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Wireless Charging Technologies: Fundamentals, Standards, and Network Applications

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

arXiv:1509.00940v2cs.NI

TL;DR

Wireless charging offers cord-free energy transfer, but integrating it with communication networks raises implementation, scheduling, and power-management challenges. The paper surveys charging technologies, standards, and network strategies, finding broad progress while identifying underexplored coordination, beamforming, deployment, and access-control issues. It also identifies limited scalability in some existing charging strategies as an important boundary.

  • Problem

    Integrating wireless charging into communication networks requires addressing implementation, scheduling, power management, and resource-allocation challenges.

  • Method

    The paper comprehensively surveys wireless charging fundamentals, standards, hardware, and network applications including charger scheduling, dispatch, and deployment strategies.

  • Results

    The survey presents existing solutions for static charger scheduling, mobile charger dispatch, and wireless charger deployment, while identifying several emerging issues as less explored.

  • Takeaways & Limitations

    Future work should investigate online mobile-charger dispatch, near-field energy beamforming, mobile-network energy provisioning, distributed deployment, and multiple access control.

  • Takeaways & Limitations

    Existing single-E-AP charging strategies apply only to small-scale networks with a limited number of devices.

Abstract

from arXiv · show

Wireless charging is a technology of transmitting power through an air gap to electrical devices for the purpose of energy replenishment. The recent progress in wireless charging techniques and development of commercial products have provided a promising alternative way to address the energy bottleneck of conventionally portable battery-powered devices. However, the incorporation of wireless charging into the existing wireless communication systems also brings along a series of challenging issues with regard to implementation, scheduling, and power management. In this article, we present a comprehensive overview of wireless charging techniques, the developments in technical standards, and their recent advances in network applications. In particular, with regard to network applications, we review the mobile charger dispatch strategies, static charger scheduling strategies and wireless charger deployment strategies. Additionally, we discuss open issues and challenges in implementing wireless charging technologies. Finally, we envision some practical future network applications of wireless charging.

I. INTRODUCTION

Wireless charging transmits electromagnetic energy across an air gap and is advancing from theory toward commercial products. This survey reviews its technologies, standards, communication-network applications, implementation challenges, and future directions.

  • Scope and motivation: Wireless charging transfers electromagnetic energy from a power source to an electrical load across an air gap without interconnecting cords.The technology is being applied from low-power toothbrushes to high-power electric vehicles and is increasingly incorporated into commercial mobile devices.
  • Benefits and costs: Wireless charging improves user-friendliness, supports smaller devices, enhances durability, and offers flexibility for devices where wired charging is costly, hazardous, or infeasible.Examples include waterproof or dustproof contact-free devices and body-implanted sensors.
  • Benefits and costs: Wireless charging can deliver power on demand and improve flexibility and energy efficiency, but it normally incurs higher implementation costs than wired charging.Costs include installing chargers, implanting receivers, and addressing additional heat through materials or design.
  • Technology directions: Radiative wireless charging uses RF or microwave electromagnetic waves to deliver energy through radiation, while coupling-based charging uses non-radiative transfer.RF exposure creates safety concerns for radiative systems.
  • Survey scope: The survey covers inductive coupling, magnetic resonance coupling, and RF/microwave radiation from fundamental principles through applications.It also reviews international standards, commercialization, implementations, and transmitter-side strategies for network applications.
  • Survey scope: The paper organizes network applications around static charger scheduling, mobile charger dispatch, and wireless charger deployment.It additionally discusses open research directions and practical future network applications.

C. Commercialization

Wireless charging progressed from early experimental systems toward commercial products and a broad taxonomy of coupling-based and RF-based technologies. The section outlines representative system models and practical limitations that shape technology selection.

  • Commercialization: Wireless charging research recently accelerated in response to demand from portable electronic devices, while both far-field and near-field approaches continued advancing.
  • Commercialization: Commercial standards from WPC, PMA, and A4WP were adopted in market products such as smartphones and wireless chargers.
  • Technology classification: Wireless charging technologies are classified into non-radiative coupling-based charging and radiative RF-based charging.Coupling-based charging includes inductive, magnetic-resonance, and capacitive coupling; RF-based charging includes directive beamforming and non-directive transfer.
  • Inductive coupling: Inductive coupling transfers energy between two coils through a varying magnetic field and generally operates within 20cm.The technique is typically tuned in the kilohertz range and uses low quality factors because transferred power attenuates quickly for larger values.
  • Inductive coupling: Inductive coupling offers easy implementation, convenient operation, high close-distance efficiency, and ensured safety, supporting mobile-device applications.MagMIMO was reported to charge a wireless device from up to 30cm away by detecting and directing a cone of energy toward the phone.

