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1Globecom 2016
Wireless Powered Communication: From Theory to Applications
Rui Zhang
ECE Department, National University of Singapore
Globecom 2016, Washington, DC USA
Rui Zhang, National University of Singapore
Agenda
• Part I: Wireless Power Transfer (WPT): Introduction & Applications
• Part II: Communications and Signals Design for Microwave WPT
• Part III: WPT and Wireless Communication Co-design
Globecom 2016 2
Part I: WPT: Introduction & Applications 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)
Part I: WPT: Introduction & Applications
Globecom 2016 4
Rui Zhang, National University of Singapore
Inductive coupling
Magnetic resonant coupling
Electromagnetic (EM) radiation
Laser power beaming
Overview of Main WPT Technologies
Part I: WPT: Introduction & Applications
Globecom 2016 5
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
Part I: WPT: Introduction & Applications
Globecom 2016 6
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
Part I: WPT: Introduction & Applications
Wireless Power Transfer via Magnetic Resonant Coupling in 2000s
Globecom 2016 7
Demonstration of magnetic coupling to power light bulb (Intel Corp.) and charge mobile phones (Witricity Corp.)
Rui Zhang, National University of SingaporePart I: WPT: Introduction & Applications
Globecom 2016 8
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
Part I: WPT: Introduction & Applications
Radiative Wireless Power Transmission
Globecom 2016 9
Rui Zhang, National University of SingaporePart I: WPT: Introduction & Applications
Globecom 2016 10
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 BeamingPart I: WPT: Introduction & Applications
NASA’s Wireless Power Transfer Project Using Laser Beam
Globecom 2016 11
Rui Zhang, National University of SingaporePart I: WPT: Introduction & Applications
Wireless Power Transfer: State-of-the-Art Technology
Range
Efficiency
inductive coupling
magnetic resonant coupling
Electromagnetic (EM) radiation
<5cm pre-determined distance, e.g., 15-30cm
>1m
Globecom 2016 12
Rui Zhang, National University of SingaporePart I: WPT: Introduction & Applications
Application Examples
Globecom 2016 13
Inductive Coupling
Magnetic ResonantCoupling
EM Radiation
The Qi wireless mobile device charging Standard Electric tooth brush
Wireless powered medical implants
Qualcomm eZonewireless charging
Qualcomm Halo electric vehicle powered by charging pad
Haier wireless powered HDTV
Intel WISP RFID tags harvest energy from RF radiation
Powercast RF harvesting circuit for sensor networks
The SHARP unmanned plane receives energy beamed from the ground
Rui Zhang, National University of SingaporePart I: WPT: Introduction & Applications
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 14
Rui Zhang, National University of Singapore
This tutorial will focus on EM radiation WPT technology and its applications in wireless powered communication
Part I: WPT: Introduction & Applications
15
Energy-Aware Wireless Communications: An Overview Rui Zhang, National University of Singapore
Wireless power transfer
Green communications
Smart grid
Energy harvesting
Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Communication Powered by Batteries (Conventional)
Rui Zhang, National University of Singapore
Need manual battery recharging/replacement Costly, inconvenient, abruption to use Inapplicable in some scenarios, e.g., implanted medical devices,
sensors built in cement structures
16Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Communication Powered by Energy Harvesting (More Recent)Rui Zhang, National University of Singapore
External energy source: solar, wind, vibration, ambient radio power, etc. Inexpensive, green, renewable Intermittent and uncontrollable, costly/bulky harvesting and storage devices
17Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Communication Powered by Wireless Power Transfer (Emerging)Rui Zhang, National University of Singapore
Wireless charging fully controllable Wide coverage, low production cost, and small receiver Main challenges: low efficiency of wireless power transfer, wireless
information and power transfer co-design
18Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Powered Communication Applications (1)Rui Zhang, National University of Singapore
19Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Powered Communication Applications (2)Rui Zhang, National University of Singapore
20Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Powered Communication Applications (3)Rui Zhang, National University of Singapore
21Globecom 2016
Part I: WPT: Introduction & Applications
Wireless Powered Communication Applications (4)Rui Zhang, National University of Singapore
Energy transfer
Information transfer
Energy Receivers
Information Receivers
Hybrid Information and Energy Access Point
22Globecom 2016
Part I: WPT: Introduction & Applications
A Generic Model
Hybrid Access point
Energy and/or InformationReceiver
Information flowEnergy flow
Downlink (DL)
Uplink (UL)
Three “Canonical” Models/Modes Wireless Power Transfer (WPT) in DL
Wireless Powered Communication Network (WPCN): DL WPT and UL wireless information transmission (WIT)
