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Wireless Charger Networking for Mobile Devices: Fundamentals, Standards, and Applications

Xiao Lu, Dusit Niyato, Ping Wang, Dong In Kim, Zhu Han

arXiv:1410.8635v2cs.NI

TL;DR

Wireless charging standards largely focus on communication between chargers and charging devices, leaving communication among chargers and other entities underdeveloped. The paper reviews wireless charging, Qi and A4WP protocols, proposes wireless charger networking, and demonstrates user–charger assignment as an application that minimizes users’ cost in identifying suitable chargers.

  • Problem

    Existing wireless charging standards mainly focus on charger–device communication and overlook communication among chargers and other entities.

  • Method

    The paper reviews wireless charging fundamentals and Qi and A4WP communication protocols, then proposes a network enabling inter-charger data communication and information transfer.

  • Results

    The demonstrated user–charger assignment application minimizes users’ cost in identifying the best charger to replenish device energy.

  • Takeaways & Limitations

    Wireless charger networking supports information collection and control among chargers and can aid charger selection for mobile-device energy replenishment.

Abstract

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Wireless charging is a technique of transmitting power through an air gap to an electrical device for the purpose of energy replenishment. Recently, the wireless charging technology has been significantly advanced in terms of efficiency and functionality. This article first presents an overview and fundamentals of wireless charging. We then provide the review of standards, i.e., Qi and Alliance for Wireless Power (A4WP), and highlight on their communication protocols. Next, we propose a novel concept of wireless charger networking which allows chargers to be connected to facilitate information collection and control. We demonstrate the application of the wireless charger network in user-charger assignment, which clearly shows the benefit in terms of reduced cost for users to identify the best chargers to replenish energy for their mobile devices.

I. INTRODUCTION

Wireless charging transfers power across an air gap and has moved toward commercial adoption, but existing standards mainly address charger–device communication. The paper proposes wireless charger networking to enable inter-charger communication and demonstrates reduced assignment cost for identifying suitable chargers.

  • Motivation: Wireless charging transfers power from a source to a load across an air gap, improving convenience and enabling contact-free devices.Benefits described include removing cable hassle, supporting cross-brand charging, improving durability, and enabling charging where cables or battery replacement are costly or infeasible.
  • Motivation: Wireless charging was advancing from theory toward standards and adoption in commercial mobile and portable devices.The passage notes built-in wireless charging in products from Samsung, Apple, and Huawei and cites market projections for 2016 and 2020.
  • Scope: The article reviews wireless charging fundamentals, Qi and A4WP standards, and their data communication protocols.It frames these reviews as groundwork for examining communication beyond the charger–device link.
  • Research gap: Existing standards mainly focus on communication between a charging device and charger while overlooking communication among chargers and other entities.This gap motivates networking chargers rather than treating each charger as an isolated charging point.
  • Contribution: Wireless charger networking connects chargers to facilitate data communication and information transfer among them.The concept extends charger functionality toward networked information collection and control.
  • Application: The paper demonstrates the network through user–charger assignment, reducing the cost of identifying the best charger for replenishing device energy.The application is presented as evidence of the networking concept’s benefit for charger selection.

A. Wireless Charging Techniques

Wireless charging techniques differ in field regime, distance behavior, efficiency, and application constraints. The section covers magnetic inductive coupling, magnetic resonance coupling, and microwave radiation, emphasizing their operating trade-offs.

  • Overview: The three major wireless charging techniques are magnetic inductive coupling, magnetic resonance coupling, and microwave radiation.Inductive and resonant coupling operate in the near field, whereas microwave radiation operates in the far field.
  • Operating regimes: Near-field power attenuates with the reciprocal cube of distance, while far-field power decreases according to the reciprocal of distance.The passages also distinguish the effects of radiation absorption on transmitters in near-field and far-field techniques.
  • Magnetic inductive coupling: Magnetic inductive coupling uses field induction between two coils and is popular for mobile devices because of close-range efficiency and safety.Its effective range is typically less than a coil diameter.
  • Magnetic resonant coupling: Magnetic resonance coupling transfers energy through strongly coupled resonant coils, with efficiency influenced by coupling tightness and coil quality factor.Alignment, distance, coil geometry, materials, and operating frequency affect these properties.

3) Microwave Radiation:

Microwave radiation transfers energy through electromagnetic waves, using rectennas at receivers and optionally beamforming for directional delivery. Its longer range and communication compatibility are balanced by safety, regulatory, deployment, and harvesting-efficiency constraints.

