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1Globecom 2016
Communications and Signals Design for Wireless Power Transmission
Rui Zhang
ECE Department, National University of Singapore
Globecom Workshop on Wireless Energy HarvestingWashington, DC USA, 2016
Rui Zhang, National University of Singapore
Agenda
Overview of main WPT technologies
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 2
Agenda Rui Zhang, National University of Singapore
3Globecom 2016
Why Wireless Power?Rui Zhang, National University of Singapore
Wireless power transfer (WPT): deliver power without wires Advantages over traditional energy supply methods:
Convenient: without the hassle of connecting wires and replacing batteries Cost-effective: on-demand power supply with uninterrupted operations Environmental friendly: avoid battery disposal
Extensive applications: Consumer electronics wireless charging Biomedical implants wireless charging Wireless sensor/IoT devices charging Backscatter/RFID communications Simultaneous wireless information and power transfer (SWIPT) Wireless powered communications (WPC)
Overview of main WPT technologies
Globecom 2016 4
Rui Zhang, National University of Singapore
Near-field technique based on magnetic induction Main advantage: Very high efficiency (e.g. >90%) Main limitations
Require precise tx/rx coil alignment, very short range, single receiver only Example Applications
Electric vehicle charging, smart phone charging, RFID, smart cards, … Industry standard: Qi (Chee) Representative companies: Powermat, Delphi, GetPowerPad,
WildCharge, Primove, …
Inductive Wireless Power Transfer
Overview of main WPT technologies
Globecom 2016 5
Rui Zhang, National University of Singapore
Near-field technique based on magnetic resonant coupling Main advantages: high efficiency and mid-range, one-to-many (multicast) charging Main limitations: sensitive to tx/rx coil alignment, large tx/rx size Applications
Similar to inductive coupling, but target for longer range and multicasting Industry standard: Qi, AirFuel,… Representative companies: Intel, PowerbyProxi, WiTricity, WiPower,….
Magnetic Resonant Wireless Power Transfer
Overview of main WPT technologies
Globecom 2016 6
Rui Zhang, National University of Singapore
Far-field WPT technique via EM/microwave radiation Main advantages:
long range, small tx/rx form factors, flexible deployment, support power multicasting with mobility, applicable for both LoS and Non-LoS environment, integration with wireless communication (backscatter, SWIPT, WPCN)
Main limitations: low efficiency, safety and health issues Extensive Applications
Wireless sensor/IoT devices charging, RFID, solar power satellite,… Representative companies: Intel, Energous, PowerCast, Ossia,…
Radiative Wireless Power Transmission
Energy flow
Overview of main WPT technologies
Globecom 2016 7
Rui Zhang, National University of Singapore
WPT via highly concentrated laser emission Main advantages
long range, compact size, high energy concentration, no interference to existing communication systems or electronics
Main limitations laser radiation is hazardous, require LoS link and accurate rx focusing,
vulnerable to cloud, fog, and rain Applications
Laser-powered UAVs, laser-powered solar power satellite,… Representative company: LaserMotive, …
Laser Power BeamingOverview of main WPT technologies
Comparison of the Main WPT Technologies
Strength Efficiency Distance Multicast Mobility Safety
Inductive Coupling Very high Very high Very short No No Yes
Magnetic Resonant Coupling
High High Short Yes Difficult Yes
EM Radiation
Omni-directional
Low Low Long Yes Yes Yes
Beamforming (microwave)
High High Very long(LOS)
Yes Yes Safety constraints may apply
Laser beaming High High Long No Difficult Safety constraints may apply
Globecom 2016 8
Rui Zhang, National University of Singapore
