IEEE COMMUNICATIONS SURVEYS AND TUTORIALS, TO APPEAR 1 Wireless Charging Technologies: Fundamentals, Standards, and Network Applications Xiao Lu†, Ping Wang‡, Dusit Niyato‡, Dong In Kim§, and Zhu Han≀ † Department of Electrical and Computer Engineering, University of Alberta, Canada ‡ School of Computer Engineering, Nanyang Technological University, Singapore § School of Information and Communication Engineering, Sungkyunkwan University (SKKU), Korea ≀ Electrical and Computer Engineering, University of Houston, Texas, USA. Abstract—Wireless charging is a technology of transmitting Firstly, it improves user-friendliness as the hassle from • power through an air gap to electrical devices for the pur- connecting cables is removed. Different brands and dif- pose of energy replenishment. The recent progress in wireless ferent models of devices can also use the same charger. charging techniques and development of commercial products Secondly, it renders the design and fabrication of much have provided a promising alternative way to address the energy • bottleneck of conventionally portable battery-powered devices. smaller devices without the attachment of batteries. However, the incorporation of wireless charging into the existing Thirdly, it provides better product durability (e.g., water- • wireless communication systems also brings along a series of proof and dustproof) for contact-free devices. challenging issues with regard to implementation, scheduling, and Fourthly, it enhances flexibility, especially for the devices power management. In this article, we present a comprehensive • overview of wireless charging techniques, the developments for which replacing their batteries or connecting cables in technical standards, and their recent advances in network for charging is costly, hazardous, or infeasible (e.g., body- applications. In particular, with regard to network applications, implanted sensors). we review the static charger scheduling strategies, mobile charger Fifthly, wireless charging can provide power requested by • dispatch strategies and wireless charger deployment strategies. charging devices in an on-demand fashion and thus are Additionally, we discuss open issues and challenges in imple- menting wireless charging technologies. Finally, we envision some more flexible and energy-efficient. practical future network applications of wireless charging. Nevertheless, normally wireless charging incurs higher implementation cost compared to wired charging. First, a Index terms- Wireless Charging, wireless power transfer, wireless charger needs to be installed as a replacement of inductive coupling, resonance coupling, RF/Microwave radia- traditional charging cord. Second, a mobile device requires im- tion, energy harvesting, Qi, PMA, A4WP, simultaneous wire- plantation of a wireless power receiver. Moreover, as wireless less information and power transfer (SWIPT), energy beam- chargers often produce more heat than that of wired chargers, forming, wireless powered communication network (WPCN), additional cost on crafting material may be incurred. Magnetic MIMO, Witricity. The development of wireless charging technologies is ad- I. INTRODUCTION vancing toward two major directions, i.e., radiative wireless charging (or radio frequency (RF) based wireless charging) Wireless charging [1], [2], also known as wireless power and non-radiative wireless charging (or coupling-based wire- arXiv:1509.00940v2 [cs.NI] 14 Nov 2015 transfer, is the technology that enables a power source to less charging). Radiative wireless charging adopts electro- transmit electromagnetic energy to an electrical load across magnetic waves, typically RF waves or microwaves, as a an air gap, without interconnecting cords. This technology medium to deliver energy in a form of radiation. The energy is attracting a wide range of applications, from low-power is transferred based on the electric field of an electromagnetic toothbrush to high-power electric vehicles because of its wave, which is radiative. Due to the safety issues raised by convenience and better user experience. Nowadays, wireless RF exposure [5], radiative wireless charging usually operates charging is rapidly evolving from theories toward standard in a low power region. For example, omni-directional RF features on commercial products, especially mobile phones radiation is only suitable for sensor node applications with and portable smart devices. In 2014, many leading smartphone up to 10mW power consumption [6], [7]. Alternatively, non- manufacturers, such as Samsung, Apple and Huawei, began to radiative wireless charging is based on the coupling of the release new-generation devices featured with built-in wireless magnetic-field between two coils within the distance of the charging capability. IMS Research [3] envisioned that wireless coils’ dimension for energy transmission. As the magnetic- charging would be a 4.5 billion market by 2016. Pike Research field of an electromagnetic wave attenuates much faster than [4] estimated that wireless powered products will triple by the electric field, the power transfer distance is largely limited. 