics Measurements of Indoor Wireless Link in D-Band
###### Abstract  
For the millimeter wave (mm-Wave) and terahertz (THz) indoor wireless communication system, the reflection channels need to be characterized and modeled. In this paper, the reflection measurements of the parallel polarized wave are carried out under multiple incident angles and five kinds of materials in the D-band (110-170 GHz). A modified reflection model with two parameters estimated by the minimum mean square error (MMSE) criterion is proposed. The results show that the measurements are in good agreement with the proposed model. Furthermore, a set of measured properties is demonstrated and it can be concluded that both the reflection coefficients and relative permittivity gradually decrease, whereas the surface roughness increases slightly with the increasing frequency, indicating a weak frequency dependence. Interestingly, the concrete board with high surface roughness, which means more power loss in a specular direction, has the lowest reflection loss at a certain frequency and incident angle. It implies that the reflection characteristics of indoor building materials are determined not only by surface roughness, but also by many other factors, such as relative permittivity, frequency, and incident angle. Our work suggests that the reflection measurements of indoor D-band wireless links have a prospective application for future indoor wireless communication systems. D-band; indoor wireless link; MMSE criterion; reflection characteristics; relative permittivity; surface roughness 202  
To establish the realistic models of indoor reflection channels, it is necessary to obtain the dielectric properties of typical building materials such as relative permittivity, surface roughness, absorption coefficient, and refractive index. Hardened concrete relative permittivity in 1.0-95.9 GHz is measured as 6.2-7.0 [24]. Other related work is summarized in Table 2. Table 3 shows the extracted permittivity of five building materials from previous research. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Year** & **Frequency** & & **Contributions** \\ \hline
1995 & 57.5, 78.5 and 95.9 GHz & 1. & The authors put forward layer models for estimating the refractive indices; & \\  & 2. & The authors used a network-analyzer-based step-frequency radar system to measure the reflection coefficients of multiple construction materials [18]. & \\ \hline
1997 & 60 GHz band & 1. & The authors measured the reflection characteristics of some typical materials; & \\  & 2. & The authors confirmed that the circular polarization wave will reduce the reflection energy compared with linear polarization even though the building materials have complicated structures [19]. & \\ \hline
2007 & 100–1000 GHz band & 1. & The authors introduced a Rayleigh roughness factor calculated from the measured surface height distribution of the sample and derived modified Fresnel equations using Kirchhoff scattering theory; & \\  & 2. & The authors derived reflection coefficients based on material parameters and surface measurements in propagation models and compared them with the measured results. Both of them shows good agreement [20]. & \\ \hline
2008 & 100–500 GHz & 1. & The authors provided measurements and modeling results of multiple reflections in building materials; & \\  & 2. & In contrast to bulk materials, the reflection losses show strong oscillations over the frequency which result from interference [21]. & \\ \hline
2010 & 110–135 GHz & 1. & The authors studied the measured and simulated wideband reflection properties of different objects and showed good agreement [22]. & \\ \hline
2018 & 100, 200, 300 and 400 GHz & 1. & The authors characterized THz wireless links using a 1 Gbit/s data flow in both indoor and outdoor environments; & \\  & 2. & The authors establish the feasibility of using THz carrier waves for data transmission [23]. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Reference Review of Exploring Reflection Characteristics.