rui zhang, national university of singapore recent ... · december 2017 6 rui zhang, national...

1December 2017

Recent Advances in Far and Near Field Wireless Power Transfer:

Power Waveform Design and Magnetic MIMO Optimization

Rui Zhang, National University of Singapore

Rui Zhang (e-mail: [email protected])

WEHCN, 2017 December 4 2017, Singapore

Acknowledgement to Mohammad Reza V. Moghadam for helping slides preparation

mailto:[email protected]

Agenda

Introduction

Main WPT technologies

Wireless information and power transfer

Far-Field WPT

Power waveform design with non-linear rectifiers

Near-Field WPT

Magnetic MIMO optimization

Conclusions and future work

December 2017 2

Agenda Rui Zhang, National University of Singapore

3December 2017

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 (WPCN)

Introduction

December 2017 4


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

Introduction

December 2017 5


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

Introduction

December 2017 6


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

Introduction

December 2017 7


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 BeamingIntroduction

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

December 2017 8

Rui Zhang, National University of SingaporeIntroduction

Wireless Information and Power Transfer:Prior Work and Future Trend


Wireless power transfer (WPT)

Wireless poweredcommunication network

(WPCN)

Simultaneous wireless information and power transfer

(SWIPT)Energy

Energy

Information

Energy

Information

9December 2017

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 needs to be made, e.g., time switching, power splitting, harvest-then-transmit, etc.

Compared to Wireless Information Transfer (WIT), WPT usually dominates the performance trade-off

Recent research trend: apply communications & signal processing techniques to achieve high-efficiency WPT

Introduction

Agenda

Introduction



Far-Field WPT


Near-Field WPT



December 2017 10


Far-field Wireless Power Transmission: A Fresh New Look

December 2017 11


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 microwave 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

Far-field WPT

Far-field Wireless Power Transmission: End-to-End Efficiency

December 2017 12


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 beamforming e3: RF-to-DC conversion efficiency at energy receiver (ER)

Require efficient (non-linear) rectenna design & power waveform optimization

Y. Zeng, B. Clerckx, and R. Zhang, “Communications and signals design for wireless power transmission,” IEEE Transactions on Communications, May 2017. (Invited Paper)

Far-field WPT

December 2017 13

Rui Zhang, National University of SingaporeFar-field WPT

Power Waveform Design: System Model

Multisine transmit signal: periodic with 𝑇𝑇 = 1Δ𝑓𝑓

Received signal:

𝑁𝑁: number of subcarriers 𝑤𝑤𝑛𝑛( 𝑓𝑓𝑛𝑛): 𝑛𝑛-th subcarrier frequency Δ𝑓𝑓: frequency spacing

𝐿𝐿: number of paths 𝜏𝜏𝑙𝑙 ,𝛼𝛼𝑙𝑙 , 𝜉𝜉𝑙𝑙: delay, amplitude and phase for 𝑙𝑙-th path

December 2017 14


Circuit Analysis of Non-linear Rectifier

Output DC voltage:

Output DC power:

Remark: maximizing output DC voltage/power ≡ maximizing the integral term

December 2017 15


Power Waveform Design: Problem Formulation

Maximizing �̅�𝑣𝑜𝑜𝑜𝑜𝑜𝑜 by jointly designing amplitudes and phases, 𝑠𝑠𝑛𝑛’s and 𝜙𝜙𝑛𝑛’s,subject to maximum transmit sum-power 𝑃𝑃𝑇𝑇

Optimal phases: 𝜙𝜙𝑛𝑛= −𝜓𝜓𝑛𝑛, 𝑛𝑛 = 1, … ,𝑁𝑁

Optimal amplitudes:

Non-convex problem, since maximizing convex objective function

December 2017 16


Conventional Approach: Truncated Taylor Approximation

Step1: approximate the inner exponential function as

with

𝑄𝑄 = 2: commonly used in SWIPT/WPCN (linear model)

𝑄𝑄 = 4 [Clerckx16]: better accuracy, but needs Sequential Convex Programming and Geometric Programming (SCP-GP)

,

[Clerckx16] B. Clerckx and E. Bayguzina, “Waveform design for wireless power transfer,” IEEETrans. Signal Process., Dec. 2016.