Simultaneous wireless information and power transfer (SWIPT): DL WPT and WIT at the same time
Energy and/or InformationReceiver
Rui Zhang, National University of Singapore
23Globecom 2016
Part I: WPT: Introduction & Applications
General Network Model
Rui Zhang, National University of Singapore
Three canonical operating modes Wireless power transfer (WPT): AP2 -> WD5 Wireless powered communication (WPC): AP1 <-> WD3, AP2->WD6->AP3 Simultaneous wireless information power transfer (SWIPT): AP1->WD4, AP1->WD1/WD2
24Globecom 2016
Part I: WPT: Introduction & Applications
Agenda
• Part I: Wireless Power Transfer (WPT): Introduction & Applications
• Part II: Communications and Signals Design for Microwave WPT
• Part III: WPT and Wireless Communication Co-design
Globecom 2016 25
Rui Zhang, National University of Singapore
Outline of Part II
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 26
Part II: Communications & Signals Design for Microwave WPT 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 27
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Radiative 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 28
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
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 29
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Solar Satellite with Microwave Power Transmission (1970s-current)
NASA Sun Tower
Target at GW-level power transfer with more than 50% efficiency
Globecom 2016 30
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Globecom 2016 31
Rui Zhang, National University of Singapore
2.411 GHz288 elements phased array on
the roof of the car120 rectennas on the fuel-free
airplane DC output power ~88W
Microwave Power Transfer Field Experiment with Phased Array (1992)
Part II: Communications & Signals Design for Microwave WPT
Microwave Wireless Power Transmission: A Fresh New Look
Globecom 2016 32
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
Part II: Communications & Signals Design for Microwave WPT
Outline of Part II
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 33
Part II: Communications & Signals Design for Microwave WPT Rui Zhang, National University of Singapore
Wireless Power Transmission: A Generic Model
Globecom 2016 34
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
Part II: Communications & Signals Design for Microwave WPT
Narrowband Wireless Power Transmission: Channel Model
Globecom 2016 35
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Modulated vs. Unmodulated Energy Signal
Globecom 2016 36
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 SingaporePart II: Communications & Signals Design for Microwave WPT
Wireless Power Transmission: Receiver Model (1)
Globecom 2016 37
Rui Zhang, National University of Singapore
Only keep the second-order term since y(t) is typically small
Part II: Communications & Signals Design for Microwave WPT
Wireless Power Transmission: Receiver Model (2)
Globecom 2016 38
[4]
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 considered [47]
Part II: Communications & Signals Design for Microwave WPT
Outline of Part II
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 39
Part II: Communications & Signals Design for Microwave WPT Rui Zhang, National University of Singapore
Single-User Multi-Band MIMO WPT
Globecom 2016 40
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= ≤ ≤
Part II: Communications & Signals Design for Microwave WPT
Energy Beamforming for Multi-Band MIMO WPT
Globecom 2016 41
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
Part II: Communications & Signals Design for Microwave WPT
Channel Acquisition for MIMO WPT
Globecom 2016 42
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
Part II: Communications & Signals Design for Microwave WPT
Antenna Architecture of ER
Globecom 2016 43
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
Part II: Communications & Signals Design for Microwave WPT
CSI Acquisition (1): Forward-Link Training with CSI Feedback
Globecom 2016 44
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
Part II: Communications & Signals Design for Microwave WPT
CSI Acquisition (2): Reverse-Link Training via Channel Reciprocity
Globecom 2016 45
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
Part II: Communications & Signals Design for Microwave WPT
Example of Reverse-Link Training
Globecom 2016 46
Rui Zhang, National University of Singapore
Single-band WPT over Rayleigh fading MIMO channel with average gain β Quasi-block fading channel with channel coherence time T Reverse-link training with Mr’≤Mr ER antennas trained Average harvested energy at ER [28]:
Net harvested energy:
( ) ''max
: training power by ER
( , ) , with having i.i.d. Gaussian entries of unit variancer t
r
M MHt r
p
M M Cλ × Λ = ∈ XE X X X
Part II: Communications & Signals Design for Microwave WPT
Net Harvested Power versus Number of Trained ER Antennas
Globecom 2016 47
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=
Part II: Communications & Signals Design for Microwave WPT
CSI Acquisition (3): Power-Probing with Energy Feedback
Globecom 2016 48
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
