A Novel Quasi-Static Channel Enhancing Technique for Body ...

A Novel Quasi-Static Channel Enhancing Technique for Body Channel Communication ∗

Bo Zhao∗ , Huazhong Yang∗ , and Yong Lian+

Department of Electronic Engineering, Tsinghua University, Beijing, China. Email: ∗ zhao [email protected] + Department of Electronic Engineering & Computer Science, Lassonde School of Engineering, York University, Toronto, Canada.

Abstract—Body channel communication (BCC) is a most power efficient way for communications among sensors in a wireless body-area network (WBAN). In BCC, the forward signal of the quasi-static field is conducted by the body surface, whereas the backward path is formed by the electrostatic coupling between the GND electrodes (GEs) of transmitter and receiver. As a result, the transmission loss is dominated by the backward path, which has high impedance due to small air capacitance between two compact GEs. Conventional backward path enhancement techniques make use of a large inductor to resonate with the air capacitance in order to reduce the impedance. Such approach is not suitable for integrated solution and not reconfigurable for varying communication distances. In this paper, we propose a novel active channel enhancer to compensate the loss in backward path, which is integratable and reconfigurable for variable distances and frequencies. Designed with 0.13 µm CMOS process, the proposed active enhancer improves the quasi-static coupling by more than 15 dB for a wide frequency band of 40 MHz−120 MHz compared to the 4 dB enhancement of conventional method; and the power consumption is only 0.6 mW.

I. I NTRODUCTION Wireless body-area networks (WBANs) are envisaged to be a next wave of technology for preventive healthcare. Distributed in, on, or around the human body, the WBAN sensors collect vital signs and send signals to remote doctors for diagnosis. The IEEE 802.15.6 Task Group has published three main physical-layer (PHY) standards for WBAN [1]: UltraWide band (UWB), narrow band (NB), and body-channel communication (BCC). Among the WBAN standards, BCC has an advantage over other communication schemes in terms of power efficiency due to two reasons. First, the conductivity of human body is higher than that of air; and second, the carrier frequency of BCC is much lower than that of UWB and NB. Therefore, BCC is regarded as one of the most power efficient candidates for WBAN applications. Much effort has been invested in BCC research including body-channel modeling [2]–[4], communication protocols [5], and transceiver design [6], [7]. The biological tissue of body channel was modeled by measurement in [2]; whereas a simplified circuit model was derived by finite-element method (FEM) in [3]. The effect of environment coupling and interference on the BCC channel was studied in [4]. At communication level, a statistical MAC protocol was presented to improve energy efficiency [5]. At circuit level, CMOS

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implementation of BCC transceiver has been advancing over the years. The data rate increases from 8.5 Mb/s [6] to 10 Mb/s [7], resulting in an improvement on energy efficiency from 0.32 nJ/bit [6] to 0.24 nJ/bit [7]. One of the unsolved problems in BCC is the high transmission loss in the backward path, which is closely related to the signal transmission schemes. Quasi-static coupling and surface wave propagation are two main schemes used in the BCC. For a carrier frequency at tens of megahertz, the wave length is much longer than human body, so the time-variation electric field around human body can be approximated as quasi-static field. The forward signal is transmitted through the body surface by contact electrodes (CEs), while the backward path is formed by the electrostatic coupling between GND electrodes (GEs) of transmitting (Tx) port and receiving (Rx) port, as shown in Fig. 1. Although the human body serves as a good conductor for forward signals, the high impedance of small air capacitance CAIR leads to a serious transmission loss. So far there are limited solutions to deal with backward path loss. The BCC transceiver in [7] introduced a resonance matching (RM) method, which used a large-value inductor to resonate with CAIR , resulting in 4 dB channel enhancement. However, the large off-chip inductor is not possible for on-chip integration, and the 4 dB enhancement was only achievable for a fixed communication distance. CAIR

Backward Path GE

GE

Human Body CE

CE

Forward Path Fig. 1.