Step2: simplify the integral and then optimize

December 2017 17


Proposed Approach: SCP with Time Sampling

Step 1: approximate the objective function in (P1) using its first-order Taylor series (SCP) as

where

with

𝑠𝑠𝑛𝑛(𝑚𝑚) : values of

decision variablesat iteration 𝑚𝑚

Step 2: compute coefficients 𝛽𝛽0(𝑚𝑚)and 𝛽𝛽𝑛𝑛

(𝑚𝑚)’s numerically via Newton-Cotes formula → �𝛽𝛽0

(𝑚𝑚)and �𝛽𝛽𝑛𝑛(𝑚𝑚)’s (time sampling)

December 2017 18


SCP-QCLP Algorithm

Step 3: formulate the approximate problem at iteration 𝑚𝑚 as

Step 4: compute the optimal solution to (P1−𝑚𝑚) which is derived in closed-form as

Step 5: check the stopping criteria

Quadratically constrained linear programming (QCLP)

Remark: SCP-QCLP is guaranteed to return at least a locally optimal solution to (P1)

December 2017 19


Benchmark: Frequency MRT

Replacing the integral in the objective function of (P1) by the peak value of its integrand over the period 𝑇𝑇

Optimal solution to (P2) is derived in closed-form as

December 2017 20


Simulation Setup

SISO WPT: center frequency 915MHz, bandwidth 10MHz Channel model:

Path loss: 51.67dB (10 meters from Tx to Rx) NLoS: 𝐿𝐿 = 18 paths, equal power profile, uniformly distributed

delay and phase for each path Frequency amplitude response:

December 2017 21


Simulation Results (1)

Fix 𝑁𝑁 = 16 , vary 𝑃𝑃𝑇𝑇 Fix 𝑃𝑃𝑇𝑇 = 10W, vary 𝑁𝑁

December 2017 22


Simulation Results (2)

𝑃𝑃𝑇𝑇 = 10W and 𝑁𝑁 = 16

December 2017 23


For more details, please refer to

M. R. V. Moghadam, Y. Zeng, and R. Zhang, “Waveform optimization for radio-frequency wireless power transfer,” IEEE International Workshop on Signal Processing Advances for Wireless Communications (SPAWC), 2017. Available online at https://arxiv.org/abs/1703.04006

https://arxiv.org/abs/1703.04006

Agenda

Introduction



Far-Field WPT


Near-Field WPT



December 2017 24



25December 2017

Near-field WPT

Near-Field WPT via Magnetic Resonance Coupling (MRC):The “Rezence” Standard

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


26December 2017

Near-Field WPT in MISO Setup Distributed Magnetic Beamforming: Constructively combine magnetic fields at RX

by jointly optimizing amplitudes/phases of voltage/current at different TXs

(Centralized WPT)(Distributed WPT)

Example: 5 TXs with different placed locations over a disc region

Node placement optimization: Achieve uniform power coverage in a target region

Magnetic beamforming RF beamforming

Near-field WPT


27December 2017

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 multiuser SIMO charging An efficient solution by exploiting Tx-Rx coupling: jointly optimizing the load

current by adjusting 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)

Near-field WPT


28December 2017

Magnetic MIMO Optimization (1)

Power region boundary characterization (𝑁𝑁 TXs and 𝑄𝑄 RXs):

𝒊𝒊 : TX current vector (complex valued) 𝑃𝑃: sum-power deliverable to all RXs 𝛼𝛼1 …𝛼𝛼𝑄𝑄

𝑇𝑇: power-profile vector, subject to 𝛼𝛼𝑞𝑞 ≥ 0 and ∑𝑞𝑞=1

𝑄𝑄 𝛼𝛼1 = 1 𝑃𝑃𝑇𝑇: sum-power limit for all TXs 𝑉𝑉𝑛𝑛,𝐴𝐴𝑛𝑛: peak voltage/current limits for TX 𝑛𝑛 𝐁𝐁, 𝐁𝐁n, 𝐌𝐌q, 𝐖𝐖n, 𝑟𝑟𝑞𝑞, 𝜔𝜔: system parameters

