The Signal Transmission Mechanism on the Surface of Human Body for Body Channel Communication

Transcription

1 582 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 3, MARCH 2012 The Signal Transmission Mechanism on the Surface of Human Body for Body Channel Communication Joonsung Bae, Student Member, IEEE, Hyunwoo Cho,StudentMember,IEEE,KiseokSong, Student Member, IEEE, Hyungwoo Lee, Student Member, IEEE, and Hoi-Jun Yoo, Fellow, IEEE Abstract The signal transmission mechanism on the surface of the human body is studied for the application to body channel communication (BCC). From Maxwell s equations, the complete equation of electrical fieldonthehumanbodyisdevelopedtoobtaina general BCC model. The mechanism of BCC consists of three parts according to the operating frequencies and channel distances: the quasi-static near-field coupling part, the reactive induction-field radiation part, and the surface wave far-field propagation part. The general BCC model by means of the near-field and far-field approximation is developed to be valid in the frequency range from 100 khz to 100 MHz and distance up to 1.3 m based on the measurements of the body channel characteristics. Finally, path loss characteristics of BCC are formulated for the design of BCC systems and many potential applications. Index Terms Body channel communication, electric field communication, far-field propagation, general model, human body communication, intra-body communication, near-field coupling, on-body transmission, quasi-static, surface wave, transmission mechanism. I. INTRODUCTION WIRELESS BODY AREA NETWORK (WBAN) is an emerging technology that can combine health care and consumer electronic applications around the human body. By continuously connecting and sharing the information of mobile devices around the human body, WBAN allows new convenient usages and application services. There are 3 physical layer(phy) schemes discussed in the IEEE Task Group for WBAN standardization [1]: ultra-wide-band (UWB) PHY, narrowband (NB) PHY, and body channel communication (BCC) PHY. The BCC, which uses the human body as the communication channel to transmit the electric signal, has advantages over UWB and NB duetothehighconductivityofthehumanbodycomparedtothatof air. In addition, not only is most of the signal from the transmitter confined to the body area without interference from external RF devices, but also the communication frequency can be lowered without enlarging the antenna size. These reduce the power consumption of the BCC transceiver compared to the conventional RF approaches [2]. Since the introduction of the first prototype system reported by T. G. Zimmerman in 1995 [3], there have been various studies to investigate the mechanism of sending and receiving data through the human body and to model the body channel. Manuscript received April 15, 2011; revised October 10, 2011; accepted November 28, Date of publication January 16, 2012; date of current version March 02, The authors are with the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon , Korea ( joonsung@eeinfo.kaist.ac.kr; hwcho@kaist.ac.kr; sks8795@eeinfo.kaist.ac.kr; hwlee@eeinfo.kaist.ac.kr; hjyoo@ee.kaist.ac.kr). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TMTT These investigations are different from each other in terms of the operating frequency ranges and its body channel modeling approaches. Currently, two prevailing methods are used for explaining the mechanism of BCC as shown in Fig. 1. One is the capacitive coupling method [3] [6], and the other is the wave propagation method [7] [10]. At a frequency lower than tens of MHz whose wavelength is much larger than the size of the human body, the electric fieldaroundthehuman body is almost constant with time, which means its phase is nearly uniform everywhere on the body. In this condition, the time-varying electric field around the human body can be regarded as a quasi-static field. From the circuit model of [12], the complex unit impedance of the human body can be obtained by unit resistance of 70 and unit capacitance of 31 pf/m. Therefore, the quasi-static assumption simplifies the analysis by ignoring reactive contributions and only considering the resistance on the human body. Furthermore, at a low frequency, the human body has been approximated as a conducting wire, and a complete closed loop should be formed for the signal transmission [3]. Therefore, the return signal is transferred onto human body through capacitive near-field coupling mechanism. Since the closed loop signal path of the electric field is provided by electrostatic coupling to external conductive objects, such as earth ground, the body channel has been modeled with the capacitor and resistor circuits(fig.1(a)).however,the capacitive coupling model is not well matched with channel characteristics at the high frequency and long channel distance. The signal degradation along the human surface that is observed in the measurement cannot be explained in this model. On the other hand, the wave propagation method is used if the frequency is higher than tens of MHz. In this frequency range, the electrical signal attenuates as the signal propagates through the human body (Fig. 1(b)). The signal transmission mechanism has been indirectly modeled by numerical methods, such as finite-element-method (FEM) [8], finite-difference-time-domain (FDTD) method [9], and statistical method [10], to obtain the waveguide and surface wave characteristics. However, the geometric shape and the complex dielectric coefficient of the human body make it difficult to directly analyze the wave equation. Recently, phenomenological and empirical approaches have been used in the channel modeling, such as a circuit-coupled FEM model [11] and a distributed RC circuit model [12]. Such models can be conveniently applied to design the BCC transceiver and provide engineering insight into BCC mechanism, but due to their negligence in the physical mechanism behind the signal transmission on the human body, its application to optimize the BCC system is limited. In summary, previous studies covered only a limited frequency range by limited explanation method. Furthermore, /$ IEEE

