# Low-Power, High Data-Rate Digital Capsule Endoscopy Using Human Body Communication
## 3 Techniques to Improve Performance in the CEHBC System
### Selection Algorithm of the Electrode  
The capsule endoscope moves inside the body. Therefore, the reception quality of a signal received from the receiving electrode attached to the body varies. We proposed using an algorithm to select  
Figure 6: Bit error rate (BER) performance comparison between FSDT and differential Manchester coding (DMC). an optimal reception electrode pair to enable stable image data transfer despite change in the position of a capsule. The algorithm used to select the electrodes is influenced by variables such as the structure of the receiver, the number of electrodes, and the distance or position between the capsule and receiving electrodes. The CEHBC receiver presented in this paper has a differential amplifier structure to deal with noise signals. The input of the differential amplifier requires two signals corresponding to plus and minus signals. Therefore, this CEHBC receiver uses a method of selecting two electrodes. Also, the number of electrodes used in this system affects the number of combinations of electrode pairs. Plus or minus differential amplifier input signals could reverse the polarity of the differential amplifier output signal by the order of selection of the electrode pair. However, the effect on the selection order of the electrode pair is not considered in the electrode selection algorithm to reduce the complexity by half. The polarity problem of the signal is solved in the LineSync block. Table 2 shows the combinations of electrode pairs when using the eight electrodes (E1-E8) in the CEHBC system. In this case, the combination of electrode pairs generates 28 cases (=\(i\)th case). Based on these combinations, the number of electrode pair combinations (ECnum) used in the actual system is expressed by Equation (1). The algorithm for selection of the electrodes is affected by the position between the capsule and the electrode. Figure 7 displays the receiving condition according to the position of the capsule and electrode. While in the case of matching the capsule and the electrode pairs (E3-E7, \(i\)th = 16), each of the electrodes (without interfering) is capable of receiving the signal with the highest quality, but in the case where the capsule and the electrode pairs form at right angles (E1-E5, \(i\)th = 3), each of the electrodes receives the low quality signal offset by the opposite polarity. In addition, even in the case of \(i\)th = 16 described above Table 2, the capsule influences the polarity of the receiving signal in accordance with the location towards E3 or E7, due to the differential amplifier. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Case \(i\)th** & **Selected Electrode 1** & **Selected Electrode 2** & **Case \(i\)th** & **Selected Electrode 1** & **Selected Electrode 2** \\ \hline
0 & E1 & E2 & 14 & E3 & E5 \\
1 & E1 & E3 & 15 & E3 & E6 \\
2 & E1 & E4 & 16 & E3 & E7 \\
3 & E1 & E5 & 17 & E3 & E8 \\
4 & E1 & E6 & 18 & E4 & E5 \\
5 & E1 & E7 & 19 & E4 & E6 \\
6 & E1 & E8 & 20 & E4 & E7 \\
7 & E2 & E3 & 21 & E4 & E8 \\
8 & E2 & E4 & 22 & E5 & E6 \\
9 & E2 & E5 & 23 & E5 & E7 \\
10 & E2 & E6 & 24 & E5 & E8 \\
11 & E2 & E7 & 25 & E6 & E7 \\
12 & E2 & E8 & 26 & E6 & E8 \\
13 & E3 & E4 & 27 & E7 & E8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Combinations of electrode pairs when using the eight electrodes in the CEHBC system. Figure 7: The received condition according to the position of the capsule and the electrode. Figure 8 shows a flow chart of the electrode selection algorithm considering certain effects. For the ECnum = 28 (\(n\) = 8, \(r\) = 2), the \(i\)th variable is generated up to 0-29. During the case \(i\)th < ECnum, each pair of electrodes in each combination shown in Table 2 receives a signal from the capsule endoscope. The signal received from the electrode pair is used to calculate the correlation value. At this time, if the correlation value is bigger than a threshold Vth value, it updates the maximum correlation (= MaxCorr) and selects the \(i\)th number (= SelEP).