This talk will focus on EM radiation WPT technology and the main communication and signal design techniques for improving its performance
Overview of main WPT technologies
Agenda
Overview of main WPT technologies
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 9
Microwave WPT: Historical development and contemporary design Rui Zhang, National University of Singapore
Microwave Wireless Power Transmission: Historical MilestonesYear Main activity and achievement1888 Heinrich Hertz demonstrated electromagnetic wave propagation in free space
1899 Nicola Tesla conducted the first experiment on dedicated WPT
1901 Nicola Tesla started the Wardenclyffe Tower project
1964 William C. Brown invented rectenna
1964 William C. Brown successfully demonstrated the wireless-powered tethered helicopter
1968 William C. Brown demonstrated the beam-positioned Helicopter
1968 Peter Glaser proposed the SPS concept
1975 Over 30kW DC power was obtained over 1.54km in the JPL Goldstone demonstration
1983 Japan launched the MINIX project
1987 Canada demonstrated the free-flying wireless-powered aircraft 150m above the ground
1992 Japan conducted the MILAX experiment with the phased array transmitter
1993 Japan conducted the ISY-METS experiment
2008 Power was successfully transmitted over 148km in Hawaii
2015 Japan announced successful power beaming to a small device
Globecom 2016 10
Rui Zhang, National University of SingaporeMicrowave WPT: Historical development and contemporary design
Microwave Wireless Power Transmission: Nikola Tesla and his Wardenclyffe Project in early 1900
150 KHz and 300 kW. Unsuccessful and never put into practical use.
Globecom 2016 11
Rui Zhang, National University of SingaporeMicrowave WPT: Historical development and contemporary design
The Invention of ``Rectenna” for Microwave Power Transmission:the Microwave Powered Helicopter by William C. Brown in 1960s
2.45 GHz and less than 1kW. Overall 26% transfer efficiency at 7.6 meters high.
Globecom 2016 12
Rui Zhang, National University of SingaporeMicrowave WPT: Historical development and contemporary design
Solar Satellite with Microwave Power Transmission (1970s-current)
NASA Sun Tower
Target at GW-level power transfer with more than 50% efficiency
Globecom 2016 13
Rui Zhang, National University of SingaporeMicrowave WPT: Historical development and contemporary design
Microwave Wireless Power Transmission: A Fresh New Look
Globecom 2016 14
Rui Zhang, National University of Singapore
Historical microwave WPT: Targeting for long distance and high power Mainly driven by the wireless-powered aircraft and SPS applications Requires high transmission power, huge tx/rx antennas, clear LoS link
Contemporary WPT systems: Low-power delivery over moderate distances Reliable and convenient WPT network for low-power devices (sensors, IoT
devices, RFID tags, smart phone, etc.) New design challenges and requirements:
Range: a few meters to hundreds of meters Efficiency: a fractional of percent Non-LoS: closed-loop WPT with channel state information Mobility support: device tracking Ubiquitous and authenticated accessibility Inter-operate with wireless communication systems Safety and health guarantees
Microwave WPT: Historical development and contemporary design
Research in Wireless Power Transmission : A Shift of Paradigm
Rui Zhang, National University of Singapore
Wireless power transfer (WPT)
Wireless poweredcommunication network
(WPCN)
Simultaneous wireless information and power transfer
(SWIPT)Energy
Energy
Information
Energy
Information
15Globecom 2016
Extensive research efforts have been devoted to co-designing the wireless power and communication systems, e.g., WPCN & SWIPT
A trade-off between rate & power maximization needs to be made, e.g., time switching, power splitting, harvest-then-transmit, etc.