  • Operation: Microwave charging converts electricity to RF energy for transmission and rectifies captured microwaves back into electricity at the receiver.Typical microwave frequencies range from 300MHz to 300GHz.
  • Beamforming: Microwave energy can be radiated isotropically for broadcast or directed through beamforming for point-to-point transmission.Power beamforming improves transmission performance by shaping the beam toward a target.
  • System design: Microwave systems use antenna arrays to sharpen power beams, and commercial transmitters and receivers can provide 1W or 3W isotropic power.Beam sharpness improves with the number of transmit antennas.
  • Applications: Microwave radiation offers longer transmission distance and compatibility with communication systems, including simultaneous wireless information and power transfer.Amplitude and phase can carry information while microwave radiation and vibration carry energy.
  • Constraints: Microwave harvesting efficiency depends significantly on received power density, while infrared and X-ray alternatives are excluded because of safety issues.These constraints limit the practical design space for far-field wireless charging.

B. Standards

Qi and A4WP are leading wireless charging standards with distinct positioning, power-transfer, and communication approaches. Qi emphasizes inductive charging with alignment procedures, while A4WP targets spatial freedom through resonant coupling.

  • Qi: Qi uses magnetic inductive coupling, typically within 40 millimetres, and defines low-power and medium-power categories up to 120W.The low-power category transfers within 5W at 110–205 kHz; the medium-power category delivers up to 120W at 80–300 kHz.
  • Qi: Qi requires device alignment for tight coupling and specifies guided positioning, a movable primary coil, and a coil array as alignment approaches.The coil-array approach can charge multiple devices irrespective of position but incurs more implementation cost.
  • Qi: Qi supports in-band communication, allowing the charger to adjust power output to device demand and disable transfer when charging finishes.Its protocol includes start, ping, identification and configuration, and power-transfer stages.
  • A4WP: A4WP aims to provide spatial freedom through magnetic resonance coupling, without requiring precise alignment.A4WP uses a PTU power transmitter and PRU power receiver, with feedback signaling for charging control.

2) Alliance for Wireless Power (A4WP):

A4WP uses resonant power transfer and separate BLE-based control communication to support spatial freedom and concurrent charging. Its PTU–PRU protocol coordinates discovery, parameter exchange, and power regulation.

  • A4WP characteristics: A4WP generates a larger electromagnetic field through magnetic resonance coupling and does not require precise alignment.It can also tolerate separation between the transmitter and receiver and allows foreign objects on an operating charger without adverse effect.
  • Charging control: A4WP can charge multiple devices concurrently with different power requirements when the PTU has sufficient power.The PRU periodically updates dynamic parameters so the PTU can adjust control and receive alerts for errors or completed charging.
  • A4WP architecture: A4WP transfers power from a PTU to a PRU and uses feedback signaling to control charging.The PTU and PRU contain power, control, communication, and conversion functions organized into defined operating states.
  • Communication protocol: A4WP operates wireless power at 6.78 MHz and control signaling at 2.4 GHz through out-of-band communication.A4WP-compliant systems adopt a Bluetooth Low Energy link and support charging distances up to several meters.
  • Communication protocol: The A4WP protocol has three steps: device detection, information exchange, and charging control.The PTU and PRU exchange static and dynamic parameters, including status, capabilities, current, voltage, temperature, and functional status.
  • Motivation: Existing wireless charging standards support charger-to-device communication but not information exchange among multiple chargers.The paper identifies this limitation as motivating wireless charger networking to improve charger usability and efficiency.

III. WIRELESS CHARGER NETWORKING

The paper introduces wireless charger networking, in which chargers communicate with charging devices and a server. It applies this architecture to user–charger assignment and reports reduced energy replenishment cost.

  • III. WIRELESS CHARGER NETWORKING: Wireless charger networking enables chargers to communicate with charging devices and exchange information with a server.The paper first presents the network architecture and features, then focuses on user–charger assignment.
  • III. WIRELESS CHARGER NETWORKING: The user–charger assignment application demonstrates that wireless charger networking can reduce energy replenishment cost.The reported application concerns matching users with chargers for replenishing mobile-device energy.

A. Wireless Charger Network

The wireless charger network connects smart chargers, users, wireless access points, and a server for information collection, communication, and centralized control. Its functions include real-time charger selection, authentication, payment, status reporting, assignment, and add-on services.

