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Measuring the Transmission Characteristic of the Human Body in an Electrostatic-Coupling Intra Body Communication System Using a Square Test Stimulus

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(1)IEICE TRANS. FUNDAMENTALS, VOL.E93–A, NO.3 MARCH 2010. 664. LETTER. Measuring the Transmission Characteristic of the Human Body in an Electrostatic-Coupling Intra Body Communication System Using a Square Test Stimulus Yuhwai TSENG†∗a) , Chauchin SU†† , Nonmembers, and Chien-Nan Jimmy LIU† , Member. SUMMARY This study employs a simple measurement methodology that is based on the de-convolution of a square test stimulus to measure the transmission characteristics of the human body channel in an electrostaticcoupling intra body communication system. A battery-powered square waveform generator was developed to mimic the electrostatic-coupling intra body communication system operating in the environment of the ground free. The measurement results are then confirmed using a reliable measuring method (single tone) and spectral analysis. The results demonstrate that the proposed measurement approach is valid for up to 32.5 MHz, providing a data rate of over 16 Mbps. key words: square waveform, deconvolution, intra-body communication. 1.. Introduction Fig. 1. Intra-body commutation (IBC) is a wireless scheme in which the human body functions as a transmission medium [1], [2]. IBC systems are divided into two categories - electrostatic coupling (ESC) and electromagnetic waveguide (EMW) systems. The ESC system is a ground free system in which the environment provides the signal return path. An EMW system produces electromagnetic waves using two electrodes, and treats the human body as a waveguide through which to transmit signals. Figure 1 schematically depicts a simplified circuit model of the ESC IBC system. The transmitter and receiver with different battery-powered sources employ the electrode of a positive terminal to connect on the human body, respectively, and the electrode of a negative terminal remains ground free. Where RL denotes the load resistor of the receiver and the output resistor of the transmitter is sufficiently small to be neglected. Additionally, GndT and GndR are the ground of the transmitter and the receiver, respectively. Since GndT  GndR , a signal return path from the GndT and GndR through the environment to the earth ground is represented by using capacitors CGT and CGR . Accordingly, the capacitors CGT and CGR degrade the transmitted signal quality, especially for low frequency band signal. Here, assume Manuscript received October 2, 2009. Manuscript revised November 16, 2009. † The authors are with Electrical Engineering Department, National Central University, No.300, Jung-Da Rd, Jhong-Li City, Taiwan 320, R.O.C. †† The author is with the Department of Electrical and Control Engineering, National ChiaoTung University, No.1001, Ta-Hsueh Rd., Hsin-Chu City, Taiwan 300, R.O.C. ∗ Corresponding author. a) E-mail: s0541004@cc.ncu.edu.tw DOI: 10.1587/transfun.E93.A.664. Simplified circuit model of the ESC IBC system.. that CGT = CGR = CG . Since the capacitor CG [1] typically range between several hundreds fF and several pF, which are smaller by several hundred times than most of all the capacitors associated with the human body [3], [4], in which the human body can be replaced by a simplified equivalent impedance RB [4]. Hence, the ESC IBC system becomes a high pass system with a transfer function shown in Eq. (1a) and a high pass 3 dB frequency fh3 dB shown in Eq. (1b). H(s) =. RL VL (s)  × VT (s) RL + RB. fh3 dB =. 1 . πCG (RL + RB ). 1 1+. 2 sCG (RL + RB ). (1a). (1b). Figure 2 depicts the simplified model of two measurement methods conventionally used to determine the channel characteristics of the ESC IBC system. Figure 2(a) shows a grounded measurement [5]–[7], in which both the instrument waveform generator and the measuring instruments share a common ground from the power line ground. Figure 2(b) illustrates an ungrounded measurement [8]–[11], in which the output of the instrument waveform generator is isolated from the power line ground by using a high series impedance (typically 1 MΩ in parallel 45 nF smaller than the reactance of the capacitor CG in Fig. 1). All of the above measuring approaches have a signal return path provided from the power line of the instrument. Such a signal return path is not present in the ESC IBC systems shown in Fig. 1. Therefore, certain model mapping must be done to transform the channel model with physical metal wire as the ground return loop to the one that uses the. c 2010 The Institute of Electronics, Information and Communication Engineers Copyright .

