2013 13th International Symposium on Communications and Information Technologies (ISCIT)
Human Body Transfer Function Model for Ultra Wideband Body Area Network Sathaporn PromwongÝ, Jiraphan SahakitÝ, Sanit TeawehimÝ, and Bundit RuckverathamÝÝ Ý Department of Telecommunication Engineering, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand ÝÝ MCOT Public Company Limited, 63/1 Rama IX Road, Huaykwang, Bangkok 10310, Thailand E-mail:
[email protected],
[email protected],
[email protected]
Abstract— A waveform distortion on human body of an ultra wideband body area network (UWB-BAN) system can be extremely distorted through a channel even for free-space transmission because of antenna dispersion. Therefore, the understand of antenna characteristics, which effects on waveform distortion, is necessary. This paper presents the waveform distortion due to human body for wireless meadical applications based on measurement data. The template waveform is considered at the receiver side to maximize the SNR for evaluation. In this results are evaluate based on the extended Friis’ transmission formula. This technique gives very accurate results and is very useful for the design and evaluation of UWB-BAN transmission waveform for wireless medical applications, especially focusing on the effect of template waveform.
I. I NTRODUCTION The antennas usually act as significant pulse-shaping filters and cause extreme waveform distortion. Consequently, this will increase the complexity of the detection mechanism at the receiver [1]. Moreover, low cost, geometrically small and still efficient structures are required for the typical wireless applications. Therefore, the knowledge of waveform distortion due to antenna is preponderant to design and improve the performances of UWB-BAN system. Even if the channel is in line of sight (LOS), Friis’ transmission formula cannot be directly applied to the UWB radio as the bandwidth of the pulse is extremely wide. Furthermore, simple comparison between waveforms of the transmitter and the receiver is not significant because of the distortion of the waveform caused by the frequency response of the antenna. The purpose of this paper is to propose a new link budget model for studying the waveform distortion due to antenna on free space transmission in UWB-BAN system. We develop the free space link budget evaluation scheme in the term of frequency transfer function for UWB-IR system that takes into account the transmitted waveform, its distortion due to the antennas, the channel and the correlation receiver. This model is based on the Friis’ transmission formula, adapted to the UWB-BAN transmission system, in the sense that we derive the equivalent frequency transfer function of UWBBAN system [2]- [3]. The distortion quantities in the terms of magnitude, phase and waveform distortions, and transmission gain are defined and shown. This scheme provides some useful physical insights and optimized design procedure with clear and accessible description of the UWB-BAN link budget comprised of practical antennas. Furthermore, these distortion
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quantities can be used as reference for performance evaluation of UWB-BAN antennas. In this paper, we consideration the characterization of the waveform distortion due to human body with correlation receiver for UWB-BAN transmission. This scheme is based on the Friis’ transmission formula, adapted for UWB, in the sense that we would like to derive the equivalent antenna gain for UWB systems. The transmission waveform and the correlation receiver are key for the extension of the Friis’ formula to UWB systems. An experiment is carried out using broadband antennas for UWB-BAN operation in an anechoic chamber. II. T HOERY OF UWB-BAN T RANSMISSION A. UWB-BAN Transmission Analysis In this study, we focus on the experimental evaluation of human body effect for UWB-BAN transmission with correlation receiver for wireless medical applications. In narrowband systems, the link budget of the free space transmission loss is usually estimated by using Friis’ transmission formula [4]. However, it is not directly applicable to the UWB-BAN transmission system, as the formula is expressed as a function of the frequency. Moreover, the waveform may be distorted due to the frequency characteristics of the antenna. Ref. [5] treats the special cases of the constant gain and the constant aperture, but no general discussion had been made although it suggested the use of the time-domain antenna effective length. The Friis’ transmission formula [4] has been widely used, and can be applied to the calculation of these LOS channels. Friis
r t
where
r
and
t
f
r t
(1)
are Rx and Tx antenna gain, f
¾
(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 between transmitter and receiver antennas. It is noted, however, that Eq. (1) is satisfied only at some certain frequency, and is not directly applicable to UWB systems. The Friis’ transmission formula shall be extended
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Tx antenna
Rx antenna d
Input waveform Template waveform
AWGN
Fig. 1.
