applied sciences

Article Low-Power, High Data-Rate Digital Capsule Using Human Body Communication

Mi Jeong Park, Taewook Kang, In Gi Lim, Kwang-Il Oh, Sung-Eun Kim, Jae-Jin Lee and Hyung-Il Park * SoC Design Research Group, Intelligent SoC Research Department, ICT Materials & Components Research Laboratory, ETRI, Daejeon 34129, Korea; [email protected] (M.J.P.); [email protected] (T.K.); [email protected] (I.G.L.); [email protected] (K.-I.O.); [email protected] (S.-E.K.); [email protected] (J.-J.L.) * Correspondence: [email protected]; Tel.: +82-42-860-1316

 Received: 13 July 2018; Accepted: 17 August 2018; Published: 21 August 2018 

Abstract: A technology for low-power high data-rate digital capsule endoscopy with human body communication (CEHBC) is presented in this paper. To transfer the image data stably with low power consumption, the proposed system uses three major schemes: Frequency selective digital transmission (FSDT) modulation with HBC, the use of an algorithm to select electrode pairs, and the LineSync algorithm. The FSDT modulation supports high-data rate transmission and prevents the signal attenuation effect. The selection algorithm of the electrode pair finds the best receiving channel. The LineSync algorithm synchronizes the data and compensates for data polarity during the long data transmission section between the capsule endoscope and the receiver. Because all the major functional blocks of the CEHBC transmitter can be implemented as digital logics, they can be easily fabricated using the field programmable gate array (FPGA). Moreover, this CEHBC transmitter can achieve low power-consumption and can support a relatively high data rate in spite of using its clock a few tens of MHz slower. The proposed CEHBC-TXD is the digital portion of the CEHBC transmitter that provides low-power (3.7 mW) and high data-rate (6 Mbps) performance while it supports a high-resolution image (480 × 480 byte) at 3.13 fps.

Keywords: capsule endoscopy; human body communication; frequency selective digital communication; selection algorithm of electrode pair; synchronization

1. Introduction Recently, information technology (IT) developments in the medical area have been in the spotlight in the new field of biomedical engineering. Wireless capsule endoscopy (WCE) is a good example. It has been considered to be an innovative technology for the diagnosis of issues in the (e.g., in the , , , , and colon) because it is possible to avoid the disadvantages of the inspection techniques used currently for digestive organs. These disadvantages include side-effects caused by taking sleep-inducers or analgesics, and the possibility of medical accidents, such as organ perforation due to incorrect insertion of the endoscope. In addition, there is the inconvenience and fear of patients related to inserting the endoscope (a long tube) into the body. In contrast, it is possible to observe the digestive tract comfortably and without the inconvenience and risk described above when people use WCE [1–3]. Effective WCE was first introduced by Given Imaging Inc. (Yogneam, Israel) in 2000. The PillCam SB made by Given Imaging was approved by the food and drug administration (FDA) in 2003. It has been used as an actual medical diagnostic apparatus since then. More recently, several other kinds of WCEs have been developed and manufactured to look into the small intestine; for example, the EndoCapsule produced by Olympus Inc. (Tokyo, Japan, 2005) and the MiRoCam made by Intromedic Co. (Seoul, South Korea, 2006) [1,4].

Appl. Sci. 2018, 8, 1414; doi:10.3390/app8091414 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1414 2 of 17

A variety of studies have been conducted regarding WCE. One study area focuses on active capsule endoscopy, in which the posture or the motion of capsule is controlled to make observations not only in the small intestine, but also in other internal organs (the stomach and large intestine) with interior spaces that are larger than the capsules. Because the endoscope capsule is moved involuntarily through open spaces, it is difficult to obtain accurate images. Therefore, studies were conducted on active capsule endoscopy technologies to provide stable posture and movement in the various spaces to be examined [1,5–8]. Some other studies on image encoders employing image compression techniques to overcome the problem of a transceiver with limited data transfer capability have been progressed in capsule endoscopes image [1,9,10]. However, since there is a limit to the image compression method, in order to transmit the high resolution image, the transmitter supporting the high data rate is needed. Another area of WCE study has been the exploration of several communication standards: radio frequency (RF), ultra-wide band (UWB), and human body communication (HBC). The RF systems have a disadvantage in that a significant amount of the transmitted signal is absorbed and attenuated by the human body. Therefore, the use of a high-gain antenna is required to receive the weak signals [2,3,11–13]. Especially, the capsule endoscope is a device that uses the human body as a signal transmission channel, rather than a general RF air channel. Since the human body is made up of various materials with different conductivities and permittivities, such as skin, muscle, fat, bone, and various organs, the speed and direction of the transmission signal varies depending on the position of the capsule [14,15]. Thus, signals transmitted outside the human body do not have isotropic characteristics. In this channel environment, complex 2D/3D array receiving antennas are used to receive signals anywhere outside the body [16–18]. Therefore, a technique that can select a channel more optimally while reducing the complexity is required. On the other hand, HBC is a new communication approach for transferring baseband data using the human body as the channel medium without the need for a cable channel or RF communication channel. HBC systems have an advantage in that they can be implemented using only digital logic without complex RF blocks. This means that it is possible to manufacture a system that can be driven with low power consumption, and that should become smaller in the future by continuing the development process [19–23]. In this paper, we propose a new CEHBC (capsule endoscopy with HBC) system that provides low power use and a high data-rate. The CEHBC system architecture is presented in Section2, techniques to improve performance of the CEHBC system in Section3, and measurement and performance of the new system are described in Section4. Discussion and conclusions are presented in Sections5 and6, respectively.

2. CEHBC System Architecture

2.1. CEHBC System In general, the WCE system is composed of one transmitter (Tx) and one receiver (Rx) with multiple receiving electrodes, because the WCE Tx capsule moves in the human body in real time. Figure1 shows an example of the use of the WCE system. The capsule endoscope transmits the image data while moving in the body, and the receiver continually requests data reception utilizing the receiving electrodes that are attached to the human body. There are three system limitations. First, the capsule endoscope must be operable for a long time without supply of additional external power. Second, high-resolution images are required for accurate diagnosis. Third, because of the signal transmission in the body, the characteristics of the reception signal are changed by the distance and position between the capsule endoscope and the electrodes. To overcome these limitations, we proposed the following three methods for the new CEHBC system. First, we used the frequency selective digital transmission (FSDT) modulation method in the HBC to implement ultra-low power consumption and to transfer the image data stably at a high data rate. Second, we proposed using the selection algorithm to choose the best combination of electrodes among the plurality of receiving Appl. Sci. 2018,, 8,, xx 3 of 17 Appl. Sci. 2018, 8, 1414 3 of 17 selection algorithm to choose the best combination of electrodes among the plurality of receiving electrodes.electrodes. Third, Third, we we applied appliediedthe the LineSync LineSync algorithm to to synchronize synchronize and and rematch rematch the the polarity polarity of ofthe the signalsignal for for stable stable data data transfer. transfer.

