Statistical Channel Modeling 2013

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AbstractA great amount of work on antennas and

propagation for body-centric wireless communication has been

studied at frequencies up to X band; however, on-body radio

propagation at millimeter/sub-millimeter wave frequencies still

remains largely unexplored. This paper presents a study of on-

body radio propagation at 94 GHz, particularly focusing on the

analysis of specific channels such as waist-to-torso and head-to-

shoulder links. Measured data are compared with results

obtained with numerical simulations emphasizing the sensitivity

of the simulated path loss to the positioning of the receivers with

respect to the human body.

I ndex TermsOn-body propagation, millimeter/sub-

millimeter waves, path loss, ray tracing.

I. INTRODUCTIONODY-CENTRIC wireless communications are now regarded

as a well-established subject area, with many published

papers addressing different aspects related to antennas and

propagation [1]-[3]. In particular, on-body radio propagation is

related to the Body Area Networks (BANs), in which human

subjects are considered as a communication medium, in

contrast with conventional indoor/outdoor communications,

where they are just considered as electromagnetic scatterers

and sources of fading [4]-[6]. BANs are relevant to a wide

range of applications, of both military and civil relevance,

including augmented reality, vital signs monitoring and

interactive entertainment [7], [8]. One of these is the

development of advanced dismounted infantry, which includes

various wireless devices integrated in the soldiers gear

(clothes, weapons, armor) [9], [10], aimed at increasing its

performance and effectiveness. Many BAN antennas and on-

body propagation studies have been focused on 2.4 GHz,

5.8 GHz and UWB bands [1], [11]-[13], while various groups

have devoted themselves to the study of network topology andtransmission protocols [14]-[16].

Although antennas and radio propagation for on-body

communications at frequencies up to X band have been

studied extensively, research interests in higher frequency

Manuscript received June 15, 2012. This work was supported by the U.K.

Engineering and Physical Sciences Research Council (EPSRC), under Grant

EP/I009019/1.

A. Pellegrini, A. Brizzi, L. Zhang and Y. Hao are with the Antennas and

Electromagnetics Group, School of Electronic Engineering and Computer

Science at Queen Mary, University of London, London, E1 4NS UK, (e-mail:[email protected] ).

bands are still growing, particularly at mm-wave/sub-mm-

wave frequencies. Indeed, on-body propagation at 60 GHz has

been recently studied [17]-[19]. This interest is justified by the

following reasons. First of all, the use of higher frequencies

would enable broadband mobile communications with an

extremely high data rate, required, for instance, in case of real-

time audio and video streaming. Moreover, a shorter

wavelength allows the realization of more compact devices,

which is of paramount importance in the design of wearabledevices. MM/sub-mm waves also demonstrate higher free-

space attenuation with respect to microwaves: from the point

of view of the security, this is an important feature, which

allows to confine the wave propagation in the proximity of the

human body, thus limiting the possibility of interference with

other systems and, in military and defense, reducing the

probability of interception by hostile forces [20].

A further advantage of mm-waves is the limited interaction

with biological tissues, reducing possible concerns relevant to

electromagnetic exposure of the human body [21]. This is

particularly due to the fact that, at these frequencies, the

penetration depth into the human tissues gets smaller and,

thus, it minimizes the absorption of waves generated by on-body devices. However, this is an area subject to further

studies and it is out of the scope of this work.

Finally, the possibility of avoiding requesting licenses is

likely to be a strong incentive to the widespread

implementation of mm-wave BAN systems: for this reason,

the frequencies around 60 GHz and 94 GHz raise particular

interest. In fact, many countries allow the unlicensed use of a

portion of the spectrum of up to 7 GHz around 60 GHz [22],

which allows data rates greater than 2 Gbit/s. Moreover, in the

USA the Federal Communications Commission (FCC) allows

the use of the frequencies between 92 and 95 GHz for

unlicensed indoor applications [23], [24] and the European

Telecommunications Standards Institute (ETSI) was invited to

follow this indication.

An additional advantage of these two bands is that they do

not seem to be good candidates for point-to-point long range

multi-Gbit/s links [25], [26]: this reduces the risk that such

frequency bands get overcrowded by an excessive amount of

applications.

In addition, the high free space loss also contributes to the

reduction of the BAN-to-BAN interference. Table I reports the

calculation of path losses in free space for various distances

Statistical Path-Loss Model for On-body

Communications at 94 GHz

Alessio Brizzi, Student Member, IEEE, Alice Pellegrini, Lianhong Zhang, Member, IEEE,and Yang Hao,Fellow, IEEE

B


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relevant to body-centric systems at 94GHz. On the other hand,

although the atmospheric absorption increases with the

frequency [27], its contribution to the on-body link is still

negligible at the considered distances.

