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938 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

Multiple Human Effects in Body Area NetworksGeorge Koutitas , Member, IEEE 

 Abstract—This letter investigates the channel variations inwireless body area networks (WBANs). For the purpose of the in-vestigation, a ray solution for multiple closed surfaces is proposedthat utilizes a modified slope uniform theory of diffraction (UTD)technique and can be applied to model scattering, radiation, cou-pling effects, or a combination of the three in a single formulation.The solution considers all possible ray paths and is valid at thetransition boundaries of the scenario. The human bodies arerepresented as closed cylindrical surfaces with constitutive pa-rameters based on the “Muscle” model. The proposed algorithmpredicts channel variations in a ray format that requires less CPUand is characterized by lower complexity compared to full-wavetechniques. Body-to-access point (BAP) and body-to-body net-works are investigated.

 Index Terms—Body-to-access point (BAP) communications,

multiple closed surfaces, ray tracing, uniform theory of diffrac-tion, wireless body area networks (WBANs).

I. INTRODUCTION

USER-CENTRIC and personal networks incorporate

sensors and actuators on a body that can communicate

with an access point, forming an off-body or body-to-access

point (BAP) network, or they can communicate between dif-

ferent bodies or parts of the same body forming a body-to-body

or an on-body network [1], [2]. Channel modeling in a wireless

body area network (WBAN) is usually obtained with exten-

sive measurement campaigns that yield empirical estimations,or with advanced electromagnetic codes that consider field

strength calculations, angle of arrival, and delay spreads.

Empirical channel models for on-body multiple-input–mul-

tiple-output (MIMO) and ultrawideband (UWB) systems are

presented in [1]–[3]. It is concluded that human body shad-

owing is a prominent factor for short-range body area networks.

Deterministic techniques can provide accurate results and

avoid extensive measurement campaigns. The most suitable

deterministic electromagnetic algorithms are the ray formula-

tion of UTD and the finite-difference time domain (FDTD).

The human body is modeled as a circular cylinder for the

cylindrical UTD ray formulation, or it can be modeled with ad-

vanced geometrical configurations to represent human anatomy(FDTD solution). Based on FDTD analysis, the authors in

[4] and [5] derive path-loss formulations. In [6], a cylindrical

UTD ray theory that utilizes the   Fock   coupling functions is

compared to a FDTD solution and measurements for on-body

channel characterization. For the purpose of the investigation,

the authors apply a mechanistic multiplication of the UTD

Manuscript received June 12, 2010; revised August 23, 2010; acceptedSeptember 25, 2010. Date of publication September 30, 2010; date of currentversion October 11, 2010.

The author is with the School of Science and Technology, International Hel-lenic University, 57001 Thermi, Greece (e-mail: g.koutitas@ihu.edu.gr).

Digital Object Identifier 10.1109/LAWP.2010.2082485

coupling coefficients of perfectly electrical conducting (PEC)

surfaces [7] to provide field predictions on a human body that is

modeled by three cylindrical surfaces, each one representing the

torso and the arms, respectively. The field predictions assumed

only the creeping waves around the human body, neglecting

cross-propagating rays over the cylinders, and this bounds

the applicability of the solution around the transition zones

of the scenario [8]. For the case of body-to-access point and

body-to-body networks and in the presence of multiple humans,

diffraction mechanisms occur in the transition boundaries of 

the scenario, and slope terms are of major importance.

This letter presents a UTD ray solution for a cascade of 

closed cylindrical surfaces that can be applied to multiple

scattering, radiation, coupling, and a combination of thosescenarios in a single formulation. In order to achieve this goal,

the UTD scattering coefficients of [7] are modified to model

radiation and coupling mechanisms (surface-mounted antennas

with height over the surface), according to [9]. The

modified scattering coefficients are then incorporated within

the multiple slope cylindrical UTD formulation of [8]. Finally,

the algorithm is extended to incorporate cross-propagating

rays around the cylinders to provide field continuity in all the

transition boundaries of the scenario. The proposed solution is

applied to WBAN channel predictions, and simulation results

are validated with the method of moments (MoM) and mea-

surements in the anechoic chamber of the Aristotle Universityof Thessaloniki, Greece.

II. RADIATION FROM HUMAN BODY

The presence of a human body in a propagation channel can

be modeled as a circular cylinder with a typical radius of cur-

vature cm, and UTD is proven to be accurate

compared to real measurements [10]. At high frequencies, above

500 MHz, penetration of radio waves through the human tissue

is of minor importance, and creeping waves are the dominant

field components [5], [10]. For a body-to-access point WBAN

scenario, the transmitter or the receiver is surface-mounted (or

placed very close 1–5 cm) on the human body, resulting ina radiation problem. In this section, the UTD scattering coeffi-

cient of [7] is modified according to [9] in order to approximate

surface-mounted antennas on a body. The solution is validated

with the UTD radiation case, the MoM, and measurements in

an anechoic chamber.

