Wireless Power Data Transmission

8/18/2019 Wireless Power Data Transmission

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Data Transmission for Resonant-type WirelessPower Transfer

Shinpei Noguchi, Mamiko Inamori, Yukitoshi SanadaDepartment of Electronics and Electrical Engineering, Keio University, Yokohama, 223-8522 Japan

Email:[email protected], {inamori,sanada }@elec.keio.ac.jp

Abstract —Wireless power transfer research has been receivinga great deal of attention in recent years. In resonant-typewireless power transfer, energy is transferred via LC resonantcircuits. However, system performance is dependent on thecircuit components. To transfer power safely, information, suchas frequency, required power and element values, need to betransmitted initially in the system. This paper investigates datacommunication using orthogonal frequency division multiplexing(OFDM) modulation in resonant-type wireless power transfersystems. The equivalent circuit used in the transmitting andreceiving antennas is a band pass lter (BPF) and its bandwidth is

evaluated through circuit simulations and experimental measure-ments. Numerical results obtained through computer simulationshow that the bit error rate (BER) performance is affected bythe mismatch of resonant frequency.

I. INTRODUCTION

Recent interest in wireless power transfer has been attractinga great deal of attention. Wireless power transfer will enableadvances in the use of electronic devices such as mobilephones, portable computers, etc.. The wireless power transferis currently achieved via three techniques, each system hasdifferent characteristics in terms of distance and power transferefciency.

The three techniques are electromagnetic induction, coupledradio frequency power transmission, and resonant coupling.In electromagnetic induction, the magnetic ux induces theelectric current, thus power is transferred wirelessly to the re-ceived coil [1]. The efciency of power transfer varies between60-98% over a distance of several millimeters. To achievecoupled radio frequency power transmission, electromagneticwaves are converted to direct currents, which provides power[2]. The efciency of power transfer is less than 50% over adistance of several meters. In the resonant coupling technique,two coils are tuned at the same resonant frequency, the powertransfer is expected to be very efcient. The efciency of power transfer is approximately 50% over a distance of severaltens of centimeters [3]. In 2006, MIT has released “WiTricity”,which applies this resonant induction [4]. In this paper, themagnetic resonant coupling system is modeled for wirelesspower transfer.

The transmitting and receiving antennas in the coupledresonances need to create non-radiative and induced magneticeld easily. As a practical realization, the loop antennas canbe applied. The antennas tend to change the induced magneticeld with the number of turns [5]. However, the self-resonantcoils rely on the interplay between distributed conductance

and distributed capacitance, which results in the effect on thepower transfer efciency. Therefore, the circuit information inthe receiver such as frequency, required power and elementvalues need to be transmitted to the transmitter according tothe request of the receiver [6][7]. It becomes very importantfor wireless power transfer systems to make sure that thesedata are transmitted reliably. The equivalent circuit used in thetransmitting and receiving antennas is a band pass lter (BPF)and its bandwidth is evaluated through circuit simulations and

experiments. In this paper, the transfer function |S 21 | as mea-sured experimentally and as calculated from the circuit modelare evaluated. The bandwidth to transmit the data informationthen is decided. Orthogonal frequency division multiplexing(OFDM) is applied as a modulation scheme and bit error rate(BER) is calculated through MATLAB simulation.

This paper is organized as follows. Section II introduces thesystem model and Section III outlines the experimental setup.In Section IV, numerical results obtained through computersimulation are presented. Section V gives our conclusions anddirections for future work.

I I . S YSTEM M ODEL A. Single Antenna

Thickness

Diameter

Fig. 1. Single antenna.

r R

1C

1 L0

C

Fig. 2. Equivalent circuit of antenna.

In this paper, 3-turn coil is applied as shown in Fig. 1 asa transmitting and receiving antenna, whose equivalent circuit


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is shown in Fig. 2 [5]. In the single antenna in Fig. 2, L1represents the self inductance, Rr represents the radiationresistance of a coil, C 0 represents the stray capacitance, andC 1 represents the load capacitance. The conductor losses areignored in the circuit model. From Neumann’s formula, theself inductance, L 1 , is given as

L1 = µ0

4π 1

2

d 1 ·d 2r 12

, (1)

where d 1 and d 2 are small line elements on a coil, and r 12is the thickness of the coil. The radiation resistance of a coil,R r , is given as

R r = 20 π2 q 2 (βp)4 , (2)

where β is the phase constant, q is the number of turns, and p is the radius of a coil. The diameter of the coil, D , is 27cmand the thickness is 4.5mm in experimental model. From theself-resonant frequency f 0 in the experimental model, the straycapacitance between lines, C 0 , is given from

f 0 = 1

2π√ L1 C 0 . (3)In this paper, the load capacitance, C 1 , is determined since theresonant frequency f c is set to 10[MHz]. The values of eachparameter of this model are shown in Table I.