Closed loop for quasi-static coupling.

In this paper, we propose a novel technique to enhance the quasi-static coupling of BCC. Instead of the conventional RM method proposed in [7], we introduced an active channel enhancer to compensate the air capacitance in a wide frequency band of 40 MHz−120 MHz. The equivalent inductance of

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proposed enhancer is tunable for both flexible communication distances and GE heights over external grounds. The circuit is designed with 0.13 µm CMOS technology, and improvement on quasi-static coupling is validated by simulation results. The rest of this paper is organized as follows. In Section II, the basic idea is presented. The circuit design is detailed in Section III. Simulation results are presented and analyzed in Section IV. Finally, Section V concludes the paper.

Tunable Channel Enhancer VIN IIN

Gm

C0

1 − 4π 2 fC2 L(CAIR + CGE,Rx ) . 2πfC CAIR (1 − 4π 2 fC2 LCGE,Rx )

S1

S0

S2

…...

Sn

Digital Controller fC

d

h

Equivalent Circuit VIN

GE

L

IIN

CPAR External Ground

(1)

where CO is the output capacitance of the capacitor array, gm1 and gm2 are the transconductances of the Gm cells. The enhancer acts as an inductive component L with a small input parasitic capacitance CP AR , as illustrated in Fig. 2. It should be noted that L and CP AR has different ground terminals, i.e. L with GE, and CP AR with the external ground. The enhancer provides a high Q-factor inductor through the active Gm circuit, thus eliminates the need of off-chip high-Q inductor. At the same time, the enhancer can be tuned by a digital controller according to the carrier frequency fC , communication distance d, and the node height h, as given in Fig. 2. In this way the air capacitance CAIR can be effectively compensated to reduce the loss of backward path over different communication distances and a wide range of frequencies. By inserting the proposed enhancer at the Rx port, as shown in Fig. 3, we can improve the conductance of backward path. The equivalent circuit is given in Fig. 3, where CGE,T x and CGE,Rx represent the capacitances from Tx GE and Rx GE to external ground, respectively. Besides the air capacitance CAIR , the backward loss is also significantly affected by the capacitance CGE,Rx . Therefore, the backward loss can be reduced by minimizing the impedance ZB in Fig. 3, whose magnitude can be expressed as: ZB =

Cn

Gm

The conventional RM method in [7] faces the dilemma of selecting high-Q or low-Q inductor, i.e. a high-Q inductor is better for the improvement on channel loss but only works for a narrow frequency band while a low-Q inductor certainly provides compensation in a wide frequency range but is less effective in terms of channel loss. The proposed active channel enhancer solves this problem with the help of a tunable active inductor, as shown in Fig. 2. The enhancer consists of two Gm transconductors and a capacitor array. The classical Nauta structure is utilized to realize the transconductors [8] in low power. A binary capacitor array, which is controlled by MOS switches, is used for inductance tuning. The equivalent inductance L can be expressed as: CO , gm1 gm2

C2 …...

II. D ESIGN C ONSIDERATIONS

L=

C1

(2)

Fig. 2.

Tunable quasi-static channel enhancer.

CAIR

Backward Path GE

GE

ZB Tx

Rx L

CAIR CGE,Tx

CGE,Rx

Fig. 3.

CPAR

Equivalent circuit of impedance compensation.

According to the analysis in [4] and [9], both CAIR and CGE,Rx can be estimated by K(.) function:

In theory, ZB becomes zero when the capacitor array is adjusted to the corresponding value of CO : gm1 gm2 . (3) CO = 4π 2 fC2 (CAIR + CGE,Rx )

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( d ) επR2 , 2R d ( h ) επR2 =K , R 2h

CAIR = K

(4)

CGE,Rx

(5)

where ε is the dielectric constant, R is the radius of each GE, d is the communication distance between two GEs, and h is the GE height over external ground. K(.) is the empirical function determined by a classical experiment in [9]:  1 + 2.367b0.867 , for 0.005 ≤ b < 0.5 (6)     0.982 1 + 2.564b , for 0.5 ≤ b < 5.0 K(b) =  1 + 2.5465b, for 5.0 ≤ b < 10.0    4εR. for b ≥ 10.0

4επ 3 R2 fC2

[

gm1 gm2 d 1 d K( 2R )

].