Near-field WPT


29December 2017

Magnetic MIMO Optimization (2)

Benchmark: equal current allocation over all TXs

Pareto boundary: achievable via time sharing (TS)

Example: TXs 1 and 2 are active only, 𝑃𝑃𝑇𝑇 = 100W, 𝑉𝑉𝑛𝑛 = 50 2V, 𝐼𝐼𝑛𝑛 = 5 2A,𝑓𝑓0 = 6.78 MHz

Near-field WPT


30December 2017

Magnetic Channel Estimation

On-off estimation (benchmark): 𝑇𝑇 = 𝑁𝑁𝑄𝑄 training slots needed; inefficient for 𝑁𝑁,𝑄𝑄 ≫ 1 Simultaneous estimation: 𝑇𝑇 = 𝑄𝑄 training slots required only

Apply randomly generated TX voltages over different slots (𝑮𝑮 matrix) Measure the resulted RX currents over different slots (𝒁𝒁 matrix) Estimate the magnetic MIMO channel as �𝑴𝑴 = 𝑮𝑮𝒁𝒁−1

𝛾𝛾: SNR of RX current knowledge at TX

Measurement error, quantization error, feedback error, etc.

𝑇𝑇 > 𝑄𝑄 training slots needed in general

Optimal estimator: maximum likelihood (ML), difficult to solve due to non-linearity

Suboptimal estimator: least-square (LS)

Perfect RX current knowledge:

Imperfect RX current knowledge:

Near-field WPT


For more details, please refer to

SIMO Magnetic WPT: M. R. V. Moghadam and R. Zhang, “Multiuser wireless power transfer via magnetic resonant coupling: performance analysis, charging control, and power region characterization,” IEEE Transactions on Signal and Information Processing over Networks, vol. 2, no.1, pp. 72-83, March 2016.

MISO Magnetic WPT: M. R. V. Moghadam and R. Zhang, “Node placement and distributed magnetic beamforming optimization for wireless power transfer,” IEEE Transactions on Signal and Information Processing over Networks, accepted and available online at https://arxiv.org/abs/1608.00304

Magnetic MIMO and Channel Estimation: G. Yang, M. R. V. Moghadam, and R. Zhang, “Magnetic MIMO signal processing and optimization for wireless power transfer,” IEEE Transactions on Signal Processing, vol. 65, no. 11, pp. 2860-2874, June 2017.

31December 2017

Near-field WPT

https://arxiv.org/abs/1608.00304

Conclusions

December 2017 32


Wireless Power Transfer Two main paradigms: Far-Field vs. Near-Field

Communications and Signal Processing Advances for Far-field WPT Circuit analysis for non-linear RF energy harvesting Power waveform design with non-linear rectifiers Higher-order Taylor approximation (SCP-GP) versus time sampling (SCP-QCLP)

Communications and Signal Processing advances for Near-field WPT Distributed energy beamforming, node placement optimization Multiuser charging control by exploiting load coupling, near-far fairness Magnetic MIMO optimization, multi-user power region, time-sharing Magnetic channel estimation

Conclusions

Future Work

December 2017 33


Far-field power waveform design with non-linear rectifiers Extension to MIMO and/or multi-user WPT More advanced rectifiers (e.g., transistor-based) End-to-end efficiency optimization (consider non-linear DC-RF efficiency

at energy transmitter) Channel estimation and feedback SWIPT with non-linear energy receivers ….

Near-field magnetic MIMO optimization Magnetic MIMO channel measurement and modelling Fundamental limits characterization (with receiver coil coupling) Coil antenna design, fabrication, and placement Prototype development and performance improvement over “Rezence” ….

Future work

rui zhang, national university of singapore recent ... · december 2017 6 rui zhang, national...

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