FEASIBILITY STUDY ON

MICROWAVE POWER TRANSMISSION TO A ZIGBEE DEVICE

FOR WIRELESS SENSOR NETWORK

Tomohiko Mitani, Takuya Ichihara, Nozomu Suzuki, and Naoki Shinohara

Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto, Japany

mitani@rish.kyoto-u.ac.jp

Keywords: Wireless power transmission, ZigBee, Wireless sensor network

Abstract: The objective of the present study is to drive or charge a ZigBee device wirelessly by microwave power

transmission. Wireless sensor network is expected to monitor several systems in order to control various

infrastructures, such as electric power consumption, actively. However, power supply for sensor terminals is

a critical problem to realize a fruitful wireless sensor network system. Primary batteries needs to be changed

soon or later, wired power supply confines their installation location, and natural energy utilization like

solar cells limits their regular operation. We therefore suggest wireless power supply for the sensor

terminals by microwave power transmission. We adopt a ZigBee device as a wireless sensor terminal

because of its low power consumption. We experimentally investigated electromagnetic compatibility

between ZigBee and microwave power transmission, and found that there were some frequencies and power

levels of microwave power transmission not to interrupt ZigBee. We also developed a microwave power

receiving system which consists of a receiving antenna, a rectification circuit, a dc-dc converter, and a

power storage circuit or a secondary battery. Finally we succeeded establishment of ZigBee network while

driving a ZigBee device without batteries by microwave power transmission. Through the experiments, we

found out intermittent microwave power transmission was preferable to CW microwave power transmission

with respect to electromagnetic compatibility and rf-dc efficiency.

1 INTRODUCTION

Wireless sensor network is becoming an attractive

application for monitoring systems such as energy

conservation systems of buildings and houses, traffic

management systems, environment monitoring

systems etc. A vast number of wireless sensor

terminals are scattered over a wide area, and send

and receive monitoring information through ad hoc

network. The collected information will be utilized

to control various infrastructures, such as electric

power consumption, actively.

One of the critical issues of the wireless sensor

network is the way to supply electric power for

sensor terminals. Primary batteris need to be

changed soon or later even though power

consumption of the sensor terminals is quite small.

That will make the running cost of the wireless

sensor network expensive. Wired power supply can

provide stable operation of the sensor terminals;

whereas it confines their installation location and

burdens the wireless sensor network with wired cost.

Natural energy utilization like solar cells with a

charging system will drive the sensor terminal

permanently; however their installation location and

regular operation are limited because the natural

energy is quite unstable.

We therefore suggest wireless power supply to

the sensor terminals by microwave power

transmission (MPT), in order to realize a fruitful

wireless sensor network. MPT is able to provide a

stable power for the sensor terminals, transmits

power even for a long distance, supplies the power

for multiple terminals simultaneously, and even

drives the terminals without batteries. By utilizing

MPT for the wireless sensor network, we can

provide new applications of the wireless sensor

network as well as the envisioned ones. A great

potential application is a wireless sensor network

system for disaster relief, as shown in Figure 1.

Wireless sensor terminals located in any places

normally sleep but work in emergency situations by

receiving the power from vehicles or helicopters via

Mitani T., Ichihara T., Suzuki N. and Shinohara N.

FEASIBILITY STUDY ON MICROWAVE POWER TRANSMISSION TO A ZIGBEE DEVICE FOR WIRELESS SENSOR NETWORK.

DOI: 10.5220/0005413500290034

In Proceedings of the First International Conference on Telecommunications and Remote Sensing (ICTRS 2012), pages 29-34

ISBN: 978-989-8565-28-0

long-range MPT. The activated terminals can collect

and send disaster information via short-range

wireless communication. Another application is

“energy harvesting” from radio communication

(Kawahara, 2009). The energy harvesting system by

receiving VHF or UHF energy from TV towers were

reported in 2009 (Sample, 2009). Mobile

communication systems and wireless local area

network systems, as shown in Figure 2, will be

potential candidates as energy harvesting source in

microwave band.