Part II: Communications & Signals Design for Microwave WPT
Power-Probing with One-Bit Feedback: Case Study [6]
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 49
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Two-Phase WPT Protocol (1)
Phase 1 Channel Learning: feedback intervals each of length In the nth interval ET sends one or more energy beams with covariance Each ER measures its harvested energy Each ER feeds back one bit to ET based on
ET updates based on ’s After channel learning phase, ET estimates as , and obtains
estimated dominant eigenvector of as
Globecom 2016 50
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Two-Phase WPT Protocol (2)
Phase 2 Energy transmission: ET sends one single energy beam with covariance
Total harvested energy over two phases
Globecom 2016 51
energy harvested in Phase 1
energy harvested in Phase 2
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
ACCPM Based Channel Learning
Objective: find any point in target set Analytic center cutting plane method (ACCPM) [24] : 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 52
1n−P
X
Cutting planenH
1n n n−= P P H
X
G(n) G(n)~ ~
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave 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 53
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Simulation Result: Setup
6 ERs, all 5 meters from ET ET with antennas, each ER with antennas Rician fading channels Transmit power: Energy transfer efficiency:
Globecom 2016 54
ET
ER 1ER 2
ER 3
ER 4
ER 5
ER 6
5 meters
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Simulation Result: Baseline Schemes
Globecom 2016 55
Partial CSIT: existing one-bit feedback based channel learning schemes Cyclic Jacobi technique (CJT) [25]
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 [26] 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 [27] One-bit feedback: larger or smaller than prior highest received power Usage of feedback: adjust phase of transmit beam Application: MISO, one receiver only
Perfect CSIT: optimal EB
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Simulation Result
Globecom 2016 56
CJT: discrete error points Gradient sign and distributed beamforming: larger step size yields faster
convergence but more fluctuations ACCPM: best accuracy & convergence
Consider only ER 1 is activeAbsolute error of harvested power versus No. of feedback intervals
Rui Zhang, National University of SingaporePart II: Communications & Signals Design for Microwave WPT
Outline of Part II
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 57
Part II: Communications & Signals Design for Microwave 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
58Globecom 2016
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 59
Rui Zhang, National University of Singapore
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 60
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
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 61
Rui Zhang, National University of Singapore
1
0 : weight for ER
1
kK
kk
kµ
µ=
≥
=∑
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 62
Rui Zhang, National University of Singapore
1
0 : weight for ER
1
kK
kk
kα
α=
≥
=∑
Simulation Results
Globecom 2016 63
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
Spatial Power Distribution with Max-Min Solution
Globecom 2016 64
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
Achievable Power Region with Co-located vs Distributed Antennas
Globecom 2016 65
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
Outline of Part II
Microwave WPT: Historical development and contemporary design
WPT and energy receiver model
Single-user WPT
Multi-user WPT
Extensions and future work
Globecom 2016 66
Part II: Communications & Signals Design for Microwave WPT 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
67Globecom 2016
Part II: Communications & Signals Design for Microwave WPT
Harvested Power vs. Input Power with Curve Fitting
The non-linear model can be obtained via curve fitting based on measured data [48]
Rui Zhang, National University of Singapore
68Globecom 2016
Part II: Communications & Signals Design for Microwave WPT
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
69Globecom 2016
Part II: Communications & Signals Design for Microwave WPT
Harvested Power versus Signal PAPR
Waveform optimization by exploiting non-linear energy harvesting model [47]
Rui Zhang, National University of Singapore
70Globecom 2016
Part II: Communications & Signals Design for Microwave WPT
Other Extensions & Future Work
Globecom 2016 71
Rui Zhang, National University of Singapore
Channel acquisition in frequency-selective [49] and/or multi-user channels
WPT with retrodirective amplification [50]
Energy outage minimization in delay-sensitive applications
Distributed channel training and energy beamforming [51]
Massive MIMO and mmWave WPT [52][53][54]
WPT with safety and health related constraints
Coexisting with wireless communication and interference management [55]
Higher layer (MAC, Network, etc.) design issues in WPT [56]
Hardware development and applications
Part II: Communications & Signals Design for Microwave WPT
