# Analysis of Human Body Shadowing Effect on Wireless Sensor Networks Operating in the 2.4 GHz Band
## 2 Materials and Methods
### Analysis of Shadowing Effect with Finite-Difference Time-Domain Based Program
Simulations were performed for the two cases, presented in Figure 1. In the first case (presented in Figure 1a), it was assumed that there were no obstacles between the antennas while in the second case (Figure 1b), a human body was placed between the antennas. The detailed geometry of the numerical experiment is presented in Figure 2. The antennas were placed 1.3 m above the feet of the model body. The transmitting antenna was placed at distance \(a\) from the body axis. The receiving antenna was located at distance \(b\). Half-wave dipoles were used, shortened for resonance at 2.45 GHz. The absorbing boundary condition was applied to model the location of the antenna and the human body in free space, without any obstacles or reflecting planes that might have influenced the path loss between the antennas. The absorbing boundary condition that limits the simulation domain in the vertical direction touched the feet of the model body and was placed 40 cm above its head. This models the case of a person standing on the floor, but in this case, only the body and the distance between the antennas have any influence on path loss. No diffraction from the soles of the feet or reflections from the floor are included in the model. Figure 2: Geometry of the numerical experiment performed in XFdtd(tm) software. The first analysis of the body shadowing effect simulated the electric field distribution in the area behind the human body. The results of the simulations of electric field intensity generated by the transmitting antenna placed at a distance, \(a=1.5\) m, are presented in Figure 3. As can be seen, in the shadow region on the opposite side of the body to the transmitting antenna, the electric field intensity is significantly lower than on the side of the transmitting antenna. Another important factor influencing electric field intensity in the shadow region is the distance of the transmitting antenna from the body. Assuming that the transmitting antenna is placed in front of the body and the receiving antenna is behind, the shadowing effect will influence the path loss between the antennas. To analyze this effect, a series of simulations was performed for different distances between the body and the receiving antenna. The path loss between the two antennas was simulated in the XFdtd(tm) program with the transmitting antenna in three fixed positions (parameter \(a\) in Figure 2), with the receiving antenna at different distances (parameter \(b\) in Figure 2). The 3 fixed positions for the transmitting antenna (parameter \(a\)) were 1 m, 2 m, and 3 m from the axis of human body model. As the reference case, the free space path loss was simulated with no human body model between the antennas and parameter \(a\) equal to 2 m.  
The results of the body shadowing simulations are presented in Figure 4. The body shadowing effect was the most intense when the receiving antenna was in close proximity to the human body (0.3-0.4 m from the axis of the body). Free space path loss was approximately 44 dB while in the shadowed case, it increased to approximately 65 dB (depending on the position of the transmitting antenna). With the receiving antenna at a distance of 1 m from the body, the difference between the free space and the shadowed case was still high (approx. 10 dB). At larger distances, the differences reduced, reaching approx. 6 dB at 4 m. These results show the influence that the presence of a human body can have on the operation of a wireless sensor network. When a human body is close to the wireless sensor, the path loss may increase (from 10 dB to 20 dB) and could exceed the fading margin of the link. Figure 3: Electric field intensity in proximity to a human body simulated in XFdtd. Little difference was noted between the path loss simulated for transmitting antennas located at distances of 2 m and 3 m from the axis of the body. Only with the transmitting antenna located at a distance of \(a=1\) m was the path loss slightly lower in proximity to the human body.