The objective of the present study is to drive or

charge a wireless sensor terminal by MPT. We adopt

a ZigBee device as wireless sensor terminal because

of its low power consumption. In this paper we

describe our current status of MPT to a ZigBee

device, including our previous studies on

development of a microwave power receiving circuit

(Suzuki, 2010), and feasibility of intermittent MPT

(Ichihara, 2012).

Microwavepowertransmission

Short‐rangewirelesscommunication

Figure 1: A conceptual image of a wireless sensor network

system for disaster relief.

WLAN

routeretc.

・・・SensorTerminal

Celltower etc.

Figure 2: A conceptual image of energy harvesting from

radio communications in microwave band.

2 OUTLINE OF MPT SYSTEM TO

ZIGBEE DEVICE

2.1 Zigbee Device

ZigBee is one of the radio communication standards

and appropriate for the sensor network for the

following reasons: its power consumption is lower

than wireless LAN and Bluetooth, its production

cost is low, and its network capacity is large. Its low

power consumption is beneficial for a MPT system

from the viewpoint of electromagnetic compatibility

between MPT and ZigBee.

In the present study, we are using

IEEE802.15.4/ZigBee Evaluation and Development

Kit TWE-EK-001 produced by Tokyo Cosmos

Electric Company (TOCOS) as ZigBee devices.

ZigBee network consists of the coordinator, a router

and an end device. The coordinator is only one in a

ZigBee network system and it coordinates the

network. The router has the function of relaying data

from other routers and end devices as well as

monitoring. The end device only has the function of

monitoring and data sending. Therefore the power

consumption of the end device is lower than the

coordinator and the router. Table 1 shows measured

average power consumptions of a router and an end

device of this kit. The power consumption of the

router is stable whether or not it joins in the

network; whereas the power consumption of the end

device is quite small when it joins in the network.

We have studied on MPT to an end device in the

present paper.

In our configuration, the router and the end

device sent data for nearly 2 milliseconds every 1.14

seconds. The coordinator decided to drop them from

its network if it had not received data from them for

15 seconds. When it had dropped out of the network,

the device was requested to join in the network again.

Table 1: Measured average power consumption of ZigBee

devices.

Device type Joining Not joining

Router 57.4 mW 57.1 mW

End device 9.46 mW 61.8 mW

2.2 Microwave Power Receiving

Circuit

Figure 3 shows a schematic of a microwave power

receiving circuit. The microwave power receiving

First International Conference on Telecommunications and Remote Sensing

circuit consists of a receiving antenna, a rf-dc

rectifier, a power storage circuit, and a dc-dc

converter. The combination of the receiving antenna

and the rf-dc rectifier is called “rectenna”. The

power storage circuit can be omitted when the

transmitted microwave power is large enough to

drive a ZigBee device directly. The dc-dc converter

converts the rectenna output voltage to a regulated

voltage for a stable ZigBee device operation.

We adopt 2.4 GHz ISM-band, which is the same

frequency band as ZigBee, as MPT frequency, from

the viewpoint of efficient frequency usage. In this

study, continuous or intermittent microwave without

modulation transmits power to the receiving circuit.

Microwave

power

Rf‐dc

rectifier

Receivingantenna

Powerstorage

circuit

Dc‐dc

converter

ZigBee

(enddevice)

ZigBee

communication

From/to

coordinator

orrouter

From

transmitting

antenna

Figure 3: A schematic of microwave power receiving

circuit.

3 EXPERIMENTAL STUDY ON

COMPATIBITITY BETWEEN

MPT AND ZIGBEE

We firstly conducted experimental measurements of

electromagnetic compatibility between MPT and

ZigBee, in order to investigate how MPT affected

ZigBee. Also the measurements contributed to fix

the frequency and maximum transmitting power of

MPT in our study.