Block diagram of UWB transmission model for BAN
to take into account the transmission signal waveform and its distortion as well [2]. Input signal i at the transmitter port is expressed as the convolution of an impulse input and the pulse shaping filter i as i iÆ i (3)
where
i ¾
i
¾
(4)
Friis’ formula is extended taking into account the transmission waveform as
e-Friis r f i
r
i
t
(5)
where
(6)
r or t
is a complex transfer function vector of the antenna relative to 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 template waveform. The correlator output o is therefore expressed as o r w (9)
where r is the receiver input waveform which is inverse Fourier transform, and w is the template waveform. corresponds to the timing of the template waveform, and the optimum timing o is chosen as
o o
(10)
Hereafter
where
w is normalized as
w
¾
(11)
is the signal bandwidth, so that the output noise
o
is power spectral density power is constant as ¼ , where 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 (12) r ¾
where o is usually chosen so that wm for to satisfy the causality. wm is called the optimum template waveform hereafter. It is noted that the link budget evaluation is identical to that when wm is used as the receiver template. C. Isotropic Correlation Receiver It is obvious from Eq. (12) that the optimum template waveform is not the simple time-reversed version of the transmitter waveform, but the channel characteristics including the antennas and the free space propagation. Therefore, it is not always feasible to adapt the template waveform to the angulardependent antenna characteristics, since the waveform shall be generated at the clock rate of tens of gigahertz. Therefore, we consider a canonical template waveform wc . In this paper we have chosen wc that is optimum for the isotropic and the constant gain antennas, i.e. wc r-iso o (13) ¾
r-iso where
r-iso
f t
(14)
is the receiver input voltage for isotropic antenna including. The difference between the optimum and the isotropic templates indicates quantitatively the distortion of the waveform.
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TABLE I E XPERIMENTAL SETUP PARAMETERS . Parameter Frequency range Number of frequency points Dynamic power range Tx antenna height Rx antenna height Distance between Tx and Rx Rx rotate range Rx rotate step
Rotate Rx-antenna from 0 to 360 degree Fix Tx-antenna at 0 degree 1-5 m Tx-antenna Rx-antenna
Value GHz to GHz
dB m m m Æ to Æ Æ
VNA Port 1
Port 2
1 0.8
Fig. 2.
The instrument setup.
0.6
Amplitude
0.4 0.2 0 −0.2 −0.4 −0.6 −0.8 −1 0.6
Biconical antenna
Fig. 3.
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
Time (ns)
Meander line antenna
Fig. 4.
Ultra wideband antennas.
III. UWB-BAN E XPERIMENT S YSTEMS A. Experimental Evaluation Scheme By using the vector network analyzer (VNA), complex transfer functions can be measured. However, this transfer function is a product of transfer functions of Tx and Rx antennas 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 the receiver port (Rx), respectively. The measurement was done in an anechoic chamber. Both Tx and Rx antennas were fixed at the height of m and separated by a distance of m. The setup is sketched in Fig. 2. ¾½ , measures the transfer function between Tx and Rx antennas. The Tx antenna is fixed at pointing angle Æ and the Rx antenna is rotated from pointing angle Æ to Æ with each step at Æ . In this study, we considered a broadband antenna that was suitable for the operation with pulsed waveforms. The structure of the UWB antennas is shown in Fig. 3 the Tx antenna 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 profile antenna developed by Skycross Lnc.,(USA) [7] used as the AUT. C. Parameters of Experiment The important parameters for the experiments are listed in Table I. It is noted that calibration is done at the connectors
The transmitted waveform of UWB-BAN signal.