However, even designing efficient WPT system alone is challenging and new Focus of this talk: introduce the main communication & signal processing
techniques for achieving efficient WPT
Microwave WPT: Historical development and contemporary design
Agenda
Overview of main WPT technologies
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 16
WPT and energy receiver model Rui Zhang, National University of Singapore
Wireless Power Transmission: A Generic Model
Globecom 2016 17
Rui Zhang, National University of Singapore
End-to-end efficiency:
e1: DC-to-RF conversion efficiency at energy transmitter (ET) e2: RF-to-RF transmission efficiency, main bottleneck
Require highly directional transmission with multi-antenna and accurate channel knowledge at ET
e3: RF-to-DC conversion efficiency at energy receiver (ER) Require efficient rectenna design and power waveform optimization
WPT and energy receiver model
Narrowband Wireless Power Transmission: Channel Model
Globecom 2016 18
Rui Zhang, National University of SingaporeWPT and energy receiver model
Modulated vs. Unmodulated Energy Signal
Globecom 2016 19
Use pseudo-random modulated energy signal to avoid the spike in the power spectral density (PSD) caused by constant unmodulated energy signal
Rui Zhang, National University of SingaporeWPT and energy receiver model
Wireless Power Transmission: Receiver Model (1)
Globecom 2016 20
Rui Zhang, National University of Singapore
Only keep the second-order term since y(t) is typically small
WPT and energy receiver model
Wireless Power Transmission: Receiver Model (2)
Globecom 2016 21
Rui Zhang, National University of Singapore
The harvested DC power is proportional to the input RF power (linear model) Nonlinear model if higher-order terms are included (to be considered later)
WPT and energy receiver model
Agenda
Overview of main WPT technologies
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 22
Single-user WPT Rui Zhang, National University of Singapore
Single-User Multi-Band MIMO WPT
Globecom 2016 23
Rui Zhang, National University of Singapore
Single-user MIMO WPT with Mt antennas at ET and Mr antennas at ER N frequency sub-bands, with MIMO channel gains H1,…,HN The received power is (assuming linear model):
Power maximization problem:
: transmit covariance matrix at sub-band n nS
rf
rf
: sum-power limit: per-subband power limit
' , where 1 '
t
st
s
PP
P N P N N= ≤ ≤
Single-user WPT
Energy Beamforming for Multi-Band MIMO WPT
Globecom 2016 24
Rui Zhang, National University of Singapore
Optimal solution:
: dominant eigenvector of Hn n nv H H
[ ] : permutation of sub-bands with theirdominant eigvenvalues in decreasing order•
Concentrate power to the N’strongest sub-bands
For each sub-band, concentrate power to the strongest eigen-direction
In contrast to multi-band MIMO communication systems
Optimal value:
Exploit both frequency-diversity gain and spatial energy-beamforming gain
max,[ ] : the dominant eigenvalue of the th strongest sub-bandn nλ
Single-user WPT
Channel Acquisition for MIMO WPT
Globecom 2016 25
Rui Zhang, National University of Singapore
Energy beamforming requires channel state information (CSI) at the ET Unique considerations for CSI acquisition in WPT in contrast to conventional
wireless communication: CSI at (energy) receiver: not required for WPT Net energy maximization: to balance the energy overhead for CSI acquisition and
the energy harvested with CSI-based energy beamforming Hardware constraint: no/low signal processing capability for low-cost ERs
Candidate solutions depending on the antenna architecture at the ER Forward-link training with CSI feedback Reverse-link training via channel reciprocity Power probing with limited energy feedback
Single-user WPT
Antenna Architecture of ER
Globecom 2016 26
Rui Zhang, National University of Singapore
For enabling CSI acquisition, each ER must have a communication module, in addition to the energy harvesting module
Shared-antenna architecture The same set of antennas used for both energy harvesting and communication Energy harvesting and communication take place in a time-division manner Compact receiver form factor, easy channel estimation But require communication and energy harvesting at the same frequency, and
new frontend design of ER Separate-antenna architecture
