## 5 Results and Discussion
### Link Budget Analysis  
The smartwatch on the human wrist experiences complex three-dimensional movement during various day-to-day activities. Therefore, error-free transmission and reception are essential for seamless, smooth communication between smartwatches and mobile/IoT devices during these movements. A link budget was studied between the mobile and proposed smartwatch antenna under a line-of-sight (LOS) environment at various rotational degrees. A standard dipole antenna was considered as a reference antenna (transmitting antenna) inside the mobile case with a gain (\(G_{td}\)) of 1 dBi. The receiving antenna is the proposed antenna (antenna under test (AUT)). The receiver antenna gain (\(G_{tr}\)) is 9.9 and 10.9 dBi at 25.5 and 38 GHz (placed inside the watch case, as discussed in the previous section). In our case, the reference antenna and AUT were separated by a 50 cm distance. The AUT was stationary but rotated over the \(x\)-axis from 0\({}^{\circ}\) to \(\pm\)180\({}^{\circ}\), as depicted in Figure 20. The received signal at various rotational angles was computed using the shooting and bouncing ray technique in HFSS Savant, considering the losses such as the path loss exponent effect and the shadowing effect (\(LS\)) [26]. Therefore, Equation (2) [26; 27] can calculate the received power as  
\[P_{rw}=P_{td}+G_{td}+G_{tr}-LS \tag{2}\]  
In the analysis, the power of the transmitting antenna (\(P_{td}\)) considered is 30 mW. The required power (\(R_{Pw}\)) to estimate the link budget for an ideal binary-phase-shift-keying (BPSK) with an energy-to-noise ratio (\(E_{b}/N_{0}\)) of 9.6 dB is given by Equation (3):  
\[R_{Pw}\ (dB)=\frac{E_{b}}{N_{0}}\ (dB)+KT+B_{r}\ (dB) \tag{3}\]  
where \(K\) is the Boltzmann constant with a value of \(1.38\times 10^{-23}\), \(T\) is the temperature in Kelvin (in our case, 290 K), and \(B_{r}\) represents the various bit rates supported for reliable communication. The link margin is the difference between the actual received power \(P_{rw}\) and required power \(R_{Pw}\) given by Equation (4) [27; 28]:  
\[LM\ (dB)=P_{rw}\ (dB)-R_{Pw}\ (dB) \tag{4}\]  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{2}{c}{**25.5 GHz**} & \multicolumn{2}{c}{**38 GHz**} \\ \hline
**Tissue** & \begin{tabular}{c} **Density** \\ **(kg/m\({}^{3}\))** \\ \end{tabular} & **Permittivity** & \
\begin{tabular}{c} **Electrical** \\ **Conductivity** \\ **(s/m)** \\ \end{tabular} & **Permittivity** &
\begin{tabular}{c} **Electrical** \\ **Conductivity** \\ **(s/m)** \\ \end{tabular} \\ \hline Skin & 1109 & 18 & 24 & 12.3 & 31 \\ \hline Fat & 911 & 6.34 & 4.66 & 5.33 & 6.36 \\ \hline Muscle & 1090 & 26.2 & 31.1 & 19.1 & 41.8 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Human tissue properties at different frequencies.  
Figure 19: SAR analysis results (**a**) at 25.5 GHz and (**b**) at 38 GHz.  
Assuming that a link margin of 20 dB is sufficient for error-free communication, the proposed antenna with the highest data rate of 1 Gbps is supported with a link margin of \(>\)20 from 0\({}^{\circ}\) to \(-\)180\({}^{\circ}\) angle orientation, as depicted in Figure 21a,b. At these angles, both the reference and AUT antenna have good LOS conditions; in fact, direct LOS occurs at \(-\)90\({}^{\circ}\). From 0\({}^{\circ}\) to 180\({}^{\circ}\), specifically from 45\({}^{\circ}\) to 135\({}^{\circ}\), the AUT antenna experiences a shadowing effect due to the metallic case of the watch. As a result, the link margin significantly deteriorates, as shown in Figure 21a,b. Nonetheless, the performance of the proposed antenna is satisfactory, with a data rate support of 1 Gbps.