We defined and evaluated two indices on MPT

power density: communicable power density (CPD)

and joinable power density (JPD). CPD is the

threshold of MPT power density which does not

affect ZigBee. A ZigBee device can communicate

with the other one under the CPD when it has

already joined in the network. JPD is the threshold

of MPT power density under which a ZigBee device

can join in the network. The CPD is generally larger

than the JPD.

3.1 Measurement Setup

Figure 4 shows the experimental configuration. The

ZigBee frequency was set to 2.46 GHz (22 ch), and

the network had just two devices of the coordinator

and an end device. First, the coordinator, which was

put behind the transmitting horn antenna, established

communication with the end device. Then, the end

device was irradiated with non-modulated

microwave as alternative to MPT. We measured

CPD with increasing the non-modulated microwave

power until the communication was disabled. After

that, we measured JPD with reducing the non-

modulated microwave power until the coordinator

established communication with the end device

again.

We investigated two types of non-modulated

microwave irradiation: CW microwave irradiation

and intermittent microwave irradiation. During the

intermittent microwave irradiation, the non-

modulated microwave turned on and off under the

conditions of a pulse frequency and a duty ratio.

78.0cm

575cm

ZigBee

enddevice

Directional

coupler

Power

meter

43dB

Amplifier

(20W max)

ZigBee

coordinator

Signal

Generator

Figure 4: Experimental configuration of electromagnetic

compatibility between MPT and ZigBee.

3.2 Measurement Results

3.2.1 CW microwave irradiation

We conducted CW microwave irradiation to a

ZigBee end device, with changing the frequency

from 2.4 GHz to 2.5 GHz (20 MHz step). We

measured the maxima of the CPD and JPD at all the

frequency points.

Experimental results of the CPD and JPD are

shown in Figure 5. At all the frequency points, the

CPDs were lower than 1 mW/cm

– the maximum

power density determined by International

Commission on Non-Ionizing Radiation Protection

(ICNIRP, 1998). Around the ZigBee frequency, the

power density of 5 pW/cm

even interrupted ZigBee.

From these results, CW MPT is quite difficult to be

compatible with ZigBee, in order to supply enough

power wirelessly for a ZigBee device. Although we

have not checked yet, CW MPT at another

frequency band except 2.4 GHz band might be

compatible with ZigBee.

Feasibility Study Onmicrowave Power Transmission to a Zigbee Devicefor Wireless Sensor Network

1.E-9

1.E-8

1.E-7

1.E-6

1.E-5

1.E-4

1.E-3

1.E-2

1.E-1

1.E+0

2.4 2.41 2.42 2.43 2.44 2.45 2.46 2.47 2.48 2.49 2.5

Power density / (mW/cm²)

Frequency / GHz

Maximum of

communicable power

density (CPD)

Maximum of joinable

power density (JPD)

Figure 5: Measurement results of the CPD and JPD when

the ZigBee end device was irradiated with CW microwave.

3.2.1 Intermittent microwave irradiation

Under the same condition as shown in Figure 4, we

conducted intermittent microwave irradiation to a

ZigBee end device. We fixed the intermittent

microwave frequency of 2.46 GHz, at which we

obtained the lowest maximum of the CPD and JPD

in the CW microwave irradiation case. The end

device was irradiated with intermittent microwave,

within the pulse frequency range from 1 Hz to 200

Hz and the duty ratios of 0.1, 0.5 and 0.9.

From experimental results, 1.91 mW/cm

of the

peak power density with any duty ratios allowed the

end device to participate in the network and

communicate with the coordinator. This means the

CPD and JPD of the intermittent microwave is more

than 10

times larger than those of CW microwave,

when the MPT frequency is the same as the ZigBee

frequency.