FigureFigure 1. 1.Example Example of of the the use use of of the the wireless wirelessireless capsulecapsulecapsule endoscopy endoscopy ((WCE (WCE))) system system to to explore explore the the state state of of thethethe small smallsmall intestine. intestineintestine..

2.2.2.2. CEHBC CEHBC Physical Physical Structure Structure FigureFigure2 shows2 shows a a block-diagram block-diagram of of the the proposed proposed system system consisting consisting of a CEHBC of a CEHBC transmitter transmitter (Tx), (Tx),electrocardiogram electrocardiogram (ECG) (ECG) electrodes, electrodes, and andthe theCEH CEHBCBC receiver receiver (Rx). (Rx). Figure Figure 2a 2isisa is the the the CEHBC CEHBC CEHBC Tx Tx Tx includingincludingincluding the thethe lens, lens,lens, an anan image imageimage sensor, sensor,sensor, light lightlight emittingemittingemitting diodesdiodes (( (LEDs),LEDs),, CEHBCCEHBC CEHBC TxTx Tx interfaceinterface interface (CEHBC(CEHBC (CEHBC-TXIF),-TXIF), thethe CEHBC CEHBC Tx Tx digital digital block block (CEHBC-TXD), (CEHBC-TXD), aa signal electrode, electrode, a a ground ground (GND) (GND) electrode, electrode, and and batteries. batteries. Figure 2b is the CEHBC Rx comprising a CEHBC analog front end (CEHBC-AFE), a CEHBC Rx digital FigureFigure2b 2 isb theis the CEHBC CEHBC Rx Rx comprising comprising a a CEHBC CEHBCanalog analog frontfront end (CEHBC (CEHBC-AFE),-AFE), a aCEHBC CEHBC Rx Rx digital digital block (CEHBC-RXD), a CEHBC Rx interface (CEHBC-RXIF), and a micro controller unit (MCU). block (CEHBC-RXD), a CEHBC Rx interface (CEHBC-RXIF), and a micro controller unit (MCU).

(a)

(b)

Figure 2. Block-diagram of the proposed capsule endoscopy with a human body communication (CEHBC) system: (a) CEHBC transmitter; (b) CEHBC receiver. Appl. Sci. 2018, 8, 1414 4 of 17

The CEHBC Tx is inside the capsule that is inserted into the human body. The CEHBC Rx and the receiving ECG electrodes are placed on the exterior of the human body. The CEHBC Tx is capsule –shaped, and it is swallowed. On one end, the CEHBC Tx is a transparent dome-shaped lens that is able to observe the appearance of internal organs. A sensor captures the image showing the internal condition while LEDs illuminate the space near the sensor. The CEHBC-TXIF stores the image data from the image sensor in the memory and converts the raw data to the input data formats that are available in the CEHBC-TXD. The CEHBC-TXD transmits the signal that is modulated by the FSDT technique using HBC. The signal electrode radiates the Tx signal. The GND electrode serves as the reference ground for the system. Batteries supply the voltage that is needed to operate the entire endoscope. The CEHBC-TXD transmits the image data, configuring a one-frame signal. The CEHBC-TXD is composed of a preamble generator, a LineSync generator, a cyclic redundancy check (CRC) encoder, a serial-to-parallel (S2P), a frequency selective (FS) spreader, and a multiplexor (MUX) block. The preamble generator creates the signals that are needed to acquire synchronization of the frame signal, and it selects the optimal electrode pair. The LineSync generator makes the signals that are needed to acquire synchronization, and it compensates for the polarity of each line-by-line signal that carry the image data. The CRC encoder generates parity bits to determine whether errors have occurred during the transfer of the image data. The S2P block changes the data rate to apply the FSDT techniques. The FS spreader enables the spread of data and movement of the frequency band. The multiplexer (MUX) block routes the generated data values into a specified transmission frame period. The receiver of the capsule endoscope receives signals from a plurality of ECG electrodes attached to the exterior of the human body. The signal received from an electrode is a low-amplitude signal and it should be converted into a digital signal via the CEHBC-AFE block. The converted digital signal is input into the CEHBC-RXD block, which performs reverse digital signal processing (such as the de-spreading and demodulation) of the CEHBC-TXD block. The CEHBC-RXIF transmits the demodulated image signal to the MCU or to the PC software. It is possible to observe the image of the organ interior via the image viewer. The CEHBC-AFE consists of a switch block, a differential amplifier, a filter block, a clock, and data recovery (CDR). The switch block changes the channel of the electrode combination to select the optimal combination of receiving electrodes. The differential amplifier amplifies the low amplitude signal. The filter block removes the noise, and the CDR processes the asynchronous data. The CEHBC-RXD is made up of a preamble processing unit, LineSync block, FS de-spreader, parallel-to-serial (P2S) convertor block, and a CRC decoder. The preamble processing unit finds the start of the frame data received and selects the best electrode pair. The LineSync block decides the sign of the data and adjusts the synchronization of each line from the specific information data and the image data. The FS de-spreader de-spreads the data using Walsh code. The P2S converts the parallel bits to serial bits. The CRC decoder block performs error detection. Table1 shows the main parameters used in the CEHBC system that are presented in this paper. The CEHBC system generates the signal from the capsule inside the body and receives signals via the receiving electrodes attached to the body exterior. The CEHBC system is an example of in-body to on-body communication. The main clock of the CEHBC system uses a 32 MHz signal and the FSDT modulation method with 64 bit Walsh code. Therefore this system could spread the spectrum and generate a signal, of which the center frequency varies from direct current (DC) to 16 MHz. However, the CEHBC system utilizes the 8–16 MHz frequency band, which has less signal attenuation effect in the body. The resolution of the transfer image data is 480 × 480 byte, and it is transmitted at 3.13 f/s. One frame requires 319,440 µs of transmission time. It is possible to generate data rates up to 6 Mbps with FSDT high-rate mode modulation. The CEHBC system takes the combination structure formed with the short preamble generator output (SP) and the long preamble generator output (LPn). The SP takes on the properties of the 7-bit pseudo-random noise (PN) code; whereas the LPn has Appl. Sci. 2018, 8, 1414 5 of 17 the properties of the 12-bit PN code. The combination structure has the advantage of increasing the accuracyAppl. Sci. of 2018 the, 8 receiving, x synchronization while having less hardware complexity. This system5 of 17 uses the LineSync generator, which keeps signal synchronization per line to maintain performance for data accuracy of the receiving synchronization while having less hardware complexity. This system uses that is received over long transmission times. The LineSync uses the properties of the 5-bit PN code. the LineSync generator, which keeps signal synchronization per line to maintain performance for It alsodata exploits that is thereceived CRC over formed long from transmission a 12-bit times. polynomial The LineSync to find uses out whetherthe properties an error of the has 5-bit occurred. PN code. It also exploits the CRC formed from a 12-bit polynomial to find out whether an error has Table 1. Parameters of capsule endoscopy with a human body communication (CEHBC) system. occurred.

