Wearable Antennas for Body-Centric Wireless Communications

Koichi Ito and Nozomi Haga

Graduate School of Engineering, Chiba University, Japan E-mail: n_haga@graduate.chiba-u.jp

Abstract In recent years, a study on body-centric wireless communications has become an active and attractive area of research because of their various applications such as e-healthcare, support systems for specialized occupations, personal communications, and so on. Whereas UHF bands are subjects of interest especially in Europe and USA, relatively low frequency bands below several megahertz are of great interest especially in Japan. Hence, all of the prospective frequencies are in an extremely wide range, and an objective idea on how to select a right frequency band for individual applications is required. Currently in our laboratory, we have been studying on frequency dependence of basic characteristics of wearable antennas as well as body-centric wireless communication channels in the range of HF to UHF (3 MHz – 3 GHz). There are experimental, analytical, and numerical ways to clarify the basic characteristics of the antennas and communication channels. In experiments, we have to ensure impedance matching at certain frequency points because of sensitivity limitations; therefore, it is hard to obtain such broadband characteristics. Theoretical analysis is useful to understand physical mechanism; however, complex geometry and motion of the human body cannot be modeled. By contrast to them, numerical simulation can solve complex problems with relative ease. For example, Hall et al. have shown that the dynamics of the human body can be modeled by dividing the motion into several frames, and the simulated results agree with the measured results. Also in our study, observations of channel characteristics are conducted by employing numerical simulations. In this paper, firstly, electric field distributions around the human body wearing a small top-loaded monopole antenna are numerically calculated and compared in a wide range of HF to UHF bands. Then, received open voltages at receiving antennas which are equipped at several different points on the human body are numerically investigated. The received open voltages are also numerically calculated and compared with several different postures of the human body. Statistic characterization and experimental validation will be necessary in further studies.

Introduction

In recent years, body-centric wireless communications have become an active area of research because of their various applications such as e-healthcare, support systems for specialized occupations, and personal communications [1]–[7]. Whereas UHF bands are subjects of interest especially in Europe, relatively low frequency bands below several megahertz are of great interest especially in Japan [4], [5]. Hence, all of the prospective frequencies are in an extremely wide range, and an objective idea on how to select frequency for individual applications is required. There are experimental, analytical, and

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numerical ways to clarify the channels. In experiments, we have to ensure impedance matching at certain frequency point because of sensitivity limitations; therefore, it is hard to obtain broadband characteristics. Theoretical analysis is useful to understand physical mechanism; however, complex geometry and motion of the human body cannot be modeled. By contrast to them, numerical simulation can solve complex problems with relative ease. For example, Hall et al. have shown that the dynamics of the body can be modeled by dividing the motion into several frames, and the simulated results agree with the measured results [6]. Also in our study, observations of channel characteristics are conducted by employing numerical simulations.

Numerical Modeling of Antennas and Human Body

Since extremely wide frequency range is used in this study, an antenna should remain as simple and identical as possible over the range. For this requirement, a typical top-loaded monopole antenna shown in Fig. 1 is used both for transmitting and receiving. The antenna has low-profile dimension (4-mm thick) and has no matching structure; thus, impedance matching is not considered here. Below the first resonant frequency at around 4 GHz, only the capacitor-like mode is dominant, and so the EM field outside the antenna is mostly determined by current flowing on the feeding probe. Therefore, the radiation pattern is omnidirectional for any frequency over the range. The metallic bodies are represented by perfect electric conductor. A transmitting antenna is excited by use of a voltage source with 50-Ω internal resistance. Terminals of receiving antennas are open-ended in order to observe open voltages. Fig. 2 shows the postures we prepared this time. They were created by employing the 3-D graphics software Poser® [8], and have 1.72-m tall 2-mm resolution. A transmitting antenna is equipped at abdomen, and seven receiving antennas are equipped together at back, right and left chests, ankles, wrists, and ears, respectively. There is a 4-mm separation between the antennas and the body surface. We assume that the human body model has homogeneous and muscle-equivalent tissue. The dielectric dispersion of the tissue was approximated by the single-pole Debye equation

01

)(ωεσ

ωτεεεωε

jjs

r ++

−+= ∞∞ (1)

where ε0 is the free-space permittivity, εs is the static permittivity at zero frequency, ε∞ is the optical permittivity at infinite frequency, τ is the relaxation time, and σ is the static conductivity. Since the Debye equation cannot fully approximate the actual value of muscle over the range, the parameters are individually defined on three ranges of 3–30 MHz, 30–300 MHz, and 300 MHz–3 GHz, so as to be continuous at boundaries. Discrepancy between the literature [9] and approximated values is within ±10% over the whole frequency range. Some other parameters were also individually determined at each range as summarized in TABLE I. A time step of 3.84 × 10−12 s is used to satisfy the stability condition. Then, the FDTD calculations were conducted separately at each range.

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Top view 12

12

4

6

Unit: [mm]

Feed point

4

Human body

Side view

yx

Standing Walking Hand-raising

TX

RX

RX

RX

RX

z

Fig. 1 Geometry of the antenna. Fig. 2 Postures of the human body model.