We also measured error rates of ZigBee during

the intermittent microwave irradiation. Experimental

results under the peak power density of 1.91

mW/cm

are shown in Figure 6. Since we fixed the

peak power density in the measurements, the

average power density was dependent on the duty

ratio: 0.191 mW/cm

, 0.955 mW/cm

and 1.72

mW/cm

at the duty ratios of 0.1, 0.5 and 0.9,

respectively. The experimental results show that the

ZigBee end device could communicate with the

coordinator almost perfectly at the duty ratio of 0.1;

whereas it seemed difficult for the ZigBee end

device to send data stably at the duty ratio of 0.9.

In the measurements, interference would occur

stochastically because we made the period of

intermittent microwave irradiation irrelevant to that

of ZigBee. Therefore, it is important to build a

scheduling rule between intermittent MPT and

ZigBee for a robust wireless sensor network with

MPT.

100

1 10 100

Error rate / %

Pulse frequency / Hz

0.1 0.5 0.9

Duty ratio

Figure 6: Measurement results of error rates when a

ZigBee end device was irradiated with intermittent

microwave under the peak power density of 1.91 mW/cm

4 DEVELOPMENT OF A

RECEIVING CIRCUIT

4.1 Rectenna

Rectenna consists of a receiving antenna and a rf-dc

rectifier. As with the experiments in Section 2, we

focused on the MPT frequency of 2.46 GHz.

Circular patch antenna was adopted as receiving

antenna. Figure 7 shows a photograph of the circular

patch antenna. The measured antenna gain was 6.5

dBi.

Figure 7: Photograph of the circular patch antenna.

We developed a single-shunt type rf-dc rectifier,

whose photograph is shown in Figure 8. We adopted

Schottky barrier diode (Avago HSMS-2860) as

rectifier diode. Measurement results of rf-dc

conversion efficiency are shown in Figure 9. The

rectifier provided the maximum rf-dc conversion

efficiency of 65 % at 2.46 GHz when the output load

was 138.1 ohms, in the CW microwave case (Suzuki,

2010). The rf-dc conversion efficiency started to be

dropped at an input power of 180 mW. This drop in

conversion efficiency is related to the breakdown

voltage of the diode.

First International Conference on Telecommunications and Remote Sensing

We also investigated rf-dc conversion efficiency

when the intermittent microwave was input to the

rectifier. The duty ratio of the intermittent

microwave was changed from 1 (CW) to 0.1, the

pulse frequency was 1 kHz, and the average input

power was 16 mW, 65 mW and 101 mW. Since we

fixed the average input power, the peak input power

was dependent on the duty ratio. The peak input

power was equal to the average input power at the

duty ratio of 1; whereas it became 10 times larger

than the average input power at the duty ratio of 0.1.

Figure 10 shows measurement results of rf-dc

conversion efficiency in the intermittent microwave

case. Of great interest is that the rf-dc conversion

efficiency depended on the peak input power. Even

if the average power is small, one can obtain the

maximum rf-dc conversion efficiency by adjusting

the duty ratio of intermittent microwave. The rf-dc

conversion efficiency dropped down at low duty

ratios when the average input power was 65 mW and

101 mW, because the peak input power became over

the input power of 180 mW, where the rf-dc

conversion efficiency started to be dropped.

Figure 8: Photograph of the rf-dc rectifier (Suzuki, 2010).

0.1 1 10 100

Rf-dc conversion efficiency / %

Input power / mW

Figure 9: Rf-dc conversion efficiency of the rectifier in the

CW microwave case.

0 0.2 0.4 0.6 0.8 1

Rf-dc conversion efficiency / %

Duty ratio

101 mW

65 mW

16 mW

Figure 10: Rf-dc conversion efficiency of the rectifier in

the intermittent microwave case.

4.2 Dc-dc converter

The rectenna output voltage became over 4 V at the

input microwave power of 180 mW from the

measurement results; whereas the ZigBee input

voltage should be in the range from 2.7 V to 3.6 V.

We therefore adopted a step-down dc-dc converter

(Texas Instruments TPS62120) as dc-dc converter.

