A Compact UWB Antenna for On-Body Applications

A Compact UW

B Antenna for On-Body Applications Nacer Chahat,Student Member,IEEE,Maxim Zhadobov,Member,IEEE,Ronan Sauleau,Senior Member,IEEE,

and Koichi Ito,Fellow,IEEE

Abstract—A new compact planar ultrawideband(UWB)

antenna designed for on-body communications is presented.

The antenna is characterized in free space,on a homogeneous phantom modeling a human arm,and on a realistic high-resolu-

tion whole-body voxel model.In all con?gurations it demonstrates

very satisfactory features for on-body propagation.The results are

presented in terms of return loss,radiation pattern,ef?ciency,and E-?eld distribution.The antenna shows very good performance within the3–11.2GHz range,and therefore it might be used

successfully for the3.1–10.6GHz IR-UWB systems.The simula-

tion results for the return loss and radiation patterns are in good

agreement with measurements.Finally,a time-domain analysis

over the whole-body voxel model is performed for impulse radio

applications,and transmission scenarios with several antennas

placed on the body are analyzed and compared.

Index Terms—Body-area network(BAN),body-centric wireless

communications,compact antenna,printed antenna,ultrawide-

band(UWB)antenna.

I.I NTRODUCTION

B ODY-AREA NETWORKS(BAN)are wireless commu-

nication systems that enable communications between wearable and/or implanted into the human body electronic devices.Such systems are of great interest for various appli-cations including sport,multimedia,health care,and military applications[1],[2].Ultrawideband(UWB)antennas have been identi?ed as a highly promising solution for BAN.One of the major advantages of the UWB systems at3.1–10.6GHz is their high data-rate-transmission capabilities(typically100 Mbps)with low power spectral densities(41.3dBm/MHz) [3],ensuring thereby low interference with other narrow-band wireless devices.

Designing an antenna for UWB body-centric communica-tions is a challenging task,as the antenna needs to ful?ll several fundamental requirements,such as:1)optimized characteristics in frequency-and time-domains;2)small size and low pro?le;3) good on-body propagation.

Indeed,both frequency-and time-domain responses should be considered and characterized[4]–[7],and the interaction with the human body,i.e.,the changes in the antenna performance

Manuscript received April12,2010;revised August04,2010;accepted September24,2010.Date of publication January28,2011;date of current ver-sion April06,2011.This work was supported in part by the“Agence Nationale de la Recherche”(ANR),France by Grants ANR-09-VERS-003(META VEST project)and ANR-09-RPDOC-003-01(Bio-CEM project),“Région Bretagne”(Dose_ULB project),and in part by the“Centre National de la Recherche Scienti?que”(CNRS),France.

N.Chahat,M.Zhadobov,and R.Sauleau are with the Institute of Electronics and Telecommunications of Rennes(IETR),UMR CNRS6164,University of Rennes1,35042Rennes,France(e-mail:nacer.chahat@univ-rennes1.fr).

K.Ito is with the Graduate School of Engineering,Chiba University,Chiba 263-8522,Japan(e-mail:ito.koichi@faculty.chiba-u.jp).

Digital Object Identi?er10.1109/TAP.2011.2109361due to the presence of the body and power losses in the tissues, should be carefully taken into account[8].

Besides,the miniaturization of UWB antennas is particu-larly important for wearable applications.Signi?cant research efforts have been undertaken to reduce the size of the radiating structures,and some interesting miniaturization techniques have been proposed[9]–[11].

Finally,for on-body applications,the antenna needs to ex-hibit suitable on-body propagation features.However,in most of the proposed body-centric UWB communication scenarios,om-nidirectional planar antennas are placed parallel to the human body,and,as a result,the ef?ciency of these antennas is signi?-cantly reduced[12],and the on-body propagation is not optimal. Indeed,this con?guration is more suitable for off-body commu-nication,i.e.,for communication between an antenna mounted on the body and a remote device or base station,as demonstrated in several studies introducing textile antennas with high poten-tial for off-body communications[13]–[15].It was shown that the antenna-?eld polarization needs to be normal to the body surface in order to improve the on-body propagation[16].In par-ticular,it was demonstrated that a quarter-wavelength monopole antenna is appropriate for on-body communication for the fol-lowing reasons:1)it has an omnidirectional pattern with max-imum radiation along the body surface;2)-?eld is normal to the body surface[16].Furthermore,a comparison between two different UWB antennas has been performed showing that the planar inverted cone antenna(PICA)with an omnidirec-tional monopole-like pattern demonstrates very good perfor-mances for on-body communications[17].Nevertheless,the quarter-wavelength monopole antenna and the PICA have a rel-atively large ground plane and heights.

