Three Dimensional Motion Analysis of Antennas on Body

for BAN Channel Modeling

Iswandi†, Minseok KIM†, Yuuki TERAO†, Jun-ichi TAKADA†

† Department of International Development EngineeringGraduate School of Science and Engineering, Tokyo Institute of Technology

S6-4, 2-12-1,O-okayama, Meguro-ku, Tokyo, 158-0094 Japan

Abstract Knowledge of the on-body antenna movement due to the body motion is an important key toward auseful channel model for body area network (BAN). This paper presents the three-dimensional motion analysis of on-body antenna based on the real human body motion captured by specific motion capture equipments. The variationof distance between transmitter and receiver antennas and the antenna rotation during the the body motion derivedfrom the measured data.

Key words body area network, BAN, channel modeling, 3D-motion analysis, motion capture

1 Introduction

As a candidate technology that has abundant promis-ing application especially in medical healtcare [1], bodyarea network (BAN) has attracted much attention froma lot of researchers recently. The rapid growth in BANresearches has been acknowledged by IEEE by forming astandardization committee namely IEEE 802.15.6 TaskGroup [2]. The committee classified BAN into threecategories: communication from the body surface tonearby base station (off-body), both antennas are onthe body surface (on-body), and at least one antennabe implanted within the body (in-body). In the nextdiscussion, this paper will focus on the on-body as theinterest of the study.

Regarding its application especially in medical health-care, the committee also concerned about the reliability,low power consumption, low power emission, and com-pactness of the BAN communication systems. There-fore, since those characteristics are inextricably linkedto the radio channel, channel model is significantly re-quired to design the network architecture and devices.IEEE 802.15.6 Task Group has finalized the policy forchannel modeling in BAN that was documented in [3].These documents consist of all statistical and theoreti-cal channel models in various frequencies, transmitter-receiver positions, body postures, static and dynamicscenarios, narrowband and wideband, etc. summarizedfrom all models that have been submitted to the group.

The radio wave propagation in BAN has much differ-ent with those for general wireless communications dueto the effect of the body surface nearby the channel andon which the antennas are mounted. The existence ofbody in the near field antenna cause some power absorp-tion, pattern distortion, and additional return losses.The radio wave propagates over the body surface bycreeping or surface wave and possibly propagate throughhuman body (especially in low frequency). Moreover

in the dynamic scenario, multipath propagation has tobe taken into account due to reflection and diffractionby body part and surrounding environment. The bodymotion also produces temporal variation of distance be-tween transmitter and receiver antennas. Furthermore,the antennas suffer from a temporal rotation that candegrade the channel performance.

Some researchers have made some efforts to character-ize the effect of body motion to channel by analyzing thestatistical behavior of receiving power on the dynamicscenario [4, 5, 6]. The temporal position and rotation ofantennas have a important aspect to the channel fluctua-tion. However, the motion behavior of on-body antennain the BAN channel has not been studied due to thedifficulties of the facilities for observation. This paperpresents the analysis of the on-body antenna motion inthe dynamic BAN scenario based on the measurementon the real human body with dedicated sensors equip-ment namely to capture the human body motion.

2 Measurement Campaign

In this research, the motion of on-body antenna on thedynamic BAN channel has been observed in the mea-surement by employing the motion detector namely Mo-tion Analysis [7]. To capture the motion of the body,the system was equipped by 10 Eagle-4 digital real timecameras [8]. The cameras can sense a infra-red reflec-tive marker on its coverage area. The sensing data forall cameras were collected and analyzed by the Evart5.0 Motion Analysis to compute the coordinate positionof each marker. The software also has ability to traceeach marker along the measurement time. The systemconfiguration of the motion analysis equipment can beseen in Fig.1.

To capture the temporal antenna position during thebody motion, three infra-red reflective markers are at-

Figure 1: Configuration of motion analysis system.

(a) Transmitter antenna (b) Receiver antenna

Figure 2: Placement of marker on antennas.

tached on both transmitter and receiver antenna respec-tively. The chosen number of markers was considered tocalculate the rotation of antenna with smallest numberof marker. Since the markers are located closely eachother, the large number of marker can give difficultieson tracing process. The marker positions on both trans-mitter and receiver antennas are depicted in Fig.2. Themarkers were attached not in the radiating part of an-tenna to avoid the destruction of antenna structure. Toget the similarity of antenna dimension in the previousand probably future BAN channel measurements, theantennas for this measurement were MD1103-ST01 sur-face mountable dielectric chip antennas [9]. In addition,the measurement as presented in this paper was withoutradio transmission.

As initial result of the research, this paper presentsmeasurement on 4 channel scenes (navel-wrist, navel-ear, navel-chest, and navel-ankle) on the body postureof walking on the spot. The antennas were attached onthe body with the belt as depicted in Fig. 3. The motionof antennas were captured for 30 seconds duration and100 samples per second sample rate for each scene.

3 Result of Measurement

The motion analysis was used in the measurement toobserve the on-body antenna motions by sensing theposition of each marker on the antennas and calculat-ing the position of markers. The software provided theposition data in the Cartesian coordinate system.