The problem under investigation is presented in Fig. 1. A re-

ceiver (RX) is placed at a distance away from the center

of the body, where . For the scattering case, the field can

be modeled according to [7]

(1)

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KOUTITAS: MULTIPLE HUMAN EFFECTS IN BODY AREA NETWORKS 939

Fig. 1. Proposed modified UTD scattering solution (         ) compared tothe UTD radiation case, the MoM for cylinder of radius        m,        ,      S/m (based on the “Muscle” model) and measurements in an anechoicchamber with a human body.

where is an   ON–OFF parameter indicating if there is line of 

sight (LOS) or non-line of sight (NLOS) between the transmitter

and the receiver, respectively. is the LOS field; and

represent the incident field at the reflection and diffraction

points, respectively; parameter represent the spreading factor

(that depends on distance ); and operators , represent the

left- and right-side field reaching the receiver around the closed

surface. and are the diffraction and reflection coeffi-

cients for the soft and hard case, respectively

(2)

(3)

where , and is the transition function. For

the diffraction case, is the external angle of the diffracting and

incident ray, , ,

, is the distance parameter that depends on the dis-

tances ( ) of incident and diffracted or reflected rays, and isthe radius of curvature of the cylindrical surface. For the reflec-

tion case, is the angle of reflection, ,

and . The terms , in (2) and

(3) are the so-called Fock  scattering functions. They depend on

the polarization and the electrical characteristics of the curved

obstacle. For any surface impedance material, they are related

to the surface field function [11], [12]

(4)

where and represent the Fock-type Airy functions,

and depends on the impedance of the surface and the localradius of curvature. For the radiation problem of an impedance

boundary cylinder, the  Fock  radiation functions are presented

in [13].

In [9], a heuristic modification of the diffraction and reflection

coefficients is presented, capable to approximate radiation or

coupling phenomena, causedby surface mounted antennas, with

the scattering coefficients of (2) and (3), providing acceptable

accuracy for engineering applications. For the case of antennaheights (Fig. 1) and radius of curvature , the

scattering coefficients are accurate enough. The inaccuracies are

observed for antenna heights or and for

a receiver placed into the deep shadow region. This is because

the UTD scattering formulas do not reduce to  Keller’s  form in

the deep shadow since the transition function in (2) does

not approach unity fast enough. Based on this observation, the

term when for the hard case, and

when for the soft case, representing field points that are

sufficiently into the deep shadow region. By employing these

modifications, the errors never exceeded 1.5 dB [9] for the case

of and . These conditions sat-

isfy accurately enough surface-mounted antennas on a humanbody in the frequency range of 900 MHz and above.

The modified UTD solution is compared to the case of the

UTD radiation problem , presented in [7]. For the radi-

ation case, the UTD coefficients incorporate the  Fock  radiation

functions. The comparisons for a PEC surface of radius of cur-

vature m, frequency 2.4 GHz, and the hard case is

presented in Fig. 1. It can be observed that there is an excellent

agreement. Similar observations were observed for the soft case

as well. The case of an impedance boundary condition cylinder

with constitutive parameters based on the “Muscle” model [14]

and for a frequency of 5 GHz is compared to the MoM solu-

tion that employed a line source with rectangular pulses as basisfunctions and a point-matching technique with points spaced at

. In addition, measurements of a surface-mounted antenna

on a real human body in the anechoic chamber show an accept-

able agreement. The values were obtained as the mean of three

measurements of the field strength, from 0 to 180 with a step

of 10 . A signal generator and a spectrum analyzer were uti-

lized. The antenna used is a commercial UWB antenna with

an efficiency of 85% at 5 GHz, a return loss of 15 dB, and

swept gain of 4 dBi. In order to provide minimum return loss

from the antenna ( 9 dB), the antenna was placed on an

anechoic foam material of size 10 10 1 cm to model the

ground plane. A similar setup is also presented in [15]. One can

use an inverse Fourier transformation to model a line source ex-

citation to a point source, similar to [15]. For comparison rea-

sons, the known antenna pattern was extracted from the mea-

sured values. The agreement of the results validates the accu-

racy of the proposed single UTD solution to model humans as

cylinders that incorporate surface-mounted antennas.

III. RAY SOLUTION OF MULTIPLE EFFECTS

This section presents a ray solution based on UTD that in-

corporates a single formulation to model the radiation, scat-

tering, coupling, and combination of the three phenomena in a

multiple-cylinder configuration. The superiority of the ray so-

lution compared to analytical models [16] is that it provideschannel predictions, like the field strength, angles of arrival,

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940 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 9, 2010

Fig. 2. Ray representation of the field and shadow boundaries for the case of abody-to-access point WBAN with two humans modeled as closed cylinders.

and delay spreads with less CPU and complexity. Furthermore,

it can be easily converted in the time domain, to model UWB

with Laplace transformations. The scenario under investigationis presented in Fig. 2.