2) Magnetic Resonance Coupling:

Magnetic resonance coupling extends near-field charging through resonant coils, while RF radiation supports far-field delivery and can integrate energy transfer with communications. The reviewed applications span implants, appliances, vehicles, sensors, and communication networks, with efficiency and implementation trade-offs.

  • Magnetic Resonance Coupling: High quality factors in magnetic resonance coupling mitigate the decrease in coupling efficiency as distance increases, enabling meter-range transfer.
  • Magnetic Resonance Coupling: Magnetic resonance coupling can charge multiple devices concurrently, but mutual coupling between receiving coils can cause interference requiring proper tuning.
  • RF Radiation: RF-to-DC efficiency depends on received power density, impedance matching, and voltage-multiplier efficiency; one implementation achieved 62% at -10dBm and 84% at 5.8dBm.
  • RF Radiation: RF energy can be radiated isotropically for broadcast applications or directionally through energy beamforming for point-to-point transmission.Beam sharpness improves with the number of transmit antennas, supporting more efficient directed transfer.
  • RF Radiation: Microwave radiation can simultaneously transfer information and energy through simultaneous wireless information and power transfer.Microwave amplitude and phase modulate information while radiation and vibration carry energy.
  • Applications: Near-field applications include industrial automation, electric vehicles, biomedical implants, household devices, portable products, wearables, RFID, and smart grids.
  • Applications: Far-field charging uses non-directive RF radiation or directive RF beamforming; non-directive systems tolerate orientation changes but generally have low charging efficiency.

2) Far-field Charging:

Wireless charging systems span radiative RF/microwave transfer and non-radiative coupling-based architectures, with applications ranging from body sensors to electric vehicles and portable devices. Non-radiative systems combine power-conversion stages with inductive or magnetic-resonance hardware, including two-coil, four-coil, relay-resonator, and domino-resonator designs.

  • Radiative Charging: RF-powered body sensors typically consume tens of milliwatts, while reported charging efficiency can be only several percent.The passage gives 1.2% as an example efficiency.
  • Radiative Charging: Directive microwave beamforming has supported remote electric-vehicle charging and portable-device power delivery, including a 10kW rectenna exceeding 80% RF-DC efficiency.The Cota system can deliver a power beam up to 30 feet without line-of-sight, although further experimental evaluation is required for some applications.
  • Non-radiative System Architecture: A general non-radiative charger converts AC to DC, raises or changes the voltage, and converts DC to high-frequency AC before transmission through a coil.The receiver reverses this process through rectification and voltage conversion for the load.
  • Non-radiative System Architecture: Inductive coupling commonly uses two-coil architectures, whereas magnetic-resonance systems include four-coil, relay-resonator, and domino-resonator configurations.Domino resonators can form straight, circuit, curved, or Y-shaped patterns, with power paths that may split or combine.

2) Hardware Design and Implementation:

Non-radiative charging performance depends on coil geometry, mutual inductance, quality factors, load matching, and operating distance. The paper formulates SISO transfer relationships and surveys hardware implementations whose performance varies with coupling architecture and range.

  • Hardware Design: Magnetic-field intensity increases with transmit-coil turns and radius, but both must be optimized against transmission frequency and resistance.The receive coil should also be designed with low impedance to capture transferred energy effectively.
  • Hardware Design: Power-transfer efficiency depends strongly on mutual inductance, quality factor Q, and load matching between the coils.Mutual inductance reflects the influence of one coil on the induced current in the other.
  • Hardware Design: Higher Q indicates lower energy loss during transmission, while Q depends on inductance, resistance, intrinsic frequency, and fabricated materials.A high-Q system exhibits a slower decline in oscillation or resonance.
  • Hardware Design: Changing coil separation shifts resonance frequencies, motivating coupling manipulation, frequency matching, impedance matching, and resonator-parameter tuning.These methods are used to maintain resonance-frequency matching as distance varies.
  • Implementation Results: 50%-80% efficiency is reported within several centimeters for inductive systems, versus 50%-90% over several decimeters for magnetic-resonance systems.The figures summarize recently developed hardware implementations in Tables IV and V.
  • Propagation Modeling: The near-field model progresses from SISO to MISO, SIMO, and MIMO configurations for characterizing magnetic-wave propagation.The SISO formulation uses coil radii, separation distance, and resonance angular frequency, with circuit equations derived from Kirchhoff’s voltage law.

