## 3 Results and Discussion
### Measurement Results
#### 3.2.4 On Body Link Budget Calculation  
It is very important to examine the link budget of the wearable devices in body-centric communications to provide a stable link budget, as shown in Figure 25. We tested \(\mathsf{S}_{21}\) in free-space and on-body communication cases in an open environment (or residential area). This test allowed the research work to demonstrate the range of expected values around the path loss. The path loss (_PL_) is defined as the ratio of the input power (at  
Figure 24: Simulated and measured radiation patterns of E- and H-planes. Left side: the lower frequency band (2.45 GHz), right side: the higher frequency band (5.6 GHz), (**a**) free space, (**b**) on-chest, (**c**) on-arm. point 1) to the power received (at point 2) that is measured in terms of the transmission coefficient (\(\left\{\left.\mathrm{1S_{21}}\right.\right\}\)). \[PL_{dB}(d)=-\left|S_{21}\right|_{dB} \tag{5}\]  
Free-Space Communication  
Path loss measurements were first performed in free space as a baseline, as illustrated in Figure 26. The BSA was placed on a tripod at the height of 1 m above the ground, with the WiFi router vertically aligned. Furthermore, the distance (\(d\)) was chosen at 2 m to 40 m in the \(\mathrm{S_{21}}\) measurement to estimate the path loss in LOS and NLOS. Please note that a full-wave simulation was not possible; therefore, we used the open space area in an inside and outside-the-house measurement to precisely calculate the path loss exponent. The log-distance path-loss model was used, which can be written as:  
\[PL_{dB}(d)=PL(d_{0})+10 nlog\left(\frac{d}{d_{0}}\right) \tag{6}\]  
where \(n\) is the path loss exponent, \(d_{0}\) is the close-in reference distance and \(d\) is the far point representing the separation between transmitter (Tx) and receiver (Rx) [67]. After substituting the values \(d_{0}=2\) m and \(d=40\) m, we obtained an average range for \(n\in\left[2.7\text{ to }3.5\right]\) for a distance of 10 m to 11 m, and a range of \(n\in\left[4.0\text{ to }6.0\right]\) for the dis  
Figure 25: Measurement scenarios of wireless connectivity setup for on-body communication. tances of 11 m to 40 m, which were well matched with the urban areas and obstructed building paths, respectively [67]. On-Body Communication  
In the cases of an on-body situation, measurements were taken on a 2.45 band. As shown in Figure 27, the BSA was located on body locations (chest and arm) to calculate S\({}_{21}\). To explore the influence of distance (d) on S\({}_{21}\), the separation between the BSA and WiFi router was adjusted to 2, 5, 8, and 40 (in m), with a 5 mm gap was created between the BSA and the top skin layer. The BSA was located on the chest of male volunteers with a weight of 72 kg and a height of 1.58 m, both in LOS and NLOS, inside and outside of the building. In the first sets of measurements, the subject's arms were spread across the subject's body. However, due to the power absorption of the body tissues, the path losses were higher in the presence of the body, in LOS and NLOS, compared to free-space measurements. For the on-body chest communications, the distance (\(d\)) between the design and WiFi router was raised from 2 m to 40 m, similar to the free-space measurements. We computed the path loss values (\(n\)), which are slightly greater than the free-space case, using (8) and \(d_{0}=2\) m and \(d=40\) m. Because of the restriction caused by the bodies, as well as the losses and power absorbed by the body tissues, the increase in losses were expected. The BSA was then mounted to the male subjects' arms in LOS and NLOS, and path loss was measured. The path loss was found to be 72 dB. The BSA was tested in the NLoS in the outdoor environment in the following situations, where it was put on-body chest and the arm of a male subject, at the distance of 40 m, 84 dB path loss was measured.