2 BAE et al.: SIGNAL TRANSMISSION MECHANISM ON THE SURFACE OF HUMAN BODY 583 Previous BCC Mechanism. (a) Capacitive coupling; (b) Wave propa- Fig. 1. gation. there was not a clear understanding of the on-body electrical signal transmission mechanism. It is because the only phenomenological behavior models were used for the body channel analysis. In this paper, we study the theoretical background to unify all mechanisms and also generally explain all of the mechanisms of electrical signal transmission through the human body. Based on the physical and electromagnetic theory, a general BCC model is developed to analyze the path loss according to frequency and distance for on-body transmission, and to get an engineering insight about BCC. The rest of this paper is organized as follows. Section II gives the general equation about the electrical field strength at any point above the human body, which has finite conductivity and permittivity. From the equation, the on-body transmission mechanism will be explained. Then, the measurement setup and results are shown in Section III. For the general BCC model, the near-field and far-field approximation are found to be valid in all of the frequency and distance range from the measurements in Section IV. The dominant transmission mechanism in terms of wavenumber and communication distance is also discussed. In addition, path-loss characteristics for on-body transmission will be formulated to obtain an engineering insight on the BCC. Finally, Section V concludes the paper. II. ELECTRICAL SIGNAL PROPAGATION MECHANISM ON THE HUMAN BODY As shown in Fig. 1, the concept of BCC is that the signal is applied onto the surface of the human body through signal electrode and GND electrode for sending the information, and then the potential difference, which is generated by the electric field from signal source, is sensed by electrodes contacted on the other side of the body for receiving the information. The potential difference from magnetic field can be ignored since there is no closed loop in the receiving electrodes that magnetic field passes through. The signal source is generalized by a vertical Fig. 2. Electric field from dipole and its geometry. (a) in free space; (b) on the human body. electrical dipole over the human body, and the detector measures the intensity of electrical signal at the remote location on the body. The on-body communication is based on the principle of electric field propagation from dipole source. A. Near-Field and Far-Field Fig. 2(a) shows the electric field from infinitesimal dipole in free space and its geometry. The received electric field is given as follows [13]: where is the wire current in amps, is the wire length in meters, isthewavenumber, is the angular frequency in radians per second, and is the permittivity of free space. Equation (1) contains terms in 1/r,,and. In the near field, the term dominates the equation. As the distance increases, the and terms attenuate rapidly and, as a result, the 1/r term dominates in the far field. In general, 1/r,,and terms correspond to the dominant electric fieldinthefar-field, induction-field, and near-field of the dipole, respectively. These basic concepts are useful for the understanding of the analysis in the electric signal propagation on the human body. B. Theoretical Analysis for BCC To investigate physics behind the BCC, Fig. 2(b) shows the simplified model of BCC and the electric field from infinites- (1)

3 584 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 3, MARCH 2012 imal dipole for calculating electric field intensity at the point above the surface of the human body, which has finite conductivity and permittivity.the,,a,r,x,y,zareadequately definedinfig.2(b).,,,. For the generality and simplicity of the analysis, we assume the surface of the human body as an infinite half-plane with an imperfectly conducting property. To obtain a general solution of Maxwell s equations in Fig. 2(b), the wave potential of a unit vertical dipole and its image dipole is derived by the continuity property of the tangential components from the electric and the normal components from the magnetic field with the boundary surface of the plane. By differentiating the wave potential, the vertical component of electric field intensity from a vertical dipole is given by Norton in [14] and [15] as follows: The first term and the second term of (2) correspond to direct wave (path 1 in Fig. 2(b)) and reflected wave (path 2 in Fig. 2(b)) from dipole (TX) to RX. These two terms are a space wave, given by the inverse-distance terms. In addition, the wave from image dipole to RX (path 3 in Fig. 2(b)) is expressed as third term of (2), and it is a surface wave that contains the additional attenuation function. The last remaining terms correspond to the induction and electrostatic fields of the dipole and its image. The space wave predominates at large