+

1 h 2h K( R )

0.80 µm/0.13 µm

1.46 µm/0.50 µm

0.28 µm/0.13 µm

0.35 µm/0.13 µm

1.23 µm/0.50 µm

0.64 µm/0.13 µm

0.80 µm/0.13 µm

1.46 µm/0.50 µm

0.28 µm/0.13 µm

0.35 µm/0.13 µm

1.23 µm/0.50 µm

+

-

-

To minimize the backward path impedance, the output value of capacitor array should be adjusted to: CO =

0.64 µm/0.13 µm

+

(7) (a)

This is achievable through the digital controller in order to enhance the backward channel coupling at different fC , d, and h.

Transconductance (µS)

343.50968

III. C IRCUIT D ESIGN The proposed circuit is implemented using 0.13 µm CMOS technology. In our simulation, the size of GEs is chosen to be 2 cm in radius for all the BCC nodes. The transconductor circuit and its transistor sizes are given in Fig. 4(a). Fig. 4(b) reports the simulated transconductance of both Gm cells (gm1 and gm2 ), which indicates that the transconductance maintains at about gm1 = gm2 = 343.5 µS in the 40 MHz−120 MHz frequency band. The reduction of backward-path impedance ZB using the proposed enhancer is quantified by simulation. For each carrier frequency fC , communication distance d, and GE height h, the impedances of backward path under two conditions are simulated: 1) Without the enhancer and 2) with the enhancer. The size of normal human body is less than 2.5 m, whereas CAIR and CGE,Rx tend to be constant when d ≥ 0.4 and h ≥ 0.2, according to equations (4), (5) and (6). Several sets of typical value of d and h are selected to evaluate the effectiveness of the enhancer, e.g. d =0.1 m, 0.2 m, 0.3 m, and ≥0.4 m, whereas h =0.1 m and ≥0.2 m. The results are shown in Fig. 5. For all typical values, the proposed technique has reduced the impedance of backward path by at least 44 dB. The largest reduction in impedance is 53 dB for d = 0.1 m and all h values. The reduction in impedance of the backward path improves the quasi-static channel condition as illustrated in next section. IV. S IMULATION R ESULTS The effectiveness of the enhancer is evaluated by simulation using an existed model of human body surface. The simulation platform is shown in Fig. 6, where VS is the signal source of the transmitter, VO represents the magnitude of received signal at the receiver, and RT x = RRx = 50 Ω are set as the port resistance of transceivers. The switch SW is included for two settings: With the enhancer (SW is off) and without the enhancer (SW is ON). For the forward path, the FEM model

343.50966 343.50964 343.50962 343.50960 343.50958 343.50956 343.50954 40

50

60

70

80

90

100

110

120

Frequency (MHz) (b) Fig. 4.

Nauta transconductor. (a) Transistor size. (b) Transconductance.

in [3] is adopted in the simulation, which is a cascade of πshaped unit-length arm circuit model and torso circuit model. The values of communication distance d and node height h are the same as in section III for these simulations. The simulations are conducted for two cases, i.e. (1) the switch SW is turned on to bypass the channel enhancer and (2) SW is turned off to evaluate the enhancer. The total BCC transmission loss is simulated for different d and h under both cases. Transmission losses for both cases are plotted in Fig. 7 for comparison. It can be seen that the proposed enhancer improves the quasi-static coupling by at least 15 dB, compared to the 4 dB improvement of conventional RM method [7]. The best compensation is 17.5 dB for the carrier frequency of 40 MHz. It is also interest to note that the proposed enhancer achieves almost same level of compensation regardless of communication distances and ground panel height. The power consumption of the proposed enhancer is about 500 µA under 1.2 V supply, i.e. 0.6 mW, which is acceptable for most WBAN applications. V. C ONCLUSION In this paper, we have presented a novel quasi-static channel enhancing technique for BCC. Instead of using a large off-