TABLE IPARAMETERS IN SINGLE ANTENNA

R r [m Ω] L 1 [µH ] C 1 [ pF ] C 0 [ pF ]1.731 7.6 15 20

B. Resonant Coupling System

dz

dy

Receiving antenna

Transmi ng antenna

mm5.4

cm27

Fig. 3. Resonant coupling system with two coils

r R

r R

0C 0C

1C

1C

0 Z

0 Z

M L −1

M L −1

M

Fig. 4. Equivalent circuit of the power transfer system

The equivalent circuit of this system is shown in Fig. 4. Z 0is set to 50 Ω. From Neumann’s formula, the mutual inductancebetween the antennas is given as

M = µ04π 1 2 dL 1 ·dL 2dz , (4)

and

M = k√ L1 L2 , (5)where dz is the distance between antennas, k is the couplingcoefcient, and L2 is the self inductance of the receivingantenna [8]. From Eqs. (4) and (5), the coupling coefcient,k, is calculated as shown in Fig. 5. With the value for thecoupling coefcient, k, and Eq. (5), the transfer function, |S 21 |,which represents the power transfer efciency, is calculatedfrom the circuit model.

Fig. 5. Coupling coefcient vs. distance between two antennas

C. Communication Model

In the communication model for data transmission, theequivalent circuit used in the transmitting and receiving an-tennas is regarded as a BPF, which has to be custom designednot to cause the interference. To satisfy this constraints, OFDMis applied for data transmission in the power transfer system.Suppose the information symbol on the kth subcarrier is s[k](k = 0 ,...,N

−1), the OFDM symbol is given as

u[n] = 1√ N

N − 1

k =0

s[k]ej 2 πnk

N , (6)

where n (n = 0 ,...,N −1) is time index and N is the numberof subcarriers. The guard interval is added before the datatransmission. The baseband signal at the output of the lteris given by x(t) = ∑

P − 1n =0 u[n]C t (t −nT s ), where C t (t) isthe impulse response of the transmitting lter, P is the length

of the impulse response, and T s is the symbol duration. In


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Fig. 6. Experimental single antenna.

Port 1

dz

Transmi ngantenna

Receivingantenna

Port 2

A enuator A enuator

Fig. 7. Experimental setup.

this system, the antennas are xed and multipath fading is notassumed. The received signal is given as

y(t) =P − 1

n =0u[n]h(t −nT s ) + v(t), (7)

where v(t) is the additive white Gaussian noise (AWGN), h(t)is the impulse response of the composite channel and is givenby

h(t) = C t (t)⊗C r (t), (8)

where ⊗ denotes convolution and C r (t) is the impulse re-sponse of the receiving lter. The frequency response of channel in the communication model, H , is equivalent to |S 21 |in the power transfer system.

III . E XPERIMENTAL M EASUREMENT

The experimental single antenna with 3-turn coil is shownin Fig. 6 and measurement setup is shown in Fig. 7. The mea-surement equipment is shown in Table II. In this experimentalsystem, |S 21 | was measured with the vector network analyzer.

It is assumed that the antenna is only moved toward the z-axis.The measured inductance of the coil both at the transmittingand receiving antenna experimentally, L̂1 and L̂2 , have thesame values as the calculated values in the circuit model, L1and L 2 .

TABLE IIM EASUREMENT EQUIPMENTS

Equipment SpecicationVector network analyser Agilent 8753ETVNA control software Agilent technology

Intuilink (Version 1.3)Circuit simulator PSpice circuit simulatorTx antenna Loop antenna

(D =27cm)Rx Antenna Loop antenna

(D =27cm)

IV. S IMULATION R ESULTS

Fig. 8. Transfer function | S 21 | (dz = 10 cm).

A. |S 21 | CharacteristicFigure 8 shows |S 21 | characteristic given as measured ex-perimentally and calculated from the equivalent circuit model.

The distance between the transmitting and receiving antennaon the experimental measurements, dz, is set to 10[cm], thecoupling coefcient, k, on the circuit simulator is set to 0.176from Fig. 5. In Fig. 8, both the theoretical curve based onthe circuit model and the experimental measurement curveshow the splitting of resonant peak. As the coupling betweenthe coils at the transmitting and receiving antenna becomesstronger, the peak splits into two. Moreover, the theoreticalcurve based on the circuit model does not t the experimentalmeasurement curve. It is due to the mismatch of derived valueson the experimental measurement model and equivalent circuitmodel, which are chosen from parameters such as resistances,stray capacitances and self inductances.


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Fig. 9. Impulse response of | S 21 | from the circuit model ( dz =10cm).

Fig. 10. Impulse response of |S 21 | from the experimental measurement

(dz =10cm).