distances above the surface, whereas the surface wave is the larger near the surface. Since in the BCC application, we mainly consider the electric signal near the surface of the human body, we can assume,,and, therefore the direct wave and reflected wave cancel each other, which makes space wave to be zero. We can obtain the magnitude of electric field intensity near the human body as follows: (10) (2) (3) (4) (5) (6) (7) (8) (9) where, is an attenuation function, is the coefficient of reflection for a wave with its electric vector in the plane of incidence, wavenumber is, relative conductivity is,and is the dielectric constant of the human body referred to air as unity while is the conductivity of the human body, is the operating frequency, and is the corresponding wavelength. In regard to (1), the electric field intensity in (10) consists of terms of the first order in 1/r, second order in 1/r, and third order in 1/r. The firsttermof(10)correlateswithfar-field propagation in combination with attenuation factor of surface wave,, which is an inherent property of electric field at the surface of the half-plane with finite conductivity. On the other hand, second and third terms correspond to the induction-field radiation and near-field coupling of the dipole, respectively. Consequently, the mechanism of BCC can be divided into three parts: the surface wave far-field propagation of the first term, the reactive induction-field radiation of the second term, and the quasi-static near-field coupling of the third term. The intensity of the electric field is a function of communication distance and wavenumber. As the wavenumber or frequency, and the distance increase, the surface wave propagation term starts to have significant effect on the overall electric field intensity whereas the quasi-static coupling term is negligible, which agrees well with the previous studies in frequency range and operation mechanism (capacitive coupling at a low frequency in [3] and wave propagation at a high frequency in [9]). In addition, surface wave attenuation factor of varies not only with distance and frequency but also with complex permittivity of the human body. The numeric value of attenuation factor is given as a function of frequency in Table I by using conductivity and dielectric properties of human body s dry skin [16] when is fixedto1m.aslistedintablei,from 100 khz to 500 MHz, for the surface of the human body, if value of is far smaller than 1, the numeric value of is almost 1, but otherwise it starts to exponentially decreases, which will be discussed in Section IV.

4 BAE et al.: SIGNAL TRANSMISSION MECHANISM ON THE SURFACE OF HUMAN BODY 585 TABLE I NUMERIC VALUE OF ATTENUATION FACTOR OF SURFACE WAVE C. Path Loss According to Frequency From (10), the electric field intensity is a function of the channel distance and wavenumber which is proportional to the frequency. The path loss of BCC can be expressed as the ratio of received signal to transmitted signal as follows: (11) where, is the communication distance, is the wavenumber of operating frequency, and is the reference distance regarded as transmitting point which is determined by the physical size of the electrode. To represent the path loss characteristics according to the frequency, Fig. 3 shows the frequency response in the frequency range from 100 khz to 1 GHz with respect to various channel distance and by plotting the value obtained from the numerical analysis in Table I. For the infinitesimal dipole, the channel distance is large enough to neglect the reference distance, and the graph in Fig. 3(a) describes the path loss characteristics with to model the infinitesimal dipole. The graph resembles high-pass filter response since the first surface wave term of (10) is proportional to the frequency, and the third quasi-static term of (10) is inversely proportional to the frequency. At a low frequency the third term of (10) is dominant over the firsttermof(10)andvice versa at a high frequency. Therefore, the path loss characteristics could be approximated by means of the frequency range as the following equation: at a low frequency at a high frequency (12) Since is far smaller than 1, as frequency increases, frequency response looks like high-pass filter. In addition, the effect of channel distance is negligible in the low frequency. Meanwhile, as described in Section IV-B, the attenuation factor exponentially decreases with channel distance. Consequently, the frequency response begins to be affected by channel distance at a high frequency. To verify the effect of the attenuation factor at a high frequency and long channel distance, Fig. 3(b) plots the frequency response with various comparable to channel distance. In the case of Fig. 3(b), the graph resembles bandpass filter response due to the effect of the attenuation factor. It is noted that the frequency response of the body channel goes gradually downward with the large and the small and its cutoff frequency is lowered with the large. In brief, for the high frequency and low frequency, surface wave propagation and quasi-static coupling mechanism is dominant, respectively, as (10) shows. In order to represent the amount of contribution of each term to overall terms in (10) as frequency increases, the ratios of near-field part, induction-field part, and far-field part to overall response are shown in Fig. 4 with the channel distance of 1 m. The far-field surface wave term becomes equal to the sum of other two terms