of the cables to be connected to the antennas. Therefore, all impairments of the antenna characteristics are included in the measured results. D. UWB-BAN Signal Model The effect of the signal distortion is more obvious when the bandwidth is wider. We considered the impulse radio signal that fully covers the FCC band GHz [8]. The center frequency and the bandwidth were therefore set to be ¼ GHz and b GHz, respectively. The transmit waveform assumed in the simulation was a single ASK pulse with the carrier frequency ¼ . To satisfy the bandwidth requirement of b , the pulse length was set to
b . Then the signal was band-limited by a Nyquist rolloff filter with roll-off factor (rectangular window) and b b . Figure 4 shows the transmit passband be
¼
¼
pulse waveform. The transmission process of the pulse waveform is simulated based on the measured transfer function of the antennas. IV. E XAMPLE R ESULTS AND D ISCUSSION This section, decribes the graphical compilation of the experiment results. Figure 5 shows the magnitude of the measured channel transfer function and its phase is also shown in Fig. 6. We can particularly see the frequency characteristic of the channel transfer function at each pointing angle. As the AUT is the broadband biconical antenna, the ideal linear phase is almost
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−5
Power delay profile (W)
x 10
1.2 1 0.8 0.6 0.4 0.2 0 10 300
5
Time (ns)
Fig. 5.
The channel transfer function: magnitude.
200 100 0
0
Angle (degree)
Fig. 7. Power delay profiles of the UWB-BAN channel without human body.
−5
Power delay profile (W)
x 10
1.2 1 0.8 0.6 0.4 0.2 0 10 300
5
Time (ns)
Fig. 6.
The channel transfer function: phase.
Fig. 8.
realized, except for the null directions, which change with frequency. The UWB signal shown in Fig. 4 is used as the transmission waveform. The received waveforms at the output of the Figure 7 shows the without human body case (free space) of power delay profiles of the measured antenna transfer function and its with human body case is also shown in Fig. 8. We can particularly see the frequency characteristic of the antenna transfer function and delay spread at each pointing angle. The effects of human body shadowing on the UWB antenna propagation, the nulls are observed at Æ to Æ pointing angles. Figure 9 shows the comparison of correlation between two waveforms corresponding to the free space or without body and with human budy transfer functions. It has higher distortion from to degree and to degree, from to degree the distortion is small. Figures ?? and 10 shows the comparison of UWB transmission gain versus antenna pointing angle that uses the optimum and isotropic receiver for without human body (free space) and with human body. In the without human body case, the peaks are found at Æ to Æ , and Æ pointing angles which
200 100 0
0
Angle (degree)
Power delay profiles of the UWB-BAN channel with human body.
corresponds to the broadside of the antenna. The nulls are observed at Æ and Æ pointing angles. For with human body case, the peaks are found at Æ to Æ, and Æ pointing angles which corresponds to the broadside of the antenna. The nulls are observed at Æ pointing angles. V. C ONCLUSION In this paper, we presented the characterization of human body transfer function model for UWB-BAN transmission with without body and with human budy for BAN applications by using an extension of Friis’ transmission formula in order to take into account the transmit waveform and the template waveform into the system. The experimental examples using the biconical antenna as the transmitter and the maender line antenna as the receiver are presented. This scheme may be effective especially to evaluate the deployable antenna with non-ideal frequency characteristics of return loss and directivity, as the overall performance can be evaluated only by the term of the UWB-BAN transmission gain. The formulation presented in a special case for the UWB-BAN optimum template receiver.
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Fig. 9. The comparison of correlation between two waveforms corresponding to without body and with human budy transfer functions.
Fig. 10.
The comparison of power gain with body for UWB-BAN.
R EFERENCES [1] K. Siwiak, “Impact of ultra wide band transmissions on a generic receiver,” 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’ transmission formula for ultra-wideband systems,” IEICE Tech. Rep., WBS20038/MW2003-20, May 2003. [3] S. Promwong, W. Hachitani, and J. Takada, “Free Space Link Budget Evaluation of UWB-IR Systems,” in Proc. 2004 Int. Workshop Ultra Wideband 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-Wideband Signals 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 Budget Evaluation of UWB-IR Systems,” 2004 International Workshop on Ultra Wideband Systems Joint with Conference on Ultra Wideband Systems and Technology (Joint UWBST&IWUWBS2004), pp. 312-316, May 18-21, 2004. [7] Skycross, Inc., “3.1-10 GHz UWB Antenna for Commercial UWB Applications” http://www.skycross.com/ [8] Federal Communications Commission, “Revision of Part 15 of the Commission’s Rules Regarding Ultra-Wideband Transmission Systems,” First Report and Order, FCC 02–48, Apr. 2002.
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