TABLEI

ATMOSPHERIC ABSORPTION AND FREE SPACE PATH LOSS AT 94GHZ FORBODY CENTRIC COMMUNICATION LINKS

Distance[m]

AtmosphericAbsorption [dB]

Free Space Path Loss[dB]

0.3 1.5e-4 61.4

1 5e-4 71.9

5 2.5e-3 85.910 5e-3 91.9

However, the use of mm-waves for on-body applications

presents significant challenges. If the high free space

attenuation helps in confining the energy around the human

body and mitigates the risk of interference, it also results in

high attenuation on the transmitter-receiver link. The free

space attenuation over a 50 cm link increases from 34 dB at

2.45 GHz to 62 dB and 66 dB at 60 GHz and 94 GHz

respectively. The wavelength, which is equal to 5 mm at60 GHz and 3.2 mm at 94 GHz, also makes the human body a

scatterer with extremely large electrical dimensions: this may

introduce heavy fades due to shadowing effects, as the loss of

the line-of-sight (LOS) link is highly possible in relation to

movements of human body parts.

A further point that differentiates BANs at mm-waves from

those at lower frequencies is that the presence of clothing

cannot be considered negligible, as the majority of

investigations at lower frequencies have implicitly or

explicitly implied [1], [28], [29]. The typical thickness of

clothes, ranging from tenths of millimeter up to a few

millimeters, is comparable to the wavelength at V and W

bands, and it has been demonstrated how their presence can

affect the power transmission coefficient between air and skin

at 60 GHz [21].

For the above mentioned reasons, the on-body environment

is potentially hostile to the propagation of millimeter waves.

Therefore, the characterization of the propagation channel is

urgently needed for the development of reliable mm-wave

BANs. So far, modeling of the propagation channel up to

frequencies at the X band has been relying mainly on

measurement campaigns and full-wave simulations based on

the Finite-Difference Time-Domain (FDTD) method [28],

[30]-[34]. If at these frequencies the FDTD has the advantage

of being able to take into account both in-body and off-bodypropagation, at mm-wave frequencies such an approach does

not seem to be feasible: the electrical dimensions of the human

body (in the order of hundreds of wavelengths) make the

computational burden of an FDTD simulation extremely high.

In addition, the penetration depth is small [35], [36], resulting

in a remarkable fraction of the computational time being spent

to calculate the negligible field inside the human body.

Therefore, the use of ray-based methods has been considered

[17], [18].

The aim of this paper is to provide a preliminary path loss

characterization for on-body communication at 94 GHz and to

evaluate the tradeoff between experimental investigation and

numerical prediction. This is achieved through the

investigation of the on-body propagation channel over the

head-shoulder and waist-torso links at 94 GHz. A

measurement campaign has been carried out, considering

various positions of the receiver on the shoulder and the chest

area and the collected data have been compared with the ones

obtained by using ray-based methods applied to a similar bodycentric scenario. In addition, measured data have been

compared with similar results obtained at lower frequencies

[37].

Finally, in order to evaluate the sensitivity of the

methodology to the position of the receivers, two different

grids of receivers were considered for the waistto-chest link:

a planar and a conformal one. The commercial software

Remcom XGTD [38], which implements a combination of GO

(Geometrical Optics) and UTD (Uniform Theory of

Diffraction), has been used to analyze the aforementioned

links.

The rest of the paper is organized as follows. Section II

describes the measurement campaign carried out on the

investigated channels. In Section III the numerical simulation

has been described and in Section IV a comparison between

the simulated data and the measured ones has been shown.

The conclusions are drawn in Section V.

II. EXPERIMENTAL CHARACTERIZATIONTwo standard flanged rectangular waveguides (WR-10) at

94 GHz have been used as transmitter and receiver by placing

them in proximity of a human subject in different reciprocal

positions. These antennas present a less directive pattern

compared to the commercially available W-band hornantennas and, therefore, they appear to be less sensitive to the

misalignment between the transmitter and the receiver.

The path loss relative to two different links has been

measured: the head-shoulder link and the waist-torso link. In

the former case, the transmitter antenna has been fixed on the

head of the subject, above the ear; the receiver has been placed

near the shoulder, on the same side of the head, in

correspondence of several distances from the transmitter

(Fig. 1a). In the latter case, the transmitter has been placed on

the left side of the belt and the receiver has been moved in

order to scan a grid of different positions placed parallel to the

body in the chest area (Fig. 1b). The dimensions of the grid

have been chosen in order to cover the torso area; in

particular, the grid is set to be 30 cm by 36 cm. The spacing

(equal to 3 cm) has been fixed in order to obtain a good trade-

off between having a sufficient amount of data and limiting

the measurement time. The distance between the center of the

waveguide and the body surface is 1cm, resulting from the

flange of the waveguide plus a thin spacer to avoid direct

contact between the waveguide and the human subject. The

measured data were collected on a conformal grid due to the

body curvature. Both the transmitter and the receiver have

been placed on top of the cloths. Fig. 2 shows the


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measurement setup without the presence of the human subject

(a), and with the human subject for the waist-to-torso link (b-

c) and for the head-to-shoulder link (d). The absorbers used to

minimize scattering from the measurement system were

removed in (d) for a clearer visualization of the set-up.

