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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
2 MCRG seminar 2012/11/8
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
3 MCRG seminar 2012/11/8
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
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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
2012/11/8 MCRG seminar
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
MCRG seminar 10
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
MCRG seminar 13
Tx antenna #1
Rx antenna
Tx antenna #2
Positioner
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Phase shifter
MCRG seminar 14
• 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
MCRG seminar 21
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