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The Era of Wireless Information and Power Transfer

Derrick Wing Kwan Ng1*, Trung Q. Duong2, Caijun Zhong3, and Robert Schober4

1School of Electrical Engineering and Telecommunications, The University of New South Wales, Australia

2School of Electronics, Electrical Engineering and Computer Science, Queens University Belfast, United Kingdom

3Institute of Information and Communication Engineering, Zhejiang University, China

4Institute for Digital Communications, Friedrich-Alexander-University Erlangen-Nuremberg (FAU), Germany

1.1 Introduction

In recent decades, the rapid development of wireless communication technologies has triggered a massive growth in the number of wireless communication devices for various practical applications, including e-health, autonomous control, logistics and transportation, environmental monitoring, energy management, safety management, etc. It is expected that in the era of the Internet of Things (IoT), there will be 50 billion wireless communication devices connected together worldwide via the Internet with a connection density of 1 million devices per [1]. In particular, small wireless sensor modules will be unobtrusively and invisibly integrated into clothing, walls, and vehicles at locations which are inaccessible for wired/manual recharging. However, battery-powered wireless communication devices have limited energy storage capacity and their frequent replacement can be costly, cumbersome, or even impossible (e.g., biomedical implants), which creates a serious performance bottleneck for realizing reliable and ubiquitous wireless communication networks. A promising approach to prolong the lifetime of traditional wireless communication systems is to let the wireless communication devices harvest energy from the environment [2-4]. For example, solar, wind, and geothermal are the major renewable energy sources for generating electricity. Unfortunately, these conventional natural energy sources are usually climate and location dependent, which may be problematic for mobile devices. Also, the intermittent and uncontrollable nature of natural energy sources makes the use of energy harvesting in wireless communication systems, where providing a continuous and stable quality of service (QoS) is of paramount importance, challenging.

Wireless power transfer (WPT) offers a viable solution for facilitating efficient and sustainable communication networks serving energy-limited communication devices [5-8]. Specifically, in practical systems, wireless devices communicate with each other via electromagnetic (EM) waves in the radio frequency (RF) band. Indeed, RF signals carry both information and energy simultaneously. Thus, the RF energy of propagating signals radiated by transmitters can be recycled at receivers for prolonging the lifetime of networks and supporting the energy consumption required for information transmission. This technology eliminates the need for power cords and any physical contact for manual recharging. Moreover, the broadcast nature of wireless channels facilitates one-to-many wireless charging, which is crucial for wireless networks with large numbers of energy-limited devices. On the other hand, compared to natural renewable energy sources generating intermittent energy, RF-based energy harvesting enables a stable and controllable wireless energy supply for energy-limited communication receivers. More importantly, WPT technology enables simultaneous wireless information and power transfer (WIPT). It is expected that WIPT will serve as a building block for realizing self-sustained communication networks and as the key to unlock the potential of IoT networks. However, despite the conveniences introduced by WIPT technology, the integration of WIPT technology into communication networks also introduces many challenges. For instance, the WPT efficiency is usually low. In practice, wireless power has to be transferred via a carrier signal with a high carrier frequency such that antennas of reasonable size can be used for harvesting energy at handheld devices. However, the associated path loss severely attenuates the signal such that only a small amount of power can be harvested at the receiver. For example, for a communication distance of 10 m in free space, the attenuation of a wireless signal can be up to 50 dB for a carrier frequency of 915 MHz. Moreover, traditional communication networks were optimized for pure data communications. Therefore, it is expected that existing network protocols, resource allocation algorithms, and receiver structures will not be able to meet the unique challenges incurred by the nature of WPT. This book addresses these challenges and provides a comprehensive reference for various solutions for realizing efficient WIPT in practice. In the following sections, we will provide some background information on WPT and discuss exciting research directions. The specific details will then be covered in the subsequent chapters.

1.2 Background

The concept of WPT was first proposed by Nikola Tesla in 1899. The initial efforts on WPT focused on high-power-consumption applications. This raised serious public health concerns about strong electromagnetic radiation which prevented the further development of WPT in the late twentieth century. As a result, this area developed slowly until recent advances in silicon technology and multiple-antenna technology made WPT attractive once again. In fact, the use of WPT avoids the potentially high costs in planning, installing, displacing, and maintaining power cables in buildings and infrastructure. Hence, it is expected that innovative WPT networks are the key enabler of the IoT to connect all devices together via wireless powered sensors for the development of smart cities. The continued study of WPT in both industry and academia will produce frontier technologies by developing novel and cost-effective designs to enable breakthroughs in WPT in the information and communication technology industry sector. For example, it is estimated that the development of IoT for logistics and transportation has a total potential economic impact of 1.9 trillion per year in the next decade [9]. In order to seize the rising business opportunities, recently different companies, e.g., Samsung Electronics and Huawei Technologies, have begun to launch various research and study groups to facilitate the development and standardization of WPT for powering small wireless communication devices.

1.2.1 RF-Based Wireless Power Transfer

The existing WPT technologies can be categorized into three classes: inductive coupling, magnetic resonant coupling, and RF-based WPT. The first two technologies rely on near-field EM waves, which do not provide any mobility to energy-limited wireless communication devices due to the limited wireless charging distances (a few meters) and the required alignment of the EM-field with the energy harvesting circuits. In contrast, RF-based WIPT exploits the far-field properties of EM waves, which enable concurrent wireless charging and data communication over long distances (hundreds of meters). Moreover, RF energy is omnipresent and can be harvested from the signals in the environment transmitted by Wi-Fi access points, TV base station towers, cellular communication base stations, etc. Also, RF-based WIPT utilizes the RF spectrum and the radiation is regulated by the government to ensure safety. More importantly, RF signals can serve as a dual-purpose carriers for conveying both information and power simultaneously.

Nowadays, prototype RF-based energy harvesting circuits are able to harvest microwatts to milliwatts of power over the range of 10 m for a transmit power of 1 W (typical transmit power of a Wi-Fi router) and carrier frequencies of less than 1 GHz [10]. The harvested energy is sufficient to power not only wireless sensors (e.g., fire alarm sensors), but also digital clocks mounted on the wall, which reduces the inconvenience of battery replacement. Although WIPT is critical to the design and implementation of sustainable communication networks, existing system models and resource allocation algorithms have only been proposed and optimized for pure information transfer. In practice, network designers need to strike a balance in the non-trivial trade-off between information and power transfer, leading to significantly different resource allocation algorithms, system models, and interference management schemes, compared to conventional wireless data communications. The introduction of RF-based WIPT imposes new challenges for the design of communication networks since traditional techniques used for the design of data communications cannot solve the fundamental problems in WIPT networks. Hence, there is an emerging need for the development of novel design theories, hardware circuit architectures, and signal processing techniques to unlock the potential of WIPT networks.

Figure 1.1 Three commonly adopted WIPT network architectures: (a) a SWIPT network, (b) a WPCN, and (c) a WPBC network.

1.2.2 Receiver Structure for WIPT

RF-based energy harvesting technology enables the possibility of simultaneous WIPT (SWIPT), wireless-powered communication (WPC), and wireless-powered backscatter communication (WPBC), e.g., [11-15]. Specifically, in SWIPT networks, cf. Figure 1.1a, a...