The waist-torso link has been investigated considering the

human subject wearing two different types of clothes: a thin

cotton T-shirt and a thick wool sweater.

The measurements of the path loss for the described linkshave been carried out by using the system setup depicted in

Fig. 3. A Continuous Wave (CW) generator has been used to

generate a signal at 10.4 GHz, which represents the input for

the frequency multiplier.

(a) (b)

Fig. 1. Position of transmitter and receiver: head-shoulder link (a) and waist-

torso link (b).

The signal at 94 GHz, output of the multiplier, is decoupled

by the 20 dB directional coupler. This module provides both

the feeding for the open-end rectangular waveguide WR-10

and the input signal for the mixer.

(a) (b)

(c) (d)

Fig. 2. Measurement setup: no human subject (a), waist-to-torso link (b-c)

and head-to-shoulder link (d).

This module extracts the 9th harmonic of the 94 GHz signal

and combines it with the one generated by the Local Oscillator

(LO) in order to obtain a reference signal at a frequency of

20 MHz, which represents the input for the Vector Network

Analyzer (VNA). On the receiving path, the second

waveguide (WR-10) is remotely controlled by a mechanical

scan with a precision of 0.1 mm. The 9 th harmonic of the

received signal is combined with the one generated by the LO

in order to obtain a test signal again at 20 MHz. Finally, theVNA allows comparing the received signal with the reference

one, both at 20 MHz, in order to obtain the path loss in terms

of S21 parameter.

Fig. 3. Measurement system for evaluation of path loss over the waist-torso

and head-shoulder links.

Firstly, the head-shoulder link has been considered; in order

to scan the shoulder area, the receiving antenna has been

moved covering four different distances from the head. The

measured path loss has been reported in Table II.

TABLEII

HEAD-SHOULDERMEASURED PATH LOSS FORDIFFERENT DISTANCES

Head-shoulder

distance [mm]

Measured Path

Loss [dB]

24.0 36.5

26.0 3928.0 40.8

30.0 41

For what concerns the waist-torso link, the propagation

channel has been investigated in terms of path loss exponent

and shadowing factor according to the following model [39],

[40]:

() () () (1)

where PL(d0) is the estimated path loss at the reference

distance d0 between transmitter and receiver, is the path loss

exponent and N(0,) is a normal distribution which has zero

expectation and standard deviation , representing the

shadowing factor. Actually, several statistical models have

been considered for the latter, and the t-location scale

distribution reveals to be the best fit according to the Log-

likelihood criterion. However, the discrepancy between

RX

TX

RX

TX

CW

10.4 GHzLO

10.4 GHz

Multiplier

94 GHz

Directional

Coupler

9th Harmonic

Mixer

9th Harmonic

Mixer

Vector Network Analyser

S21


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this distribution and the normal one is less than 5%.

Therefore, a normal distribution of the shadowing factor

has been here considered to avoid increasing the

complexity of the model.

By observing the previous formula, it can be noticed that

the path loss is assumed to be dependent on the logarithm of

the distance d between the transmitter and the receiver,

normalized to the minimum distance d0 between the two

antennas. According to the linear model expressed in (1), thepath loss exponent , which allows taking into account the

propagation in a complex environment, is equal to 2 for

propagation in free space and is expected to be higher for

propagation in presence of scatterers and obstacles. Two sets

of measured data have been obtained considering the human

subject wearing firstly a thin cotton T-shirt and then a wool

sweater. The measurements have been carried out considering

a minimum distance d0 of 10 cm. The two sets of measured

data have been plotted in Fig. 4. According to the sensitivity

of the instruments, path loss contributions higher than 80 dB

have been neglected.

By considering the above mentioned figure, in order to

obtain the path loss exponent of the measured data, the linear

regression has been calculated.

In particular, in the case of the data shown in Fig. 4, relative

to the measurements performed on a human subject wearing a

thin cotton T-shirt, the path loss exponent is 4.4; in the case of

the human subject wearing a thicker wool sweater (Fig. 4), the

coefficient in (1) is equal to 4.5. The higher value in the case

of a wool sweater is probably due to the combination of

different dielectric and geometrical properties with respect to

the previous scenario. The higher electrical conductivity of

wool with respect to cotton, and the larger thickness of the

sweater in comparison with the T-shirt make the space close to

the human trunk more adverse to propagation. The CumulativeDistribution Function (CDF) of the shadowing factorN(0,,),

obtained according to (1), is shown in Fig. 5 for the two

measured sets of data.