Different antennas for energy harvesting and communication, independent and concurrent operations, and commercial off-the-shelf hardware available
Single-user WPT
CSI Acquisition (1): Forward-Link Training with CSI Feedback
Globecom 2016 27
Rui Zhang, National University of Singapore
Applicable for shared-antenna architecture only Similar to conventional wireless communications, pilot signals sent by the
ET to the ER for channel estimation ER then feeds back the estimated channel to ET Limitations:
Training overhead scales with the number of antennas at ET, not suitable for massive MIMO WPT
Requires channel estimation and/or feedback by ER, though it does not require CSI for energy harvesting
Single-user WPT
CSI Acquisition (2): Reverse-Link Training via Channel Reciprocity
Globecom 2016 28
Rui Zhang, National University of Singapore
Applicable for shared-antenna architecture only Exploits channel reciprocity: ER sends pilot signals to ET for channel estimation Advantages:
No channel estimation or feedback required at ER Time/energy training overhead independent of number of ET antennas, suitable for
massive MIMO WPT Limitations: Critically depends on channel reciprocity (holds in practice?) New design trade-offs:
Too little training: coarsely estimated channel, reduced energy beamforming gain Too much training: consumes excessive energy at ER, less time for energy transfer
Maximize net energy at ER: harvested energy – energy consumed for training
Single-user WPT
Net Harvested Power versus Number of Trained ER Antennas
Globecom 2016 29
Rui Zhang, National University of Singapore
rf
Number of ET antennas: =5Number of ER antennas: =10
1 Watt, =-60 dB
t
rt
MM
P β=
Single-user WPT
CSI Acquisition (3): Power-Probing with Energy Feedback
Globecom 2016 30
Rui Zhang, National University of Singapore
Applicable for separate-antenna architecture ET sends energy signals with online designed transmit covariance matrices ER measures the amount of harvested energy during each interval ER sends a finite-bit feedback based on its present and past energy measurements ET obtains refined CSI estimation based on the feedback bits
Advantages: Low signal processing requirement at the ER, no need for hardware change Simultaneous energy harvesting not interrupted
Limitations: Training overhead increases with the number of ET antennas
Single-user WPT
Power-Probing with One-Bit Feedback: Case Study
ER k feeds back one-bit information indicating increase or decrease of harvested energy between time slots n and n-1
With one-bit feedbacks, ET Adjusts transmit beamforming for next slot Obtains improved estimation of channels
Globecom 2016 31
Rui Zhang, National University of SingaporeSingle-user WPT
ACCPM Based Channel Learning
Objective: find any point in target set Analytic center cutting plane method (ACCPM): Iteratively shrink working set towards target set. In the nth iteration Find analytic center of working set Find cutting plane whose boundary passes (neutral cutting plane) Cut away half space according to cutting plane to obtain new working set
Q: How to cut half-space? A: Based on one-bit feedback (energy increase or decrease at ER)
Globecom 2016 32
1n−P
X
Cutting planenH
1n n n−= P P H
X
G(n) G(n)~ ~
Rui Zhang, National University of SingaporeSingle-user WPT
Convergence Analysis
ACCPM based single user channel learning algorithm obtains estimation for with in at most intervals
Convergence speed only depends on No. of transmit antennas , but not on No. of receive antennas
Reason: Dimension of :
Theoretical bound only, faster convergence is often observed in simulation
Globecom 2016 33
Rui Zhang, National University of SingaporeSingle-user WPT
Simulation Result: Baseline Schemes
Globecom 2016 34
Partial CSIT: existing one-bit feedback based channel learning schemes Cyclic Jacobi technique (CJT)
One-bit feedback: increase or decrease in received power Usage of feedback: perform blind estimate of EVD of MIMO channel Application: MIMO, one receiver only
Gradient sign One-bit feedback: increase or decrease in received power Usage of feedback: adjust transmit beam with random perturbation Application: MIMO, one receiver only
