1 Fundamentals of Wireless Information and Power Transfer: From RF Energy Harvester Models to Signal and System Designs Bruno Clerckx, Senior Member, IEEE, Rui Zhang, Fellow, IEEE, Robert Schober, Fellow, IEEE, Derrick Wing Kwan Ng, Senior Member, IEEE, Dong In Kim, Senior Member, IEEE, and H. Vincent Poor, Fellow, IEEE Abstract—Radio waves carry both energy and information I. INTRODUCTION simultaneously. Nevertheless, Radio-Frequency (RF) transmission of these quantities have traditionally been treated separately. Wireless communications via Radio-Frequency (RF) radi- Currently, we are experiencing a paradigm shift in wireless ation has been around for more than a century and has sig- network design, namely unifying wireless transmission of in- formation and power so as to make the best use of the RF nificantly shaped our society in the past 40 years. Wireless is spectrum and radiations as well as the network infrastructure however not limited to communications. Wireless powering of for the dual purpose of communicating and energizing. In devices using near-field Inductive Power Transfer has become this paper, we review and discuss recent progress on laying a reality with several commercially available products and the foundations of the envisioned dual purpose networks by standards. However, its range is severely limited (less than one establishing a signal theory and design for Wireless Information and Power Transmission (WIPT) and identifying the fundamental meter). On the other hand, far-field Wireless Power Transfer tradeoff between conveying information and power wirelessly. (WPT) via RF (as in wireless communication) could be used We start with an overview of WIPT challenges and technologies, over longer ranges. It has long been regarded as a possibility namely Simultaneous Wireless Information and Power Transfer for energising low-power devices but it is only recently that (SWIPT), Wirelessly Powered Communication Network (WPCN), it has become recognized as feasible due to reductions in the and Wirelessly Powered Backscatter Communication (WPBC). We then characterize energy harvesters and show how WIPT power requirements of electronics and smart devices [1], [2]. signal and system designs crucially revolve around the underlying Indeed, in 20 years from now, according to Koomey’s law energy harvester model. To that end, we highlight three different [3], the amount of energy needed for a given computing task energy harvester models, namely one linear model and two will fall by a factor of 10000 compared to what it is now, nonlinear models, and show how WIPT designs differ for each of thus further continuing the trend towards low-power devices. them in single-user and multi-user deployments. Topics discussed include rate-energy region characterization, transmitter and Moreover, the world will see the emergence of trillions of receiver architecture, waveform design, modulation, beamform- Internet-of-Things (IoT) devices. This explosion of low-power ing and input distribution optimizations, resource allocation, devices calls for a re-thinking of wireless network design. and RF spectrum use. We discuss and check the validity of Recent research advocates that the future of wireless net- the different energy harvester models and the resulting signal working goes beyond conventional communication-centric theory and design based on circuit simulations, prototyping and experimentation. We also point out numerous directions that are transmission. In the same way as wireless (via RF) has dis- promising for future research. rupted mobile communications for the last 40 years, wireless Index Terms—Wireless information and power transfer, wire- (via RF) will disrupt the delivery of mobile power. However, arXiv:1803.07123v1 [cs.IT] 19 Mar 2018 less power transfer, wireless powered communications, wireless current wireless networks have been designed for communica- energy harvesting communications, rate-energy region, linear and tion purposes only. While mobile communication has become nonlinear energy harvester modeling, signal and system design, a relatively mature technology, currently evolving towards its prototyping, experimentation. fifth generation, the development of mobile power is in its B. Clerckx is with the Electrical and Electronic Engineering De- infancy and has not even reached its first generation. Today, partment at Imperial College London, London SW7 2AZ, UK (email: not a single standard on far-field WPT exists. Wireless power [email protected]). will bring numerous new opportunities: no wires, no contacts, R. Zhang is the Department of Electrical and Computer Engi- neering, National University of Singapore, Singapore 117583 (e-mail: no batteries, genuine mobility and a perpetual, predictable, [email