Agenda
• Part I: Wireless Power Transfer (WPT): Introduction & Applications
• Part II: Communications and Signals Design for Microwave WPT
• Part III: WPT and Wireless Communication Co-design
Globecom 2016 72
Rui Zhang, National University of Singapore
Outline of Part III
Wireless powered communication network (WPCN)
Simultaneous wireless information and power transfer (SWIPT)
Extensions and conclusions
Globecom 2016 73
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Wireless Powered Communication Network (WPCN)
Harvest-then-transmit protocol [2] Phase I: AP broadcasts energy to all wireless devices (WDs) to harvest energy in DL Phase II: WDs transmit individual messages with harvested energy to the AP in UL
TDMA-based multiple access Orthogonal transmission in the UL
SDMA-based multiple access Spatial multiplexing in the UL
Globecom 2016 74
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
“Doubly” Near-far Problem
Doubly Near-Far Problem Due to distance-dependent signal attenuation in both DL and UL “Near” user harvests more energy in DL but transmits less power in UL “Far” user harvest less energy in DL but transmits more power in UL Results in unbalanced energy consumptions in the network
Globecom 2016 75
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Potential Solutions to Doubly Near-far Problem
(a): Joint communication and energy scheduling, transmit (energy)/receive (information) beamforming
(b): Wireless powered cooperative communication
Rui Zhang, National University of Singapore
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Part III: WPT and Wireless Communication Co-design
Wireless Powered Communications: Various Setups
Rui Zhang, National University of Singapore
77
(a): Separate energy/information access point (AP) (b): Co-located energy/information AP
(c): Out-band half-duplex energy/information (d): In-band full-duplex energy/information
Globecom 2016
Part III: WPT and Wireless Communication Co-design
Throughput Comparison of Different Setups
Rui Zhang, National University of Singapore
1 2 3 4 5 6 7 8 92
4
6
8
10
12
14
16
18
d (meters)
Thro
ughp
ut (b
ps/H
z)
Half duplex, co-locatedHalf duplex, separatedFull duplex, co-locatedFull duplex, separated
For full-duplex: 80 dB self-interference
cancellation at AP 10% self-energy recycling [46]
at wireless device (WD)
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Part III: WPT and Wireless Communication Co-design
Outline of Part III
Wireless powered communication network (WPCN)
Simultaneous wireless information and power transfer (SWIPT)
Extensions and conclusions
Globecom 2016 79
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Rui Zhang, National University of Singapore
SWIPT: Rate-Energy Tradeoff at Transmitter Side
Wireless Power Transfer vs. Wireless Information Transfer Power Transfer :
Information Transfer :
Optimal transmit power allocation in frequency-selective channel
Maximize data rateMaximize energy transfer
Q hPTζ∝
( )2log 1R T hP∝ +
80
Part III: WPT and Wireless Communication Co-design
MIMO SWIPT with Two Separate EH and ID Terminals
Rate-energy region: all the achievable rate and energy pairs under a given transmit power constraint P
Globecom 2016 81
Each terminal has 4 antennas, P = 1W, energy receiver 1m/information receiver 10m from transmitter
eigenmode transmission + WF
energy beamforming
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Rui Zhang, National University of Singapore
SWIPT: Rate-Energy Tradeoff at Receiver Side
Practical receiver cannot harvest energy and decode information from the same signal
Time switching receiver
Power splitting receiver
Integrated EH/ID receiver
Antenna switching receiver
82
Part III: WPT and Wireless Communication Co-design
Rui Zhang, National University of Singapore
Rate-Energy Region of SWIPT in Point-to-Point AWGN
0 1 2 3 4 5 60
10
20
30
40
50
60
Rate(bits/channel use)
Ene
rgy
Uni
t
Ideal RxPower Splitting RxTime Switching RxIntegrated Rx
?
83
Part III: WPT and Wireless Communication Co-design
Rui Zhang, National University of Singapore
Dual Role of Interference in SWIPT
Interference is harmful to information receiver but useful to energy harvesting Opportunistic EH and ID in fading channel via receiver mode switching In general, this opens a new paradigm for interference management
84
Part III: WPT and Wireless Communication Co-design
Dynamic Power Splitting vs. Dynamic Antenna Switching
Globecom 2016 85
Dynamic antenna switching is a special case of power splitting with on/off (two-level) power splitting ratio per receive antenna
Dynamic power splitting achieves better R-E region than antenna switching, but antenna switching has much lower hardware complexity
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Joint Information and Energy Beamforming for SWIPT
SWIPT with Separate EH/ID Receivers SWIPT with Co-located EH/ID Receivers
Energy transferInformation transfer
1U
h1
hK
g1
gK
UK
UK +1
UK +K
E
E
E
E
I
I
Energy transferInformation transfer
1U
2U
KU
h1
g1=h1h2
hK
g2=h2
gK=hK
Rui Zhang, National University of Singapore
Joint transmit beamforming and receiver design optimization to maximize transferred energy and information under heterogeneous power/rate requirements of the users