(2) LETTER. 665. To obtain H(s), divided the output VO (s) by the input VI (s). Such an operation in the time domain is called deconvolution. H(s) =. VO (s) . VI (s). (4). 2.2 Square Test Stimulus A square waveform comprises multiple frequency components or harmonics. It allows information to be obtained over a wider frequency range from a single measurement. A square waveform of amplitude A0 , period T p , and a duty cycle xd can be expressed as in Fourier series: |V(n)| = xd × Ao × sinc (nπxd ) , (a) Fig. 2. (b). (a) Grounded measurement. (b) Ungrounded measurement.. environment as the ground return loop. This work develops the use of a square wave as the test stimulus to simplify the measuring procedure. A batterypowered square waveform generator is applied to generate the test stimulus that imitates the environment of the ESC IBC system. De-convolution skill measures channel transmission characteristic, obtaining both amplitude and phase responses. Twenty-two single-tone signals with fundamental harmonics with high SNR are employed to confirm the proposed measurement method. The current study utilizes the obtained and confirmed channel characteristic in a wideband IBC transmission system that directly transmits the baseband signal to simplify the circuit design and reduce the power consumption. 2.. De-Convolution of Square Test Stimulus. Sinusoidal waveforms of various frequencies are conventionally adopted to measure the frequency response of a channel. However, generating numerous sinusoidal waveforms of various frequencies is difficult, especially in a ground-free environment. This study proposes the use of a square wave as the test signal because it contains multiple frequency components and is easy to implement. It employs de-convolution to extract the frequency response.. n = 1, 3, 5, 7, . . . .(5). where |V(n)| is the amplitude of the nth harmonic. For an ideal square wave with a 50% duty cycle, Eq. (5) is simplified as |V(n)| =. Ao , nπ. n = 1, 3, 5, 7 . . . .. (6). The human body is a noisy channel. To ensure that measurements are sufficiently accurate, the effect of noise must be taken into account. A human body can be modeled as an impulse response h(t) with an additive noise source vBn (t). The output signal is expressed as vo (t) = vi (t) ⊗ h (t) + vBn (t).. (7). where vi (t) is the applied square wave. In frequency domain it is VO (n) = VI (n) H (n) + VBn (n) .. (8). Here, where VBn (n) is the noise amplitude at the frequency of the nth harmonic. The corresponding signal-to-noise ratio (SNR) is SNR(n) =. Ao /nπ VI (n) H (n) = . VBn (n) VBn (n). (9). For an acceptable SNR level and a known noise floor VBn (n), the maximum number of applicable harmonics is n= 3.. Ao /π . SNR(n) · VBn (n). (10). Experimental. 2.1 De-Convolution In the frequency domain, the output is the product of the input and the transfer function. Given the system transfer function H(s), the output is VO (s) = VI (s) × H(s).. (2). In the time domain, the corresponding operation is the convolution operation. vo (t) = vi (t) ⊗ h(t).. (3). Figure 3 depicts the experimental setup. A battery-powered square wave generator is placed on the left wrist (Point 1) and an Agilent 54382D oscilloscope is connected to either the left arm 40cm away (Point 2) or the right wrist 1.5 m away (Point 3). The measured human body sample is 1.75 m height and weights 70 kg. The stainless steel electrodes connect the human body to both the square wave generator and the oscilloscope. A load resistor switch is employed to change the resistance of the load resistor RL from 50 Ω to 50 kΩ..

