Human Body Transfer Function Modelfor Ultra Wideband Body Area Network

Sathaporn Promwong�, Jiraphan Sahakit�, Sanit Teawehim�, and Bundit Ruckveratham��

�Department of Telecommunication Engineering, Faculty of Engineering,King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand

��MCOT Public Company Limited, 63/1 Rama IX Road, Huaykwang, Bangkok 10310, ThailandE-mail: s1060921@kmitl.ac.th, bundit1973@hotmail.com, kpsathap@kmitl.ac.th

Abstract— A waveform distortion on human body of an ul-tra wideband body area network (UWB-BAN) system can beextremely distorted through a channel even for free-space trans-mission because of antenna dispersion. Therefore, the understandof antenna characteristics, which effects on waveform distortion,is necessary. This paper presents the waveform distortion dueto human body for wireless meadical applications based onmeasurement data. The template waveform is considered at thereceiver side to maximize the SNR for evaluation. In this resultsare evaluate based on the extended Friis’ transmission formula.This technique gives very accurate results and is very useful forthe design and evaluation of UWB-BAN transmission waveformfor wireless medical applications, especially focusing on the effectof template waveform.

I. INTRODUCTION

The antennas usually act as significant pulse-shaping filtersand cause extreme waveform distortion. Consequently, thiswill increase the complexity of the detection mechanism atthe receiver [1]. Moreover, low cost, geometrically small andstill efficient structures are required for the typical wirelessapplications. Therefore, the knowledge of waveform distortiondue to antenna is preponderant to design and improve theperformances of UWB-BAN system.

Even if the channel is in line of sight (LOS), Friis’ trans-mission formula cannot be directly applied to the UWB radioas the bandwidth of the pulse is extremely wide. Furthermore,simple comparison between waveforms of the transmitter andthe receiver is not significant because of the distortion of thewaveform caused by the frequency response of the antenna.

The purpose of this paper is to propose a new link budgetmodel for studying the waveform distortion due to antennaon free space transmission in UWB-BAN system. We developthe free space link budget evaluation scheme in the term offrequency transfer function for UWB-IR system that takesinto account the transmitted waveform, its distortion due tothe antennas, the channel and the correlation receiver. Thismodel is based on the Friis’ transmission formula, adaptedto the UWB-BAN transmission system, in the sense that wederive the equivalent frequency transfer function of UWB-BAN system [2]- [3]. The distortion quantities in the terms ofmagnitude, phase and waveform distortions, and transmissiongain are defined and shown. This scheme provides someuseful physical insights and optimized design procedure withclear and accessible description of the UWB-BAN link budgetcomprised of practical antennas. Furthermore, these distortion

quantities can be used as reference for performance evaluationof UWB-BAN antennas.

In this paper, we consideration the characterization of thewaveform distortion due to human body with correlationreceiver for UWB-BAN transmission. This scheme is based onthe Friis’ transmission formula, adapted for UWB, in the sensethat we would like to derive the equivalent antenna gain forUWB systems. The transmission waveform and the correlationreceiver are key for the extension of the Friis’ formula toUWB systems. An experiment is carried out using broadbandantennas for UWB-BAN operation in an anechoic chamber.

II. THOERY OF UWB-BAN TRANSMISSION

A. UWB-BAN Transmission Analysis

In this study, we focus on the experimental evaluation of hu-man body effect for UWB-BAN transmission with correlationreceiver for wireless medical applications.

In narrowband systems, the link budget of the free spacetransmission loss is usually estimated by using Friis’ transmis-sion formula [4]. However, it is not directly applicable to theUWB-BAN transmission system, as the formula is expressedas a function of the frequency. Moreover, the waveform maybe distorted due to the frequency characteristics of the antenna.Ref. [5] treats the special cases of the constant gain and theconstant aperture, but no general discussion had been madealthough it suggested the use of the time-domain antennaeffective length.

The Friis’ transmission formula [4] has been widely used,and can be applied to the calculation of these LOS channels.

�Friis��� ��r���

�t���� �f����r����t���� (1)

where �r and �t are Rx and Tx antenna gain,

�f��� �

��

���

��

(2)

is the free space propagation gain (less than unity in practice),� �

�

�is the wavelength, � is the velocity of the light, �

is the operating frequency, and � is the separation betweentransmitter and receiver antennas.

It is noted, however, that Eq. (1) is satisfied only at somecertain frequency, and is not directly applicable to UWBsystems. The Friis’ transmission formula shall be extended

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Tx antenna Rx antenna

d

Input waveform

Templatewaveform

AWGN

Fig. 1. Block diagram of UWB transmission model for BAN

to take into account the transmission signal waveform and itsdistortion as well [2].

Input signal i�� at the transmitter port is expressed as theconvolution of an impulse input and the pulse shaping filter�i�� as

i�� � �i�� � �i��� (3)

where ��

��

��i ��� �

��

��

� i����� �� � �� (4)

Friis’ formula is extended taking into account the transmissionwaveform as

e-Friis��� ��r���

�i� f��� i����r��� ��t���� (5)

where

����� � ������ ��� ��

� ��� ������ ��� �� � ��� ������ ��� ���(6)

� � r or t�

is a complex transfer function vector of the antenna relativeto the isotropic antenna,

f��� ��

����������� (7)

is the free space transfer function where

� ���

�� (8)

is the propagation constant.

B. Received signal Correlation Receiver

Let us consider a correlation receiver shown in Fig. 1.The output SNR is dependent on the choice of the templatewaveform. The correlator output o��� is therefore expressedas

o��� �

��

��

r���w�� ���� (9)

where r�� is the receiver input waveform which is inverseFourier transform, and �w�� is the template waveform. �corresponds to the timing of the template waveform, and theoptimum timing �o is chosen as

�o � ��� �

o���� (10)

Hereafter �w�� is normalized as��

��

��w����� � ��� (11)

where � is the signal bandwidth, so that the output noise

power is constant as ���, where�o

�is power spectral density

of AWGN.Under the constraint of Eq. (11), �wm�� maximizes o��o�

when �wm�� is a time-reversed and scaled version of r��,i.e.

�wm�� �

���r��o � ����

���r�����

� (12)

where �o is usually chosen so that �wm�� � � for � � tosatisfy the causality. �wm�� is called the optimum templatewaveform hereafter. It is noted that the link budget evaluationis identical to that when �wm�� is used as the receivertemplate.

C. Isotropic Correlation Receiver

It is obvious from Eq. (12) that the optimum templatewaveform is not the simple time-reversed version of thetransmitter waveform, but the channel characteristics includingthe antennas and the free space propagation. Therefore, it is notalways feasible to adapt the template waveform to the angular-dependent antenna characteristics, since the waveform shall begenerated at the clock rate of tens of gigahertz. Therefore, weconsider a canonical template waveform �wc��. In this paperwe have chosen �wc�� that is optimum for the isotropic andthe constant gain antennas, i.e.

�wc�� �

���r-iso��o � ����

���r-iso�����

� (13)

where

r-iso�� �

��

��

f����t��� ��������� (14)

is the receiver input voltage for isotropic antenna including.The difference between the optimum and the isotropic tem-plates indicates quantitatively the distortion of the waveform.

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VNA

1-5 m

Port 1 Port 2

Tx-antenna

Rx-antenna

Fix Tx-antennaat 0 degree

Rotate Rx-antennafrom 0 to 360 degree

Fig. 2. The instrument setup.

Biconical antenna Meander line antenna

Fig. 3. Ultra wideband antennas.

III. UWB-BAN EXPERIMENT SYSTEMS

A. Experimental Evaluation Scheme

By using the vector network analyzer (VNA), complextransfer functions can be measured. However, this transferfunction is a product of transfer functions of Tx and Rxantennas as well as the free space channel.

B. Instrument Setup

The VNA was operated in the response measurement mode,where Port-� was the transmitter port (Tx), and Port-� was thereceiver port (Rx), respectively. The measurement was donein an anechoic chamber. Both Tx and Rx antennas were fixedat the height of ���� m and separated by a distance of � m.The setup is sketched in Fig. 2. ���, measures the transferfunction between Tx and Rx antennas. The Tx antenna isfixed at pointing angle �Æ and the Rx antenna is rotated frompointing angle �Æ to ���Æ with each step at �Æ.

In this study, we considered a broadband antenna thatwas suitable for the operation with pulsed waveforms. Thestructure of the UWB antennas is shown in Fig. 3 the Txantenna is a biconical antenna with maximum diameter of���� mm and length of �� mm used as the standard antenna [6]and the Rx antenna is a commercial, small-size, low profileantenna developed by Skycross Lnc.,(USA) [7] used as theAUT.

C. Parameters of Experiment

The important parameters for the experiments are listed inTable I. It is noted that calibration is done at the connectors

TABLE IEXPERIMENTAL SETUP PARAMETERS.

Parameter ValueFrequency range � GHz to �� GHzNumber of frequency points ����

Dynamic power range �� dBTx antenna height ���� mRx antenna height ���� mDistance between Tx and Rx � mRx rotate range �

Æ to ���Æ

Rx rotate step �Æ

0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

−1

−0.8

−0.6

−0.4

−0.2

0

0.2

0.4

0.6

0.8

1

Time (ns)A

mp

litu

de

Fig. 4. The transmitted waveform of UWB-BAN signal.

of the cables to be connected to the antennas. Therefore, allimpairments of the antenna characteristics are included in themeasured results.

D. UWB-BAN Signal Model

The effect of the signal distortion is more obvious whenthe bandwidth is wider. We considered the impulse radiosignal that fully covers the FCC band ��� � ���� GHz [8].The center frequency and the bandwidth were therefore setto be �� � ���� GHz and �b � ��� GHz, respectively.The transmit waveform assumed in the simulation was asingle ASK pulse with the carrier frequency ��. To satisfythe bandwidth requirement of �b, the pulse length was set to

be�

�b. Then the signal was band-limited by a Nyquist roll-

off filter with roll-off factor � � � (rectangular window) and

passband

��� � �b

�� �� �

�b

�

�. Figure 4 shows the transmit

pulse waveform. The transmission process of the pulse wave-form is simulated based on the measured transfer function ofthe antennas.

