Experimental Study on Multi-point Wireless Energy Transmission

at 950MHz Band Daiki Maehara (M2) Araki-Sakaguchi Lab.

1 MCRG seminar 2012/11/8

Contents • Background

•  Wireless sensor networks •  Wireless energy transmission •  950MHz & 920MHz

• Multi-point wireless energy transmission • Carrier shift diversity •  Indoor experiment

•  Experimental environment •  Power distribution •  Coverage evaluation

• Conclusion

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Wireless sensor networks • Application

•  Health monitoring of buildings •  Reduction of wire harness in airplanes •  BEMS(Building Energy Management System)

• Problems

•  Plug •  Limit the place of installation

•  Battery •  Needed to be replaced

•  Environmental energy •  Lack of stability

Wireless energy transmission

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Wireless grid

4

Sensor node

Sense environmental data Activated w/ wireless energy

Wireless grid

Grid node

Supply wireless energy to sensors

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Data communication

Wireless power

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Wireless energy transmission 5 MCRG seminar

Inductive coupling Resonant coupling Radio wave emission

Principle

1.  Generation of magnetic field

2.  Change of current at Rx by mutual inductive coupling

3.  Generation of electric power

1.  Electric or magnetic resonance at Tx

2.  Generation of near field

3.  Generation of electric or magnetic field at Rx

1.  Matching between Tx and air

2.  Propagation in air 3.  Matching between

air and Rx

Distance ~ a few cm 1/10λ～1/2λ ～ several km

Application

RFID(13.56MHz) Qi

WiTricity RFID(950MHz)

SSPS

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950MHz Band •  For wireless energy transmission

•  non-LBT (Listen Before Talk), 1W, 4 channels

•  For wireless sensor network •  1mW, 33 channels

6

Sensor network

Wireless Energy Transmission

MCRG seminar 2012/11/8

920MHz Band

Wireless energy transmission

Sensor network

7

•  950MHz band → LTE mobile systems •  920MHz band → RFID systems (2012.7~) •  For wireless energy transmission

•  non-LBT (Listen Before Talk), 1W, 4 channels

•  For wireless sensor network •  1mW, 77 channels

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Multi-point wireless energy transmission

8

• Single-point wireless energy transmission •  Limited coverage by the maximum transmit power •  Standing-wave problem due to reflected waves

• Multi-point wireless energy transmission •  Can extend supply area •  Standing-wave problem due to multiple wave sources

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dead spot

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•  Frequency of n-th Tx point

• Cycle of power fluctuation

Carrier shift diversity

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fn = fc +BN

n− N +12

"

#$

%

&'

Allocate N orthogonal subcarriers to N different points

(n =1,2,...,N )

T =1/Δf = N / B

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Carrier shift diversity • Signal from n-th point

• Received power passing through the rectenna

•  Average received power •  w/o carrier shift diversity

•  w/ carrier shift diversity

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wn t( ) = 2Pn cos ωnt +θn( )

Ptotal (t) = LPF wn t( )n=1

N∑

2"

#$

%

&'= Pnn=1

N∑ + PnPm cos (ωn −ωm )t +θn −θm[ ]

m≠n

N∑n=1

N∑

1T

Ptotal (t)0

T∫ = Pn + PnPmm≠n

N∑ cos θn −θm[ ]

n=1

N∑n=1

N∑

= Pnn=1

N∑ Summation of the powers from the N single-points.

Fluctuate depending on the position of the sensor. ( standing wave)

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Multi-point wireless energy transmission

11

• Single-point wireless energy transmission •  Limited coverage by the maximum transmit power •  Standing wave problem due to reflection wave

• Multi-point wireless energy transmission •  Can extend energy supply area •  Standing-wave problem due to multiple wave source •  Carrier shift diversity

Time average

2πΔft = 02πΔft = π / 42πΔft = π / 22πΔft = 3π / 42πΔft = π2πΔft = 5π / 42πΔft = 3π / 22πΔft = 7π / 4

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dead spot

Single-point Simple multi-point Proposed multi-point

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Environment

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Reader / Writer

Rx antenna

Tx antenna

Phase shifter

Positioner

Tx power: 30dBm Center frequency: 952.4MHz

Measurement interval: 3cm

Carrier offset: 50Hz

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Environment

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Tx antenna #1

Rx antenna

Tx antenna #2

Positioner

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Phase shifter

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• Phase shift controlled by D/A board •  Control voltage 0V→Phase shift 0 •  Control voltage 10V→Phase shift 2π

• Experimental value •  Voltage step：0.1V（phase step 2π/100） •  Phase shift cycle：20ms（50Hz）

RF Input

RF Output

Control voltage

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Power distribution (Single-point)

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Power distribution (Single-point)

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Path loss

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Power distribution (Single-point)

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Due to reflected wave from opposite side wall

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Power distribution (Multi-point w/o carrier shift diversity)

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Due to multiple wave sources

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Power distribution (Multi-point w/ carriers shift)

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Resolve dead spots

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• Coverage : Location probability of sensor activation 　Activation point　 Coverage

Coverage evaluation

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S(l,Preq ) =1 P(l) ≥ Preq0 P(l)< Preq

"#$

%$C(Preq ) =

S(l,Preq )dl0

L∫

L

Required power

Distance between Tx antenna　

L

S(l,Preq )P(l)

Preq

Power distribution Activation point

C(−10dBm) = 96%

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Coverage evaluation

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Maintain the coverage of 100%

IC power required in real RFID systems

Single: 96% Multi w/o CS: 96% Multi w/ CS: 100%

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Battery-less sensor (Collaborated with Nihon Dengyo Kosaku Co., Ltd.)

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Energy transmitter

Sensor

Data receiver

Data monitor

It is technically possible to activate sensors via wireless energy transmission

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Conclusion •  Wireless energy transmission for sensor networks

•  Standing-wave problem due to reflection wave and multiple sources •  Resolve the dead spot by multi-point wireless energy transmission with

carrier shift diversity

•  Future works •  Design of the transmitter and the transmission points in order to enhance

the coverage •  System design for realizing inter-grid communication and grid-sensor

communication

23 MCRG seminar

Show the standing wave problem and the benefits of the proposed scheme by indoor experiment and verify the seamless wireless energy supply

2012/11/8