5 586 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 3, MARCH 2012 Fig. 5. Path loss according to distance using theoretical analysis. D. PathLossAccordingtoDistance To describe the path loss according to the channel distance, Fig. 5 shows the path loss graph as channel distance increases from 0.5 m to 2.5 m in regard to frequencies of 30 MHz and 300 MHz whose wavelength is 10 m and 100 m in the free space, respectively, by using numerical value in Table I. The path loss falls off as a function of channel distance as the following equation: at a short distance Fig. 3. Path loss according to frequency using theoretical analysis. (a) with fixed ; (b) with fixed. Fig. 4. Contribution ratio of each mechanism in terms of frequency. when frequency is about 70 MHz, and corresponding value of is about 1.5. Furthermore, induction-field term contributes maximum 30% in the frequency of 50 MHz or value of 1. at a long distance. (13) In terms of (13), for the short channel distance and long channel distance, quasi-static coupling and surface wave propagation mechanism is prevailing, respectively. The graph plotted by shows that in the long distance, the decrement at the high frequency is more remarkable than at the low frequency since as becomes larger than the magnitude of attenuation factor significantly decreases. The signal loss at the low frequency decreases with, and it is found that the ratio of to has significant effect on the path loss at a low frequency. In addition, the path loss with logarithmic scale is superimposed on Fig. 5. It shows that first, the slope for signal attenuation is steeper in short channel distance than long channel distance, and second, the slope in decibel scale is linear, which means that the signal attenuates exponentially, at a long channel distance. The contribution of each mechanism in terms of channel distance is plotted in Fig. 6, which shows the ratio of quasi-static coupling part, reactive radiation part, and surface wave propagation part to overall electric field intensity of (10) by using the frequency of 50 MHz. When is 1.3 m or is about 1.4, the surface wave part makes the same contribution as other two parts. This agrees well with the result of frequency response in the previous section. The induction field term is inversely proportional to square of whereas the quasi-static coupling term is inversely proportional to cube of. Therefore, it prevails against quasi-static coupling term at the long channel distance as shown in Fig. 6.

6 BAE et al.: SIGNAL TRANSMISSION MECHANISM ON THE SURFACE OF HUMAN BODY 587 Fig. 6. Contribution ratio of each mechanism in terms of distance. connected to the battery ground. The signal and ground electrodes constitute the vertical dipole. In the RX board, signal electrode and ground electrode are linked to differential inputs of balun for connecting the signal electrode to the spectrum analyzer and for isolating the ground electrode from the earth ground, respectively. The height of the human subject is 1.8 m. The RX board is attached to the left hand of the human body, and it is apart from the external ground by more than 0.9 m. The spectrum analyzer is connected with a balun at the RX electrode through 50 -matched coaxial cable to measure the signal. The distances between the TX and RX are varied from 10 cm to 130 cm by changing the TX locations from the left arm to the right arm across the chest with 5 cm interval. The length of the coaxial cable is kept short to reduce the antenna effect of the cable. During the measurement, various poses of the subject standing with two arms outstretched, folded at his side, sitting down, etc. are considered, and multiple data are collected repeatedly for the same frequency and distance. III. ELECTRICAL SIGNAL PROPAGATION MEASUREMENT ON THE HUMAN BODY A. Measurement Setup The measurement is carried out to characterize the BCC channel and validate the theoretical BCC analysis in the previous section. To measure the channel characteristics, TX should be able to sweep the frequency band of interest, 100 khz to 100 MHz, with accurate and tunable frequency source. RX needs to quantitatively detect the received power and store the data according to the transmitted signals. Meanwhile, a careful setup is required to measure the path loss according to the frequency and distance of BCC because both of the TX and RX should be isolated from each other and the earth ground or any devices connected or heavily coupled to the earth ground. There exist two possible methods to isolate the TX and RX. One method is to utilize battery-powered TX and RX, which have an independent power source of its own [6]; however, it is hard to sweep the precise frequencies in TX and detect the received signal in RX by using battery sources without measurement equipment, heavily coupled to the earth ground. The other method is to employ a balun with measurement equipment, such as signal generator, spectrum analyzer, and network analyzer. The balun transforms differential signals into single-ended signal for the sake of de-coupling the DC component [11]. Though a balun can conveniently separate TX and RX from earth ground and each other, it can be coupled to the earth ground with capacitance of approximately 2 pf, which is more heavily coupled than real environment. From these observations, to simulate an actual BCC application, a measurement setup in Fig. 7 is used. It consists of battery-powered signal generator with replaceable crystal oscillator as a TX, and a spectrum analyzer with a balun as a RX. Fig. 7(a) shows the overall experimental configuration and