In the case of the measurements performed on the human

subject wearing a cotton T-shirt, the standard deviation of the

curve which fits the data is equal to 6.4, while in the case of

the wool sweater the same parameter is equal to 8.7. The

increase in the shadowing factor confirms that the propagation

channel is significantly affected by the presence of clothes. A

comparison with the results obtained by Sani et al. [37], [43]

at 2.4 GHz in a similar scenario is shown Table III. Due to the

dependence of the electromagnetic properties of the human

tissues on the frequency, in the two referred cases, thedielectric permittivity and the electrical conductivity are

respectively higher and lower than the correspondent ones at

94 GHz (Table IV) [36].

Paper [37] analyses four different antennas (microstrip

rectangular patch, planar monopole, point source and inverted

L), and the mean values of and have been reported in the

Table III. In [43] various subjects are considered: out of them,

Male 02 is very similar to the subject of the measurements of

the present investigation (1.78 m height per 74 Kg weight),

and the relevant values are reported in the same table. It can be

noticed how both the path loss exponent and the shadowing

factor are higher in the 94 GHz scenario, even if the antenna

considered in this work presents a greater gain and a good

omnidirectionality over the area of interest. This can be

ascribed to the bigger electrical dimensions of the obstacles

over the body surface (curvature of waist, stomach and chest),

which increase the scattering and obstruct the propagation.

TABLEIIICOMPARISON OF PATH LOSS EXPONENT AND SHADOWING FACTOR AT

2.4 GHZ AND 94GHZ

Analyzed ScenarioFrequency

(GHz)Pathloss

ExponentShadowing

Factor

Hugo Model [37] 2.4 3.8 5.2

Male 02 [43] 2.4 2.8 3.9

Cotton t-shirt 94 4.4 6.4

Wool sweater 94 4.5 8.7

TABLEIV

ELECTROMAGNETIC PROPERTIES OF DRY SKIN AND MUSCLE AT 2.4 GHZ AND94GHZ[36]

Frequency Dry Skin rDry Skin

[S/m]Muscle r

Muscle

[S/m]

2.4 GHz 38.06 1.44 52.79 1.7094 GHz 5.79 39.18 9.01 61.46

Fig. 4. Measured path loss for the waist-torso link: human subject wearing a

cotton T-shirt (blue) and human subject wearing wool sweater (red).

Fig. 5. Cumulative Distribution Function of the shadowing factor for

measured data.

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8

0

10

20

30

40

50

60

70

80

90

0 2 4 6 8

Path

Loss[dB]

10log(d/d0)

Measured data (Wool Sweater)

Linear regression (=4.51)

Measured data (Cotton T-shirt)

Linear regression (=4.36)


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III. NUMERICAL ANALYSISThe numerical analysis has been performed by using the

software RemCom XGTDv2.5 which implements a

combination of Geometrical Optics and Uniform Theory of

Diffraction (GO-UTD).

The modifications of the radiation patterns of the antennas

due to the proximity of the human body have been taken into

account.

Fig. 6. Digital phantom of a male human body.

A. Numerical ModelThe investigated numerical model, shown in Fig. 6, is

shaped to have dimensions similar to the subject of the

measurement campaign, and consists of a 3-dimensional

surface composed by triangular facets. This model has been

obtained by means of the statistical analysis of MRI (Magnetic

Resonance Imaging) scans of a population of several human

subjects which present different ranges of shape [41].

In order to take into account the electromagnetic properties

of the tissues and the fabrics, the model has been considered

as a stratified structure. In particular, three different areas can

be distinguished according to the clothes worn by the human

subject used for the measurements. Considering the small

penetration depth at the investigated frequency of 94 GHz[30], the body has been assumed as having the dielectric

characteristics of dry skin: hence the fictitious thickness of

30 mm assigned to the skin layer. Following this assumption,

the parts uncovered by the clothes, such as the head, are

associated to a single layer of skin.

(a) (b)

Fig. 7. Layered model: according to the position of the clothes, three differentareas of coverage can be distinguished.

In order to have as much similarity as possible between the

simulated scenario and the measured one, two models wearing

thin cotton T-shirt and a wool sweater have been considered.

As indicated in Fig. 7, the former is represented by a layer of

dry skin, a layer of air (1 mm of thickness) and a layer of

cotton (1 mm of thickness). In the latter model a layer which

has the electromagnetic properties of the wool replaces the one

which represents the cotton fabric. In addition, a model

entirely made of dry skin has been considered for a

comparison.