2) MISO:

In the MISO model, multiple transmitter coils couple to one receiver at resonance. Each coil contributes power according to its distance, geometry, angle, quality factor, and efficiency, and the receiver aggregates these contributions.

  • MISO: Each of the Nt transmitter coils couples to the single receiver at resonant frequency, with its delivered power determined by the coil-specific geometry and separation.The model defines dn as the distance between transmitter coil n and the receiver coil.
  • MISO: The aggregate received power is additive across the charger’s individual coils.The formulation sums the power contributions from the separate transmitter coils.
  • MISO: For identical charger coils, transmit power, quality factor, efficiency, and radius can be replaced by common values PT, QT, ηT, and rT.This produces a simplified transferred-power expression for the MISO configuration.
  • SIMO: The SIMO model reverses the arrangement: one charger coil couples to Nr receiver coils, each capturing a portion of the transmitted energy.The total transferred power is then calculated across the receiving coils.

3) SIMO:

The SIMO configuration distributes power from one charger coil to multiple receivers, while the broader standards discussion contrasts Qi’s inductive, alignment-sensitive operation with A4WP’s resonance-based spatial freedom. Qi also specifies communication and control procedures for power delivery.

  • SIMO: In SIMO, each of Nr receiver coils captures part of the energy from a single charger coil, and total transferred power sums across receivers.The power contribution for each receiver depends on its distance from the charger coil.
  • MIMO: In MIMO, each receive coil obtains power from each transmit coil separately, with small crosstalk between transmit and receive coils.The total transferred power is derived by summing the individual transmit-receive contributions.
  • Qi Standard: Qi combines wireless power transfer with in-band communication, allowing the device to request power and the charger to stop transfer when charging finishes.Its protocol includes start, ping, identification and configuration, and power-transfer stages.
  • Qi Standard: Qi uses inductive coupling, typically within 40 millimetres, with low-power transfer up to 5W and medium-power delivery up to 120W.The cited frequency ranges are 110-205kHz for low power and 80-300kHz for medium power.
  • Qi Standard: Qi supports guided positioning, a movable primary coil, and coil-array free positioning to manage alignment and charging placement.The coil-array approach can charge multiple devices simultaneously irrespective of their positions.
  • A4WP Standard: A4WP uses magnetic resonance coupling to avoid precise alignment, permit charger-device separation, and charge multiple devices concurrently with different power requirements.The maximum charging distance is described as up to several meters.

2) A4WP:

A4WP defines a wireless-charging architecture built around PTU–PRU power transfer, BLE-based control, and feedback-driven charging management. Prototype work primarily targets Qi-compatible implementations, alignment, conversion control, and efficiency.

  • A4WP architecture: A4WP systems comprise a power transmitter unit and power receiving unit, with PTU-to-PRU transfer controlled by a charging-management protocol and PRU feedback.The system operates in the 6.78MHz ISM band.
  • Functional units and states: A4WP PTUs integrate resonator and matching, power conversion, and signaling/control components, while PTUs operate across configuration, power-save, low-power, and transfer states.PRUs similarly manage energy reception, conversion, control, communication, and error states.
  • Communication protocol: A4WP communication uses BLE for power-level control, valid-load identification, and protection of non-compliant devices through device detection, information exchange, and charging control.PRU dynamic parameters report current, voltage, temperature, and functional status for charging adjustment.
  • Prototype studies: Qi-compatible implementations dominate prototype studies because of their ease of implementation and early announcement.Reported prototypes include remote supervision, integrated transmitters and receivers, alignment control, and several power-conversion approaches.
  • Prototype performance: 70% charging efficiency was achieved at 5W output power over a 5mm charging distance in one integrated Qi-compliant prototype.A separate phase-shift-control study reported 72% overall system efficiency for 5W wireless charging, with higher circuit cost.
  • System comparison: Table VI compares wireless-charging systems by source power, frequency, and effective charging distance.The supplied passage identifies the comparison dimensions but does not provide the table entries.

