# Analysis of Human Body Shadowing Effect on Wireless Sensor Networks Operating in the 2.4 GHz Band
## 2 Materials and Methods
### Human Body Model for Shadowing Analysis in UTD-Based Program
These results were obtained for different numbers of sidewalls approximating the main body cylinder and with three distance ranges: 0.5-1 m, 1-2 m, and 2-4 m. The smallest differences between the methods were obtained for the model with six wall prisms. For the first distance range, in which the body shadowing effect had the strongest influence on path loss, the results obtained with the six-wall model were on average 1.9 dB below the reference results. For this model, in the considered range of the receiving antenna distances, the difference was approximately \(-2\) dB compared to the results obtained with the FDTD method. This model configuration was assumed to be the most suitable for simulating the body shadowing effect in the ray-based XGtdTM program. The influence of the number of sidewalls used to model the body on path loss is significant. This was further investigated by the analysis of ray-paths identified by the program for two different body models and two locations of the receiver. In the abovementioned numerical experiment, the transmitter has a fixed position and the receiver is moved from the direct proximity of the body model to a 4 m distance. In the XGtdTM software, at each receiver location, contributions from arriving ray-paths are combined and evaluated to determine the predicted quantities of path loss. The calculation engine finds ray-paths that carry most of the energy of the electromagnetic wave from the transmitter to the receiver. The ray-path direction and properties for a particular location of the receiver depend on the geometry of the object. In the considered case, the number of prism sidewalls is critical because it influences the orientation of the top edges of the prism that are identified by \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Number of Sidewalls**} & \multicolumn{3}{c}{**Difference between Median Path Loss [dB] Obtained with UTD**} \\ \cline{2-4}  & \multicolumn{2}{c}{**and FDTD Across Particular Distance Ranges**} \\ \cline{2-4}  & **0.5 m–1 m** & **1 m–2 m** & **2 m–4 m** \\ \hline
5 & \(-2.7\) & \(-12.6\) & \(-8.3\) \\
6 & \(-1.9\) & \(-1.7\) & \(-2.3\) \\
7 & \(-11.6\) & \(-9.2\) & \(-4.1\) \\
8 & \(-3.2\) & \(-13\) & \(-13.7\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Differences between median values for path loss obtained using UTD and FDTD with different numbers of sidewalls approximating the main body cylinder across three distance ranges. Figure 6: Path loss simulated in the XGtdTM program for different numbers of prism sidewalls compared to path loss simulated in FDTD based program. the program as diffracting edges. The path loss at each point is then influenced by the diffraction coefficient of the object edges along the ray paths. Those depend on the angle between the ray and the edge of the object. This causes for several locations of the receiver the dynamic changes of path loss between the points for which the diffraction coefficient along the path changed significantly. To present this feature of XGtd(tm) software, the analysis of ray-paths identified by the program was made for two different body models and two locations of the receiver. In Figure 7, the ray-paths are presented for five walls' model. For the receiver located at the distance, \(b=1\) m, there are three ray-paths identified by the program (Figure 7a) and in this case, the path loss was equal to 55 dB. The slight increase of the receiver distance to \(b=1.1\) m resulted with the five ray-paths identified by the program (Figure 7b) and the path loss decreased to 38 dB. In this case, not only the number of ray-paths is different, but also the diffraction coefficients change on the points where the path interacts with the object. This causes a rapid change in the path loss for small changes of the receiver to body distance. A similar study was made for the model with six sidewalls. In Figure 8, the ray-paths for the human body model in this configuration are presented. The receiver was located at the distances, \(b=1\) (Figure 8a) and \(b=1.1\) m (Figure 8a).