86
Part III: WPT and Wireless Communication Co-design
Outline of Part III
Wireless powered communication network (WPCN)
Simultaneous wireless information and power transfer (SWIPT)
Extensions and conclusions
Globecom 2016 87
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Wireless Power Meets Energy Harvesting
Rui Zhang, National University of Singapore
Hybrid energy supplies via both environmental energy harvesting and dedicated wireless power transfer
Wireless powered communication needs to be jointly designed with energy harvesting communication
88
Part III: WPT and Wireless Communication Co-design
Wireless Powered Cognitive Radio Network
Rui Zhang, National University of Singapore
Conventional cognitive radio (CR): secondary user is idle when nearby primary user is transmitting
Wireless powered CR: secondary user harvests energy from nearby active primary transmitters
89
Part III: WPT and Wireless Communication Co-design
Wireless Information and Power Transfer Coexisting
Rui Zhang, National University of Singapore
Wireless power transfer/wireless powered communication coexists with existing communication systems
New spectrum sharing models and techniques needed to maximize spectrum/energy efficiency [55]
90
Part III: WPT and Wireless Communication Co-design
Secure Communication in SWIPT
Security issue in SWIPT ER can easily eavesdrop IR’s information Two conflicting goals: Energy transfer: received power at each ER should be large Secure information transfer: received power at each ER should be small How to resolve this conflict? Exploiting artificial noise [32]
energy signal artificial noise
AP
ERs
IRs
information signal
Rui Zhang, National University of Singapore
Alice
BobEve
91
Part III: WPT and Wireless Communication Co-design
Multi-Transmitter Collaborative SWIPT
An 2×2 interference channel for SWIPT with TS receivers
Receivers use time switching (TS) or power splitting (PS) Transmitters cooperate in joint information and energy transmission Interference channel rate-energy tradeoff
Rui Zhang, National University of Singapore
92
Part III: WPT and Wireless Communication Co-design
SWIPT with Energy/Information Relaying
Time Switching Relay
Rui Zhang, National University of Singapore
Power Splitting Relay
Source Relay Destinationh g
energy transmissionA relay-assisted link, where the relay is wirelessly charged by RF signals from the source
Information transmission
Full Duplex Relay
Globecom 2016 93
Part III: WPT and Wireless Communication Co-design
Globecom 2016 94
SWIPT in Multi-User OFDM
OFDMA with PS receivers : PS is performed before digital OFDM demodulation. Thus, all subcarriers would have the same PS ratio at each receiver.
TDMA with TS receivers : Each user performs ID when information is scheduled for that user, and performs EH in all other time
R-E tradeoff characterizations for multi-carrier SWIPT
Multi-user OFDM OFDM Receiver with Power Splitting (PS)
Rui Zhang, National University of SingaporePart III: WPT and Wireless Communication Co-design
Conclusions
Rui Zhang, National University of Singapore
Wireless power transfer (WPT)
Wireless poweredcommunication network
(WPCN)
Simultaneous wireless information and power transfer
(SWIPT)
Energy beamforming
Energy feedback
Energy/Communication full-duplex
Joint energy and communication scheduling
Joint information and energy beamforming
Separated vs. Integrated receivers
Nonlinear energy receiver model
Doubly near-far problem
Rate-energy tradeoff
Waveform optimization
Self-energy recycling Secrecy SWIPT
Energy
Energy
Information
Energy
Information
Multiuser power region
Wireless information and power transfer coexisting
Harmful vs. useful interference
Energy multicasting
95Globecom 2016
Conclusions
Rui Zhang, National University of Singapore
Future Work Directions
Nonlinear energy harvesting model, waveform design for WPT/WPCN/SWIPT Near-field WPT/WPCN/SWIPT Fundamental limits and signal processing methods for WPT/WPCN/SWIPT Backscatter/re-scatter communications Massive MIMO/Millimeter wave based WPT/WPCN/SWIPT Small-cell, C-RAN, and distributed antennas for WPT/WPCN/SWIPT Imperfect CSIT and practical channel acquisition for WPT/WPCN/SWIPT Full-duplex WPCN/SWIPT Coexistence of wireless communication and power transfer systems Higher layer (MAC, Network, etc.) design issues in WPT/WPCN/SWIPT Safety/security/economic issues in WPT/WPCN/SWIPT Hardware development, applications, …
96
Conclusions
Rui Zhang, National University of Singapore
For more details, please refer to
S. Bi, C. K. Ho, and R. Zhang, “Wireless powered communication: opportunities and challenges,” IEEE Communications Magazine, vol. 53, no. 4, pp.117-125, April, 2015.
S. Bi, Y. Zeng, and R. Zhang, “Wireless powered communication networks: an overview,” IEEE Wireless Communications, vol. 23, no. 4, pp. 10-18, April 2016.
Y. Zeng, B. Clerckx, and R. Zhang, “Communications and signals design for wireless power transmission,” submitted to IEEE Trans. Commun. (Invited Paper), Nov., 2016.
97Globecom 2016
References
Globecom 2016 98
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