V. STATIC WIRELESS CHARGER SCHEDULING STRATEGIES

Static wireless charger scheduling in WPCNs is organized around four network models and resource-allocation problems that coordinate harvested energy with information transmission. Reviewed methods address throughput, spatial throughput, outage, beamforming, and imperfect channel knowledge.

  • System models: Static WPCN studies distinguish H-AP, dedicated E-AP, relay-based H-AP, and multi-antenna E-AP models.The models separate or combine charging, data reception, relaying, and spatial energy steering.
  • H-AP scheduling: H-AP research primarily optimizes resource allocation to maximize wireless-powered-device throughput.Harvest-then-transmit protocols schedule downlink energy harvesting before uplink TDMA information transmission.
  • H-AP scheduling: Full-duplex H-AP studies jointly optimize downlink power and time allocation with uplink user scheduling, but imperfect self-interference cancellation makes the problem non-convex.Perfect cancellation yields a convex formulation in the cited work.
  • H-AP scheduling: Stochastic-geometry analysis extends H-AP scheduling to randomly located devices by jointly optimizing uplink power and downlink–uplink time partitioning.The framework characterizes successful information-transmission probability and spatial-throughput optimization.
  • Beamforming: Multi-antenna H-AP methods tune energy-beamforming weights and combine time, beamforming, and uplink-power optimization to improve fairness or throughput.Massive-MIMO protocols also estimate downlink channels using reciprocity before broadcasting RF energy.

B. Charging Strategies for Dedicated Energy Access Point

Dedicated E-AP research controls charging power, time allocation, and beamforming to optimize throughput or energy consumption, while relay-based systems coordinate charging and data transmission. The literature remains limited in scale, receiver multiplicity, practical validation, and communication-aware implementation.

  • Dedicated E-AP: Dedicated E-AP studies focus on controlling wireless-charging power to achieve an optimization objective.Throughput maximization commonly balances energy harvesting and information-transmission time under system constraints.
  • Dedicated E-AP: Multiple-device, multi-antenna E-AP scheduling jointly optimizes time allocation and energy beamforming through a relaxed convex formulation with global-optimality guarantees.A fast semiclosed-form solution was numerically shown to reduce implementation complexity.
  • Dedicated E-AP: Opportunistic multi-E-AP power control minimizes energy consumption subject to a desired distortion level at the data access point.The setting shares a channel opportunistically between wireless charging and information transmission.
  • Relay-based WPCN: Relay-based WPCNs design protocols that coordinate wireless charging and data transmission for throughput maximization.Proposed protocols include harvest-then-cooperate, energy cooperation, and dual cooperation with joint power and time allocation.
  • Multi-antenna E-AP: Multi-antenna E-AP research emphasizes energy beamforming and CSI acquisition, without considering information-transmission issues.Studies examine imperfect CSI, training-duration tradeoffs, and frequency-diversity gains.
  • Multi-antenna E-AP: MagMIMO uses load-based channel estimation and non-radiative magnetic-field beamforming; experiments found comparable power consumption to existing chargers with substantially longer effective charging distance.The result is reported for a coupling-based multicoil near-field system.
  • Discussion and open issues: Existing dedicated-E-AP strategies mainly address small-scale networks, point-to-point charging, or theoretical and numerical settings, leaving multiple-E-AP coordination and multi-receiver beamforming open.The paper calls for practical protocols and experimental evaluation tied to communication performance.

VI. MOBILE WIRELESS CHARGER DISPATCH STRATEGIES

Mobile charger dispatch schedules one or more chargers to visit and recharge target devices, typically to prolong network lifetime. The problem spans charging locations, travel, dwell time, data routing, charger count, and centralized or distributed planning.