In Table V the electromagnetic properties and the

thicknesses of each material have been considered [36], [42].

TABLEVELECTROMAGNETIC PROPERTIES AND THICKNESS OF THE LAYERED

STRUCTURE

MaterialRelative Dielectric

ConstantElectric

Conductivity [S/m]Thickness

[mm]

Dry Skin 5.79 39.18 30

Cotton 1.5 0.01 1

Wool 2 0.1 5

Air 1 0 1

As described in the previous section, the path loss of two

different sets of reciprocal positions of the transmitter and the

receivers has been evaluated.

In order to simulate a scenario as similar as possible to the

measurement setup, the transmitter and receiver positioningdescribed in Section II (Fig. 1) has been replicated. For the

waist-to-torso link, both a flat and a conformal grid of

receivers have been taken into account. It is worth mentioning

that in the case of the flat grid, the transmitter does not lay on

the plane defined by the receivers; therefore the LOS

condition is not guaranteed for all the receivers. The two grids

were considered in order to evaluate how the agreement

between simulated and measured results changes according to

the accuracy of the simulated scenario. Setting a conformal

grid is, in fact, a very time-consuming process, and it has to be

repeated every time when a different numerical phantom is

considered. If it is accurate enough, the use of a flat grid

would otherwise be useful to speed up the analysis process.

B. Pattern Evaluation in proximity of the human bodyIn a typical body-centric scenario, the antennas are required

to operate in proximity of the human body. Therefore,

depending on both the shape and the electromagnetic

properties of the human body at the investigated frequencies,

the performance of the antennas, in terms of radiation pattern,

can be affected. In order to take into account this effect, a

simulation of the flanged rectangular waveguide, used in the

measurement setup, operating in proximity of a slice of skin,

has been performed in CST Microwave Studio.

As described in Section II, for both the investigated links,

the antenna can assume two different positions with respect tothe human body. In particular, the E plane of the rectangular

waveguide is orthogonal to the body when the transmitter is

placed on the head. Transmitters and receivers placed in the

area of the shoulder, of the chest and of the belt, exhibit the E

plane parallel to the body. In order to address this issue, the

flanged rectangular waveguide has been simulated in

proximity of the human body according to the two above

described reciprocal positions. Therefore, a flat digital

phantom which presents the properties of the dry skin

(Table V) has been placed parallel and orthogonal to the E

Wool

Dry Skin

AirCotton

Dry Skin

Air


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plane of the waveguide, as indicated in Fig. 8 (a) and Fig. 8

(b), respectively.

In order to evaluate whether additional losses related to the

impedance mismatch due to the proximity of the human body

should be considered, the distance between the antenna and

the body has been set to different values. An S11 lower than

-10dB has been observed in every case; therefore the

impedance mismatch of the flanged waveguide used in this

study does not affect the path loss evaluation.The radiation patterns, evaluated on the E and H plane, of

the flanged waveguide operating near a vertical and a

horizontal digital phantom are shown in Fig. 9 and Fig. 10

respectively. By referring to the these figures, it is important

to note that the symmetry of the radiation pattern is not

preserved on the E or on the H plane, according to the

operative conditions of the antenna in proximity of the human

body.

(a) (b)

Fig. 8. Flanged rectangular waveguide operating in proximity of a slice

of human body: E plane parallel to the skin (a) and E plane orthogonal

to the skin (b).

(a) (b)

Fig. 9. Gain of the flanged rectangular waveguide operating in proximity of avertical slice of dry skin: E plane (a) and H plane (b).

(a) (b)

Fig. 10. Gain of the flanged rectangular waveguide operating in proximity ofan horizonthal slice of dry skin: E plane (a) and H plane (b).

The evaluated radiation patterns have been imported in

Remcom XGTD and then assigned to both the transmitter and

the receiver. The transmitting and receiving antenna have been

aligned according to the polarization of the electric field, as

indicated in Fig. 11 (a)-(b) for both head-shoulder and waist-

torso link. In order to achieve a good trade-off between

numerical accuracy and computational burden, in addition to

the direct ray, 4th order contributions for reflected rays and 1 st

order contribution for the transmitted ray have been taken into

account. Moreover, contributions due to wedge and surfacediffraction have also been considered.

(a) (b)

(c)

Fig. 11. Antenna position for head-shoulder link (a) and for waist-torso link(b). Head of digital phantom with ear (c).

IV. COMPARISON OF RESULTSTo evaluate the reliability of the ray-tracing technique for

predicting on-body radio propagation, a comparison between

the measured and simulated data has been carried out.