ParametersTable 1. Parameters of capsule endoscopy with a human body Description communication (CEHBC) system. application Inbody-to-Onbody (capsule endoscopy) Parameters Description main clock frequency 32 MHz application Inbody-to-Onbody (capsule endoscopy) modulation direct digital frequency selective digital transmission (64 b Walsh code) main clock frequency 32 MHz frequency band 8–16 MHz modulation direct digital frequency selective digital transmission (64 b Walsh code) resolution image data 480 × 480 byte (3.13 f/s) frequency band 8–16 MHz frame Length 319,440 µs resolution image data 480 × 480 byte (3.13 f/s) data rate maximum 6 Mbps frame Length 319,440 µs preamble combination of short preamble and long preamble data rate maximum 6 Mbps linesync 5-bit pseudo-random noise (PN) code preamble combination of short preamble and long preamble cyclic redundancy check (CRC) CRC-12 polynomial linesync 5-bit pseudo-random noise (PN) code cyclic redundancy check (CRC) CRC-12 polynomial 2.3. CEHBC Data Frame Structure 2.3. CEHBC Data Frame Structure Figure3 presents the data frame structure of the CEHBC system. One frame signal requires the transmissionFigure time 3 presents of at least the 484data× frame660 µstructures (= 319,440 of theµ CEHBCs) to transmit system. the One 480 frame× 480 signal byte requires video graphicsthe arraytransmission (VGA) image time signal. of at least Before 484 × the660 transferµs (= 319,440 of the µs) specificto transmit information the 480 × 480 data byte and video the graphics image data, array (VGA) image signal. Before the transfer of the specific information data and the image data, the the first two lines perform the functions of driving the receiver CDR, finding the best combination of first two lines perform the functions of driving the receiver CDR, finding the best combination of receptionreception electrodes, electrodes, and and acquiring acquiring the the frameframe synchronization. The The CEHBC CEHBC Rx uses Rx uses the CDR, the CDR, so it so it needsneeds a CDR a CDR lock lock time time section section of 100of 100µs, µs, which which is is configured configured with with pattern pattern datadata repeatedrepeated with with “1” “1” and “0” bitsand for “0” accurate bits for accurate operation operation of the of CDR. the CDR.

100μs 32μs 8μs 32μs 8μs LockTime SWT SWT Preamble (0) Preamble (13) 0101.. 0101.. 0101.. 1024bits 1024bits 3,200bits 256bits 256bits 2 line SWT SWT SWT Preamble (14) Preamble (28) Preamble* (29) 0101.. 0101.. 0101.. 1024bits 1024bits 1024bits 256bits 256bits 896bits LineSync LineNum CRC Tail Fixed Pattern (“F055 F055 ...”) + Expose Time 128bits 256bits 64bit 0101.. 2 line LineSync LineNum CRC Tail Frame Number 128bits 256bits 64bit 0101..

VGA Image (480x480) Tail LineSync LineNum CRC 0101.. 480 line 128bits 256bits Data 64bit 480 byte (320 Symbol) x 480 Line 192bits =20,480bits x 480 Line

Interval (0~32 Lines)

4μs 8μs 640μs 2μs 6μs 660μs (495byte)

FigureFigure 3. 3.Data Data frameframe structure of of the the CEHBC CEHBC system system.. Appl. Sci. 2018, 8, 1414 6 of 17

The CEHBC data frame is different from that of a conventional communication system. The CEHBC uses preamble sections to generate a plurality of preamble signals. This method requires selection of the optimal combination of electrodes while varying the combination of receiving electrodes. At this time, the number of preamble transmissions is determined by the number of electrodes attached to the body, and it can be expressed in Equation (1). Here, ECnum is the number of electrode pair combinations, n is the total number of electrodes, and r is the number of electrodes selected:

n! ECnum = (1) r!(n − r)!

For example, if the number of the receiving electrode is eight, the CEHBC data frame uses 30 preamble sections that contain the 28 preamble sections (n = 8, r = 2) to test the receiving environment, one preamble section for confirmation of the selected electrode pair, and one last preamble section to indicate the last preamble transmission. In addition, switch time (SWT) sections are the physical time needed to change the reception electrodes according to the chosen combination out of the 30 options. By comparing the preamble correlation values for 28 pieces of the received signal, the CEHBC system picks the electrode pair with the maximum correlation output and is able to select the electrode pair that has the most excellent reception accuracy and sensitivity. The LineSync section that generates the signal for synchronizing the line data is located in the first part of the line to prevent missing the synchronization during long frame intervals (3–484 lines). Then each line includes a LineNum section to indicate the line number of the transmission data, the data section, and the CRC section to determine whether an error occurred in the transmission data. The tail section indicates the last zone of the line. The data section in the third line is with the fixed pattern information to find the start of the image signal and exposure information of the camera. The data section in the fourth line is placed with the frame data information, indicating the number of the transfer frame. The data sections in the other lines are filled with 484 × 480 byte image data.

3. Techniques to Improve Performance in the CEHBC System 3.1. 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) [19–21]. 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. Figure4 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. Figure4a exhibits the chosen two frequency bands and Walsh code ranges when using the Low-rate FS Spreader presented in this paper. Figure4b also presents the chosen three frequency band and Walsh code ranges when using the High-rate FS Spreader. Figure4 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 Figure4, 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 Figure4a. 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 Figure4b. 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 presented Appl. Sci. 2018, 8, 1414 7 of 17 Appl. Sci. 2018, 8, x 7 of 17 Appl. Sci. 2018, 8, x 7 of 17 in thethe existing existing WBAN WBAN standard.standard. When When the the transmission transmission section section of the of theLineNum LineNum data dataindicating indicating the the transmissiontransmissionthe existing line lineWBAN number number standard. utilizes utilizes When the the Low-rate Low the -transmissionrate FS FS spreader spreader section block, block, of there the there LineNum is a is disadvantage a disadvantage data indicating in terms in terms theof of datadata transfertransmission transfer rate, rate, line but numberbut there there isutilizes is better better the bit bit Low errorerror-rate rate FS ( (BER)spreaderBER) performance. performance. block, there On is Onathe disadvantage theother other hand, hand, inwhen terms when the of the transmission section of the image data employs the High-rate FS spreader block, there is relatively transmissiondata transfer section rate, ofbut the there image is better data bit employs error rate the (BER High-rate) performance. FS spreader On the other block, hand, there when is relatively the poorertransmission BER performance section of the than image with data the employs Low-rate the FS High spreader,-rate FS but spreader it is possible block, tothere transf is errelatively larger poorer BER performance than with the Low-rate FS spreader, but it is possible to transfer larger amountspoorer BER of data. performance than with the Low-rate FS spreader, but it is possible to transfer larger amountsamounts of data. of data.

