19-10-2016, 02:40 PM
WIRELESS CHARGER NETWORKING FOR MOBILE DEVICES: FUNDAMENTALS, STANDARDS AND APPLICATIONS
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Abstract: 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 static charger scheduling strategies, mobile charger dispatch 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
. INTRODUCTION
Wireless charging technology [1, 2] enables wireless power transfer from a power source (e.g., a charger) to a load (e.g., a mobile device) across an air gap. The technology provides convenience Wireless charging technology [1, 2] enables wireless power transfer from a power source (e.g., a charger) to a load (e.g., a mobile device) across an air gap. The technology provides convenience and better user experience. Recently, wireless charging is rapidly evolving from theories toward standards, and being adopted in commercial products, especially mobile phones and portable devices. Using wireless charging has many benefits. First, it improves user friendliness as the hassle from connecting cables is removed. Different brands and models of devices can also use the same charger. Second, it provides better product durability (e.g., waterproof and dustproof) for contact-free devices. Third, it enhances flexibility, especially to devices for which battery replacement or cable connection or charging is costly, hazardous, or infeasible (e.g., body-implanted sensors). Fourth, wireless charging can provide on-demand power, avoiding an overcharging problem and minimizing energy costs. In 2014, many leading smart phone manufacturers (e.g., Samsung, Apple, and Huawei) released their products equipped with built-in wireless charging capability.
In this article, we first describe a brief history of wireless power transfer technologies. Then we present an overview and fundamentals of wireless charging technologies. This is then followed by an introduction of two leading international wireless charging standards, Qi and the Alliance for Wireless Power (A4WP). We describe data communication protocols used in these standards. Furthermore, we find that the existing standards mainly focus on the data communication between a charging device and a charger, and overlook the communication among chargers and other entities as a network. Therefore, we propose the concept of wireless charger networking to facilitate data communication and information transfer functions among the chargers.
II. OVERVIEW OF THE WIRELESS CHARGING TECHNIQUES
Nikola Tesla, the founder of alternating current electricity, was the first to conduct an experiment on wireless charging. He achieved a major breakthrough in 1899 by transmitting 108 V of high-frequency electric power over a distance of 25 mi to light 200 bulbs and run an electric motor. In 1901, Tesla constructed the Wardenclyffe Tower to transfer electrical energy globally without cords through the ionosphere. However, due to technology limitations (e.g., low system efficiency), the idea has not been widely further developed and commercialized. Later, during 1920s and 1930s, magnetrons were invented to convert electricity into microwaves, which enables wireless power transfer over long distance. However, there was no method to convert microwaves back to electricity until 1964, when W. C. Brown realized this through a rectenna. Brown demonstrated the practicality of microwave power transfer by powering a model helicopter, which inspired a series of research efforts in microwave-powered airplanes during the 1980s and 1990s in Japan and Canada [3]. More recently, different consortia, for example, Wireless Power Consortium [4], Power Matters Alliance [5], and Alliance for Wireless Power [6], have been established to develop international standards for wireless charging. Nowadays, the standards have been adopted in many products in the market.
WIRELESS CHARGING TECHNIQUES
Three major techniques for wireless charging are magnetic inductive coupling, magnetic resonance coupling, and microwave radiation. The magnetic inductive and magnetic resonance coupling work in the near field, where the generated electromagnetic field dominates the region close to the transmitter or scattering object. The near-field power is attenuated according to the cube of the reciprocal of the distance. Alternatively, microwave radiation works in the far field at a greater distance. The far-field power decreases according to the reciprocal of the distance. Moreover, for the far-field technique, the absorption of radiation does not affect the transmitter. In contrast, for the near field techniques, the absorption of radiation influences the load on the transmitter.
Magnetic inductive coupling — Magnetic inductive coupling [4] is based on magnetic field induction, which delivers electrical energy between two coils. Figure 1a shows the reference model. Magnetic inductive coupling happens when a primary coil of an energy transmitter generates a predominant varying magnetic field across the secondary coil of the energy receiver within the field, generally less than the wavelength. The near-field power then induces voltage/current across the secondary coil of the energy receiver within the field. This voltage can be used by a wireless device. The energy efficiency depends on the tightness of coupling between two coils and their quality factor. The tightness of coupling is determined by the alignment and distance, the ratio of diameters, and the shape of two coils. The quality factor mainly depends on the materials, given the shape and size of the coils as well as the operating frequency. The advantages of magnetic inductive coupling include ease of implementation, convenient operation, high efficiency at close distance (typically less than a coil diameter), and safety. Therefore, it is applicable and popular for mobile devices. Very recently, MIT scientists announced the invention of a novel wireless charging technology, called MagMIMO [7], which manages to charge a wireless device from can detect and cast a cone of energy toward a phone even when the phone is inside a pocket.