Firstly, the head-shoulder link has been examined. The

simulated path loss has been compared with the measured one

(Table II) for four different transmitter-receiver distances; the

results are shown in Table VI. Both the cases with and without

the presence of the ear(Fig. 11 (c)) have been considered.

Moreover, a comparison between the data obtained by

considering a model entirely made of dry skin and a model

wearing woolen fabrics, described in Section III (a), has been

shown in Table VI, as well.By referring to Table VI, it can be noticed that, for the

considered link there are no significant differences between

the path loss values obtained by considering human body

models made of different materials.

For what concerns the comparison with the measured data,

it is important to note that for the head-shoulder link, the

transmitter and the receiver are in Line of Sight (LoS) for each

investigated reciprocal position, therefore the direct ray

represents the main contribution to the path loss calculation.


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TABLEVI

COMPARISON BETWEEN SIMULATED AND MEASURED PATH LOSS FOR THEHEAD-SHOULDERLINK AT DIFFERENT DISTANCES FORDIFFERENT CLOTHES

WITH AND WITHOUT THE PRESENCE OF THE EAR

Analyzed scenariodistance

[cm] 24.0distance

[cm] 26.0distance

[cm] 28.0distance

[cm] 30.0

Measured Path Loss 36.5 39.0 40.8 41.0

Simulated Path Loss

(wool sweater

model)

52.4 46.7 38.9 45.4

Simulated Path Loss

(dry skin model)52.5 46.8 41.0 45.3

Simulated Path Loss

(wool sweater

model) + ear

25.7 36.8 41.8 41.0

Path Loss (dry skin

model)+ear25.7 38.2 40.7 41.3

The presence of the ear makes the model more realistic,

indeed diffraction from the ear affects the path loss evaluated

at each receiver providing very accurate results compared with

the measured ones. On the other hand, this analysis confirms

that the 24 cm link is more critical than the others. Thedisagreement in the data could be due to mis-shaped antenna

radiation pattern associated with the transmitter and the

receiver. In the specific case of the minimum distance here

considered (24 cm), the difference in the measured and

simulated path loss is mainly due to the proximity of the head.

Indeed the local orientation of the facets can strongly affect

the propagation direction of the rays. In this latter case, the

contribution due to multiple bounces or diffracted rays

become more significant, and a small difference between the

real and simulated scenario can bring to a remarkable punctual

discrepancy between the two. However it is worth mentioning

that the head-to-shoulder link has validity in the light of a

preliminary point-to-point analysis. In fact, this link isstrongly affected by the movements of the head: therefore it

requires a detailed statistical analysis, which is out of the

scope of the present paper and will be the subject of future

investigations.

Subsequently, results for the waist-torso link have been

compared with the measured ones in terms of path loss

exponent as described in Section II. Fig. 12 shows path loss

values, obtained as a function of the logarithm of the

normalized distance between the transmitter and the receiver,

for both a conformal and flat grid, relative to three different

cases: the model has the properties of dry skin, cotton T-shirt

and woolen sweater.

In Fig. 13 the CDF of the shadowing factor, expressed in

(1), evaluated in three simulated cases, is shown.

The comparison between simulated and measured data is

summarized in Tables VII and VIII in terms of path loss

exponent and shadowing factor of data. The simulated data

have been obtained considering both a flat and a conformal

grid of receivers.

By observing the data shown in Tables VII and VIII, it can

be noticed that the presence of thin cotton T-shirt does not

significantly affect the estimation of the path loss exponent

with respect to the case of the model characterized only by dry

skin.TABLEVII

PATH LOSS CALCULATION FORMEASURED AND SIMULATED DATA

Considered

surfacesMeasured

Simulated

(flat grid)

Simulated

(conformalgrid)

Dry skin N.A. 3.7 4.5

Dry skin + cottonT-shirt

4.4 3.7 4.7

Dry skin + wool

sweater4.5 4.2 4.8

TABLEVIII

SHADOWING FACTORCALCULATION FORMEASURED AND SIMULATED DATA

Considered

surfacesMeasured

Simulated

(flat grid)

Simulated

(conformalgrid)

Dry skin N.A. 8.0 8.9

Dry skin + cotton

T-shirt6.4 8.8 9.1

Dry skin + wool

sweater8.7 7.1 8.3

In addition, by comparing the simulation results, obtained

with a flat and a conformal grid, higher values of the path loss

exponent can be noticed in the latter case. This phenomenon

is, as expected, mainly due to the higher number of shadowed

receivers.

In the case of a flat grid, the discrepancy between simulated

and measured path loss exponent is 16% and 7% for the T-

shirt and wool case respectively. In the case of a conformal

grid the percentage of discrepancy is now reduced to about

7% for both fabrics.