Fig. 7(b) represents the configuration of the TX and RX board. The TX board provides frequencies from 100 khz to 100 MHz with 50% duty cycle sine wave by discrete crystal oscillators. A rectangular metal electrode is used to interface the electric signal to the human body in the TX and RX boards, and the ground electrode is B. Path Loss According to Frequency To obtain the frequency response of BCC, the ratio of the received signal to the transmitted signal through the body channel is measured by sweeping the frequency of the crystal oscillator with the spectrum analyzer connected to a balun. The graph in Fig. 8 shows the measured path loss in decibel with respect to the frequency. The effects of the channel distance of 10 cm, 40 cm, and 120 cm are considered together. As explained in Section II-C, the graph shows that below 10 MHz, the body channel is relatively deterministic with a slope of 20 db/dec regardless of the distance since in the low frequency the electric field is relatively constant across the entire human body. In this region, the body channel looks like a high-pass filter caused by near-field quasi-static coupling mechanism. However, beyond 50 MHz, the channel distance has a great effect on the overall path loss because far-field surface wave propagation mechanism starts to influence the channel characteristics. As the channel length increases, the signal attenuation of the surface wave becomes larger and induces larger signal loss. This attenuation becomes more evident at the high frequency. As a result, the path loss of the body channel goes gradually downward and its cutoff frequency is lowered with the distance. The difference between measurement results in Fig. 8 and theoretical analysis in Fig. 3 comes from various reasons because the dielectric parameter (conductivity and permittivity) of the human body and the reference distance could shift the frequency at which the path loss is the maximum. In addition, another reason of discrepancy between measurement and analysis is that we derive the intensity of the electric field through infinitesimal dipole, but the real dipole source in the measurement has a finite dimension. In Section II-B, we derived the electric field on the surface of the human body by neglecting the space wave since the space wave is much smaller than the surface wave near the human body. We also assumed that the surface of the human body is a plane, and therefore surface signal path is the same as spatial signal path between TX and RX. However, as shown in Fig. 7, two signal paths are independent from each other in the real BCC situation. The spatial path can be shorter than the surface

7 588 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 3, MARCH 2012 Fig. 7. Measurement setup. (a) Experimental configuration; (b) TX & RX board. and especially at the channel distance of 120 cm, the wide fluctuation of the measurement data is observed. It comes from the different poses of the human body, which leads to variation of the surface and spatial channel distances between TX and RX. The measurement result in Fig. 9 shows the effect of spatial distance between TX and RX, in the static surface distance of 150cm,withfrequencyof16MHz,50MHz,and80MHz.If the frequency is higher than 50 MHz, as spatial distance decreases from distance of 150 cm, itisfoundthatatleast20db channel variation occurs. However, it should be noticed that surface wave component is more predominant than spatial wave component when two channel distances are same. On the other hand, at the frequency of 16 MHz, spatial distance has negligible effect on the path loss. Fig. 8. Measured path loss according to frequecy. path. In that case, we are not able to ignore the space wave anymore if the operating frequency and surface distance increases. For example, when the surface and spatial distance are the same as 1 m, we can neglect the space wave, however when the spatial distance decreases to 10 cm with the same surface distance at the high frequency, the space wave becomes larger. It can be scattered on the surface wave and may arrive at the RX with shifted phases. The space wave can be superimposed on the surface wave through the body constructively or destructively, and it causes path loss variation at the high frequency and long surface distance. In Fig. 8, at the frequency higher than 50 MHz, C. Path Loss According to Distance The path loss according to the distance of BCC is measured by increasing the surface channel distance from 10 cm to 130 cm with 5 cm interval. Fig. 10 shows the measured path loss in decibel in terms of channel distance. The effects of the operating frequencies, 1 MHz, 33 MHz, and 80 MHz, are depicted together. As mentioned in Section II-D, at the frequency of 33 MHz and 80 MHz, the graph shows two features of attenuation in the near-field and far-field region. It agrees fairly well with (13), which shows the signal attenuation depends on the third order of 1/r in the near-field and the first order of in the far-field. Especially, in the far-field region, the attenuation slope is almost linear, which means the signal attenuates exponentially with channel distance. The graph also shows that the