(a) (a)

(b) (b) FigureFigure 4. Example 4. Example of selectingof selecting the the frequency frequency bands bands used used in in the the CEHBC CEHBC system: system: ((aa)) TwoTwo frequencyfrequency band andband WalshFigure and code4. WalshExample range code of chosen selectingrange whenchosen the frequency usingwhen theusing bands low-rate the lusedow- frequencyrate in the frequency CEHBC selective selective system: (FS) (aFS)spreader; )Two spreader frequency; ( b(b)) Three frequencyThreeband frequencyand band Walsh and band Walshcode and range code Walsh chosen range code when chosen range using chosen when the when using low- rateusing the frequency High-rate the High -selectiverate FS spreader.FS s preader(FS) spreader. ; (b) Three frequency band and Walsh code range chosen when using the High-rate FS spreader. Figure 5 is a block-diagram of FSDT that supports low data rate mode (Figure 5a) and high data Figure5 is a block-diagram of FSDT that supports low data rate mode (Figure5a) and high data rate modeFigure (Figure 5 is a block 5b) to-diagram increase of the FSDT data that rate supports to be transmitted. low data rate The mode CRC (Figureparity bits5a) andadded high to datathe rateimage moderate mode data (Figure perform(Figure5b) to5 theb) increase toS2P increase conversion. the the data data The rate rate Low to to-rate be betransmitted. FStransmitted. spreader carries The The CR CRCoutC parity1:4 parity S2P bits conversion bits added added to and the to the imageconvertsimage data data perform the perform data therate the S2P of S2P 2 conversion. Mbps conversion. to the Thesymbol TheLow-rate Low rate-rate of FS FS0.5 spreader spreaderMsps. The carries carries High out-rate out 1:4 1:4FSS2P spreader S2P conversion conversion fulfills and and converts1:12converts S2P the conversion, datathe data rate rate ofand 2of Mbpsconve 2 Mbpsrts to tothe the the maximum symbol symbol rate ratedata ofrate 0.5 of Msps.Msps. 6 Mbps The The into High High-ratea symbol-rate FS rate spreader FS of spreader 0.5 Msps.fulfills fulfills 1:12 S2PThe1:12 S2P conversion,S2P outputconversion, data and rateand converts conveis definedrts the the with maximum maximum the symbol data rate,rate rate ofhaving of 6 6Mbps Mbps 0.5 into Msps into a symbolin a symbolboth ratelow rateof-speed 0.5 of Msps. 0.5and Msps. The S2PhighThe - outputS2Pspeed output mode. data data rate rate is definedis defined with with the the symbolsymbol rate, having having 0.5 0.5 Msps Msps in both in both low low-speed-speed and and high-speedhigh-speed mode. mode.

(a) (b) (a) (b) Figure 5. Block-diagram of the dual-mode frequency selective digital transmission (FSDT): (a) Low-rate FS spreader; (b) High-rate FS spreader. Appl. Sci. 2018, 8, x 8 of 17 Appl. Sci. 2018, 8, 1414 8 of 17 Figure 5. Block-diagram of the dual-mode frequency selective digital transmission (FSDT): (a) Low- rate FS spreader; (b) High-rate FS spreader. In the sub-FS spreader, the combination of Walsh codes is determined by the band selection parametersIn the (FSS1–FSS5), sub-FS spreader, which the determines combination the of frequencyWalsh codes band is determined to be used. by The the Sub-FSband selection spreader calculatesparameters the exclusive(FSS1–FSS5), or (XOR) which operation determine ofs the the frequency chosen Walsh band code to be according used. The to Sub the- convertedFS spreader S2P datacalculates values. the exclusive or (XOR) operation of the chosen Walsh code according to the converted S2P dataThe values. Low-rate FS spreader selects the output value (either Sub-FS SpreaderL1 or Sub-FS SpreaderL2)The Low according-rate FSto spreader the most selects significant the output bit (MSB) value of (either the S2P Sub output-FS SpreaderL1 bits. The High-rate or Sub-FS FS spreaderSpreaderL2) will only according output to a the signal most with significant at least twobit (MSB) identical of the values S2P withoutput the bits. majority The High selector-rate blockFS spreader will only output a signal with at least two identical values with the majority selector block among the three input values received from the Sub-FS SpreaderH1, Sub-FS SpreaderH2, and Sub-FS among the three input values received from the Sub-FS SpreaderH1, Sub-FS SpreaderH2, and Sub- SpreaderH3. The output value of the majority selector is the same two bit value as that presented in FS SpreaderH3. The output value of the majority selector is the same two bit value as that presented Equation (2). in Equation (2). TX_OUT = A·B + B·C + C·A (2) 푇푋_푂푈푇 = 퐴 ∙ 퐵 + 퐵 ∙ 퐶 + 퐶 ∙ 퐴 (2) By applying FSDT techniques with Walsh code, which has fundamental frequencies from DC to half theBy frequency applying of FSDT the maintechniques clock, with the CEHBC Walsh code, system which can has reduce fundamental the influence frequencies of signal from attenuation DC to to behalf the the greatest frequency near of the DC main when clock, using the the CEHBC human system body can as reduce the medium. the influence There of is signal also anattenuation advantage to be the greatest near DC when using the human body as the medium. There is also an advantage if if a user selects a specific Walsh code: the CEHBC system could shift the spread signal to a specific a user selects a specific Walsh code: the CEHBC system could shift the spread signal to a specific band band without the RF frequency conversion portion. In addition, the FSDT technique performs the without the RF frequency conversion portion. In addition, the FSDT technique performs the function function of multiplying the baseband signal by the Walsh code, so that it generates a spread spectrum of multiplying the baseband signal by the Walsh code, so that it generates a spread spectrum effect effect on the signal. Therefore, it is possible to restore the signal that was to be transmitted originally on the signal. Therefore, it is possible to restore the signal that was to be transmitted originally in the in thedispreading dispreading process process,, even even if the if the spreading spreading signal signal has has been been damaged damaged by by interference interference or or noise noise componentscomponents generated generated in in the the channel. channel. InIn addition addition to to the the FSDT FSDT method, method, therethere isis aa differentialdifferential Manchester c codingoding (DMC) (DMC) method method that that usesuses a specifica specific frequency frequency band band without without usingusing basebandbaseband signals near near DC. DC. The The Figure Figure 66 shows shows the the BER BER performanceperformance comparison comparison between between FSDTFSDT andand DMC in in the the additive additive white white gaussian gaussian noise noise (AWGN (AWGN)) environment.environment. The The DMC DMC transmits transmits data data atat halfhalf thethe rate of the the operation operation clock, clock, and and the the FSDT FSDT proposed proposed in thisin this paper paper can can transmit transmit data data with with 3/16 3/16 timestimes of the the operation operation clock. clock. When When transmitting transmitting the the 6 Mbps 6 Mbps signalsignal using using these these two two modulation modulation methods, methods, the the FSDT FSDT method method shows shows the carrier the carrier power power to noise to noise power ratiopower (CNR) ratio improvement (CNR) improvement of about of 8.4 about dB compared 8.4 dB compared with the with DMC the methodDMC method (at BER (at = BER 10− =6 ).10−6).