Furthermore, in both measurement and simulation with the

conformal grid there is only a 2% increase in whenchanging

from cotton T-shirt to wool sweater, while in the case of a flat

grid the increase is 13.5%.On the other hand, it can be noticed how, in the simulations,

the decreases when the human subject clothing changes from

the T-shirt to the wool sweater, while it demonstrates an

opposite trend from the measurements, therefore highlighting

the limitation in the accuracy of the ray-tracing methods in the

investigated scenarios.

A further limitation in the use of the ray-tracing method is

the discrepancy in the values of PL (d0): although the punctual

values at the reference distance agree with the measured ones,

the linear regression yields a lower value.

V. CONCLUSIONAn investigation of a body-centric scenario performed at

94 GHz has been shown in this paper. To this aim a campaign

of measurements has been performed in presence of a human

subject. In addition, in order to investigate the reliability of

ray-based techniques applied to the study of BANs,

simulations have been carried out by using Remcom

XGTDv2.5. The path loss obtained by the simulation has been

compared with the measured one. For what concerns the head-

shoulder link, the discrepancy between measured and


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simulated data is mainly due to the difference in reproducing

the human subject head.

(a)

(b)

(c)

Fig. 12. Simulated path loss for the waist-torso link by using both a

conformal and a flat grid: human subject made of dry skin (a), humansubject

wearing a cotton T-shirt (b) and human subject wearing wool sweater (c).

In fact, as demonstrated in the case where the ear is

modeled, a better agreement between measurements and

simulations is achieved. Additionally, the analysis points out

that the proximity of the transmitter with respect to the head

plays a key role in the accuracy of the results.

The local orientation of the facets in the model can strongly

affect the path of the rays from the transmitter and the

receiver.

Fig. 13. Cumulative Distribution Function of the shadowing factor for

simulated data

This effect is more visible when the collected data are not

enough to trace a statistical analysis while a value-to-value

comparison is required. In addition, the simulations havedemonstrated that the presence of clothes in the numerical

model, such as the wool sweater, does not significantly affect

the path loss. For what concerns the waist-torso link, the

comparison of the path loss exponent model obtained both for

the simulated data (in the case of a flat and a conformal grid of

receivers) and measured ones has been discussed. A linear

regression of the data has been evaluated in terms of path loss

exponent and shadowing factor and a normal distribution has

been considered to model the latter. A discrepancy of the path

loss exponent between 16% and 7% is obtained for the T-shirt

and wool case respectively for a flat grid of receivers. This

discrepancy is dramatically reduced to 7% for both fabrics in

the case of a conformal grid of receivers.

However, discrepancies in other statistics were observed,

such as PL (d0) and shadowing factor. In general, although the

numerical model has shape and dimensions similar to the

human subject used for the measurements, the exact geometry

of the curvatures of the body and the details of the clothes

were not exactly reproduced. These differences between the

measured and simulated scenarios, at the investigated

frequencies, contribute to perturb the propagation from the

transmitter to the receiver. In addition, another uncertainty in

on-body measurements can result from the modification of the

radiation pattern of both transmitting and receiving antennas

in proximity of the human body. This effect can be accountedin the simulation by incorporating a more realistic radiation

pattern, however small changes depending on the particular

positions are difficult to replicate.

In the light of these considerations, the statistical analysis

presented here demonstrates that a ray tracing technique is

suitable for a macroscopic description of a body centric

scenario, such as the path loss exponent calculation over the

trunk area. On the other hand, the agreement between

measured and simulated data has an extremely strong

dependency on the accuracy of the simulated scenario, in

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8

Simulated data (Dry Skin Flat Grid)

Simulated data (Dry Skin Conformal Grid)

Linear regression Flat Grid (=3.7)

Linear regression Conformal Grid (=4.5)

10log(d/d0)

Path

Loss[dB]

0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8

Simulated data (Cotton T-shirt Flat Grid)

Simulated data (Cotton T-shirt Conformal Grid)



10log(d/d0)

Path

Loss[d

B]

Simulated data (Wool Sweater Flat Grid)

Simulated data (Wool Sweater Conformal Grid)



0

10

20

30

40

50

60

70

80

90

0 1 2 3 4 5 6 7 8

10log(d/d0)

Path

Loss[dB]


9/10




terms of body shape and positioning of the antennas.

Besides, the analysis presented in this paper shows that, in

order to possibly obtain a generalized path loss model that can

provide accurate link budget evaluation for different subjects,

a complete and thorough investigation of the path loss

variation with body shape and garments would be required.

ACKNOWLEDGMENT

The authors thank EPSRC for providing the funding for this

research activity, under Grant EP/I009019/1, Dr Su-Lin Lee,

and Professor Guang-Zhong Yang at the Department of

Computing, Imperial College London, for providing the digital

phantom. The authors would like also to thank Dr. Anestis

Katsounaros and Mr. Max Munoz, both with the School of

Electronic Engineering and Computer Science at Queen Mary

University of London, for their precious support in this work.