To overcome this problem we introduce here a reduced-size UWB antenna suitable for on-body communications since the -?eld is polarized perpendicularly to the body surface.

This paper is organized as follows.The antenna design and a two-thirds muscle equivalent phantom used for numerical and experimental characterizations are introduced in Section II.The main characteristics of the proposed antenna,namely its re?ec-tion coef?cient,radiation patterns,and ef?ciency are then given in Section III.Experimental results,obtained using the phantom, are also compared to calculations.In addition,the-?eld distri-bution around a homogeneous arm model and whole-body voxel phantom is also studied numerically to investigate the on-body propagation.Finally the time-domain capabilities of the antenna are analyzed in Section IV,transmission scenarios between sev-eral antennas placed on the body are numerically studied,and the effect of modulation schemes on the on-body system perfor-mance is discussed.

All calculations are carried out using the?nite integration technique implemented in CST Microwave Studio.

0018-926X/$26.00?2011IEEE

Fig.1.(a)Geometry of the proposed antenna.(b)Antenna on the two-thirds muscle-equivalent phantom.

II.E XPERIMENTAL M ODELS

A.Antenna Design

An antenna was designed and manufactured for UWB

on-body applications.It consists of a compact microstrip-fed printed monopole [Fig.

1(a),]printed on a 1.6-mm-thick AR350

substrate .In this study,the antenna performance is evaluated and optimized in the following con?gurations:1)antenna located on a homogeneous phantom with two-thirds muscle-equivalent dielectric proper-ties;2)antenna mounted on a high-resolution nonhomogeneous human body model (Sections III-C,III-E,and IV).

To our best knowledge,the smallest UWB antennas have a signi?cant height (around 17mm)[18]–[21]and thus might be not suitable for a perpendicular con?guration (-?eld normal to the body).Here,the ground plane

size and radiator height have been reduced by 8mm and 7mm,respectively,leading to a total antenna height of only 10mm.This choice enables us to minimize distortions in the antenna performance com-pared to the original design [22].This size reduction results in a signi?cant decrease of the current path,thus in an increase of the lower band limit.To decrease the lower frequency,one so-lution consists in extending the total slot length (white dotted line in Fig.1(a)).The impact of this slot length is illustrated in Fig.2where we compare the re?ection coef?cients for three slot length values:6.9mm,11.9mm,and 16.4mm.Selecting a long slot enables to slightly enlarge the 10dB return loss bandwidth and cover the full UWB frequency range (solid line in Fig.

2).

Fig.2.Re?ection coef?cients S of the proposed antenna simulated in free space for different slot shapes.Three values for the total slot length are consid-ered:6.9mm,11.9mm,and 16.4

mm.

Fig.3.Re?ection coef?cient S of the proposed antenna in free space for different ground plane heights.g =1mm .g =5mm .g =

10mm .111111g =15mm .

TABLE I

C ROSS P OLARIZATION L EVELS ,P EAK G AINS ,AN

D

E FFICIENCY FOR THE P ROPOSED A NTENNA W ITH G ROUND P LANE H EIGHT O

F 1mm/10

mm

As a wearable antenna,the antenna ground plane is vol-untarily small to minimize the antenna height.The impact of the ground plane

size on the re?ection

coef?cient is illustrated in Fig.3.Although slightly degraded around 4and

10GHz,

the

remains below 10dB between these two frequencies for larger ground plane dimensions.