Table 1 Computational parameters for respective frequency ranges. 3–30 MHz 30–300 MHz 300 MHz–3 GHz

Static permittivity εs 706 101 58.3 Optical permittivity ε∞ 81.0 57.0 35.5

Relaxation time τ 3.98 × 10−8 3.18 × 10−9 3.54 × 10−11 Static conductivity σ 0.52 S/m 0.63 S/m 0.75 S/m

Mesh size 2–20 mm 2–5 mm Computational domain 1.4 m × 1.4 m × 2.3 m 1.0 m × 1.0 m × 2.1 m Absorbing boundary

condition 20-layer perfectly matched layer

(PML) 10-layer PML

Results and Discussion In order to grasp the total picture, Fig. 3 (a)–(d) shows the calculated electric field distributions inside and around the standing model in free space at 3, 30, 300 MHz, and 3 GHz, respectively. The observation planes are the x-z plane which includes the feed point. Here equal input power is assumed to clarify efficiency. Comparing the distributions at 3 and 30 MHz, shapes of the contour line are almost same; however, the levels at 30 MHz are lower about 20 dB. Radiation efficiencies at 3 and 30 MHz are almost zero. That is due to a fact that at low frequency, the field around the human body mainly consists of the electrical stored energy. At 300 MHz, the overall levels are similar to the levels at 30 MHz, but several dips are generated and radiation efficiency increases to about 6%. When frequency is 3 GHz, the levels at back of the body are relatively low, but radiation efficiency reaches 48%. Furthermore, many ripples are generated all over the body except around the transmitting antenna.

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0.50−0.5

−1.0

−0.5

0.0

0.5

z[m]

x [m](a) At 3 MHz (b) At 30 MHz

0.50−0.5x [m]

0.50−0.5x [m]

(c) At 300 MHz (d) At 3 GHz

0.50−0.5x [m]

−80 −60 −40 −20 0Magnitude of electric field [dB]

Fig. 3 Electric field distributions around the standing model in free space. Fig. 4 (a)–(e) plots the frequency dependence of the received open voltages at various receiving antennas. In each figure, results with the standing, walking, and hand-raising models are compared. For the symmetrical structure of the standing model, only the results at left side of the body are indicated. Regarding the other two postures, the results at right and left side of the body are indicated as “Walking (R)” and “Walking (L).” Ac-cording to these results, the received levels at the chests are the most stable at any posture and frequency. The level at the back is the worst in these figures, and there are many ripples above 100 MHz. By contrast, in the other three graphs (Fig. 4 (c)–(e)), several non-periodic patterns are found. The received levels at the ankles are relatively stable, but several nodes are generated at around 100 MHz. In addition, when the model is walking, one more dip is generated at around 2.5 GHz. According to Fig. 4 (d), the received levels at the wrists significantly vary with the posture movement especially around several hundreds of megahertz. Lastly the levels at the ears are relatively stable; however, one dip is found at 300 MHz when the model is raising the hand.

100 1000Frequency [MHz]

10

Rec

eive

d op

en v

olta

ge [d

B]

−60

−80

−100

−40

−120

−1403 30 300 3000

StandingWalking (L)Walking (R)

Hand-raising (L)Hand-raising (R)

(a) At chest

−60

−80

−100

−40

−120

−140100 1000

Frequency [MHz]103 30 300 3000

StandingWalking (L)Hand-raising (L)

(b) At back

Rece

ived

open

volta

ge [d

B]

100 1000Frequency [MHz]

10

−60

−80

−100

−40

−120

−140

StandingWalking (L)Walking (R)Hand-raising (L)Hand-raising (R)

3 30 300 3000

(c) At ankle

Rece

ived

open

volta

ge [d

B]

100 1000

Frequency [MHz]10

−60

−80

−100

−40

−120

−1403 30 300 3000

StandingWalking (L)Walking (R)Hand-raising (L)Hand-raising (R)

(d) At wrist

Rece

ived

open

volta

ge [d

B]

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100 1000Frequency [MHz]

10

−60

−80

−100

−40

−120

−1403 30 300 3000

StandingWalking (L)Walking (R)Hand-raising (L)Hand-raising (R)

(e) At ear

Rece

ived

open

volta

ge [d

B]

Fig. 4 Received open voltages of respective receivers.

Conclusion

In the present paper, to bring objective and unified idea on the frequency characteristics of body-centric wireless communication channels, electromagnetic fields around a body wearing a small top-loaded monopole antenna and received open voltages at several equipped receiving antennas were numerically calculated in a wide range of HF to UHF bands. Regarding power transmission efficiency, it was suggested that the use of lower frequency would be more efficient at the HF band. Regarding the received open voltage, several non-periodic patterns are found at the levels at ankles, wrists, and ears. In addition, fluctuations of the received open voltage are significant especially for the receiver at the wrist and especially in a frequency range around several hundreds of megahertz.

References [1] P. S. Hall, and Y. Hao, Antennas and Propagation for Body-Centric Wireless

Communications. Norwood, MA: Artech House, 2006. [2] IET Seminars on “Antennas and Propagation for Body-Centric Wireless

Communications”, London, Apr. 2007 and 2009. [3] Special Issue on “Antennas and Propagation for Body-Centric Wireless

Communications”, IEEE Trans. Antennas Propag., Apr. 2009. [4] K. Fujii, M. Takahashi, and K. Ito, “Electric field distributions of wearable devices

using the human body as a transmission channel,” IEEE Trans. Antennas Propag., vol. 55, no. 7, pp. 2080–2087, Jul. 2007.

[5] Available: http://www.redtacton.com/ [6] P. S. Hall, “Diversity in on-body communications channels,” in Proc. International

Workshop on Antenna Technology 2008, pp. 5–9, Chiba, Mar. 2008. [7] N. Haga and K. Ito, “Frequency dependence of on-body channels with top-loaded

monopole antennas in the range of HF to UHF,” in Proc. Asia-Pacific Microwave Conference 2009, pp.2208–2211, Singapore, Dec. 2009.

[8] Available: http://www.e-frontier.com/ [9] S. Gabriel, R. W. Lau, and C. Gabriel. “The dielectric properties of biological tissues:

II. Measurements in the frequency range 10 Hz to 20 GHz,” Phys. Med. Biol., vol. 41, pp. 2251–2269, 1996.

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