Abstract — High PAPR signals such as multisines have been used to increase the RF-DC conversion efficiency in WPT systems. Considerable work has been dedicated to the receiver side, namely to investigate the efficiency enhancement in RF-DCconverters excited with such signals. However, less attention has been paid to the transmitter side. In this paper, we propose the use of spatial power combining technique for use in WPT, to efficiently generate/radiate a high PAPR multisine signal.

Index Terms— Wireless Power Transmission, High PAPR Signals, Multisine, Space Power Combining.

I. INTRODUCTION

Recently, there has been a growing interest in signal waveform design to increase the efficiency of WPT (Wireless Power Transmission) systems. The use of high PAPR (Peak-to-Average Power Ratio) signals has been proposed as a way of increasing the energy transfer efficiency and extending the system coverage range. Briefly, as previously demonstrated in [1], high PAPR signals are able to excite the RF-DC converter devices in a more efficient way, forcing them to generate much more DC power. An application of this concept can be found in [2] where a high PAPR signal was used for reducing the average radiated power while keeping the same communication distance of an RFID reader. This was done by switching the carrier on and off with a given duty cycle. High PAPR UWB (Ultra Wide Band) signals were also proposed in [3] and [4] for efficient low power transmission. Following the same reasoning of using high PAPR signals, chaotic signals were also proposed in [5]. Multisine signals (commonly used in Orthogonal Frequency Division Multiplex - OFDM systems) have been explored for use in WPT. In [6] and [7], the reading range of UHF RFID tags was extended by using amultisine scheme. In [3] the non-linear behavior of RF-DC converters was investigated. In such study a mathematical model and description were presented to explain the efficiency enhancement of Schottky diode detectors when excited with high PAPR signals. In the experiments conducted in [8] and [9], a multisine front-end was integrated in a commercial RFID reader in order to extend its reading range.

Although considerable research has been devoted to the receiver side of the power transfer chain, rather less attention has been paid to the transmitter side. So far, investigations have been confined to the study of (receiver) RF-DC conversion efficiency when using high

PAPR signals. However, the high PAPR scheme is only effective if the high PAPR waveform actually reaches the receiver. This fact imposes the use of an improved transmitter architecture that is able to transmit the high PAPR signal without clipping it. Moreover, the amplification of high PAPR signals is critical.

While high PAPR is the preferable characteristic that makes multisine signals beneficial on the receiver side, this same characteristic is a drawback on the transmitter side. Thus, improved architectures for high PAPR transmission is a must. Concerning multisine signals, it should be noted that the high PAPR characteristic is due to the combination of the multisine subcarriers. Considering this fact, we propose an efficient multisine generation/transmission approach based on spatial power combining: In such architecture the multisine subcarriers are amplified and transmitted separately and they will be combined in free space. Therefore, no especial challenges will be posed to the power amplification stage.

II. SPATIAL POWER COMBINING: SPATIALLY-COMBINED MULTISINES

Power combining is a technique commonly used in millimeter-wave technology to achieve moderate to high power levels [10]. Traditional approach consists of splitting the input signal into N branches, amplifying them and combining them again to obtain an amplified version of the input signal. Since splitting and combining are implemented by transmission line circuits, this approach becomes inefficient as the number of branches increases. This is due to the losses in the line and combining structures. Spatial power combining was proposed to do the same job with higher efficiency and theoretically with no limitation in the number of branches [10]. This is achieved by passively combining the signal components in free space. Spatially combined power sources can be implemented as array of oscillators in which the coupling between adjacent elements allows for frequency and phase locking [11]. In this paper, we propose the use of spatially-combined high PAPR multisine signals for WPT. Figure 1a) depicts the proposed system composed of N=2n+1 synchronized

Spatially-Combined Multisine Transmitter for Wireless Power Transmission

Alírio Soares Boaventura and Nuno Borges CarvalhoDepartamento de Electrónica, Telecomunicações e Informática, Instituto de Telecomunicações,

Universidade de Aveiro, Aveiro 3810-193, Portugal

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transmitters that generate a multisine E-field through space power combining. Assuming far-field observation [11] and considering a small spacing between adjacent elements, the total E-field at distance r is given by:

( , , ) = ( ) + + (1)

where k is the propagation constant, ωi and ϕi are the frequency and phase of each signal source, Ei is the amplitude of each E-field component and Gi(θ) is the gain of each antenna.