Magnetic Resonance Coupling — Magnetic resonance coupling [8], as shown in Fig. 1b, is based on evanescent-wave coupling, which generates and transfers electrical energy between two resonant coils through varying or oscillating magnetic fields. As the resonant coils, operating at the same resonant frequency, are strongly coupled, high energy transfer efficiency can be achieved with small leakage to non-resonant externalities. This property also provides the advantage of immunity to the neighboring environment and line-of-sight transfer requirement. Compared to magnetic inductive coupling, another advantage of magnetic resonance charging is longer effective charging distance. Additionally, magnetic resonance coupling can be applied between one transmitting resonator and many receiving resonators, which enables concurrent charging of multiple devices.
In 2007, MIT scientists proposed a high-efficient mid-range wireless power transfer technology, Witricity, based on strongly coupled magnetic resonance. It was reported that wireless power transmission can light a 60 W bulb from more than 2 m with transmission efficiency around 40 percent [8]. The efficiency increases up to 90 percent when the transmission distance is 1 m. However, it is difficult to reduce the size of a Witricity receiver because it requires a distributed capacitive coil to operate. This poses a big challenge in implementing Witricity technology in portable devices. Magnetic resonance coupling can charge multiple devices concurrently by tuning coupled resonators of multiple receiving coils [9]. This has been shown to achieve improved overall efficiency. However, mutual coupling of receiving coils can result in interference, so proper tuning is required.
Microwave Radiation — Microwave radiation [9, 10] utilizes microwave as a medium to carry radiant energy. Microwaves propagate over space at the speed of light, normally in line of sight. Figure 1c shows the architecture of a microwave power transmission system. The power transmission starts with the AC-to-DC conversion, followed by a DC-to-RF conversion through a magnetron at the transmitter side. After being propagated through the air, the microwaves captured by the receiver rectenna are rectified into electricity again. In network applications, an energy-harvesting-enabled device can either harvest microwave radiation from dedicated sources [10] or ambient environment [10]. The typical frequency of microwaves ranges from 300 MHz to 300 GHz. The energy transfer can use other electromagnetic waves such as infrared and X-rays. However, due to safety issues, they are not widely used. Microwave energy can be radiated isotropically or toward some direction through beam forming.
The former is more suitable for broadcast applications. For point-to-point transmission, beam forming transmitting electromagnetic waves, referred to as power beam forming [5], can improve the power transmission efficiency. A beam can be generated through an antenna array (or aperture antenna). The sharpness of power beam forming improves with the number of transmit antennas. The use of massive antenna arrays can increase the sharpness. Recent development has also brought commercial products into the market. For example, the power caster transmitter and Power harvester receiver [5] allow 1W or 3W isotropic wireless power transfer.
III. STANDARDS
Different wireless charging standards have been proposed. Among them, Qi and A4WP are two leading standards supported by major smart phone manufacturers. This subsection presents an overview of these two standards.
Qi — Qi (pronounced “chee”) is a wireless charging standard developed by the Wireless Power Consortium (WPC) [4]. A typical Qi-compliant system model is illustrated in Fig. 2a. The Qi standard specifies interoperable wireless power transfer and data communication between a wireless charger and a charging device. Qi allows the charging device to be in control of the charging procedure. The Qi-compliant charger is capable of adjusting the transmit power density as requested by the charging device through signaling. Qi uses the magnetic inductive coupling technique, typically within the range of 40 mm. Two categories of power requirement are specified for a Qi wireless charger:
•The low-power category, which can transfer power within 5 W in the 110–205 kHz frequency range
•The medium-power category, which can deliver power up to 120 W in the 80–300 kHz frequency range.