protected]). dedicated, on-demand, and reliable energy supply as opposed R. Schober is with the Institute of Digital Communications, Friedrich- to intermittent ambient energy-harvesting technologies (e.g. Alexander-University Erlangen-Nurnberg (FAU), Germany (email: [email protected]). solar, thermal, vibration). This is highly relevant in future D. W. K. Ng is with the School of Electrical Engineering and Telecom- networks with ubiquitous and autonomous low-power and munications, University of New South Wales, Sydney, NSW 2052, Australia energy limited devices, device-to-device communications, and (email: [email protected]). D. I. Kim is with the School of Information and Communication Engineer- the IoT with massive connections. ing, Sungkyunkwan University (SKKU), Korea (email: [email protected]). Interestingly, although radio waves carry both energy and H. V. Poor is with the Department of Electrical Engineering, Princeton information simultaneously, RF transmission of these quanti- University, Princeton, NJ 08544 USA (e-mail: [email protected]). This work has been partially supported by the EPSRC of UK, under grant ties have traditionally been treated separately. Imagine instead EP/P003885/1. a wireless network, e.g. WiFi, in which information and 2 energy flow together through the wireless medium. Wireless range of areas spanning sensors, devices, RF, communication, communication, or Wireless Information Transfer (WIT), and signal and system designs for WPT. This survey article targets WPT would then refer to two extreme strategies, respectively, challenge 8) by reviewing the fundamentals of WIPT signal targeting communication-only and power-only. A unified de- and system designs. In WPT and WIT, the emphasis of the sign of Wireless Information and Power Transmission (WIPT) system design is to exclusively deliver energy and informa- would on the other hand have the ability to softly evolve tion, respectively. On the contrary, in WIPT, both energy and compromise in between those two extremes to make and information are to be delivered. A WIPT system should the best use of the RF spectrum/radiation and the network therefore be designed such that the RF radiation and the RF infrastructure to communicate and energize. This will enable spectrum are exploited in the most efficient manner to deliver trillions of low-power devices to be connected and powered both information and energy. Such a system design requires anywhere, anytime, and on the move. the characterization of the fundamental tradeoff between how The integration of wireless power and wireless commu- much information and how much energy can be delivered in nications brings new challenges and opportunities, and calls a wireless network and how signals should be designed to for a paradigm shift in wireless network design. As a result, achieve the best possible tradeoff between them. numerous new research problems need to be addressed that As illustrated in Fig. 1, WIPT can be categorized into three cover a wide range of disciplines including communication different types: theory, information theory, circuit theory, RF design, signal • Simultaneous Wireless Information and Power Transfer processing, protocol design, optimization, prototyping, and (SWIPT): Energy and information are simultaneously experimentation. transferred in the downlink from one or multiple ac- cess points to one or multiple receivers. The Energy A. Overview of WIPT Challenges and Technologies Receiver(s) (ER) and Information Receiver(s) (IR) can be co-located or separated. In SWIPT with separated WIT and WPT are fundamental building blocks of WIPT receivers, ER and IR are different devices, the former and the design of efficient WIPT networks fundamentally being a low-power device being charged, the latter be- relies on the ability to design efficient WIT and WPT. In ing a device receiving data. In SWIPT with co-located the last 40 years, WIT has seen significant advances in RF receivers, each receiver is a single low-power device that theory and signal theory. Traditional research on WPT in the is simultaneously being charged and receiving data. last few decades has focused extensively on RF theories and • Wirelessly Powered Communication Network (WPCN): techniques concerning the energy receiver with the design of Energy is transferred in the downlink and information efficient RF, circuit, antenna, rectifier, and power management is transferred in the uplink. The receiver is a low-power unit solutions [4]–[6], but recently a new and complementary device that harvests energy in the downlink and uses it line of research on communications and signal design for WPT to send data in the uplink. has emerged in the communication literature [7]. Moreover, • Wirelessly Powered Backscatter Communication