In order to achieve a multisine E-field with maximum PAPR the conditions of equally-spaced frequencies (ωi=ω0 + iΔω) and locked phases (ϕi+1 – ϕi = Δϕ) must be met [1][11]. If the oscillators were coupled then the constant phase progression condition would be automatically enforced by an injection-lock mechanism [11]. However, in this work we are using non-coupled sources, so the phase locking is externally imposed by a 10MHz reference signal.

x-n(t)

d

x-n+1(t) x0(t) xn(t)

... ...

xn-1(t)

d

... ...Ref(10MHz)

(b)

r

(b)

Fig. 1 a) Synchronized single carrier transmitters. If the carriers are equally spaced then a high PAPR E-field will be generated, b) an alternative configuration in which the receiver is placed in between the transmitters.

III. PRELIMINARY RESULTS

In order to provide a first validation of the proposed transmitter scheme two experiments have been conducted. In the first one a three tone multisine signal is generated and measured (at the receiver) using the setup shown in Fig. 2a). Three transmitter antennas are fed with 13dBm of average power at 887.5MHz, 890MHz and 892.5MHz respectively. The spatially-combined signal is received at 1.14m by a fourth antenna. The received signal spectrum with three frequency components is depicted in Fig. 2a). In Fig. 2b) it can be observed the time domain waveform with high PAPR value. This high PAPR value will increase the efficiency of the RF-DC converter. Moreover, the PAPR (and consequently the efficiency) increases as the number of tones in the multisine is increased [1].

In the second experiment, a dipole receiver antenna is placed between two transmitters (Fig. 3a). In order to compare with the single carrier case, a 5-stage voltage multiplier is connected to the receiver antenna and the DC is measured. Fist, one of the antennas is fed with a single carrier at 879MHz with power PCW and second the two transmitter antennas are fed with two subcarriers at 878MHz and 880MHz, each with power P2T=PCW-3dB.

(a)

(b)

Transmitter antennas

Synchronized sources

Receiver antenna

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(c) Fig. 2a) Setup used to generate/radiate a three tone multisine signal. b)

signal spectrum received at the receiver antenna. The three frequency components combined in free space can be observed. c) high PAPR waveform measured in the time using a high speed RF oscilloscope.

Fig. 3 Setup used to compare two tone and single carrier results. r = 35cm.

PCW(dBm)

VDC_CW(Volt)

P2T=PCW-3dB(dBm)

VDC_2T(Volt)

10.7 1.090 7.7 1.09011.7 1.288 8.7 2.29312.7 1.501 9.7 1.50913.7 1.726 10.7 1.76214.7 1.940 11.7 2.05615.7 2.203 12.7 2.38116.7 2.480 13.7 2.731

Table 1. DC voltage at the rectifier output for single carrier and two tone signal.

Table 1 shows the preliminary results obtained in the second experiment. In the two tone case the average input power of each antenna is the same as the average power used in the single carrier experiment. This is done by setting each subcarrier power to half power of the single carrier (P2T=PCW-3dB). The DC voltages in the table are measured at the output of a 5-stage voltage multiplier (connected to the receiver antenna) at a 510KΩ load. As can be seen, in most cases the collected DC voltage is higher for the two tone case even considering the same average power being radiated. However, the obtained gains are not so expressive. This may be due two reasons: first, a multisine with only two tones is used which does not provide a very high PAPR. In order to obtain higher gains a higher order multisine should be used. Second, in this setup the two transmitter antennas are directly looking to each other which may degrade the gain with respect to the receiver antenna due to the loading effect of the antennas. The antennas loading effect is observed in table 2. Referring to Fig.3, table 2 shows the transmission coefficient (S31) between the receiver and one of the transmitter antennas with and without the second transmitter antenna in place. Reflection coefficients (S11 and S33) are also presented. As can be seen the transmission coefficient is degraded when the second antenna (acting as a parasitic load) is in place. Other antenna configurations should be tested in order to minimize such effect and optimize the energy transfer efficiency gain.

S-param (dB)

Only one TX antenna

Two TX antennas

|S31| -24.5 -25.5|S11| -32.8 -32.2|S33| -24.4 -22.8

Table 2. System parameters with and without the second transmitter antenna in place.