Furthermore,the antenna size reduction (and particularly,the small ground plane),affects the cross-polarization component and peak gain (Table I).However,along the body surface

(i.e.,

and ),the cross-polarization

ratio remains below 10dB.B.Phantom

The human arm is modeled as a two-thirds muscle-equivalent phantom,and the antenna is located 1mm above [Fig.1(b)and Fig.4].This phantom has a parallelepipedic

shape

.A water-based semisolid phantom

CHAHAT et al.:COMPACT UWB ANTENNA FOR ON-BODY APPLICATIONS

1125

Fig. 4.Antenna prototype mounted on the two-thirds muscle-equivalent phantom.

[23]was chosen as a tissue-equivalent model for UWB mea-surements.The complex permittivity and conductivity of the

phantom are adjusted using the polyethylene powder and sodium chloride,respectively.Agar is used to maintain the shape of the phantom,sodium azide is a preservative,and TX-151improves the phantom stickiness [23].

III.F REQUENCY -D OMAIN A NALYSIS

In this section,the dielectric properties of the phantom are provided in the 3–11GHz range.Then computational and mea-sured results are presented for the antenna return loss and radia-tion pattern.Finally,-?eld distributions are computed for the arm and human body models.

A.Numerical Model and Dielectric Properties of the Phantom in the UWB Band

In order to model accurately the antenna in presence of the phantom,the dielectric properties of the phantom should be de-termined carefully in the 3.1–10.6GHz range.Two-thirds of the muscle permittivity was used as a target value for the phantom.The muscle dielectric properties are well characterized up to 20GHz [24].For the numerical modeling,the complex dielectric

permittivity of the phantom is expressed as a Debye’s disper-sion equation

[25]

(1)

where is the angular

frequency,is the static

permittivity,is the optical permittivity,

and is the relaxation time.The best ?t of this theoretical model to the target values was obtained

for

,,

and .Theoretical

permittivity and conductivity model is in a very good agree-ment with target values over the considered frequency range (3–11GHz),con?rming thereby that the choice of the Debye model is appropriate (Fig.5).The phantom has been built as explained in Section II-B and characterized using the dielectric probe kit 85070E (Agilent Tech.,CA).The measured complex permittivity is in satisfactory agreement with the numerical re-sults (Fig.5).

B.Re?ection Coef?cient and Impact of the Feed Connector The re?ection

coef?cients

of the proposed UWB an-tenna are represented in Fig.6assuming the antenna is either in free space or on the arm.Here,the numerical models do not take into account the feed connector.These results show

that

the

is very slightly affected by the presence of

the Fig.5.Measured and computed characteristics of the phantom.Target.

Measurement.

Calculation.

Fig.6.Simulated re?ection coef?cients of the optimized antenna (the total slot

length equals 16.4mm).

Antenna in free space.Antenna mounted on the

phantom.

Fig.7.Re?ection coef?cient of the antenna with feed connector mounted on

the phantom.

Simulation results. Measurements.phantom and remains below 10dB within the 3–11.3GHz

range.The fabricated prototype is fed by a tiny coaxial probe connector (BL58-3123-00,Orient Microwave Corp.,Japan).Its re?ection coef?cient measured for the antenna mounted on the phantom is represented in Fig.7.The agreement with simulations (accounting this time for the feed connector)is very satisfactory.The 10dB return loss bandwidth almost covers the full 3.1–10.6GHz UWB frequency range.C.Radiation Patterns

The antenna radiation patterns

in

and planes have been computed at three frequency points

(4,7,and 10GHz)for three con?gurations:antenna alone,an-tenna on the phantom,and antenna on the whole-body model (Duke model from Virtual Family [26]).Duke represents a 34year-old 174cm-tall adult weighting https://www.360docs.net/doc/fe11150519.html,pared to the free-space scenario,the antenna mounted on the phantom shows

1126IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION,VOL.59,NO.4,APRIL

2011

Fig.8.Measured and simulated radiation patterns at 4,7,and 10GHz.(a) =90.(b)'=0.Free space.On homogeneous phantom.On

voxel phantom.

Measurement.