Generally, a Qi wireless charger has a flat surface, referred to as a charging pad, on top of which a mobile device can be laid. As aforementioned, the tightness of coupling is a crucial factor in inductive charging efficiency. To achieve tight coupling, a mobile device must be strictly placed in proper alignment with the charger. Qi specifies three different approaches for making alignment:
• Guided positioning (i.e., a one-to-one fixed positioning charging) provides guidelines for the placement of a charging device in order to attain an accurate alignment. The Qi specification achieves this by using a magnetic attractor. This approach is simple; however, it may require implementation of a piece of material attracted by a magnet in the charging device.
• Free-positioning with a movable primary coil is also a one-to-one charging that can locate the charging device. This approach requires a mechanically movable primary coil that tunes its position to couple with the charging device.
• Free-positioning with a coil array allows multiple devices to be charged simultaneously irrespective of their positions. This approach can be applied based on the three-layer coil array structure [8]. Although it offers the advantage of user friendliness, this approach incurs higher implementation cost. The Qi-compliant wireless charging model supports in-band communication. Data transmission is on the same frequency band used for wireless charging. The Qi communication and control protocol is defined to enable a Qi wireless charger to adjust its power output for meeting the demand of the charging device and to disable power transfer when charging is finished. The protocol works as follows:
•Start: A charger senses the presence of a potential charging device.
•Ping: The charging device informs the charger of the received signal strength, and the charger detects the response.
•Identification and configuration: The charging device indicates its identifier and required power while the charger configures energy transfer.
•Power transfer: The charging device feeds back the control data, based on which the charger performs energy transfer.
Alliance for Wireless Power — A4WP aims to provide spatial freedom for wireless power [9]. This standard proposes to generate a larger electromagnetic field with magnetic resonance coupling. To achieve spatial freedom, A4WP standard does not require precise alignment, and even allows separation between a charger and charging devices. The maximum charging distance is up to several meters. Moreover, multiple devices can be charged concurrently with different power requirements. Another advantage of A4WP over Qi is that foreign objects can be placed on an operating A4 charger without causing any adverse effect. Therefore, the A4WP charger can be embedded in any object, improving the flexibility of charger
Device Detection — The PRU that needs to be charged sends out advertisements. The PTU replies with a connection request after receiving any advertisement. Upon receiving any connection request, the PRU stops sending advertisements. Then a connection is established between the PTU and PRU.
Information Exchange — The PTU and PRU exchange their Static Parameters and Dynamic Parameters as follows. First, the PTU receives and reads the information of the PRU Static Parameters, which contains its status. Then the PTU specifies its capabilities in the PTU Static Parameters and sends them to the PRU. The PTU receives and reads the PRU Dynamic Parameters, which include the PRU’s current, voltage, temperature, and functional status. The PTU then indicates PRU Control to manage the charging process.
Charging Control — It is initiated when PRU Control is specified and the PTU has enough power to meet the PRU’s demand. The PRU Dynamic Parameter is updated periodically to inform the PTU of the latest information so that the PTU can adjust PRU Control accordingly. If a system error or complete charging event is detected, the PRU sends PRU alert notifications to the PTU. The PRU Dynamic Parameter includes the reason for the alert. Apart from the data communication between a charger and a charging device, all of the standards do not support information exchange among multiple chargers. Nevertheless, such information exchange can improve the usability and efficiency of the chargers. In the next section, we introduce the idea of a wireless charger network to serve this purpose.
IV. WIRELESS CHARGER NETWORKING
We introduce the concept of wireless charger networking where chargers can not only communicate with the charging devices, but also can exchange and transfer information with a server. We first present the architecture and features of the wireless charger network. Then we focus on the user-charger assignment problem, and demonstrate that the wireless charger network can help reduce energy replenishment cost for the users.
THE WIRELESS CHARGER NETWORK
We present a novel idea of a wireless charger network. The network allows multiple chargers to communicate and exchange information with a server (Fig. 3). Such information includes availability, location, charging status, and cost of different chargers. Collecting this information, the server can optimize the use of chargers for certain purposes. One of them is user-charger assignment, which allows matching users who need to replenish energy for their mobile devices with available chargers. With the location, availability, and status of each charger, the server can inform the users of the best chargers (e.g., the nearest available ones). This information on the chargers has to be updated continuously so that the assignment can be performed in an online fashion with up-to-date information. The wireless charger network can serve this purpose.