Distributed beamforming One-bit feedback: larger or smaller than prior highest received power Usage of feedback: adjust phase of transmit beam Application: MISO, one receiver only
Rui Zhang, National University of SingaporeSingle-user WPT
Simulation Result
Globecom 2016 35
ACCPM: best accuracy & convergence performance
Absolute error of harvested power versus No. of feedback intervals
Rui Zhang, National University of SingaporeSingle-user WPT
Agenda
Overview of main WPT technologies
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 36
Multi-user WPT Rui Zhang, National University of Singapore
Multi-User MIMO Energy Multicasting
Rui Zhang, National University of Singapore
Utilize the broadcast nature of microwave propagation for energy multicast Energy near-far problem: fairness is a key issue in the multi-user EB designMultiple beams are needed in general to balance the energy harvesting
performance among users
37Globecom 2016
Multi-user WPT
Multi-User WPT: Network Architecture
J distributed ETs simultaneously serve K ERs each having multiple antennas Three main networking architectures (with complexity from high to low):1. CoMP (Coordinated Multi-Point) WPT
All ETs jointly design energy signals to the K ERs based on global CSI Only requires exchange of CSI and waveform parameters among ETs, as opposed
to message exchange in CoMP communications2. Locally-coordinated WPT
Each ER is served by a subset of ETs ET-oriented association: group the ETs into clusters, with each cluster ETs
cooperatively serving a subset of ERs ER-oriented association: each ER is freely associated with a subset of ETs
3. Single-ET WPT: each ER served by exactly one ET
Globecom 2016 38
Rui Zhang, National University of SingaporeMulti-user WPT
Multi-User WPT: Power Region Characterization
Considering CoMP-based WPT, the harvested power at the ERs are
Power region: the set of all achievable power tuples by the K ERs
Pareto-boundary: the power-tuples at which it is impossible to increase the power of one ER without reducing that of others
Pareto-boundary characterization (analogous to capacity region in multi-user communications) Weighted-sum-power maximization (WSPMax) Power-profile method
Globecom 2016 39
Rui Zhang, National University of Singapore
: MIMO channel from all the ETs to ER : the covariance matrix of the signal transmitted by all ETsk J k
JHS
Multi-user WPT
Weighted-Sum-Power Maximization
The WSPMax problem for power region characterization can be formulated as
Semidefinite programming (SDP) problem, can be efficiently solved by standard convex optimization techniques or existing software toolbox
For single ET with J=1, equivalent to point-to-point MIMO WPT with an equivalent channel
For Pareto boundary with hyper-planes, WSPMax only obtains the vertex points
Time sharing is thus needed in general to attain inner points on the boundary
Globecom 2016 40
Rui Zhang, National University of Singapore
1
0 : weight for ER
1
kK
kk
kµ
µ=
≥
=∑
Multi-user WPT
Power-Profile Method for Pareto Boundary Characterization
The power-profile approach for power region characterization solves the problem
SDP problem again, thus can be efficiently solved The optimal solution has rank greater than 1 in general, i.e., multi-beam WPT The same performance can be achieved with single-beam WPT together with
time sharing
Globecom 2016 41
Rui Zhang, National University of Singapore
1
0 : weight for ER
1
kK
kk
kα
α=
≥
=∑
Multi-user WPT
Simulation Results
Globecom 2016 42
Rui Zhang, National University of Singapore
A WPT system that serves a square area of 30m x 30m with co-located versus distributed antennas
Co-located antennas: a single ET with 9-element uniform linear array (ULA) at the center of the serving area
Distributed antennas: 9 ETs each with single antenna equally spaced in the area
Two single-antenna ERs at (15m, 5m) and (18.88m, 29.49m), which are 10m and 15m away from the area center, respectively
Total transmit power of the system is 2W Simulation 1: maximize the minimum (max-min) harvested power
by the two ERs Simulation 2: find the achievable power region of the two users
Multi-user WPT
Spatial Power Distribution with Max-Min Solution
Globecom 2016 43
Rui Zhang, National University of Singapore
(a) Co-located antenna system (b) Distributed antenna system
