IMEC WIRELESS RESEARCH

FAR-FIELD RF ENERGY TRANSPORT

BITS & CHIPS HARDWARE CONFERENCE 2013

HUBREGT J. VISSER,

2

1 INTRODUCTION

Wireless Remote Battery Charging

1 Introduction

2 Rectenna

3 Propagation and Transmit Antenna

4 Example

5 Conclusions

3

CONTENTS

1 INTRODUCTION

▸ Wireless Energy Harvesting

- Reception of ambient radiofrequency signals and the conversion

of these signals into DC energy

4

Wireless Energy Harvesting and Transport

GSM900 Summed Downlink Power Density

Urban environment, 25 to 100m from base station: 0.03 – 0.3 W/cm2

Harvesting not feasible for small sensors

1 INTRODUCTION

▸ Wireless Energy Transport

- Transport of electrical energy by means of electromagnetic

principles, using dedicated sources

5

Wireless Energy Harvesting and Transport

▸ Wireless Transport Mechanisms

- Inductive

- Non-radiative, resonant coupling

- Far-field transfer

6

1 INTRODUCTION

Far-Field RF Energy Transport Subsystems

Transmit

antenna

Propagation Rectenna

7

1 INTRODUCTION

Smart Building Integration (SBI)

Drive: Cutting costs

Artificial lighting contributes

~30% of electrical consumption

in a commercial building .

Challenge: Sensor powering

Hundreds of wireless sensors needed.

100W power consumption.

Powering through cabling is too

expensive (7-11k€ per office unit).

Energy harvesting not always possible.

RF Energy Transport

8

1 INTRODUCTION

ISM frequency bands of interest

Power Restrictions

Delivering 100W over several meters is challenging

Optimize rectenna and transmit antenna

Frequency Band Power

Duty Cycle /

Tx type

Channel

Spacing / BW Region

a 2446-2454 MHz

500mW EIRP

4W EIRP

Up to 100%

≤ 15% No spacing Europe

b1 865.0-865.6 MHz 100mW ERP 200kHz Europe

b2 865.6-867.6 MHz

2W

ERP 200kHz Europe

b3 867.6-868.0 MHz 500mW ERP 200kHz Europe

902-928

MHz

4W

EIRP

FH (50

channels) or

DSSS

USA

Canada

2400-2483.5

MHz

4W

EIRP

FH ((75

channels) or

DSSS

USA

Canada

2400-2483.5

MHz

10mW

EIRP 1MHz BW

Japan

Korea

9

2 RECTENNA

The core of the rectenna is the rectifier

RK4 analysis

Anticipate input power level and design antenna for conjugate matching

Rectifier

10

2 RECTENNA

Antenna Matching Efficiency

21 z

RL

RL

ZZ

ZZ

*

11

2 RECTENNA

RF-DC Conversion Efficiency (I)

.1801

0

0

VnkT

q

R

RR

sL

incgL

sg

eIR

VPR

nkT

q

12

2 RECTENNA

RF-DC Conversion Efficiency (II)

accL

conPR

V 2

0

inczacc PP

13

2 RECTENNA

DC-DC Boost Conversion Efficiency

14

2 RECTENNA

Efficiency Example

Direct conjugate matching

to voltage doubler

Optimized for Pinc = 0dBm

Pinc = -5dBm tot = 0.03

Pinc = 0dBm tot = 0.21

f=2.45GHz

DC-DC boost converter most

critical component

15

3 PROPAGATION AND TX ANTENNA

Observations: in-corridor measurements

Measurement narrow beam

Measurement broad beam

Fit narrow beam

Fit broad beam

DC power

16

3 PROPAGATION AND TX ANTENNA

Observations

8.1

1~

rrP

5.1

1~

rrP

Optimize Tx antenna radiation pattern

Decrease EIRP and use constructive interference

17

3 PROPAGATION AND TX ANTENNA

Geometric Optics (GO) Modeling

• Ignore impedance effects of walls

• Ignore corners and edges

1

,

2

2 ( 1)0,1 ,

1 1,

,

11 ,

2

,

04

4

m

m n

N NRjm n

r t

m n m n

Direct ray Reflected rayscontributioncontrib

m

m n i

i

t r

n

ution

t

g gP P e

d d

PG G

d

18

3 PROPAGATION AND TX ANTENNA

Tx Antenna Design Strategy

• Identify constructive-interference reflection points

• Create multi-beam Tx antenna radiation pattern

Single Beam Multi (7) Beam

50m x 50m x 50m, PEC-walled room

10dB improvement

Free space attenuation

19

3 PROPAGATION AND TX ANTENNA

Switched Array

• Switched array as cheap alternative for phased array

• One driven monopole, parasitic monopoles switched to ground

• Finite ground plane must be skirted

• To increase gain switch Yagi-Uda linear arrays

20

3 PROPAGATION AND TX ANTENNA

Switched Array Design by ‘Trial and Error’

Approximate model

needed

21

3 PROPAGATION AND TX ANTENNA

Switched Array Model

• Thick wire dipole self-impedance calculation

• Thick and thin wire mutual coupling analysis

• PIN diode equivalent model

• Wire-to-strip conversion

Model based on

22

4 EXAMPLE

Commercially Available Sensor

Temperature and relative humidity sensing

• Data transmission @ 433MHz

• 55µW power consumption

• Transmission every 45s

Sensor powering up to 7m @ 2.45GHz

EIRP=10W, power consumption DC-DC boost converter is 17.9W, total: 72.9 W

23

4 EXAMPLE

COTS wireless sensor

batteries removed

Wireless battery

+

-

+, -, current monitoring

Interior wireless battery

FR4

Microstrip patch antenna, 2.45GHz

Dual Schottky diode

Capacitor DC-DC boost converter

Rechargeable Li-Ion battery (3V)

Antenna ground

plane

Sensor Powering

24

4 EXAMPLE

Sensor Powering by GSM Phone

Temperature sensor

with display

GSM phone activated

by QR code

Dual frequency

(900MHz, 2.4GHz)

rectenna powering

temperature sensor

25

5 CONCLUSIONS

• Complex conjugate antenna matching;

• Carefully choosing anticipated RF power level;

• Decreasing boost converter voltage dynamic range;

• Applying cascaded Schottky diodes;

• Characterizing propagation channel;

• Indicating constructive interference reflection points;

• Adapting Tx radiation pattern

GSM phone battery charging?

• Ongoing research (classified)

RF power transfer Efficiency Enhancement by: