# Low-Power, High Data-Rate Digital Capsule Endoscopy Using Human Body Communication
## 3 Techniques to Improve Performance in the CEHBC System  
### Frequency Selective Digital Transmission  
The FSDT is a modulation technique that is able to spread the spectrum and to selectively move the particular frequency band using various combinations of Walsh codes. The FSDT block can be designed with digital logic only, without a digital-to-analog converter (DAC) and a radio frequency (RF) processing block. The FSDT is currently the technology that has been adopted in the IEEE 802.15.6 standard for wireless body area networks (WBAN) [19; 20; 21]. FSDT modulation uses the Walsh code. The Walsh code is a repetitive sequence of \"1\" or \"0\", having a regular changing time. Therefore the Walsh code provides the fundamental frequency of each code sequence. The FSDT utilizes this frequency characteristic of the Walsh code. Figure 4 is an example of selecting the frequency bands used in the CEHBC system with the 64 bit Walsh code and FSDT modulation, and the clock operating at 32 MHz. Figure 4 exhibits the chosen two frequency bands and Walsh code ranges when using the Low-rate FS Spreader presented in this paper. Figure 4 also presents the chosen three frequency band and Walsh code ranges when using the High-rate FS Spreader. Figure 4 shows the fundamental frequency properties of the 64-bit Walsh code. In this paper, the CEHBC system used 8-16 MHz Walsh code signals intentionally because the channel attenuation effect is greatest in the vicinity of DC in the human communication channel environment. In Figure 4, the BandSel parameter is a variable that determines which band to use. The BandSel parameter comprises four bits that indicate Grp5, Grp6, Grp7, or Grp8 that represent band group parameters. If the BandSel parameter is \"1010\", then the CEHBC system selects Grp5 and Grp7, and Walsh codes 33-40 and 49-56, as shown in Figure 4a. If the BandSel parameter is \"1011\", then the CEHBC system selects Grp5, Grp6, and Grp7, and Walsh codes 33-40, 49-56, and 57-63, as shown in Figure 4b. The FSDT block presented in this paper uses the revised dual-structure to support low-speed mode and high-speed mode, to improve the data transfer rate over that structure that is presentedin the existing WBAN standard. When the transmission section of the LineNum data indicating the transmission line number utilizes the Low-rate FS spreader block, there is a disadvantage in terms of data transfer rate, but there is better bit error rate (BER) performance. On the other hand, when the transmission section of the image data employs the High-rate FS spreader block, there is relatively poorer BER performance than with the Low-rate FS spreader, but it is possible to transfer larger amounts of data. Figure 5 is a block-diagram of FSDT that supports low data rate mode (Figure 5a) and high data rate mode (Figure 5b) to increase the data rate to be transmitted. The CRC parity bits added to the image data perform the S2P conversion. The Low-rate FS spreader carries out 1:4 S2P conversion and converts the data rate of 2 Mbps to the symbol rate of 0.5 Msps. The High-rate FS spreader fulfills 1:12 S2P conversion, and converts the maximum data rate of 6 Mbps into a symbol rate of 0.5 Msps. The S2P output data rate is defined with the symbol rate, having 0.5 Msps in both low-speed and high-speed mode. Figure 4: Example of selecting the frequency bands used in the CEHBC system: (**a**) Two frequency band and Walsh code range chosen when using the low-rate frequency selective (FS) spreader; (**b**) Three frequency band and Walsh code range chosen when using the High-rate FS spreader. In the sub-FS spreader, the combination of Walsh codes is determined by the band selection parameters (FSS1-FSS5), which determines the frequency band to be used. The Sub-FS spreader calculates the exclusive or (XOR) operation of the chosen Walsh code according to the converted S2P data values. The Low-rate FS spreader selects the output value (either Sub-FS SpreaderL1 or Sub-FS SpreaderL2) according to the most significant bit (MSB) of the S2P output bits.