FigureFigure 6. 6.Bit Bit error error rate rate (BER) (BER) performance performance comparisoncomparison between FSDT FSDT and and differential differential Manchester Manchester codingcoding (DMC). (DMC).

3.2. 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 Appl. Sci. 2018, 8, x 9 of 17

3.2. Selection Algorithm of the Electrode Appl. Sci. 2018, 8, 1414 9 of 17 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 an optimal reception electrode pair to enable stable image data transfer despite change in the position to select an optimal reception electrode pair to enable stable image data transfer despite change in of a capsule. the position of a capsule. The algorithm used to select the electrodes is influenced by variables such as the structure of the 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 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 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 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, 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. 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 Plus or minus differential amplifier input signals could reverse the polarity of the differential output signal by the order of selection of the electrode pair. However, the effect on the selection order amplifier output signal by the order of selection of the electrode pair. However, the effect on the of the electrode pair is not considered in the electrode selection algorithm to reduce the complexity by selection order of the electrode pair is not considered in the electrode selection algorithm to reduce half. The polarity problem of the signal is solved in the LineSync block. the complexity by half. The polarity problem of the signal is solved in the LineSync block. Table2 shows the combinations of electrode pairs when using the eight electrodes (E1–E8) in the Table 2 shows the combinations of electrode pairs when using the eight electrodes (E1–E8) in CEHBC system. In this case, the combination of electrode pairs generates 28 cases (=ith case). Based the CEHBC system. In this case, the combination of electrode pairs generates 28 cases (=ith case). on these combinations, the number of electrode pair combinations (ECnum) used in the actual system Based on these combinations, the number of electrode pair combinations (ECnum) used in the actual is expressed by Equation (1). system is expressed by Equation (1).

Table 2. Combinations of electrode pairs when using the eight electrodes in the CEHBC system. Table 2. Combinations of electrode pairs when using the eight electrodes in the CEHBC system.

CaseCasei thith SelectedSelected ElectrodeElectrode 1 Selected Selected Electrode Electrode 2 2Case Case ithith Selected Selected Electrode Electrode 1 1Selected Selected Electrode Electrode 2 2 00 E1E1 E2 E2 14 14 E3 E3E5 E5 11 E1E1 E3 E3 15 15 E3 E3E6 E6 22 E1E1 E4 E4 16 16 E3 E3E7 E7 33 E1E1 E5 E5 17 17 E3 E3E8 E8 44 E1E1 E6 E6 18 18 E4 E4E5 E5 55 E1E1 E7 E7 19 19 E4 E4E6 E6 66 E1E1 E8 E8 20 20 E4 E4E7 E7 77 E2E2 E3 E3 21 21 E4 E4E8 E8 88 E2E2 E4 E4 22 22 E5 E5E6 E6 99 E2E2 E5 E5 23 23 E5 E5E7 E7 1010 E2E2 E6 E6 24 24 E5 E5E8 E8 1111 E2E2 E7 E7 25 25 E6 E6E7 E7 1212 E2E2 E8 E8 26 26 E6 E6E8 E8 13 E3 E4 27 E7 E8 13 E3 E4 27 E7 E8

TheThe algorithm algorithm for for selection selection of of the the electrodes electrodes is isaffected affected by by the the position position between between the the capsule capsule and and thethe electrode. electrode. Figure Figure 77 displays displays the the receiving receiving condition condition according according to to the the position position of of the the capsule capsule and and electrode.electrode. While While in in the the case case of of matching matching the the capsule capsule and and the the electrode electrode pairs pairs (E3 (E3–E7,–E7, ithith = =16), 16), each each of of thethe electrodes electrodes (without (without interfering) interfering) is iscapable capable of of receiving receiving the the signal signal with with the the highest highest quality, quality, but but in in thethe case case where where the the capsule capsule and and the the electrode electrode pairs pairs form form at at right right angles angles (E1 (E1–E5,–E5, ithith = =3), 3), each each of of the the electrodeselectrodes receives receives the the low low quality quality signal signal offset offset by the by opposite the opposite polarity. polarity. In addition, In addition, even in even the incase the ofcase ith of= 16ith described = 16 described above above Table Table 2, the2, the capsule capsule influences influences the the polarity polarity of of the the receiving receiving signal signal in in accordanceaccordance with with the the location location towards towards E3 E3 or or E7, E7, due due to to the the differential differential amplifier amplifier..

Figure 7. The received condition according to the position of the capsule and the electrode. Appl. Sci. 2018, 8, x 10 of 17

Appl. Sci.Figure2018, 8, 14147. The received condition according to the position of the capsule and the electrode. 10 of 17

Figure 8 shows a flow chart of the electrode selection algorithm considering certain effects. For the ECnumFigure =8 28 shows (n = a8, flow r = 2), chart the of ith the variable electrode is selection generated algorithm up to 0 considering–29. During certain the case effects. ith < For ECnum, the eachECnum pair = of 28 electrodes (n = 8, r = 2),in theeachith combination variable is generated shown in up Table to 0–29. 2 Duringreceives the a casesignalith from < ECnum, the capsule each endoscope.pair of electrodes The signal in each received combination from the shown electrode in Table pair2 receives is used ato signal calculate from the capsulecorrelation endoscope. value. At thisThe time, signal if received the correlation from the value electrode is bigger pair is used than toa calculate threshold the Vth correlation value, itvalue. updates At this the time, maximum if the correlationcorrelation (= value MaxCorr) is bigger and than select a thresholds the ith Vth number value, (= it updatesSelEP). theThen, maximum in the case correlation of ith = (= ECnum, MaxCorr) the and selects the ith number (= SelEP). Then, in the case of ith = ECnum, the next process is carried out next process is carried out to check the selected electrode pair. Last, for the case of ith = ECnum + 1, to check the selected electrode pair. Last, for the case of ith = ECnum + 1, the algorithm performs the algorithm performs a process to determine if it is the last preamble section or not. a process to determine if it is the last preamble section or not.

Figure 8. Flow chart of the electrode selection algorithm. Figure 8. Flow chart of the electrode selection algorithm.