IV. CONCLUSIONS AND FUTURE WORK

Spatial power combining is a suitable approach to efficiently deliver a high PAPR signal which is able to boost the RF-DC conversion efficiency of RF-DCconverter circuits. Several benefits can be achieved: no need to amplify high PAPR signals (only single sub-carriers need to be amplified. This will avoid clipping, distortion, spectrum regrowth and efficiency degradation that otherwise would appear in high PAPR multisine amplification. Since the total power is spitted by N branches, the amplification process is much more relaxed. Sub-carriers are passively combined in free space, avoiding combining losses. A major drawback may be the use of several power amplifier stages and antennas, nevertheless, such transmitter can be

r

r

Dipole receiver antenna

13

2

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efficiently implemented as an array of coupled oscillators.

Further work is needed to explore other antenna configurations and also to evaluate higher order multisine transmitters (with more than two sub-carriers). Such higher order transmitters are capable of providing much higher RF-DC conversion efficiency gain. Moreover, since the phase synchronization is critical in more than two tones signals, more work is needed to evaluate the impact of the phase synchronization on the collected DC power. Future work will also address array of coupled oscillators to efficiently implement the proposed spatially-combined multisine Transmitter.

ACKNOWLEDGEMENTS

The authors would like to thank Portuguese Science and Technology Foundation (FCT) for the financial support provided under Project PTDC/EEA-TEL/099646/2008 TACCS and for the doctoral scholarship SFRH/ BD/ 80615/ 2011.

Thanks also to Ricardo Fernandes from University of Aveiro and Hugo Mostardinha from Institute of Teleccomunications - Aveiro.

REFERENCES

[1] Alírio Soares Boaventura and Nuno Borges Carvalho “Maximizing DC Power in Energy Harvesting Circuits Using Multisine Excitation”, IMS2011 - International Microwave Symposium, Baltimore, USA, June, 2011.

[2] Hisanori Matsumoto and Ken Takei, “An Experimental Study of Passive UHF RFID System with Longer Communication Range”, Proceedings of Asia-Pacific Microwave Conference 2007.

[3] Chun-Chih Lo, Yu-Lin Yang, Chi-Lin Tsai, Chieh-Sen Lee, and Chin-Lung, Yang, “Novel Wireless Impulsive Power Transmission to Enhace the Convertion Efficiency for Low Input Power” , Microwave Workshop Series on Innovative Wireless Power Tansmission, 2011

[4] Yu-Lin Yang, Chin-Lung Yang, Chi-Lin Tsai, and Chieh-Sen Lee, “Efficiency Improvement of the Impulsive Wireless Power Transmission”, Microwave Workshop Series on Innovative Wireless Power Tansmission, 2011.

[5] A. Collado, A. Georgiadis, "Improving Wireless Power Transmission Efficiency Using Chaotic Waveforms," in Proc. IEEE MTT-S IMS 2012, Montreal, 17-22 June 2012.

[6] M. S. Trotter, J. D. Griffin and G. D. Durgin “Power-Optimized Waveforms for Improving the Range and Reliability of RFID Systems”, IEEE International Conference on RFID, 2009.

[7] M.S. Trotter, G.D. Durgin “Survey of Range Improvement of Commercial RFID Tags With Power Optimized Waveforms”, IEEE International Conference on RFID, 2010.

[8] Alírio Soares Boaventura and Nuno Borges Carvalho, “Enhanced front-end to Extend Reading Range of Commercial RFID Readers Using Efficient Multisine Signals”, IMS2012 - International Microwave Symposium, Montréal, Canada, June 2012.

[9] Alírio Soares Boaventura and Nuno B. Carvalho, “Extending Reading Range of Commercial RFID Readers”, IEEE Transactions on Microwave Theory and Techniques.

[10] J. Harvey, E.R. Brown, D.B. Rutledge, R.A. York, “Spatial Power Combining for High Power Transmitters”, IEEE Microwave Magazine, December 2000.

[11] R. A. York, R. Compton, “Coupled-Oscillator Arrays for Millimeter-Wave Power-Combining and Mode-Locking”,IEEE International Microwave Symposium, California USA, 1992.

[12] C. A. Balanis, Antenna Theory: Analysis Design, 3th

Edition, John Wiley & Sons, 2005.

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