(3) IEICE TRANS. FUNDAMENTALS, VOL.E93–A, NO.3 MARCH 2010. 666. Fig. 3. Block diagram of the experimental setup.. method with those obtained using single tone test procedures. For single tone measurement, square waves of 22 frequencies between 400 kHz and 22.5 MHz were applied. The channel response is the output fundamental harmonic divided by the input fundamental harmonic. When only the fundamental harmonics are taken into account, the SNR is much higher and the measurements are more precise. No current flowed through the tested human body when the test stimulus frequency was less than 1 kHz. The maximum power dissipated in the tested human body with a highfrequency test stimulus was 0.144 uW/kg. This experiment satisfies the limit on DC current of 50 μA (rms) at frequencies of up to 1 kHz, which was recommended by the International Electrotechnical Commission (IEC) [13] and the Association for the Advancement of Medical Instrumentation (AAMI) [14], and the basic limit on exposure of 0.08 W/kg, recommended by the World Health Organization (WHO) [15]. 4.. Fig. 4. Architecture of the battery-powered square wave generator.. Figure 4 presents the architecture of the proposed battery-powered square wave generator that is made from a ring oscillator that consists of 74AC04N inverters to imitate an ESC IBC system operating in a ground-free environment. It is buffered by CD4009UBE to produce 18 V square wave with a 46% duty cycle. A capacitor and a variable resistor are utilized to tune the oscillation frequency up to 22.5 MHz. The experimental is performed in following four steps: 1. The square wave generator output is measured to obtain the stimulus signal vi (t). 2. The body output signal vo (t) is then measured. 3. The Matlab tool is employed to translate the measured signals from the time domain to the frequency domain. 4. The de-convolution of VO (s) by VI (s) is performed to yield H(s) using Eq. (4). In this study, for an expected SNR of 30 dB under the noise floor, −67 dB, of the human body, the maximum number of harmonics n calculated using Eq. (10) was 360. In the de-convolution of square waves, 20 kHz and 100 kHz square waves are utilized to reconstruct the channel response H(s) from 20 kHz to 6.1 MHz with a frequency spacing of 40 kHz (applied harmonics n = 305) and from 6.1 MHz to 32.5 MHz with a frequency spacing of 200 kHz (the applied harmonics n = 325), respectively. The investigation compares the results of the proposed. Results and Discussion. Figures 5 and 6 plot the verification results from the left wrist to the left arm and from the left wrist to the right wrist. The load resistor varies from 50 Ω to 50 kΩ. Table 1 summarizes fh3 dB of various measurements. Measurement results reveal that a capacitor CG exists in the measurement system in order to construct a signal return path from the batterypowered waveform generator and the measuring instrument to the earth ground respectively which mimics the ESC IBC system described in Sect. 1 and Fig. 1. Comparing Fig. 5 and Fig. 6 reveals that the system gain measured from the left wrist to the left arm exceeds that from the left wrist to the right wrist. This finding implies that the body impedance RB from the left wrist to the left arm is smaller than that from the left wrist to the right wrist. Hence, the body impedance RB is proportional to the distance between the measurement points. All cases both in Figs. 5, 6 and Table 1 indicate that the human body channel exhibits a high pass function whose fh3 dB is inversely proportional to the load resistor RL and the body impedance RB . Experimental results are consistent with the description in Sect. 1 and Eq. (1). Hence, the RL value can be manipulated to transform the transmission characteristic of the ESC IBC system. The maximum variation among the verification results is less than 2 dB. The traditional measurement procedures are simplified by de-convolution of a square test stimulus. With the measured channel transmission characteristic, the implementation of baseband intra-body communication without modulation and demodulation scheme is feasible, and reduces hardware overhead and power consumption. 5.. Conclusions. This study develops a simplified method for measuring the transmission characteristic of the ESC IBC system. The square test stimulus simplifies the measurement procedures.

(4) LETTER. 667. Fig. 5. Verification results from the left wrist to the left arm.. Fig. 6. Verification results from the left wrist to right wrist.. by as it constitutes numerous sinusoidal test stimuli. A battery-powered square waveform generator is employed to mimic the operating environment of the ESC IBC system. The proposed measuring method is verified using only the fundamentals of square waves from 400 kHz to 22.5 MHz. with a load resistance from 50 Ω to 50 kΩ. The results show that the proposed model and measurement methodology are valid for 32.5 MHz and beyond, representing a data rate of over 16 Mbps..