Power beamed towards the ERs in co-located antenna system More even spatial power distribution for distributed antenna system
Multi-user WPT
Achievable Power Region with Co-located vs Distributed Antennas
Globecom 2016 44
Rui Zhang, National University of Singapore
Distributed antenna system improves the performance of ER2 at the cost of degrading the performance of ER1, thus helps mitigating the near-far problem in co-located antenna system
Multi-user WPT
Agenda
Overview of main WPT technologies
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 45
Extensions and future work Rui Zhang, National University of Singapore
Nonlinear Energy Harvesting Model (1): Efficiency vs. Input Power
In practice, the RF-DC conversion efficiency varies with input power Energy beamforming needs to take into account this non-linear model
Rui Zhang, National University of Singapore
46Globecom 2016
Extensions and future work
Harvested Power vs. Input Power with Curve Fitting
The non-linear model can be obtained via curve fitting based on measured data
Rui Zhang, National University of Singapore
47Globecom 2016
Extensions and future work
Nonlinear Energy Harvesting Model (2): Efficiency vs. Waveform
Waveform with high peak-to-average power ratio (PAPR) tends to give better energy conversion efficiency, thus new waveform design is needed for WPT
Rui Zhang, National University of Singapore
48Globecom 2016
Extensions and future work
Harvested Power versus Signal PAPR
Waveform optimization by exploiting non-linear energy harvesting model
Rui Zhang, National University of Singapore
49Globecom 2016
Extensions and future work
Wireless Information and Power Transfer Coexisting
Rui Zhang, National University of Singapore
Wireless power transfer coexists with existing communication systems New spectrum sharing models and techniques needed to maximize
spectrum/energy efficiency “Cognitive” wireless information and power transfer
50
Extensions and future work
Rui Zhang, National University of Singapore
51Globecom 2016
Extensions and future work
Near-Field WPT: ``Rezence’’ Standard via Magnetic Resonance Coupling
Main advantages Multi-user charging Real-time charging control support (via built-in Bluetooth communication)
Main limitations Single TX charging unit Near-far fairness issue Lack of efficient magnetic channel estimation technique
Rui Zhang, National University of Singapore
52Globecom 2016
Extensions and future work
Near-Field WPT: Multi-Transmitter Charging Magnetic beamforming: make TXs’ generated magnetic fields constructively
added at one or more RXs by jointly optimizing the amplitude and phase ofvoltage/current at TXs
Node placement optimization: achieve uniform power over a target region
Different model and design from far-fieldbeamforming since RXs in near-field WPTare in general “coupled” with TXs
(Centralized WPT)(Distributed WPT)
Example: 5 TXs with different placed locations over a disc region
Rui Zhang, National University of Singapore
53Globecom 2016
Extensions and future work
Near-Far Issue in SIMO Near-Field WPT
Magnetic coupling (i.e., magnetic channel) between two coils decays withthe cubic of their separating distance (∝ 1/𝑑𝑑3)
Near-far problem in multi-user SIMO charging An efficient solution by exploiting the Tx-Rx coupling: jointly optimizing the
load resistance of different RXs
Increasing load resistance at RX 1 (closer to TX) helps increase the deliverable power to RXs 2 and 3 (far users)
But this also results in increased transmit power (i.e. lower efficiency)
Other Extensions & Future Work
Globecom 2016 54
Rui Zhang, National University of Singapore
Channel acquisition for WPT in both near-field and far-field (frequency
selective and multi-user channels)
Energy outage minimization in delay-sensitive applications
Distributed channel training and energy beamforming
Massive MIMO and mmWave WPT
WPT with safety and health related constraints
Higher layer (MAC, Network, etc.) design issues in WPT
Hardware development and applications
Extensions and future work
Rui Zhang, National University of Singapore
For more details, please refer to
Y. Zeng, B. Clerckx, and R. Zhang, “Communications and signals design for wireless power transmission,” submitted to IEEE Trans. Commun. (Invited Paper), available online at arxiv/1611.06822 Nov., 2016.
55Globecom 2016
References