(a) Wrist (b) Ear (c) Chest (d) Ankle

Figure 3: Position of antenna in the measurement.

Figure 4: Transformation of coordinate systems.

3.1 Positions of on-body antenna

Since it was not aligned to the body directions, the coor-dinate system transformation was done to ease furtherdata processing and reader understanding. The positionof transmitter antenna (Navel) was used as the originpoint of the new coordinate system as shown in Fig.4.

As mentioned in the previous section, markers are notlocated in radiating point of antenna so this point wascalculated by following formulas (refers to Fig. 5).

T = m1 +m3 − c

0.5 |m3 − c|(1)

where

c = m1 −|m1 − c||m1 −m2|

(m1 −m2)

|m1 − c| =|m3 −m1|2 + |m2 −m1|2 − |m3 −m2|2

2 |m2 −m1|

the operator |a− b|means the distance between pointa and b.

The time variation of the coordinate of the antennaradiating point shows the movement of antenna duringthe body motion in three axises as shown in the Fig. 6.The figure shows only 4 seconds of 30 seconds measure-ment to ease the reader to read the plot without loosingthe information since three walking steps are includedin the plot. As a notice the number of walking step canbe seen on the hand movement in the x-axis of Fig.6b.The x-axis, the positive value shows that the antennaposition is in front of the body since the origin of axisis on the radiating point of navel-antenna. The positiveand negative value on y-axis show whether the positionon right or left side from the vertical body center andthe upper positions from the navel are shown by positivevalues of z-axis.

Figure 5: Definition of points for calculating the relativeantenna positions.

3.2 Variation of Tx-Rx antenna distance

The distance of transmitter to receiver is the key param-eter to calculate the path-loss on the BAN channel asmentioned in some path-loss model in [3]. Therefore, theinformation of this distance provide importance valuefor the channel modeling. By knowing the time varyingposition of both transmitter and receiver antenna, thedistance can be easily computed. The variation Tx-Rxantenna distance figured from the measurement can beseen in Fig. 7. In the measurement, the subject start towalking a few second after the starting time, it can beeasily seen in the figure the different of antenna move-ment on standing still and the walking posture.

3.3 Rotation of on-body antenna

Since the antennas used in some BAN measurementsare not always omni directional antenna, the antennarotation of the antenna can reduce the network perfor-mance significantly. Moreover, the existence of bodysurface in its near field of antenna can affects distortionon its pattern [10]. Therefore, it is beneficial to providethe antenna rotation on BAN.

The calculation of the temporal antenna rotation wasstarted by calculate the unit vector of antenna direc-tion. This vector is derived from the measured markercoordinate by following equations.

y =m3 − c|m3 − c|

z =m3 −m1

|m3 −m1|x = y × z (2)

The rotation of antenna is presented in three rota-tions rotations are annotated based on the relative co-ordinate plane of the movement as zy-plane, zx-plane,and xy-plane rotations. The degree of rotations can becalculated from the following equations.

Rzy(t) = arcsin(zt.yref )Rzx(t) = arcsin(zt.xref )Rxy(t) = arcsin(xt.yref ) (3)

where the subscript t means the antenna direction unitvector in time t and subscript ref means the unit vectoras the reference. The reference vector unit was takenfrom the antenna position while on the still posture.

(a) Navel

(b) Wrist

(c) Ear

(d) Chest

(e) Ankle

Figure 6: Three dimensional projection of antennamovements

Figure 7: Variation of Tx-Rx antenna distance.

The three dimensional rotation of antenna on wrist isdepicted inf Fiq.8 for 5 seconds durations. The dashedlines is the hand movement on the x-axis as used forthe reference to the physical movement. The antennaon the wrist suffered high degree of rotation due to thehand movement during walking. T

4 Discussions

In the previous sections, two parameters has been de-rived from temporal antenna position captured from thereal human body movement, that are distance of trans-mitter and receiver antennas and rotation of antenna.The first parameter provides a good accuracy becauseit is calculated from the radiating point of antenna.

The drawback of the distance information resultedfrom this measurement is the absent of informationwhether the link is the line of sight or not. Howeverit give a great beneficial to relate the distance to thepath gain variations in term of BAN channel modeling.Therefore, the measuring the distance and path gainsimultaneously will be subjected in the future investiga-tions.

The antenna rotation has been derived in three ro-tation axises referred to the initial position of antennain the body still posture. Although it have not beenpresented here, it is possible to determine the antennarotation to the transmission line. If the radiation pat-tern of antenna is available, it can be used to study thiseffect of this rotation to the channel fluctuation.

5 Conclusions

In this paper, the motion behavior of antennas attachedon the real human has been three dimensionally ob-served in walking postures and four scenes (i.e. navel-wrist, navel-ear, navel-chest, and navel-ankle) by Mo-tion Analysis. Two parameters that are useful for BANchannel modeling has been derived from the measure-ment data that are distance of transmitter to receiver

Figure 8: Rotation of antenna for Navel-Wrist scene for5 minutes duration.

antenna and three dimensional antenna rotation.

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