3.3. LineSync Algorithm 3.3. LineSync Algorithm The LineSync section is placed at the front of each line of data, as shown in Figure3. When The LineSync section is placed at the front of each line of data, as shown in Figure 3. When receiving the individual line of data in one transmission frame, the LineSync section is used to create receiving the individual line of data in one transmission frame, the LineSync section is used to create the data synchronization that corresponds to ±1 bit variation based on the ideal sampling signal. the data synchronization that corresponds to ±1 bit variation based on the ideal sampling signal. The LineSync generator produces the 5 bit PN sequence signals formed with the LSp (z) = z5 + z3 + 1 5 3 polynomial.The LineSync The generator serial signal produces of 32 bits the is 5 changed bit PN sequence to serial data signals of 64 formed bits through with the the LSp Manchester (z) = z + z + 1encoder, polynomial. which The can serial change signal “1” of to 32 “10” bits and is changed “0” to “01”. to serial The LineSync data of 64 generator bits through then the produces Manchester the encoder,final 128 which bits that can are change repeated “1” twice. to “10” Here, and the “0” initial to “01”. value T ofhe the LineSync PN generator generator polynomial then produces is “10,000”. the final 128By bits using that the are LineSync repeated generator twice. utilizing Here, the these initial two value repeating of the structures, PN generator the CEHBC polynomial system is “10has,000 the”. advantage of reducing the size of the correlators in the receiver to approximately half and couldBy using acquire the a LineSync large correlation generator value. utilizing The LineSync these two generator repeating has structures, another advantage: the CEHBC it system can cope has thewith advantage the synchronization of reducing ofthe the size varied of the data correlator that cans occurin the during receiver the to interval approximately of a long transmissionhalf and could acquireframe. a Thelarge location correlation of the value. capsule The endoscope LineSync often generator varies withinhas another the human advantage: digestive it can tract. cope Therefore, with the synchronizationthe polarity of theof the received varied signal data couldthat can be changedoccur during depending the interval on the situation.of a long Thus,transmission the LineSync frame. Thegenerator locationshould of the performcapsule endoscope real time corrections often varies that within are conducted the human using digestive the sign tract. of the Therefore, correlation the polarityvalues ofcalculated the received by the signal LineSync could of the be receiving changed block. depending on the situation. Thus, the LineSync generator should perform real time corrections that are conducted using the sign of the correlation values calculated by the LineSync of the receiving block. Appl. Sci. 2018, 8, 1414 11 of 17 Appl. Sci. 2018, 8, x 11 of 17

4.4. CEHBC CEHBC System System Measurement Measurement and and Performance Performance

4.1.4.1. Measurement Measurement Environment Environment of of the the CEHBC CEHBC System System FigureFigure 9 9indicates indicates the the measurement measurement environment environment of the proposed of the proposed CEHBC system CEHBC for performancesystem for performanceobservation. observation Figure9a is. aFigure block 9 diagrama is a block showing diagram the showing configuration the configuration of the CEHBC of the transceiver CEHBC transceiversystem. Figure system.9b displays Figure the 9b measurement displays the environment measurement of the environment CEHBC system of the implemented CEHBC system using implementedhardware devices. using hardware devices. TheThe CEHBC CEHBC system system included included t thehe CEHBC CEHBC Tx Tx capsule capsule endoscope, endoscope, human human body body channel, channel, CEHBC CEHBC RxRx board, board, and and an an image image viewer viewer software. software. A A water water tank tank filled filled with with saline saline water water was was used used as as the the communicationcommunication channel channel of of the the CEHBC CEHBC system system to realize to the realize In-body the to In On-body-body to channel On-body environment. channel environment.The CEHBC-TXIF/CEHBC-TXD The CEHBC-TXIF/CEHBC and CEHBC-RXD/CEHBC-RXIF-TXD and CEHBC-RXD/CEHBC were designed-RXIF were with designed digital with logic digitaland implemented logic and implemented using field programmable using field programmable gate array (FPGA) gate array chips. (FPGA) chips. TheThe output output signal signal of of the the CEHBC CEHBC-TXD-TXD was was connected connected to to the the electrode electrode of of the the capsule, capsule, which which was was placedplaced in in the the saline saline tank. tank. The The signal signal was was transmitted transmitted to to the the electrodes electrodes through through saline saline channels. channels. Signals Signals receivedreceived from from the the eight eight electrodes electrodes attached attached to to the the water water tank tank were were transferred transferred to to the the CEHBC CEHBC-AFE-AFE Board.Board. CEHBC CEHBC-AFE-AFE convertedconverted thethe received received analog analog signal signal into into a 1-bita 1-bit digital digital signal signal using using the CDRthe CDR (etc.) (etc.)andsupplied and supplied the input the input data to data the CEHBC-RXD.to the CEHBC The-RXD. CEHBC-RXD The CEHBC restored-RXD restored the image the signal image from signal the fromreceived the received signal and signal transferred and transferred it to the PC. it to The the PC PC. processed The PC processed the transferred the transferred data and thendata displayedand then displayedthe image the on theimage screen, on the which screen, was which captured was bycaptured the image by the sensor. image sensor. TheThe animal animal clinical clinical experiment experiment is is needed needed for for realistic realistic results. results. However However it it costs costs a a lot lot of of money money to to experimentexperiment with with animals. animals. Since Since the the performance performance of the of capsule the capsule endoscope endoscope system should system be should measured be measuredby an indirect by an experiment indirect experiment before proceeding before proceeding with such with animal such clinical animal experiment, clinical experiment, we used the we signal used thechannel signal model channel with model the salinewith the tank saline used tank in the used reference in the reference papers [ 24papers–26] below.[24–26] The below. CEHBC The CEHBC capsule capsuleendoscope endoscope uses a galvanic uses a coupling galvanic signaling coupling system signaling and system uses a 32 and MHz uses clock. a 32 This MHz environment clock. This is esimilarnvironment to the is experimental similar to the conditions experimental presented conditions in the presented reference in paper the reference [24]. paper [24].

(a)

(b)

FigureFigure 9 9.. MeasurementMeasurement environment: environment: (a ()a ) Block Block-diagram-diagram of of the the CEHBC CEHBC transceiver transceiver system system for for measurement;measurement; ( (bb)) Measurement Measurement environment environment of of the the implemented implemented CEHBC system system..

Appl. Sci. 2018, 8, 1414 12 of 17

Appl. Sci. 2018, 8, x 12 of 17 4.2. Performance of the CEHBC System 4.2.Figure Performance 10 displays of the theCEHBC operation System results transferred by utilizing the CEHBC system presented in this paper.Figure Figure 10 displays10a is the the output operation of the results AFE transferred amplifier. Thisby utilizing result showsthe CEHBC a process system of presented testing 28 in pairs of electrodesthis paper. according Figure 10a to is the the selection output of algorithm, the AFE amplifier. and displays This aresult signal shows selected a process to transmit of testing an image.28 Thepairs last preambleof electrodes was according selected to with the sel theection electrode algorithm, pair ofand preamble displays 27.a signal The signalselected received to transmit from an the selectedimage. electrode The last pairpreamble was a was signal selected of a particular with the electrode range level pair that of preamble was neither 27. tooThe large signal nor received too small. Thus,from the the selected selected signal electrode did notpairsaturate, was a signal even of aftera particular passing range through level thethat amplifier, was neither and too it large had nor a state in whichtoo small. the Thus, signal the judgment selected couldsignal did be accuratelynot saturate performed., even after passing Therefore, through it was the confirmedamplifier, and that it the had a state in which the signal judgment could be accurately performed. Therefore, it was confirmed electrode selection algorithm maintains the optimum received signal quality state. that the electrode selection algorithm maintains the optimum received signal quality state. Figure 10b is an output image that arose from the transmission of the image data. Both images in Figure 10b is an output image that arose from the transmission of the image data. Both images Figure 10b have a common hardware structure that supports the same f/s and power consumption, in Figure 10b have a common hardware structure that supports the same f/s and power consumption, withwith the the only only difference difference being being the the TX TX input input type.type. FigureFigure 1010bb ( (left)left) is is the the known known pattern pattern used used for forthe the functionalfunctional verification verification of of the the implemented implemented CEHBCCEHBC system.system. Figure Figure 10 10b b(right) (right) is isthe the output output image image utilizingutilizing the the CEHBC CEHBC system, system, for for which which the the sourcesource data is the the image image taken taken from from the the complementary complementary metal–oxide–semiconductormetal–oxide–semiconductor (CMOS) (CMOS) image image sensor. sensor.

