# A Low-Cost Electronic System for Human-Body Communication
## 4 Evaluation Methodology
### Results  
We followed the setup shown in Figure 3 to define the distances between electrodes for the best sensing amplifier voltages. Table 3 summarizes the voltages recorded at the sensing amplifier for different distances between pairs of electrodes. We kept the total distance d1 constant (10 cm), and we varied the distance between the pair of electrodes. We analyzed these distances and their variations in both scenarios: when in the phantom and on the forearm. Considering the dimensions and characteristics of a real human forearm, the scenario that presented the highest peak-to-peak voltage (3.7 V) was where we separated the sensing and driver electrodes by a 5 cm distance. We evaluate the forearm scenario, presented in Figure 5, comparing the injected bit sequence and the sequence the system can decode. Figure 6 shows the amplifier circuit output and the decoder output. The amplifier shows the sensing amplified signal without any further processing. The decoder  
Figure 4: Full experimental setup: boards, workload generator, and phantom. \begin{table}
\begin{tabular}{c c c} \hline \hline
**dtx (cm)** & **drx (cm)** & **Pulse Voltage (V)** \\ \hline
5 & 5 & \(\pm\)3.7 \\
5 & 2 & \(\pm\)2.4 \\
2 & 5 & \(\pm\)1.3 \\
2 & 2 & \(\pm\)0.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Sensing amplifier pulse voltages. output shows the signal produced by a matched-filter applied to the sensed voltage and the result from a threshold detector. The detected sequence fully matches the generated signal. When considering the complete galvanic data transmission system presented, it is possible to reach a bit time interval of 90 \(\upmu\)s, representing an optimal throughput of 11.11 kbps. This data rate supports different applications, such as the use of cryptography keys [25], user authentication [26], vital signs monitoring, and their transmission. In summary, for those reasons, the designed low-cost system is suitable for HBC. Figure 5: Battery-powered coupling system connected to the forearm using disposable electrodes. Therefore, we evaluate BER considering the bit time interval of 90 us, and collecting the data flow through the DAQ. The hardware set does not present errors when the electrodes are separated by 10 cm (i.e., \(dl=10\) cm). When we increase \(dl\) to 20 cm, the observed BER is of \(9\times 10^{-4}\). According to [20], with this value, the system provides reliable communication. Achieving this rate is also the target in [15]. We consider this a low BER \(dl\) since in most cases, practical applications are limited to lower distance ranges. Moreover, in more than 93% of cases, the error occurs in only one bit of each data packet, which can easily be corrected using simple error detection techniques. Table 4 presents a comparison of the proposed prototype with equivalent works in the literature. According to Table 4, one can note that even operating in a frequency range of up to 10 kHz, the prototype we propose in this work achieves a throughput of \(\approx 2.3\times\) higher than [PERSON] et al. [10] with equivalent bit error rate. The throughput is even higher when compared to [PERSON] et al. [14] and, in this case, the new prototype can be as much as \(\approx 4.6\times\) better. It is important to note that our proposal presents a null bit error rate for HBC channels with a communication distance of up to 10 cm. On the other hand, [PERSON] et al. [14] present a considerable bit error rate at this same condition. As in [PERSON] et al. [10], the design of the current prototype also limits the peak current to 1 mA, ensuring user safety. However, our approach has superior energy efficiency since it consumes approximately only 50.53 uW per bit, and the [PERSON] et al. solution consumes 83.33 uW.