  • A. Wireless Charger Network: A wireless charger network allows multiple chargers to communicate and exchange information with a server.The server can collect charger availability, location, charging status, and cost, then optimize charger use.
  • A. Wireless Charger Network: The server matches users needing charging services with appropriate chargers using current charger location, availability, and status.Continuous updates support online assignment and can direct users to options such as the nearest available charger.
  • A. Wireless Charger Network: Smart wireless chargers add a data transceiver, local processing, and storage for network settings, usage history, and commands.They can process data from charging devices and commands from other network components.
  • A. Wireless Charger Network: Wireless access points provide communication channels through WLAN, cellular M2M or MTC, and multihop networks such as mesh networks.Intermediate chargers can relay information from chargers outside an access point’s transmission range.
  • A. Wireless Charger Network: The server provides authentication, authorization, accounting, network data storage, user access, and charger direction.Its stored information can include individual charger status and charging price.
  • A. Wireless Charger Network: Network functions include device authentication, server-programmed charging payment, charger-status reporting, user–charger assignment, and add-on services.Add-on services can include information downloading and content distribution over short-range charger-to-device transmission.
  • A. Wireless Charger Network: Existing mobile applications can locate charging facilities but do not provide real-time charger information.Wireless charger networking is presented as a way to enhance such functionality.

C. User-Charger Assignment

Wireless charger networking supports information sharing among chargers for user-charger assignment. Compared with individual and nearest selection, optimal assignment balances delay, price, and user effort to reduce overall cost.

  • Assignment model: User-charger assignment matches users needing energy replenishment with available chargers using charger and user information.The considered costs are user effort, price, and delay.
  • Assignment model: Individual selection chooses the charger with the minimum estimated individual cost, incorporating delay, price, and distance-related user effort.The cost uses charger load, capacity, charging price, requested energy, distance, and weights for the three cost components.
  • Assignment model: Optimal assignment periodically assigns new users to chargers to minimize total cost, with each user assigned to one charger.The assignment indicator x_i,j equals one when user i is assigned to charger j and zero otherwise.
  • Results: 22.7 versus 27.9 is the average overall cost for optimal versus individual selection, while the nearest scheme reaches 30.3.Users in high-price areas may move toward lower-price chargers, increasing delay; optimal assignment balances price and delay.
  • Results: Under load variation, individual selection and optimal assignment remain unaffected, whereas the nearest scheme experiences a sharply rising overall cost.Optimal assignment achieves the lowest cost through collective user-charger assignment rather than independent decisions.

IV. OPEN RESEARCH ISSUES

The paper identifies open issues in both wireless charging technologies and their data communication systems.

  • Scope: Open issues span wireless charging technologies and data communication in wireless charging systems.The section frames these as continuing research challenges.

A. Open Issues in Wireless Charging

Wireless charging research faces challenges involving power density, distance, channel knowledge, hardware operating range, efficiency, interference, and transmitter size.

  • Charging technology: Higher wireless charging power density introduces technical issues, while long-distance charging typically requires a large receiver.The paper also notes thermal, electromagnetic compatibility, and electromagnetic field concerns.
  • Mitigation: Mitigating receiver and battery power loss requires high-efficient power conversion and modules with effective ventilation design.These measures address the stated thermal and efficiency concerns.
  • Resonance coupling: Resonance coupling offers a larger charging area and simultaneous multi-device charging but has lower efficiency and increased electromagnetic interference than inductive charging.The paper also identifies relatively large transmitters as a limitation, with charging distance generally proportional to transmitter diameter.
  • Magnetic MIMO: Magnetic MIMO beamforming depends on magnetic-channel knowledge, making channel estimation and feedback mechanisms important.Inaccurate channel estimation or absent feedback severely deteriorates charging performance.
  • Magnetic MIMO: Impedance-matching hardware operates optimally only within a certain range.This hardware constraint is identified as a limitation for magnetic MIMO charging systems.

B. Open Issues in Data Communication

Wireless charging communication protocols leave open issues in usability, security, inter-charger coordination, duplex communication, and multiple access. The paper proposes networking chargers to exchange information and applies it to lower-cost assignment.

  • Communication scope: Improving wireless charger usability and efficiency requires stronger data communication capability.The paper distinguishes communication between a charger and device from communication among chargers.
  • Duplex and multiple access: Existing protocols support simplex and one-to-one communication, motivating duplex procedures and multiple-access MAC for multiple-device charging.Examples include device power requests and charger requests for battery status.
  • Security: Current protocols are vulnerable to eavesdropping and man-in-the-middle attacks, so security features must account for wireless charging characteristics.The paper cites identity theft and manipulation or falsification of charging status as examples.
  • Inter-charger communication: Wireless charger networking is proposed to support inter-charger data communication and exchange charger and charging-device information.The paper identifies hybrid access points as a possible extension combining data communication and energy transfer.
  • Conclusion: The article reviews wireless charging fundamentals and Qi and A4WP communication protocols before proposing charger networking and demonstrating user-charger assignment.The demonstrated assignment application minimizes users’ cost in identifying a suitable charger.
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