8 BAE et al.: SIGNAL TRANSMISSION MECHANISM ON THE SURFACE OF HUMAN BODY 589 than 1, but in a low frequency, the communication distance is comparable to the dimension of TX and RX board in Fig. 7. IV. GENERAL BODY CHANNEL COMMUNICATION MODEL Fig. 9. Effect of spatial distance between TX and RX. A. Near-Field Quasi-Static Coupling Approximation Even though we performed theoretical analysis under the assumption that human body is an infinite half-plane, other factors, such as finite curved surface of human body, electrode configuration and structure, and external environment, should be also considered in real BCC situation. They could have influence on coefficient of (10) in the path loss. To verify the applicability of the theoretical analysis to real BCC environment, the data fitting based on the least square method was done to the measurement data. The fitting equation is derived from theoretical equation of (10) by approximating the each mechanism. In thecaseoflowvalueof, the third term of (10) is dominant over other terms because the value of is almost 1. Therefore, if is much smaller than 1, we could approximate electric intensity as the following equation: (14) In the near-field quasi-static approximation, the intensity of electric field is inversely proportional to wavenumber and third order of distance. B. Far-Field Surface Wave Propagation Approximation If is far larger than 1, the first term of (10) becomes dominant. In that case, the far-field surface wave propagation approximation is possible as follows: Fig. 10. Measured path loss according to distance. (15) decrement at the high frequency is dominant over at the low frequency. As explained in the previous section, more fluctuations are observed in the measurement data at the high frequency and long distance. On the other hand, according to theoretical analysis in (12) and (13), at a short channel distance and a low frequency, the path loss is inversely proportional to the third order of distance, however, at the frequency of 1 MHz, channel distance has a negligible effect on the path loss in the short channel distance ( 0.8 m). It is thought that at a low frequency the impedance between signal electrode and the human body is not well matched to spectrum analyzer, therefore more signal attenuation might be measured regardless of its physical principle. Another reason seems that we derive the intensity of electric field via infinitesimal dipole, however the real dipole source used in the measurement in Fig. 7 has a finite dimension. In addition, the path loss derivation of (11) assumes the ratio of to is much smaller Then, the intensity of electric field is now proportional to wavenumber and 1/r, and the attenuation factor can be approximated to more simple form [14] where (16) (17) (18) (19) (20) For the complex permittivity of human body and frequency of the interest, the value of is less than 4.5. In that case,

9 590 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 3, MARCH 2012 is almost 1, and empirical formula: can be approximated as the following (21) As is proportional to, attenuates exponentially with a function of. Consequently, intensity of electric field can be approximated as the following expression: (22) where is the attenuation constant of the surface wave determined by value of in (21). In the far-field surface wave approximation, the intensity of electric field is proportional to wavenumber and inversely proportional to the first order of distance, in combination with exponential decay. C. General BCC Model From the near-field and far-field approximation, the general equation is derived as a function of distance for the sake of fitting the measurement data in terms of weighted sum of each approximation as follows: (23) By means of (23), we can get the path loss characteristics as following simplified form: (24) where (25) (26) (27) It should be noted that because is constant with the distance, A is inversely proportional to wavenumber whereas B and C is proportional to wavenumber. To fit (24) into measurement results, first, the measurement data are plotted in neper scale, which is a logarithmic unit, based on Euler s number, and then linear fitting is done only with data in the far-field region by means of the least square approximation as shown in Fig. 11(a). For example, for the frequency of 30 MHz data set, the slope of linear fit is 1.74 Np/m, and this value can be the coefficient value of C in (27). Fig. 11(b) shows the graph of the path loss in linear scale. By substituting the coefficient of C for the value obtained from Fig. 11(a), data are fitted into (24) for extracting the coefficient of A and B by the least square approximation as well. Fig. 12 shows the coefficient of (24) as a function of frequency from 100 khz to 100 MHz. It is found that coefficients of C and B increase, and coefficient of A decreases as frequency increases, which have a fair agreement with (25), (26), and (27). In addition, in order to compare with the theoretical analysis in Fig. 4 and Fig. 6, the ratios of the first term and the second term of (24) to the overall value with respect to the product of Fig. 11. Data fitting with general equation. (a) Coefficient of C. (b) Coefficient of A & B. wavenumber and channel distance are shown in Fig. 13. The coefficient values in Fig. 12 are adopted to calculate the ratios. The surface wave component becomes equal to quasi-static component at the value of 1.2. This agrees well with the theoretical value in Fig. 4 and Fig. 6. In summary, from the physical analysis of BCC, the near-field and far-field approximation are found to derive general BCC model. The derived model is well matched with real measurement results and leads to the same tendency with theoretical analysis. D. Path-Loss Characteristics To provide engineering insight into the BCC, the path-loss characteristics for on-body transmission is formulated, based on (24) with the coefficient values in Fig. 12. The path-loss versus distance can be represented by (28) where is the path loss in unit of db, and are the loss per unit distance in unit of db/m in the near-field and far-field region, respectively, is the channel distance, is the boundary