IV. EXAMPLE RESULTS AND DISCUSSION

This section, decribes the graphical compilation of theexperiment results.

Figure 5 shows the magnitude of the measured channeltransfer function and its phase is also shown in Fig. 6. Wecan particularly see the frequency characteristic of the channeltransfer function at each pointing angle. As the AUT is thebroadband biconical antenna, the ideal linear phase is almost

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Fig. 5. The channel transfer function: magnitude.

Fig. 6. The channel transfer function: phase.

realized, except for the null directions, which change withfrequency.

The UWB signal shown in Fig. 4 is used as the transmissionwaveform. The received waveforms at the output of the

Figure 7 shows the without human body case (free space) ofpower delay profiles of the measured antenna transfer functionand its with human body case is also shown in Fig. 8. We canparticularly see the frequency characteristic of the antennatransfer function and delay spread at each pointing angle.The effects of human body shadowing on the UWB antennapropagation, the nulls are observed at ��Æ to ���Æ pointingangles.

Figure 9 shows the comparison of correlation betweentwo waveforms corresponding to the free space or withoutbody and with human budy transfer functions. It has higherdistortion from � to �� degree and ��� to ��� degree, from�� to ��� degree the distortion is small.

Figures ?? and 10 shows the comparison of UWB transmis-sion gain versus antenna pointing angle that uses the optimumand isotropic receiver for without human body (free space)and with human body. In the without human body case, thepeaks are found at �Æ to ���Æ, and ���Æ pointing angles which

0100

200300

0

5

100

0.2

0.4

0.6

0.8

1

1.2

x 10−5

Angle (degree)Time (ns)

Po

wer

del

ay p

rofi

le (

W)

Fig. 7. Power delay profiles of the UWB-BAN channel without human body.

0100

200300

0

5

100

0.2

0.4

0.6

0.8

1

1.2

x 10−5

Angle (degree)Time (ns)

Po

wer

del

ay p

rofi

le (

W)

Fig. 8. Power delay profiles of the UWB-BAN channel with human body.

corresponds to the broadside of the antenna. The nulls areobserved at ��Æ and ���Æ pointing angles. For with humanbody case, the peaks are found at �Æ to ���Æ, and ���Æ pointingangles which corresponds to the broadside of the antenna. Thenulls are observed at ���Æ pointing angles.

V. CONCLUSION

In this paper, we presented the characterization of humanbody transfer function model for UWB-BAN transmissionwith without body and with human budy for BAN applicationsby using an extension of Friis’ transmission formula in orderto take into account the transmit waveform and the templatewaveform into the system. The experimental examples usingthe biconical antenna as the transmitter and the maenderline antenna as the receiver are presented. This scheme maybe effective especially to evaluate the deployable antennawith non-ideal frequency characteristics of return loss anddirectivity, as the overall performance can be evaluated only bythe term of the UWB-BAN transmission gain. The formulationpresented in a special case for the UWB-BAN optimumtemplate receiver.

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Fig. 9. The comparison of correlation between two waveforms correspondingto without body and with human budy transfer functions.

Fig. 10. The comparison of power gain with body for UWB-BAN.

REFERENCES

[1] K. Siwiak, “Impact of ultra wide band transmissions on a genericreceiver,” in Proc. 2001 Spring IEEE Veh. Tech. Conf. (VTC), Rhodes,Greece, vol. 2, pp. 1181–1183, May 2001.

[2] J. Takada, S. Promwong and W. Hachitani, “Extension of Friis’ transmis-sion formula for ultra-wideband systems,” IEICE Tech. Rep., WBS2003-8/MW2003-20, May 2003.

[3] S. Promwong, W. Hachitani, and J. Takada, “Free Space Link BudgetEvaluation of UWB-IR Systems,” in Proc. 2004 Int. Workshop UltraWideband Syst. / Conf. Ultra Wideband Syst. Tech. (Joint UWBST &IWUWBS 2004), Kyoto, Japan, pp. 312–316, May 2004.

[4] H.T. Friis, “A Note on a Simple Transmission Formula,” Proc. IRE,vol. 34, no. 5, pp. 254–256, May 1946.

[5] United States of America, “Path Loss Calculations for Ultra-WidebandSignals in Indoor Environments,” ITU-R Document 3K/30-E, pp. 1–14,Nov. 2003.

[6] S. Promwong and W. Hachitani, and J. Takada, “Free Space Link BudgetEvaluation of UWB-IR Systems,” 2004 International Workshop on UltraWideband Systems Joint with Conference on Ultra Wideband Systemsand Technology (Joint UWBST&IWUWBS2004), pp. 312-316, May 18-21,2004.

[7] Skycross, Inc., “3.1-10 GHz UWB Antenna for Commercial UWBApplications” http://www.skycross.com/

[8] Federal Communications Commission, “Revision of Part 15 of theCommission’s Rules Regarding Ultra-Wideband Transmission Systems,”First Report and Order, FCC 02–48, Apr. 2002.

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