  • Problem formulation: Mobile charger dispatch schedules charger travel so one or more mobile chargers visit and recharge distributed target devices, typically to prolong network lifetime.The problem is commonly studied in wireless rechargeable sensor networks.
  • Design issues: Dispatch design must optimize charging locations, travel paths, charging durations, data rates, routing paths, and, for multiple chargers, charger count.These decisions determine coverage, device charging sufficiency, data-gathering performance, and deployment cost.
  • Reference models: Two reference models separate wireless energy provisioning from data gathering or jointly optimize energy provisioning and data collection through a hybrid charger.Hybrid chargers can forward collected data during visits using single-hop or multi-hop routing.
  • Reference models: Figure 13 contrasts separated wireless energy provisioning and data gathering with their joint execution in a hybrid-charger model.The supplied figure labels identify these two alternatives but do not state an outcome comparison.
  • Strategy taxonomy: Mobile dispatch strategies are classified by offline versus online planning, single versus multiple chargers, and centralized versus distributed control.The review presents single-charger studies before multiple-charger studies within offline and online categories.

A. Offline Charger Dispatch Strategy

Offline charger-dispatch research primarily schedules replenishment deterministically and periodically, optimizing travel, charging, energy, routing, and service objectives under network constraints. Studies increasingly address limited charger capacity, point-to-multipoint charging, hybrid charging-and-data-gathering, and target monitoring.

  • Single-Charger Strategy:: Offline replenishment scheduling is generally deterministic and periodic, with single-charger strategies minimizing total service time under cycle, energy-flow, and node-energy constraints.Equivalent objectives include maximizing charger vacation time or minimizing charger energy consumption.
  • Single-Charger Strategy:: Renewable energy-cycle studies derive conditions for unlimited network lifetime and identify the shortest Hamiltonian cycle as an optimal sustaining travel path.Other formulations jointly optimize travel path, charging duration, and data-flow routing, including dynamic time-varying routing.
  • Single-Charger Strategy:: Point-to-multipoint formulations are NP-hard, then transformed into mixed-integer linear programs with near-optimal solutions and a considerable gap from point-to-point charging.Related work also optimizes charging locations, durations, routing, and hybrid energy-and-data provisioning.
  • Target-Oriented WRSNs:: Several studies extend dispatch to target monitoring, maximizing monitored targets or quality of monitoring while coordinating charger visits, sensor activation, and duty cycles.The QoM formulation is NP-hard; one relaxed version receives a 1/6 approximation algorithm.
  • Single-Charger Strategy:: Earlier joint data-gathering work omitted charging duration and energy consumption for data receiving and sensing, while later work modeled heterogeneous consumption and time-varying charging duration.The later formulation is non-convex but can be converted into a convex two-level optimization problem using auxiliary variables.
  • Single-Charger Strategy:: Limited-capacity charger strategies optimize travel paths or the number of charged devices subject to charger energy consumption for travel and charging.The latter problem is NP-hard and is addressed with particle-swarm-optimization-based heuristics.

2) Multiple-Charger Strategy:

Multiple-charger research addresses deployment and dispatch in one- and two-dimensional networks, including energy constraints, deadlines, distributed control, and online charging requests. Online methods mainly study single chargers, while coordination remains an open challenge for multiple chargers.

  • Multiple-Charger Strategy:: One-dimensional multiple-charger studies minimize charger count under negligible charging time, while two-dimensional formulations address energy-constrained chargers and are often NP-hard.Some approximation guarantees require identical or fixed device energy-consumption rates.
  • Multiple-Charger Strategy:: Deadline-constrained multiple-traveling-salesperson formulations minimize travel cost without node outage and use heuristics based on travel time and residual sensor lifetime.The formulation is NP-hard.
  • Multiple-Charger Strategy:: Distributed dispatch using no network information achieved performance comparable to centralized control, but local knowledge produced better performance.The comparison suggests travel-path design may matter more than charger coordination when network knowledge is limited.
  • Online Charging Dispatch Strategy: Online single-charger strategies respond to sequential requests by replanning routes, selecting clusters through charging gain, or synchronizing devices with low residual energy.The ESync protocol reduces travel distance and charging delay, as verified by experiments and simulation.
  • Online Charging Dispatch Strategy: Online distributed strategies use local information or nearest-job-next preemption, while the sole reviewed online multiple-charger study maximizes on-demand charging coverage but faces invalid travel distances.The multiple-charger coverage problem is NP-complete.

C. Discussion and Conclusion

Wireless charger deployment studies cover static point, path, multihop, and mobile-landmark provisioning, but deployment research remains limited in mobile networks and often depends on global information. Online dispatch surveys likewise identify coordination and point-to-multipoint scheduling as open directions.