ARCHITECTURE
The major components of the wireless charger network are as follows.
Smart Wireless Charger — In addition to typical charging functionality, the smart wireless charger is equipped with a data transceiver (Fig. 3). It has a local processing unit to process and data storage to store data on the wireless chargers (e.g., network setting). It can process and store data from charging devices (e.g., usage statistics and history) and commands from other components in the network (e.g., a status query).
Wireless Access Point — This is a typical wireless base station. It can be a WLAN access point or a cellular base station to provide communication channels to the smart wireless charger. The communication can be a WLAN connection (e.g., Wi-Fi or Bluetooth) or machine-to-machine (M2M) or machine-type communication (MTC) for a cellular connection (e.g., Long Term Evolution, LTE). Moreover, connections among chargers are also possible through multihop networks (e.g., mesh networks). For example, chargers outside the transmission range of the wireless access point can have their information relayed by intermediate chargers.
Server — It can perform authentication, authorization, and accounting (AAA) and other centralized functions. It has network data storage for maintaining various information about the wireless charger network (e.g., status and charging price of individual-chargers). It also provides an interface with users. For example, users can contact the server to request status information on the chargers in the network. The server can also optimize and direct users to the available chargers. In the wireless charger network, which allows smart wireless chargers to communicate, the following functions can be implemented.
Authentication — The chargers can verify the identity of a charging device. For example, any chargers in the same network can serve the devices owned by registered users. The account and password information can be exchanged between a charging device and charger. The charger can locally verify the information or can remotely authenticate the device with the server.
Charging Payment — To charge the device, the charger may require payment from a user. The charger can implement different pricing schemes (e.g., time of use) programmed by the server and the mobile user.
Reporting Status — The server can collect information about the chargers in the networks (e.g., location, available charging capacity, energy level, and identity of the charging devices). Users who want to use chargers can contact the server to retrieve some of this information (e.g., to choose the best charger). The owner of the wireless charger network can collect usage statistics, for example, for accounting purpose
V. OPEN RESEARCH ISSUES
This section highlights some open issues and challenges with both wireless charging technologies and data communication in wireless charging systems.
OPEN ISSUES IN WIRELESS CHARGING
Inductive Coupling — The increase of wireless charging power density gives rise to several technical issues, such as thermal, electromagnetic compatibility, and electromagnetic field problems [20]. This requires highly efficient power conversion techniques to mitigate the power loss at an energy receiver and battery modules with effective ventilation design.
Resonance Coupling — Resonance coupling-based techniques, such as Witritiy and Magnetic multiple- input multiple-output (MIMO), have a larger charging area and are capable of charging multiple devices simultaneously. However, they also cause increased electromagnetic interference with lower efficiency compared to inductive
charging. Another limitation with resonance coupling is the relatively large size of a transmitter. The wireless charging distance is generally proportional to the diameter of the transmitter. Therefore, wireless charging over a long distance typically requires a large receiver size.
Magnetic MIMO — For multi-antenna near-field beam forming, the computation of a magnetic beam forming vector on the transmission side largely depends on the knowledge of the magnetic channels about the receivers. The design of channel estimation and feedback mechanisms is of paramount importance. With inaccuracy of channel estimation or absence of feedback, the charging performance severely deteriorates. Additionally, there is a hardware limitation in that the impedance matching hardware optimally operates only within a certain range [7].
OPEN ISSUES IN DATA COMMUNICATION
To improve the usability and efficiency of the wireless charger, data communication capability can be enhanced.
Duplex Communication and Multiple Access — The current communication protocols support simplex communication (e.g., from a charging device to charger). However, there are some important procedures that require duplex communication. For example, the charging device can request a certain charging power, while the charger may request the battery status of the charging device. Moreover, the current protocols support one-tone communication. However, multiple devices charging can be implemented and medium access control (MAC) with multiple accesses for data transmission among charging devices and charger has to be developed and implemented.
Secure Communication — The current protocols support plain communication between a charger and charging device. They are susceptible for eavesdropping attacks (e.g., to steal a charging device’s and a charger’s identity) and man-in the- middle attacks (e.g., a malicious device manipulates or falsifies charging status). Security features have to be developed in the communication protocols, taking unique wireless charging characteristics (e.g., in-band communication in Qi) into account.