(a)

(b)

FigureFigure 10. Operation10. Operation results results of the of proposed the proposed system: system: (a) Output (a) Output of the of analog the analog front end front (AFE) end amplifier; (AFE) amplifier; (b) Image results received by the CEHBC system: (left) Image data using a known pattern, (b) Image results received by the CEHBC system: (left) Image data using a known pattern, (right) (right) Image data using the CMOS image sensor. Image data using the CMOS image sensor. Table 3 exhibits the synthesized results of the CEHBC Tx digital block and CEHBC Rx digital blocks.Table 3The exhibits CEHBC the Tx synthesized digital block results was implemented of the CEHBC using Tx an digital Actel blockIGLOO and AGLN125V5 CEHBC Rx FPGA digital blocks.(IGLOO The nano CEHBC version, Tx Microsemi, digital block San Jose was CA implemented, United States) using. It used an about Actel 52.25% IGLOO (CEHBC AGLN125V5-TXIF: FPGA35.28%, (IGLOO CEHBC nano-TXD: version, 16.96%) Microsemi,of the total chip San logic Jose and CA, consumed United 6.34 States). mW of Itthe used power about (CEHBC 52.25%- (CEHBC-TXIF:TXIF: 2.63 mW, 35.28%, CEHBC CEHBC-TXD:-TXD: 3.71 16.96%) mW). The of the power total consumption chip logic and value consumed was predicted 6.34 mW using of the powercalculating (CEHBC-TXIF: tools (Libero 2.63 mW,v11.8, CEHBC-TXD: Vivado March 3.71 2015). mW). The TheCEHBC power Rx consumptionwas implemented value using was a predictedXilinx usingArtix7 calculating XC7A50T tools FPGA (Libero (ARTIX v11.8, version, Vivado XILINX, March San 2015). Jose, CA The, United CEHBC States) Rx was; it used implemented about 16% usingof a Xilinxthe total Artix7 chip XC7A50T logic and FPGAconsumed (ARTIX 114 mW version, of power. XILINX, San Jose, CA, United States); it used about 16% of the total chip logic and consumed 114 mW of power. Appl. Sci. 2018, 8, 1414 13 of 17

Table 3. Synthesis results of CEHBC digital blocks.

CEHBC Tx field Programmable Gate Array (FPGA) Block CEHBC Rx FPGA Block (CEHBC-Tx Interface (TXIF) & CEHBC-Tx Digital (TXD)) (CEHBC-RXD & CEHBC-RXIF) field programmable Actel IGLOO Xilinx Artix 7 gate array (FPGA) AGLN125V5-CS81 XC7A50T-CSG324 synthesis and Libero v11.8 Vivado 2015.3 Download tool CEHBC-TXIF Core: 1084 of 3072 (35.29%) Resource Utilization (LUT) logic size CEHBC-TXD Core: 521 of 3072 (16.96%) 5231 of 32,600 (16.05%) input/output voltage LVCMOS 3.3V LVCMOS 3.3V level CEHBC-TXIF:2.63 mW power consumption 114 mW CEHBC-TXD:3.71 mW main clock frequency 32 MHz 32 MHz

The BER results obtained from the saline tank experiment are as follows. The diameter size of the water tank is 22 cm and height is 30 cm. The signal used in the test was the known-pattern image signal of Figure 10b (right). The variables that could control the noise were set to a direction and a distance between the electrode and the capsule endoscope. The directions of the capsule varied for each electrode (E1–E8), and the BER performance was measured according to the distance between the electrode and the capsule. Each BER performance showed the error result from receiving 100 images with 480 × 480 pixels. Table4 represents the results of BER performance of CEHBC system. When the interval was 6 cm/11 cm, the image data was received without error. We confirmed that it was possible to receive images in all directions. When the gap was less than 1 cm, the distance between the electrode and the capsule was too close and led to the saturation condition of the signal, and made BER performance poor (>0.5). In actual clinical trials, such a condition may occur when the gap between the organs and the skin is small, such as for a child, and therefore algorithms developed in the future for processing a large received signal should be considered.

Table 4. BER performance of the CEHBC system.

Distance between Electrodes The Direction of the Capsule Endoscope towards the Electrode and Capsule Endoscope E1 E2 E3 E4 E5 E6 E7 E8 6 cm 0 0 0 0 0 0 0 0 11 cm 0 0 0 0 0 0 0 0

Table5 displays a comparison of the performance of the proposed CEHBC-TXD and the existing WCE transmitters. The CEHBC-TXD presented in this paper utilizes the HBC communication scheme, while the other WCE transmitters employ a RF communication scheme using another frequency band. Because it uses HBC communication implemented mostly by digital logic, the CEHBC system has low power consumption (3.71 mW). The proposed CEHBC-TXD can support a relatively high data rate while using its clock a few tens of MHz slower. The energy consumption figure associated with the power and transmission data rate also has the small value of 0.616 nJ/bit. In addition, the results show the benefits from a high-resolution image of 480 × 480 bytes. Because the proposed method does not use an image compressor, if the data rates after image compression are compared in the structure of the papers written in the table, it can be confirmed that the data rate is the highest reported, with respect to the operating carrier frequency. The total power consumption of the CEHBC capsule endoscope is 57.5 mW. The FPGA consisting of CEHBC-TXIF and CEHBC-TXD consumed 6.34mW to transmit the image data. The remaining blocks such as CMOS image sensor and LED etc. used 51.16 mW. Compared with the capsule endoscope proposed in [10], it can be seen that the total consumed power of the capsule of [10] is 90 mW, since Appl. Sci. 2018, 8, 1414 14 of 17 three 1.5 V coin batteries are used in series and the total consumed current is 20 mA. The FPGA consumed power in the capsule endoscope of [10] is 12.42 mW. This comparison confirms that the CEHBC capsule endoscope is not only capable of high resolution image transmission, but also low power operation.