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Transmission Loss (dB)

Impedance (Ω, dB)

≥ ≥ ≥ ≥ ≥

≥

Frequency (MHz)

Transmission Loss (dB)dB)

(a)

)

Impedance (Ω dB) 

≥

Frequency (MHz)

(a)

≥

≥ ≥ ≥ ≥

≥ ≥ ≥ ≥ ≥

≥ ≥ ≥ ≥ ≥

≥

Frequency (MHz)

≥

Frequency (MHz)

(b)

(b)

Fig. 5. Backward impedance. (a) Without the enhancer. (b) With the enhancer.

Fig. 7.

Transmission loss. (a) Without the enhancer. (b) With the enhancer.

CAIR

R EFERENCES CGE,Rx

-

CGE,Tx

VS

Enhancer

+

SW

-

RTX RRX

VO

+

Body Surface Model RCHEST RARM

RINJ

RINJ

RARM

RARM

RTORSO

CINJ CLEAK_ARM

CTORSO

CINJ CLEAK_ARM

CTORSO

CARM CLEAK_ARM

RTORSO

CCHEST

CARM

CARM

CLEAK_ARM

CLEAK_ARM

CLEAK_TORSO

External Ground

Fig. 6.

GND Electrode (GE)

Simulation platform.

chip inductor as in conventional RM method, we introduced an active enhancer, which can be fully integrated on-chip and tunable for flexible communication distances and electrode-toground heights. Compared to the 4 dB improvement reported for the conventional method, the proposed technique has achieved more than 15 dB channel enhancement at the cost of additional power of 0.6 mW. VI. ACKNOWLEDGMENTS

[1] IEEE 802.15.6 Task Group, Body Area Networks (BAN) [Online]. Available: http://www.ieee802.org/15/pub/TG6.html, Jun., 2013. [2] A. Kamboh and A. J. Mason, “Channel characterization for implant to body surface communication,” in IEEE International Symposium on Circuits and Systems (ISCAS), 2011, pp. 913–916. [3] R. Xu, H. Zhu, and J. Yuan, “Electric-Field Intrabody Communication Channel Modeling With Finite-Element Method,” IEEE Transactions on Biomedical Engineering, vol. 58, no. 3, pp. 705–712, 2011. [4] R. Xu, W. C. Ng, H. Zhu, H. Shan, and J. Yuan, “Equation Environment Coupling and Interference on the Electric-Field Intrabody Communication Channel,” IEEE Transactions on Biomedical Engineering, vol. 59, no. 7, pp. 2051–2059, 2012. [5] H. Chen, Z. Nie, K. Ivanov, L. Wang, and R. Liu, “A statistical MAC protocol for heterogeneous-traffic human body communication,” in International Symposium on Circuits and Systems (ISCAS), 2013, pp. 2275–2278. [6] A. Fazzi, S. Ouzounov, and J. van den Homberg, “A 2.75mW wideband correlation-based transceiver for body-coupled communication,” in IEEE International Solid-State Circuits Conference (ISSCC), 2009, pp. 204– 205,205a. [7] 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,” IEEE Journal of Solid-State Circuits, vol. 47, no. 1, pp. 310–322, 2012. [8] B. Nauta, “A CMOS transconductance-C filter technique for very high frequencies,” IEEE Journal of Solid-State Circuits, vol. 27, no. 2, pp. 142–153, 1992. [9] H. Nishiyama and M. Nakamura, “Form and capacitance of parallelplate capacitors,” IEEE Transactions on Components, Packaging, and Manufacturing Technology, Part A, vol. 17, no. 3, pp. 477–484, 1994.

This work was supported by the National Natural Science Foundation of China (NSFC) under Grant 61204032.

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