Two closed cylindrical surfaces represent the human bodies.

One antenna is mounted on the surface of body #2, and body #1

constitutes the obstruction to the transmitted by the access

point (TX) signals. For the multiple-cylinder scenario, the field

cannot be modeled only as the sum of the fields approaching the

receiver from the left and right side of the body, but cross-prop-

agating rays are of major importance to achieve field continuity

at the shadow boundaries. Compared to the case of a single

body (Fig. 1), where the scenario incorporates two shadow

boundaries ( and ), the multiple-cylinders case

incorporates shadow boundaries, as shown in Fig. 2. Theindices 0, 1, 2, 3 represent the  TX ,  body #1,  body #2, and  RX ,

respectively.

Based on the multiple-cylinder UTD solution, from open

structures [8], the received field at position for the case of 

multiple closed cylinders incorporating slope diffraction terms

can be modeled as

(5)

In (5), the first two terms characterize the received field from the

left and right side of the scenario if the cylinders were assumed

as open structures. The last term represents the cross-propa-

gating rays due to the cross-shadow boundaries. In this

case, the operator represents the cross-propagating rays andcan be either or , in any order, according to the nature of the

Fig. 3. Received field for the configuration shown in Fig. 2.

propagating ray. The amplitude diffraction or reflection coeffi-

cients, defined as a common parameter , in (5) are the coeffi-cients presented in (2) and (3), taking into account the modifica-

tions for the transition function. The slope diffraction terms

are calculated in [8]. The distance parameters that are included

in the diffraction and reflection terms are computed according

to the continuity equations of [8] and are given by

(6)

(7)

IV. RESULTS

Fig. 3 presents the importance of cross-propagating rays to

uniform field predictions. The scenario under investigation is

the same as in Fig. 2. Body #1 ( m) is placed at a

distance m and m. Body #1 is moving on a

direction normal to the line between TX and body #2 (

m) with m m relative to axis .

The receiver is surface-mounted on body #2 ( ) at

. The difference between the case where all possible rays

are considered and the case where the cross-propagating rays are

not taken into account is presented. It is obvious that without thecross-propagating field terms, the results are not uniform when

the transition boundaries and coincide with

and , respectively.

Scenario 1 of Fig. 4 describes the case of an access point

placed at a distance m from body #1, which is placed

at a distance m from body #2. Body #2 has a sur-

face-mounted antenna ( ),and for the measurements, it

is rotated with a step angle of 10 around its axis in the anechoic

chamber. The proposed solution is compared to measurements

at the frequency of 5 GHz for the hard case. There is an accept-

able accuracy between theory and measurements. The observed

ripples of the measured field at are due to the

multipaths created by the arms of body #1 that are in LOS withthe RX. At the shadow region of body #2 ( ), the

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KOUTITAS: MULTIPLE HUMAN EFFECTS IN BODY AREA NETWORKS 941

Fig. 4. Case of a body-to-access point and body-to-body WBAN with twohuman bodies modeled as cylinders        ,        S/m.

Fig. 5. Relative power delay profile for the scenario of Fig. 2 with body #1center moving from 0    1.2 to 0 m with equal steps, creating seven position IDs.

measured field is more stable as the multipaths from the arms of 

body #1 are attenuated due to surface diffraction over body #2.

Scenario 2 of Fig. 3 represents the case of a body-to-body com-

munication where the transmitting antenna is surface-mounted

( ) on body #1 at . The observation is that

both scenarios have uniform field predictions. This is because

all possible ray paths, including slope field terms and continuity

equations, are considered in the simulations.

Fig. 5 presents the simulated relative delay spread of the mul-

tipaths for the scenario of Fig. 2. It can be observed that six dom-

inant rays describe the field variations, and these are represented

as depending on their orientation. At , where the

two bodies are aligned, the relative delays are minimized due to

the similarity of the distance traveled by the multipaths.

V. CONCLUSION

A uniform ray solution for the case of multiple closed cylin-

drical bodies was presented. The proposed algorithm utilizes

slope UTD approximation and describes field variations due to

radiation, coupling, scattering, and a combination of the three,

with a single formulation. The algorithm was applied to body-

area-network channel estimations and was proven to be uniform

and accurate compared to MoM and measurements in an ane-

choic chamber. The cross-propagating rays around the bodies

of the scenario were proven to be of significant importance in

obtaining uniform results.

ACKNOWLEDGMENT

The author would like to thank Prof. D. Chrissoulidis,

Assoc. Prof. T. Youltsis, Dr. A. Goulianos, and Ch. Liontas

for their important contributions to the measurements at the

anechoic chamber of Aristotle University of Thessaloniki,

Greece.

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