This dc-dc converter provided an efficiency of over

90 % in an output voltage range around 3 V.

5 INTERMITTENT MPT

DEMONSTRATION TO

ZIGBEE DEVICE

We conducted demonstration experiments of power

supply to a ZigBee end device by intermittent MPT.

A demonstration configuration and a photograph of

the demonstration are shown in Figure 11 and Figure

12, respectively. The MPT frequency was 2.46 GHz,

and the pulse frequency was changed from 1 Hz to

50 Hz. Three rectennas were connected in series.

The ZigBee end device had no batteries, that is, it

was driven only by intermittent MPT. In this

demonstration, we fixed a peak power density of 2.4

mW/cm

at the rectenna position.

175.0cm

575cm

ZigBee

enddevice

Directional

coupler

Power

meter

43dB

Amplifier

(20W max)

ZigBee

coordinator

Signal

Generator

3Rectennas

Dc‐dc

converter

Figure 11: Configuration of intermittent MPT

demonstration to a ZigBee end device.

Feasibility Study Onmicrowave Power Transmission to a Zigbee Devicefor Wireless Sensor Network

Figure 12: Photograph of intermittent MPT demonstration

to a ZigBee end device.

From the demonstration experiments, the end

device was driven by intermittent MPT at any pulse

frequency when the duty ratio of the intermittent

MPT was above 0.4. We therefore succeeded

battery-less operation of the ZigBee device by MPT.

Moreover, when the pulse frequency was 10 Hz, the

end device could communicate with the coordinator

with no error, while its power was being supplied by

MPT.

6 CONCLUSIONS

We succeeded MPT to a ZigBee device which was

driven without batteries. Intermittent MPT was

preferable to CW MPT because of the following

reasons: ZigBee was better compatible with MPT,

and higher peak power of MPT was allowed. The

latter factor contributed to higher rf-dc conversion

efficiency of the rectifier even at the low average

power. Moreover we confirmed that the ZigBee end

device worked and communicated correctly with the

coordinator while its power was being supplied by

intermittent MPT.

As future works, we will study on scheduling

management between MPT and ZigBee. Although

we succeeded intermittent MPT demonstration to a

ZigBee device as a feasibility study, scheduling

management will be essential for realizing a fruitful

wireless sensor network. Also we will have to study

how to transmit microwave power to multiple

ZigBee devices in a wide area.

ACKNOWLEDGEMENTS

A part of the present study is supported by The

Japan Prize Foundation.

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Ichihara, T., Mitani, T., Shinohara, N., 2012. Study on

Intermittent Microwave Power Transmission to a

ZigBee Device. In Proc. of 2012 IEEE MTT-S

International Workshop Series on Innovative Wireless

Power Transmission: Technologies, Systems, and

Applications (IMWS-IWPT 2012), pp.209-212.

International Commission on Non-Ionizing Radiation

Protection (ICNIRP), 1998. Guidelines for Limiting

Exposure to Time-varying Electric, Magnetic, and

Electromagnetic Fields (up to 300 GHz). Health

Physics, 74 (4), pp.494-522.

Kawahara, Y., Lakafosis, V., Sawakami, Y., Nishimoto,

H., Asami, T., 2009. Desing Issues for Energy

Harvesting Enabled Wireless Sensing Systems. In

Proc. of Asia-Pacific Microwave Conference 2009

(APMC 2009), pp.2248-2251.

Sample, A., Smith, J. R., 2009. Experimental Results with

two Wireless Power Transfer Systems. In Proc. of

IEEE Radio and Wireless Symposium 2009

(RWS2009), pp.16-18.

Suzuki, N., Mitani, T., Shinohara, N., 2009. Study and

Development of a Microwave Power Receiving

Circuit for ZigBee Device. In Proc. of Asia-Pacific

Microwave Conference 2010 (APMC 2010), pp.45-48.

First International Conference on Telecommunications and Remote Sensing