10 BAE et al.: SIGNAL TRANSMISSION MECHANISM ON THE SURFACE OF HUMAN BODY 591 TABLE II PARAMETERS OF PATH LOSS Fig. 14. Path-loss characteristics. Fig. 12. Coefficient as a function of frequency. (a) Coefficient of C. (b) Coefficient of A & B. time, the boundary distance between near-field and far-field region decreases. The path-loss characteristics are well matched with the measurement results as shown in Fig. 14. The difference between measurement and path-loss characteristic line is within 10 db. The path-loss formulation in (28) with the parameters in Table II is useful for the BCC system design. At last, it should be noticed that the proposed BCC model, which can be expressed as (24), is general form of the previous BCC models since the general BCC model can be replaced to the previous models if a specific boundary conditions are applied. For example, the electromagnetic equation in [9] is a specific case of the general BCC model in (24) when the value of is substituted for 0, which introduces the following equation: (29) Fig. 13. Contribution ratio of each mechanism in terms of. of the two regions, and and is the path loss at the channel distance of 0.1 m (minimum channel distance) and, respectively. The boundary distance of can be determined by making the quasi-static and surface wave term of (24) equal. The is given by initial condition. Then and can be obtained from substituting the value into (24), and value is simply calculated from the product of and C in (24). Table II summarizes the list of,,,,and at the frequencyof10mhz,30mhz,50mhz,and80mhz.itisnoticed that as frequency increases, the path loss per unit distance in the near-field and far-field region increases, while at the same Likewise, empirical equation in [12] is also a specific case of (24) with the value of 0 as the following expression: (30) In order to examine the validation of the general BCC model, and to compare with previous models [9] and [12], the data fitting based on the least square method is conducted to the measurement data with (24), (29), and (30). Fig. 15 shows the fitting results with the adjacent R-square value which indicates the compatibility of fitting equation. It can be seen that the general BCC model has fair agreement with the measurement data, and is accurate, compared with other two models. In addition, the

11 592 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 60, NO. 3, MARCH 2012 Fig. 15. Comparison with previous Works. general BCC model covers all of frequency and channel distance ranges, and unifies all mechanisms, regardless of explanation methods. V. CONCLUSION The signal transmission mechanism on the surface of the human body is studied for the application to body channel communication, which is one of the most promising energy-efficient candidates to WBAN PHY. We investigate the physics behind the BCC with the theoretical background to unify and generally explain all of the previous mechanisms. We also develop the complete equation of electric fieldonthehuman body, which is composed of the quasi-static near-field coupling, induction-field radiation, and the surface wave far-field propagation terms. Based on the equation, a general BCC model is proposed to analyze the path loss according to frequency and distance for on-body transmission. To validate the theoretical analysis, the BCC characteristics are measured up to frequency of 100 MHz and channel distance of 1.3 m. In addition, the near-field and far-field approximation are found to verify the usefulness for all of the frequency and distance. The dominant transmission mechanism in terms of wavenumber and channel distance is discussed as well. Finally, path-loss characteristics for on-body transmission are formulated to give an engineering insight into BCC. ACKNOWLEDGMENT The authors would like to thank to Dr. N. Cho, Prof. J.W. Ra, and Prof. S.Y. Shin of KAIST for their helpful advices and comments. REFERENCES [1] BodyAreaNetworks(BAN),IEEE802.15,WPANTaskGroup6Nov [Online]. Available: [2] J. Bae, K. Song, H. Lee, H. Cho, and H.-J. Yoo, A 0.24 nj/b wireless body-area-network transceiver with scalable double-fsk modulation, in Proc. IEEE Int. Solid-State Circuits Conf. Dig. Tech. Papers, Feb. 2011, pp [3] T. Zimmerman, Personal Area Networks (PAN): Near-Field Intrabody Communication, Master s thesis, MIT, Cambridge, MA, [4] K.Partridge,B.Dahlquist,A.Veiseh,A.Cain,A.Foreman,J.Goldberg, and G. Borriello, Empirical measurements of intrabody communication performance under varied physical configurations, in Proc. User Interface Softw. Technol. Symp., Nov. 2001, pp [5] M. Fukumoto, M. Shinagawa, K. Ochiai, and H. Kyuragi, A near-field-sensing transceiver for intrabody communication based on the electrooptic effect, IEEE Trans. Instrum. Meas., vol. 53, pp , Dec [6] T. W. Schenk, N. S. Mazloum, L. Tan, and P. Rutten, Experimental characterization of the body-coupled communications channel, in Proc. IEEE Int. Symp. Wearable Comput., Oct. 2008, pp [7] K. Fuji, M. Takahashi, K. Ito, K. Hachisuka, Y. Terauchi, Y. Kishi, K. Sasaki, and K. Itao, Study on the transmission mechanism for wearable device using the human body as a transmission channel, IEICE Trans. Commun., vol. E88-B, pp , Jun [8] A. Nakata, K. Hachisuka, T. Takeda, Y. Terauchi, K. Shiba, K. Sasaki, H. Hosaka, and K. Itao, Development and performance analysis of an intra-body communication device, in Proc.12thInt.Conf.TRANS- DUCER, Solid-State Sensors, Actuators Microsyst. 