Finally, the authors would like to thank Prof. Peter Hall for

the fruitful discussions.

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Electronic Engineering and Computer Science, Queen Mary Universityof London, UK, 2010.

Alessio Brizzi received the degree inTelecommunication Engineering from

University of Pisa in 2003. He worked forthree years as a contractor for the Microwave

and Radiation Laboratory at University ofPisa, subsequently moving to a consulting

company as technical consultant.He is currently pursuing the Ph.D. degree inElectronic Engineering at Queen Mary,

University of London.His research focuses on millimeter waves for

body-centric communications, specifically on the characterizationand modeling of the propagation channel, and on the design ofantennas for on-body systems. His research topics also include

numerical modeling and nanocommunications.

Alice Pellegrini received the Laurea degree(cum laude) in Telecommunication

Engineering in Applied Electromagneticsfrom University of Pisa, Italy, in October

2005. She achieved the PhD in InformationEngineering at the Microwave and

Radiation Laboratory, within theInformation Engineering Department of theUniversity of Pisa, in May 2009. Her mainresearch activity concerned the study ofinnovative numerical methods and hybrid

techniques, based on Mode Matching, Finite Element Methodcombined with the Spectral Decomposition approach, for analysingFrequency Selective Surfaces and finite large phased arrays ofradiating apertures. Currently, she is enrolled as PostDoctoralResearch Assistant at Queen Mary University of London, with the

School of Electronic Engineering and Computer Science. Her mainactivities are relevant to analysis, simulation and measurements in thefield of Body Area Network (BAN) applications at millimetre waveswith particular interest in the application of high frequency ray-based

techniques. She has been co-organizer of Special Session on Body-

Centric Wireless Communications at PIERS 2013 in Stockholm.

Lianhong Zhang obtained BSc and MSc inradio physics, electronic science andengineering department, Nanjing University,

China, MSc and PhD in electronicengineering from school of electronicengineering and computer science, Queen

Mary, University of London. From 1995 to1997, he was an antenna engineer withAerospace & Aeronautical Corporation,

Shanghai, China. From 1997 to 2005 he was

a satellite communication engineer with ST Teleport, Singapore. Hehas been working as a postdoctoral research assistant in antenna andelectromagnetics lab, school of electronic engineering and computer

science, Queen Mary, University of London since July 2010. Hisresearch is in the areas of millimetre wave imaging for concealedtarget detection, body-centric wireless communications at millimetre

band, indoor radio propagation channel characterization, building

material characterization, and nanoantenna for ultrafast coherentcontrol of optical fields.

Yang Hao received the Ph.D. degree from theCentre for Communications Research (CCR) atthe University of Bristol, Bristol, U.K., in 1998.He is currently a Professor of antennas andelectromagnetics in the Antenna Engineering

Group, Queen Mary College, University ofLondon. He is active in a number of areas,including computational electromagnetics,electromagnetic band-gap structures and

microwave metamaterials, antennas and radiopropagation for body centric wireless networks,

active antennas for millimeter/sub-millimeter applications andphotonic integrated antennas. He is a co-editor and co-author of the

books Antennas and Radio Propagation for Body-Centric WirelessCommunications (Artech House, 2006), and FDTD modelling ofMetamaterials: Theory and Applications (Artech House, 2008),respectively. Prof. Hao is an Associate Editor for the IEEE

ANTENNAS AND WIRELESS PROPAGATION LETTERS, IEEETRANSACTIONS ON ANTENNAS AND PROPAGATION,International Journal of Antennas and Propagation and a honoraryeditor for the Chinese Journal of Radio Science. He was also a Co-

Guest Editor for the IEEE TRANSACTIONS ON ANTENNAS ANDPROPAGATION. He is a vice chairman of the Executive Team ofIET Antennas and Propagation Professional Network. He is also a

member of Board of the European School of Antenna Excellence, amember of EU VISTA Cost Action and the Virtual Institute for

Artificial Electromagnetic Materials and Metamaterials,Metamorphose VI AISBL. He has served as an invited (ISAP07,

LAPC07, IWAT08) and keynote speaker (ANTEM05, IWAT10), aconference General Chair (LAPC08, Metamaterials09), a SessionChair and short course organizer at many international conferences.He is a holder of the Royal Society Wolfson Research Merit Award

between 2013 and 2018. Prof. Hao was elected as a Fellow of the

ERA Foundation in 2007, a Fellow of the IET in 2010 and a Fellowof the IEEE in 2013.

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Statistical Channel Modeling 2013