  • Discussion and Conclusion: Deployment studies compare charger count, energy constraints, optimization variables, charging patterns, control methods, and evaluation methods across offline and online dispatch strategies.The online literature contains only one reviewed multiple-charger solution.
  • Wireless Charger Deployment: Static deployment includes point provisioning for static devices, path provisioning for mobile devices, and multihop provisioning where devices share transferred energy.Landmark provisioning selects and clusters locations for mobile chargers serving nearby static devices.
  • Wireless Charger Deployment: Point-provisioning studies minimize charger count or maximize network flow using greedy, heuristic, approximation, or optimization methods.One heuristic deployment achieved 85.9% of the MILP-generated optimum on average.
  • Wireless Charger Deployment: A distributed approximation algorithm for adjustable transmit power achieved around 40% average performance gain over a centralized solution in a Powercaster testbed.The deployment problem was reformulated as a conventional linear program before distributed approximation.
  • Wireless Charger Deployment: Multi-hop provisioning can require fewer chargers, especially with large charger capacity, but the tradeoff between charging efficiency and hop count remains unanalyzed.The underlying deployment problem includes a maximum-hop constraint.

B. Mobile Wireless Charger Deployment

Mobile wireless charger deployment uses landmark-based schemes, while open technical issues span electromagnetic interference, localization, heating, feedback, and conversion efficiency. These constraints define practical boundaries for scaling wireless charging systems.

  • Mobile Charger Deployment: SuReSense minimizes landmarks through integer linear programming, clusters them near docking stations, and assigns mobile chargers to visit the resulting groups.Follow-up work optimizes landmark profit or differentiates transmission according to sensor priorities.
  • Mobile Charger Deployment: Deployment research has scarcely studied multihop provisioning, multiple-transmitter harvesting, or mobile chargers operating in mobile networks.The review specifically identifies mobile-network deployment based on user-device mobility as a future direction.
  • Mobile Charger Deployment: Global-information requirements for device capacity, location, hardware parameters, and velocity create substantial communication overhead in deployment optimization.This requirement affects scalability even where proposed solutions claim low complexity.
  • Open Research Issues: Magnetic resonance coupling supports larger charging areas and simultaneous multi-device charging, but introduces greater electromagnetic interference, lower efficiency, and larger transmitter sizes than inductive charging.Longer charging distance generally requires a larger transmitter or receiver dimension.
  • Open Research Issues: Near-field beamforming depends critically on magnetic-channel estimation and feedback, while RF energy beamforming additionally requires real-time receiver localization by angle and distance.Inaccurate estimation or absent feedback severely deteriorates charging performance.
  • Open Research Issues: Metallic materials and foreign objects can cause heating and power loss near chargers, while repeated AC-to-electromagnetic-wave-to-DC conversion leaves typical efficiency between 50% and 70%.Mitigating heating and improving conversion efficiency remain technical challenges.

2) Open Issues in Data Communication:

Wireless charging networks create new resource-allocation opportunities and challenges, including protocol security, charger coordination, sustainable provisioning, and energy-information integration. The survey identifies several emerging applications and research directions while emphasizing that green provisioning remains unresolved.

  • Communication protocols: Current protocols require duplex communication and multiple-access support for charging-power requests, battery-status exchange, and simultaneous device charging.Existing protocols primarily support one-to-one communication between a charger and a charging device.
  • Communication protocols: Plain charger-device communication remains vulnerable to jamming, eavesdropping, and man-in-the-middle attacks, motivating security features in future protocols.The attacks can block communication, expose identities, or falsify charging status.
  • Wireless charger networks: Networked chargers can exchange availability, location, charging status, and cost information to provide distributed energy provisioning through coordinated charger infrastructure.The survey describes short-range inductive, mid-range resonance-based, and far-field chargers for different power, alignment, distance, and mobility requirements.
  • Sustainable provisioning: Green wireless energy provisioning remains open because mobile chargers can emit CO2 and grid-powered RF chargers can consume substantial conventional fuel under propagation losses.Renewable sources such as solar are promising but unpredictable, which complicates reliable charging service.
  • Emerging paradigms: Emerging network applications include self-energy recycling, millimeter-wave energy beamforming, near-field magnetic-induction SWIPT, and coordinated charger scheduling and deployment.The survey also highlights online mobile dispatch, distributed charger deployment, and multiple-access control as less-explored directions requiring further investigation.
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