TABLE II

P EAK AND A VERAGE G AINS OF THE A NTENNA IN F REE S PACE AND ON

THE P

HANTOM

better front-to-back ratio.This is essentially related to the ab-sorption in and re?ections from the phantom.In addition,com-parison with measurements at 4and 10GHz in

the

and planes using the two-thirds muscle-equivalent phantom con?rms the very satisfactory matching between experimental and numerical results.The average and peak values of the an-tenna gain are given in Table II.These data con?rm the gain

reduction

in

plane due to the presence of the phantom,as already mentioned in other studies (e.g.[27],[28]).However,

in

plane,the gain is reduced at 4GHz where absorptions are higher,and at 7and 10GHz,the gain increases due to the re?ection caused by the arm.D.Radiation Ef?ciency

Radiation ef?ciencies of 19.1%,38.2%,and 28.4%are es-timated on the homogeneous phantom,at 4,7,and 10GHz,respectively.As expected,the radiation ef?ciency is quite low since the gap between the antenna and the body is set to 1mm,

which corresponds to a practical situation where the antenna is mounted on clothes.However,the antenna ef?ciency can be im-proved using a larger ground plane size as shown in Table https://www.360docs.net/doc/fe11150519.html,puted E-Field Distributions

To estimate possible range of application scenarios for the proposed antenna,it is important to consider electromagnetic ?eld distribution in,on,and around the body.

Fig.9shows the computed electric ?eld distributions in the

cross-section plane of the numerical arm model

[

plane in Fig.1(b)]at 4,7,and 10GHz.It is important to highlight that,

in this con?guration,

the

component is the dominant one,which means that the major contribution to the overall electric ?eld comes from the component perpendicular to the upper side of the phantom.

Based on these results,several observations can be made:?The electromagnetic ?eld propagates along the phantom surface and,as expected,attenuates nonlinearly;

?Within the considered frequency range,higher frequencies correspond to the more localized energy absorption.

The ?eld propagation around the body is analyzed in Fig.10using Duke model (the spatial resolution

is ).The antenna is mounted on the left arm where most part of the electric ?eld is con?ned.Fig.10shows that the electromagnetic ?eld propagates in both directions along the left arm and that shadowing due to the human body plays a key role since the electric ?eld is much stronger at the source side compared to the opposite side (the attenuation is at least 70dB).Therefore,communications between antennas situated on opposite sides of the body might be dif?cult in this con?guration.However,wire-less radio links between hand/head or hand/foot are absolutely

CHAHAT et al.:COMPACT UWB ANTENNA FOR ON-BODY APPLICATIONS

1127

Fig.9.Electric ?eld distributions (root-sum-square)in x 0y cross section of a homogeneous

model.

Fig.10.Electric ?eld distribution around and within the Duke model at 4,7,and 10GHz.

achievable,and the time-domain behavior for such communica-tion scenarios is studied in Section IV.

IV .P ROPAGATION A ROUND THE B ODY

A.Time-Domain Analysis

A UW

B system can be either a traditional pulse-based system transmitting each pulse that occupies the entire UWB bandwidth,or a carrier system such as,for instance,multiband orthogonal frequency-division multiplexing (MB-OFDM)which has been adopted by WiMedia.Time-domain analyses

are of great importance to evaluate the capabilities of UWB an-tennas for impulse radio systems (IR-UWB).A UWB antenna excited by nanosecond pulses behaves as a pulse-shaping ?lter.Therefore,a suitable antenna for UWB communications has to demonstrate minimum distortion of the pulse in time-domain to reduce the complexity of the detection mechanisms at the re-ceiver terminal.The quality of the received pulse is determined

by

(2)

1128IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION,VOL.59,NO.4,APRIL

2011

Fig.11.(a)Normalized excitation signal.(b)Its spectral

density.

Fig.12.Locations of the probes around the antenna.

where the source

pulse and received

pulse are normalized by their respective energies.

The source pulse signal chosen here [Fig.11(a)]is a ?fth-order derivative Gaussian pulse satisfying the FCC power mask [Fig.

11(b)]

(3)

where

and denote the amplitude and spread of the Gaussian pulse,respectively.