2003, Jun. 2003, vol. 2, pp [9] J. Wang, Y. Nishikawa, and T. Shibata, Analysis of on-body transmission mechanism and characteristic based on an electromagnetic field approach, IEEE Trans. Microw. Theory Tech., vol. 57, no. 10, pp , Oct [10] J. Ruiz and S. Shimamoto, Statistical modeling of intra-body propagation channel, in Proc. IEEE Wireless Commun. Netw. Conf., Mar. 2007, pp [11] R. Xu, H. Zhu, and J. Yuan, Circuit-coupled FEM analysis of the electric-field type intra-body communication channel, in Proc. IEEE Biomed. Circuits Syst. Conf., Nov. 2009, pp [12] N. Cho, J. Yoo, S. Song, J. Lee, S. Jeon, and H.-J. Yoo, The human body characteristics as a signal transmission medium for intrabody communication, IEEE Trans. Microw. Theory Tech., vol. 55, pp , May [13] C. Capps, Near field or far field?, EDN (Electronic Design News: Aug. 2001, pp [14] K. A. Norton, The propagation of radio waves over the surface of the earth and in the upper atmosphere Part I, in Proc. Inst. Radio Eng., Oct. 1936, pp [15] K. A. Norton, The propagation of radio waves over the surface of the earth and in the upper atmosphere Part 2, in Proc. Inst. Radio Eng., Sep. 1937, pp [16] IFAC Dielectric Properties of Body Tissues. [Online]. Available: niremf.ifac.cnr.it/tissprop Joonsung Bae (S 07) received the B.S. and M.S. degrees from the Department of Electrical Engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2007 and 2009, respectively, where he is currently working toward the Ph.D. degree. He has worked on developing a transceiver for high speed and low power on-chip global interconnects. He also engaged in developing low energy wireless CMOS transceivers for communicating among wearable and implantable devices. His current research interests include low energy transceiver design for wireless body area networks and body coupled electric field communications. Hyunwoo Cho (S 10) received the B.S. degree from the Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2010, where he is currently working toward the M.S. degree. He has worked on developing a low power transceiver for body channel communication. His current research interests include low energy transceiver design for wireless body area networks and analysis of body channel characteristics.

12 BAE et al.: SIGNAL TRANSMISSION MECHANISM ON THE SURFACE OF HUMAN BODY 593 Kiseok Song (S 09) received the B.S. and M.S. degrees from the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2009 and 2011, respectively, where he is currently working toward the Ph.D. degree. His current research interests include developing a wirelessly powered stimulator and body channel analysis for body coupled electric field communications. Hyungwoo Lee (S 10) received the B.S. degree from the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2010, where he is currently working toward the M.S. degree. His current research interests include wirelessly powered stimulator and body coupled electric field communications. Hoi-Jun Yoo (M 95 SM 04 F 08) graduated from the Electronic Department, Seoul National University, Seoul, Korea, in 1983 and received the M.S. and Ph.D. degrees in electrical engineering from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1985 and 1988, respectively. His Ph.D. work concerned the fabrication process for GaAs vertical optoelectronic integrated circuits. From 1988 to 1990, he was with Bell Communications Research, Red Bank, NJ,whereheinventedthe two-dimensional phase-locked VCSEL array, the front-surface-emitting laser, and the high-speed lateral HBT. In 1991, he became a manager of the DRAM design group at Hyundai Electronics and designed a family of fast-1m DRAMs to 256 M synchronous DRAMs. In 1998, he joined the faculty of the Department of Electrical Engineering at KAIST and now is a full professor. From 2001 to 2005, he was the Director of System Integration and IP Authoring Research Center (SIPAC), funded by Korean Government to promote worldwide IP authoring and its SOC application. From 2003 to 2005, he was the full-time Advisor to Minister of Korea Ministry of Information and Communication and National Project Manager for SoC and Computer. In 2007, he founded System Design Innovation & Application Research Center (SDIA) at KAIST to research and to develop SoCs for intelligent robots, wearable computers and bio systems. His current interests are high-speed and low-power Network on Chips, 3-D graphics, Body Area Networks, biomedical devices and circuits, and memory circuits and systems. He is the author of the books DRAM Design (Hongleung, 1996; in Korean), High Performance DRAM (Sigma,1999;inKorean),Low-Power NoC for High-Performance SoC Design (CRC Press, 2008), and chapters of Networks on Chips (Morgan Kaufmann, 2006). Dr. Yoo received the Electronic Industrial Association of Korea Award for his contribution to DRAM technology in 1994, the Hynix Development Award in 1995, the Design Award of ASP-DAC in 2001, the Korea Semiconductor Industry Association Award in 2002, the KAIST Best Research Award in 2007, and the Asian Solid-State Circuits Conference (A-SSCC) Outstanding Design Awards in 2005, 2006 and He is an IEEE fellow and serving as an Executive Committee Member and the Far East Secretary for IEEE ISSCC, and a Steering Committee Member of IEEE A-SSCC. He was the Technical Program Committee Chair of A-SSCC 2008.