2020 to a 15 billion market. Due to safety implementation, non-radiative wireless charging Compared to traditional charging with cord, wireless charg- has been widely used in our daily appliances (e.g., from ing introduces many benefits as follows. toothbrush to electric vehicle charger [8]) by far. Dong In Kim is the corresponding author. In this article, we aim to provide a comprehensive survey IEEE COMMUNICATIONS SURVEYS AND TUTORIALS, TO APPEAR 2 TABLE I SUMMARY OF EXISTING SURVEY IN RELATED AREA. Survey Scope Main Contribution [9] Wireless Review of i) fundamentals and circuit design network with for RF energy harvesting, ii) resource allocation RF energy schemes and communication protocols for vari- harvesting ous types of RF-powered wireless network, and iii) practical challenges and future directions. [10] Wireless Review of i) information-theoretic physical layer network performance limits to transmission scheduling with energy policies and medium access control protocols, harvesting ii) emerging paradigm of energy transfer and cooperation that occur separately or jointly with information transfer, and iii) energy consump- tion models for energy harvesting communica- tion systems. [12] Sensor nodes Review of architectures, energy sources and with energy storage technologies, as well as applications of harvesting sensor nodes with energy harvesting. [13] Devices with Review of i) various types of energy harvesting ambient en- techniques, ii) different energy harvesting mod- ergy harvest- els, and ) power management and networking ing aspects of the energy harvesting devices. [14] RF/microwave Review of i) basics and designs of the RF energy energy harvesting circuit, and ii) energy conversion effi- harvesting ciency of existing implementations of RF energy circuit harvesting circuits. of the emerging wireless charging systems with regard to the fundamental technologies, international standards as well as applications in wireless communication networks. Our previ- ous work in [9] presented a review of research issues in RF- powered wireless networks with the focus on the receiver-side (i.e., energy harvester) designs. This survey differs from [9] Fig. 1. An outline of the scope of this survey. in the following aspects: this survey i) covers various ma- jor wireless charging techniques, namely, inductive coupling, magnetic resonance coupling and RF/microwave radiation, technical breakthroughs as well as recent commercialization from fundamental principles to their applications, ii) reviews development in Section II. Then, in Section III, we present an the existing international standards, commercialization and overview of existing wireless charging techniques and their implementations, and iii) emphasizes on the transmitter-side applications, followed by the introduction of magnetic-field (i.e., wireless charger) strategy designs for different types of propagation models. We also review the hardware implemen- network applications. Another recent survey in [10] provides tation of these wireless charging technologies. Subsequently, an overview of self-sustaining wireless communications with in Section IV, the specifications of the leading international different energy harvesting techniques, from the perspective of wireless charging standards are described in details. The ex- information theory, signal processing and wireless networking. isting implementations of those standards are also outlined. We Unlike [10], this survey focuses on the wireless charging then survey the network applications including static charger strategies in communication networks with wireless energy scheduling strategies, mobile wireless charger dispatch strate- harvesting capability, also referred to as wireless powered gies, and wireless charger deployment strategies in Sections V, communication networks (WPCNs) [11]. VI and VII, respectively. Furthermore, in Section VIII, we shed Existing literatures [12]–[14] also presented relevant re- light on some open research directions in implementing wire- views in energy harvesting research, mainly from the perspec- less charging technologies. Additionally, we envision some tive of device-level techniques and hardware implementations. future network applications. Finally, Section IX concludes the In [12], the authors gave an overview of the sensor nodes survey. The abbreviations used in this article are summarized powered by different