In the numerical simulations,the output signal is monitored in three

directions

using three probes located 50cm

apart from the antenna (Fig.12).The pulse received at each

probe is represented in Fig.13(a).Its

?delity

is excellent:98.8%,98.3%,and 95.2%for probes A,B,and C,respectively.A similar study has been conducted for on-body propagation.To this end,three transmission scenarios between four antennas are considered:the transmitting antenna is mounted on the

left

Fig.13.(a)Signals monitored by virtual probes.(b)Their spectra.

wrist (TX1),the three receiving antennas are placed on the left arm (RX1),left ear (RX2),and left leg (RX3)(Fig.14).The dis-tortion of the received pulses (Fig.15)depends on the antenna position,and their ?delity equals 75%,46%,and 81%,for RX1,RX2,and RX3,respectively.

The most pronounced distortion is observed for the antenna on the head,suggesting that a direct communication in this sce-nario might be delicate.

From these results,it is clear that pulses experience strong distortions in on-body scenarios.The signal ?delity can be very low (e.g.,46%for RX2),and therefore an appropriate mod-ulation scheme needs to be implemented.For instance binary phase-shift keying (BPSK)is not appropriate in this case be-cause of strong distortions of the transmitted signal.

However,in IR-UWB systems,pulse-position modulation (PPM)and on-off keying (OOK)are excellent candidates for on-body communications.These two modulation schemes can be implemented with noncoherent receivers using energy de-tection mechanisms instead of correlation for coherent systems.With a noncoherent receiver,the pulse shape is secondary and the low ?delity can be overcome.

We focus our attention here on IR-UWB system using the en-tire UWB-band.However,these modulations schemes can also be used as multiband schemes [29],[30].

CHAHAT et al.:COMPACT UWB ANTENNA FOR ON-BODY APPLICATIONS

1129

Fig.14.Transmission scenario with four antennas mounted on the

body.

Fig.15.Wave forms of the received pulses.

B.On-Body Propagation

As a non-coherent receiver is preferred,the path loss needs to be investigated.The path loss of the proposed antenna mounted on the homogeneous phantom with two different ground plane heights is presented in Fig.16.For each distance,each point represents the path loss result at one frequency point.The an-tenna with a 10-mm-height ground plane demonstrates a better on-body propagation https://www.360docs.net/doc/fe11150519.html,ing a larger ground plane improves the path loss by 7dB whereas the cross-polarization component remains fairly the same between these two antenna models.However a larger ground plane improves the

antenna

Fig.16.Path loss of the proposed antenna mounted on the homoge-neous phantom with different ground plane heights.222g =1mm . g =10mm .

TABLE III

M EAN P ATH G AIN AND P ATH G AIN V ARIABILITY (R ANGE )FOR S OME S PECIFIC

ON B ODY P ATH L

OSS

ef?ciency (Table I)and as consequences the path loss becomes higher.

The propagation path loss for this on-body scenario is pre-sented in Table III.The mean path gain and variability within the 3.1–10.6GHz UWB band are given for all receiving an-tennas.The wrist-to-arm link,being the shortest,has the lower loss,with an average loss of 53.9dB,and has a peak-to-peak variation of approximately 32.9dB within the whole UWB fre-quency band.The wrist-to-head link has the higher average loss,(this is mainly due to the longer link length),and its path loss variability is estimated around 34.4dB.In the wrist-to-calf link,an average loss of 60.5dB is evaluated,and the variability is much lower compared to the other links.

V .C ONCLUSION

A compact planar UW

B monopole antenna has been de-signed for on-body communications.The antenna shows good impedance matching and satisfactory on-body propagation fea-tures.It was shown that,in spite of the small distance between the antenna and the body,the latter does not affect signi?cantly the antenna input matching.The re?ection coef?cient and ra-diation patterns were successfully measured using a two-thirds muscle homogeneous phantom.

The electric ?eld distributions around a realistic whole-body model have been computed at different frequencies,and suitable on-body propagation features have been highlighted.

Compared to the parallel con?guration,the perpendicular one demonstrates higher gain,better on-body propagation,and the antenna impedance performance is less affected.

The time-domain behavior of the proposed antenna has been fully investigated on a realistic body model.Signi?cant pulse distortions have been observed and thus a noncoherent modulation is advised for on-body communication.Hence,for