# A Low-Cost Electronic System for Human-Body Communication
## 4 Evaluation Methodology
### Experimental Setup  
Figure 3 illustrates a model of the environment employed to conduct the experiments for the evaluation of the proposed system. We evaluated our system using (i) the phantom represented in Figure 3, which was composed of a saline solution. The recipient (plastic or glass) contained a saline solution at 0.9%. We placed three types of pin in the sponge (in the figure, pins are represented by the large dark and red dots), with two pins as driver electrodes (left dark dots), two pins as sensing electrodes (right dark dots), and one as an active common-mode rejection electrode (red dot). We directly inserted the clips in the electrodes into a cleaning sponge to improve the mechanical stability of the experimental setup, following the model defined in Figure 3. The sponge, the dimensions, and the containers' materials of manufacture do not influence the experiment since they do not modify the ionic medium's properties. The pair of driver electrodes is aligned, as well as the pair of sensing electrodes and the common-mode rejection electrode. Pins playing the role of driver electrodes are distant from each other by a distance of \(dxx\), whereas the pins playing the role of sensing electrodes are distant from each other by a distance of \(dxx\). The distance between the pair of driver electrodes and the pair of sensing electrodes is \(dl\). We aligned the common-mode rejection electrode with the sensing electrodes, and it was an equal distance from each of the sensing electrodes. We employed these distances as factors in our experiments, i.e., we varied them to analyze results for the different distances. We also used a configuration with phantom tissue to accomplish another experiment, whose objective was to determine the bit error rate (BER). BER can be affected by many factors, including signal to noise, distortion, and jitter. To measure BER, we performed 10 repetitions, increased the transmitted data payload, and varied \(dl\). The data payload consisted of 1000 filtered ECG samples [24], where each sample was 8 bits. We captured the data flow using a low-cost data acquisition board (DAQ-ADALM1000) connected to the amplifier sensor module's output. Figure 4 shows a picture of the real environment employed for evaluations. One can see the recipient and the sponge involved with the saline solution and the positioned electrodes. To determine the bit error rate (BER), we used a recipient with 20 cm \(\times\) 30 cm dimensions. Figure 4 shows the real coupling boards (differential driver and sensing amplifier boards), designed following the specifications in Section 3, and the data generator (workload generator) board. The workload generator board takes as a basis an Arduino Uno. The workload generator supplies the differential driver board with a small bit sequence. Notably, we fixed the sequence of bits in these experiments to 0101001100001111. In this case, we fixed bit duration to 1 ms, following the NRZ scheme. Positive pulses generate logic level 0, and negative pulses generate logic level 1. We continuously repeated the bit sequence during the tests after an interval of 16 ms for each of the  
Figure 3: Phantom diagram showing electrode positions and distances. 10 repetitions. We connected the sensing amplifier's output to a USB oscilloscope (Handyscope HS3, by TiePie engineering, The Netherlands), which had data recording functionality. We set a 50 kHz sample-frequency with 12-bit vertical resolution. We employed this setup to find the maximum communication band rate of our system. We varied the data size and minimal bit duration. In addition to the phantom-based experiments, we also conducted another set of experiments. We connected the coupling system directly (ii) to the human body through disposable adhesive surface electrodes. The electrodes were positioned on the forearm, as shown in Figure 5. For safety reasons, the coupling boards were powered by a battery during the human-body experiments.