B. Impulse Response

To investigate the inuence of the transmitting and receivingantennas, the impulse responses of |S 21 | is shown. Figs. 9and 10 display the impulse response of the channel in thedelay domain between the antennas. In the data transmissionsystem, OFDM is employed for the 2nd modulation, and thebandwidth of OFDM is designed to t the relatively largeimpulse response of the channel in the guard interval period.Thus, the number of the subcarriers is derived to satisfy this

condition:N/T s ≤ W. (9)

Here, W is the bandwidth of the composite lters, which ismeasured at half-power points (3dB) from the peak.C. BER Performance

1) Simulation Model: BER performance is evaluatedthrough computer simulation. The simulation model is shownin Fig. 11 and the simulation conditions are shown in Table

Signal generated

from thecircuit model orthe experimental

measurement

AWGN

DFT Phasecorrec on

BER measurement

IDFT

is adjusted to thebandwidth of OFDM signals

DFT|| 21S || 21S IDFT

Fig. 11. Simulation model.

Fig. 12. BER performance of QPSK ( dz =10cm).

III. Information bits are modulated with quadrature phase shiftkeying (QPSK) or 64 quadrature amplitude modulation (QAM)on each subcarrier. The number of discrete Fourier transform(DFT) points is set to 32, which is t to W given fromexperimentally measured |S 21 | characteristic as shown in Fig.8. The guard interval is set to 8, which is 1/4 of the numberof subcarrier N . The phase compensation is assumed to beperfect.

TABLE IIIS IMULATION C ONDITIONS

Modulation scheme 1st : QPSK/64QAM2nd : OFDM

Bandwidth 0.67 [MHz]Sampling interval 1.5 µ seconds

FFT size 32Number of data subcarriers 32Number of guard interval 8

Channel model AWGNNumber of OFDM packets 10, 000 , 000

dy 0cmdz 10cm


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Fig. 13. BER performance of 64QAM ( dz =10cm).

2) Simulation Results: Figure 12 and 13 show the BERperformance on the AWGN model with QPSK and 64QAM,respectively. The bandwidth of OFDM is set to t the narrowbandwidth given from experimental measurements as shownin Fig. 8. In these gures, BER performance is degradedcompared to the AWGN theoretical curve due to the frequencyselective channel, which is caused by the splitting resonantfrequency. Moreover, the BER curve with experimental fre-quency response is worse than that of the calculated frequencyresponse from the circuit model. This is because the largeimpulse response can be observed in Fig. 10.

V. C ONCLUSIONS

In this paper, data transmission for power transfer systemhas been investigated. Resonant coupling is used to deliverpower from one coil to another coil wirelessly. In the wire-less power transfer system, information, such as frequency,required power and element values, need to be transmittedinitially to ensure safe power transfer. The equivalent circuitof the antenna is BPF, and the transfer function |S 21 | isregarded as the impulse response of the channel in the datatransmission. In this paper, the transfer function |S 21 | is givenfrom calculation on the circuit model and from experimentalmeasurements.

In the data transmission model, OFDM is used as the 2ndmodulation and the impulse response of |S 21 | in the time do-main is designed to t within the guard interval period. As thedistance between antennas was xed, the channel is assumedto be AWGN. From simulation results, BER performance isdegraded compared to the AWGN theoretical curve due to thesplitting resonant frequency. However, it is assumed that thesystem parameters are transmitted as information in this paper.Therefore, this research is valid for low data rate transmissionwith narrow bandwidth. Further work will consider both datatransmission and power transfer.

Acknowledgments

This work is supported in part by a Grant-in-Aid for theGlobal Center of Excellence for high-Level Global Cooper-ation for Leading-Edge Platform on Access Spaces from theMinistry of Education, Culture, Sport, Science, and Technol-ogy in Japan.

R EFERENCES

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[2] K. O’Brien, R. Teichmann, H. Gueldner, “Magnetic Field Generation inan Inductively Coupled Radio-Frequency Power Transmission System,”Power Electronics Specialists Conference, 2006. PESC ’06. 37th IEEE,pp.1-7, 2006.

[3] A. Karalis, “Efcient wireless non-radiative midrange energy transfer,”Annals of Physics, 323, pp. 34-48, April 2007.

[4] A. Kurs, “Wireless Power Transfer via Strongly Coupled MagneticResonances,” Science Express, vol. 317, No. 5834, pp. 83-86, 2007.

[5] Y. Hiraiwa, N. Kikuma, H. Hirayama, K. Sakakibara, “A Considerationon Transmission Efciency Characteristics of Wireless power transferUsing Proximity Coils,” IEICE Technical Report, AP2009-86, Sept.2009.

[6] Dries van Wageningen, Toine Staring, “The Qi Wireless Power Stan-dard,” 14th International Power Electronics and Motion Control Confer-ence, EPE-PEMC 2010.

[7] W. X. Zhong, X.liu, S. Y. R. Hui, “A Novel Single-Layer Winding Arrayand Receiver Coil Structure for Contactless Battery Charging Systemswith Free-Positioning and Localized Charging Features,” IEEE Trans.on Industrial Electronics, vol. 99, October 2010.

[8] S.I.B, C. Akyel, “New analytic-numerical solutions for the mutualinductance of two coaxial circular coils with rectangular cross sectionin air,” IEEE Trans. on Magnetics, vol. 42, no. 6, June 2006.

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