Table 5. Performance comparison of wireless capsule endoscopy (WCE) transmitters.

2007 [3] 2013 [2] 2015 [10] This Application Capsule Endoscopy data rate before image 1 Mbps 3~20 Mbps 8 Mbps 6 Mbps compression image compression X (lossy) (lossless/lossy) X compression ratio – 93% (1:15) >75% (1:4) – data rate after image 1 Mbps 1.34 Mbps 2 Mbps 6 Mbps compression communication RF RF RF HBC carrier frequency 2.4 GHz 920/925 MHz 2.4 GHz 32 MHz Quadrature Gaussian Minimum Frequency Amplitude Shift Phase Shift Selective Digital modulation Keying Shift Keying Transmission (ASK) Keying (GMSK) (FSDT) (QPSK) FPGA CMOS CMOS technology – 130 nm 250 nm 180 nm (AGLN125V5) power consumption 7.925 mW 2.5 mW – 3.7 mW energy consumption 7.925 nJ/bit 1.865 nJ/bit – 0.616 nJ/bit 320 × 240 resolution – 640 × 480 160 × 120 480×480 128 × 96 frame rate – 3 f/s 55 f/s 3.13 f/s

Table6 shows the performance comparison of the capsule endoscope using HBC. Both the MiroCam capsule and the CEHBC capsule have commonalities in using the HBC to transmit baseband signals without the RF transmitter. However, the following differences exist. The main difference between the MiroCam capsule and the CEHBC capsule is that image resolution is increased. The CEHBC capsule uses a transmission method capable of image data transmission with 480 × 480 pixel, which is more than twice as much as the 320 × 320 pixel image data. This enhancement is made possible by the use of modulation methods such as the dual mode FSDT, rather than simply increasing the sampling frequency or hardware complexity. Therefore we think that the CEHBC capsule using the FSDT structure proposed in this paper is more efficient in terms of power consumption and price. The CEHBC capsules are larger and heavier than the MiroCam capsule. This is because a commercial image sensor PIXELPLUS PO8030D (digital image sensor, 2010, PIXELPLUS, Suwon-si, South Korea) is used for transmitting the 480 × 480 image data. Since it is a general-purpose image sensor product, it consumes a large amount of power and uses a large-capacity battery. Therefore, the total power of the implemented CEHBC capsule is large because of using the PO8030D image sensor. However, there is a possibility that the consumption power of the capsules will also decrease if the image sensor for capsules is used later. Appl. Sci. 2018, 8, 1414 15 of 17

Table 6. Performance comparison of the capsule endoscope using human body communication (HBC).

Specification MiroCam [1,22] CEHBC Tx Capsule communication human body communication customized CMOS image sensor PIXERPLUS PO8030D VGS single chip CMOS image sensor with 320 × 320 pixel array) image sensor with 640 × 480 pixel array pixel 102,400 (=320 × 320) 230,400 (=480 × 480) frame rate 3 frames/s 3.13 frame/s data rate 3 Mbps 6 Mbps field of view 150 degrees – battery – 3 V, 160 mAh Coin battery battery life 11 h 8.3 h lighting 6 LEDs 6 LEDs size 11 × 24 mm 13 × 27 mm weight 3.4 ± 0.05 g 6.1 g

5. Discussion The methods presented in this paper have the merit of implementing the capsule endoscope configured using only digital logic. The presented capsule endoscope was implemented using the FPGA, and has the properties that the I/O voltage was 3.3 V and core voltage was 1.5 V. Therefore, it is possible to fabricate low-power capsule endoscopes using an advanced process with an operating voltage rather lower than that of FPGA. This architecture has the additional advantage in that its processes are less affected because it is implemented as digital logic. As can be seen from comparison with other capsule endoscopes, the proposed method can support a relatively high data rate in spite of using its clock a few tens of MHz slower. In the future, it is predicted that image data with a higher data rate could be transmitted if the image compression method is used. Capsule endoscopy is currently used to diagnose the condition of the small intestine. To modern people, it is more important to judge the disease in digestive organs such as the large intestine or stomach, than to judge the disease in the small intestine. There is also a growing demand for next-generation capsule endoscopes to meet these needs. Because the large intestine or stomach has a larger space than the small intestine, this space becomes an environment in which the capsule endoscope moves rapidly and changes its position frequently [8]. As the movement time of the capsule is fast, the ability to transmit high resolution images at high speed is required. Due to a larger space than the capsule, the optimal reception channel selection and real-time acquisition function are required according to the capsule position change. The algorithm and architecture presented in this paper would be suitable for solving the requirements of next-generation capsule endoscopes. The use of the capsule endoscopy for treatment as well as diagnosis is being studied. The capsule endoscope performs phototherapeutic treatment that illuminates blue light corresponding to Helicobacter bacteria according to pH sensing, and measures the state of the stomach [27]. These capsules still use the wireless communication system. The proposed HBC methods can be used to change this system to one that is smaller size and lower in power consumption. The technique presented in the paper is also applicable to the newly proposed medication management device [28]. We consider that it can be variously applied to new devices that transmit the bio-related information in the body in real time.

6. Conclusions In this paper, we proposed a new, low-power capsule endoscopy system with high data rate, using HBC technology. The proposed CEHBC system uses three kinds of discrimination. The first technique is FSDT, which was used to support the low speed mode and high speed mode to increase the data transmission rate. The CEHBC system has benefits in that the receiving signal has a spread effect with the FSDT modulation that makes it less susceptible to wideband noise. The second technique involves Appl. Sci. 2018, 8, 1414 16 of 17 the use of an algorithm to select the best combination of electrodes among the plurality, to select the best channel condition. The third technique involves the use of the LineSync algorithm to determine data synchronization and polarization to improve the reception accuracy of the data, even for long data transfer sections. The methods presented in this paper have the merit of implementing the capsule endoscope configured only as digital logic. The presented capsule endoscope was easily implemented using the FPGA device. Therefore, it is possible to fabricate lower power capsule endoscopes using advanced processes with lower operating voltage rather than of the FPGA. This architecture has an additional advantage that it is less affected by process variation because it is implemented as digital logic. As can be seen from comparison with other capsule endoscopes, the proposed method can support a relatively high data rate in spite of using its clock a few tens of MHz slower. In the future, it is predicted that image data with a higher data rate could be transmitted if the image compression method is used. The CEHBC-TXD system provides low power (3.7 mW) and high data rate (6 Mbps) performance while it supports high resolution images of 480 x 480 bytes at 3.13 f/s.

Author Contributions: All authors contributed to developing the measurement equipment, designing and performing measurements, data analysis, scientific discussions, and writing the paper. Funding: This work was supported by Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government (18ZB1200, Development of Core Technologies for Implantable Active Devices). Acknowledgments: Thank you for the technical support of the Intromedic Corporation. Conflicts of Interest: The authors declare no conflict of interest.

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