## 4 Discussion  
The above analysis of the electric field strength distribution and current flow revealed the differences between the signal transmission behaviors of the one- and two-electrode transmitters. The results are qualitatively summarized in Figure 8a,b. In the case of the one-electrode transmitter, the electric field spread from the transmitter to the space above the arm and not inside the arm. In the case of the two-electrode transmitter, the electric field was distributed above and below the arm and penetrated the arm. Furthermore, when the two-electrode transmitter was used, the electric field was distributed more intensely between the receiver frame and the torso than when the one-electrode transmitter was used.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**X (mm)** & \(\mathbf{-150}\) & \(\mathbf{-100}\) & \(\mathbf{-50}\) & \(\mathbf{0}\) & \(\mathbf{50}\) & \(\mathbf{100}\) & \(\mathbf{150}\) & \(\mathbf{I_{R}}\) \\ \hline Current (One-electrode) (\(\upmu\)A) & \(-11.2\) & \(-12.2\) & \(-16.1\) & \(-59.3\) & \(6.63\) & \(2.95\) & \(1.79\) & \(1.50\) \\ Current (Two-electrode) (\(\upmu\)A) & 32.8 & 32.6 & 34.6 & \(-7500\) & 34.0 & 31.3 & 31.3 & 31.2 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Current through the cross-sections and the receiver resistor.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Received Signal** & **Standard** & **Transmission Characteristic (dB)** \\ \cline{2-5}  & **Amplitude (mV)** & **Deviation (mV)** & **Experiment** & **Simulation** \\ \hline One-electrode & 6.58 & 0.26 & \(-41.1\) & \(-50.4\) \\ Two-electrode & 53.9 & 0.85 & \(-22.8\) & \(-24.1\) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Received signal amplitude measured by experiment.  
Furthermore, in the one- and two-electrode transmitter cases, the current flowed in opposite directions in part of the human forearm between the transmitter and the elbow. This means that the only return path of the one-electrode transmitter is through the capacitive coupling of the floating ground of the transmitter. In contrast, in the case of the two-electrode transmitter, current flowed from the torso, through the upper arm, and to the ground electrode of the transmitter that was in contact with the arm. The capacitive coupling between the receiver ground (i.e., the receiver frame) and the torso and lower parts of the human body allowed the current to flow from the receiver ground to the torso. The capacitive coupling between the receiver ground and the underside of the arm also contributed to the return path. In effect, the area of the ground electrode of the transmitter was enlarged by putting the ground electrode of the transmitter in contact with the human body. It was concluded that this increased coupling is the mechanism producing the larger received signal when the two-electrode transmitter was used.  
The signal transmission path and mechanism of HBC revealed in this study are not readily apparent from the distributions of electric field strength that are often reported in previous studies. The method used in this study to analyze the difference between the one- and two-electrode transmitters can be used to analyze other HBC configurations. The results will be useful for the development of HBC applications in terms of device placements and practical choice of electrode configurations.