(5) IEICE TRANS. FUNDAMENTALS, VOL.E93–A, NO.3 MARCH 2010. 668 Table 1. fh3 dB of various measurements.. Acknowledgment The authors would like to thank the Veterans Affairs Commission and the National Science Council of the Republic of China, Taiwan, for financially supporting this research. Ted Knoy is appreciated for his editorial assistance. References [1] T.G. Zimmerman, “Personal area network: Near-field intrabody communication,” IBM Syst. J., vol.35, no.3&4, pp.609–617, 1996. [2] E.R. Post, M. Reynolds, M. Gray, J. Paradiso, and N. Gershenfeld, “Intra-body bus for data and power,” Proc. 1st international Symposium on Wearable Computers, IEEE Comp. Soc. Press, pp.52–55, 1997. [3] N.V. Thakor and J.G. Webster, “Ground-free ECG recording with two electrodes,” IEEE Trans. Biomed. Eng., vol.BME-27, no.12, pp.699–704, Dec. 1980. [4] J.G. Webster, Medical instrumentation: Application and design, pp.194–200, p.248, Wiley, 1998. [5] T. Handa, S. Shoji, S. Ike, S. Takeda, and T. Sekiguchi, “A very lowpower comsumption wireless ECG monitoring system using body as a signal transmission medium,” 1997 International Conference on Solid-State Sensor and Actuator, pp.1003–1006, Chicago, June. 1997. [6] J.A. Ruiz and S. Shimatoto, “Experimental evaluation of body channel response and digital modulation schemes for intra-body communication,” 2006 IEEE International Conference on Communications, vol.1, pp.349–354, June 2006. [7] J.A. Ruiz and S. Shimatoto, “Novel communication services based on human body and environment interaction: Application inside trains and applications for handicapped people,” 2006 IEEE International Conference on Communications, vol.1, pp.2240–2245, June 2006. [8] K. Hachisuka, A. Nakata, T, Takeda, Y. Terauchi, K. Shiba, K. Sasaki, H. Hosaka, and K. Itao, “Development and performance analysis of an intra-body communication device,” Tech. Digest, 12th Int. Conf. Solid-Stae Sens. Actuat. Microsystems, pp.1722–1725, Boston, MA, USA, June 2003. [9] K. Hachisuka, Y. Terauchi, Y. Kishi, T. Hirota, K. Sasaki, H. Hosaka, and K. Ito, “Simplified circuit modeling and fabrication of intrabody communication devices,” 13th International Conference on SolidState Sensors, Actuators and Microsystems, Digest of Technical Papers, TRANSDUCERS’05, vol.1, no.5-9, pp.461–464, June 2005. [10] J.A. Ruiz and S. Shimatoto, “A study on the transmission characteristic of the human body towards broadband intra-body communications,” Consumer Electronics, 2005. (ISCE 2005). Proc. Ninth International Symposium on, pp.99–104, June 2005. [11] K. Ito and K. Fujii, “Development and investigation of the transmission mechanism of the wearable devices using the human body as a transmission channel,” 2006 IEEE International Workshop on Antenna Technology Small Antennas and Novel Metamaterials, pp.140–143, March 2006. [12] N. Cho, J. Yoo, S.-J. Song, J. Lee, S. Jeon, and H.-J. Yoo, “The human body characteristics as a signal transmission medium for intrabody communication,” IEEE Trans. Microw. Tech., vol.55, no.5, pp.1–7, May, 2007. [13] International Electrotechnical Commission, Medical Electrical Equipment — Part 1: General Requirements for Safety, 2nd ed. IEC 601-1. Geneva: IEC, 1988.A. [14] Association for the Advancement of Medical Instrumentation, Safe Current Limits for Electromedical Apparatus, ANSI/AAMI ES1 — 1985. Arlington (Vir.): AAMI, 1985. ISBN 0-910275-50-5. [15] World Health Organization, “Electromagnetic fields (300 Hz to 300 GHz),” 1993, www.inchem.org/documents/ehc/ehc/ehc137.htm.

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