IEEE Microwave Magazine - September/October 2018 - 70

Wireless power via RF (as in wireless
communication) could be used for
longer ranges via two different
methodologies: wireless energy
harvesting (WEH) and wireless power
transfer/transmission (WPT).

and the Alliance for Wireless Power, such as Rezence).
On the other hand, wireless power via RF (as in wireless communication) could be used for longer ranges
via two different methodologies: wireless energy harvesting (WEH) and wireless power transfer/transmission (WPT). While WEH assumes that RF transmitters
are designed exclusively for communication purposes
(the ambient signals of which can be harvested), WPT
relies on dedicated sources designed exclusively for
wireless power delivery. Wireless power via RF has
long been regarded as a possibility for energizing
low-power devices, but only recently has this possibility been recognized as feasible. According to [1], at a
fixed computing load, the amount of required energy
falls by a factor of two every year-and-a-half due to
the evolution of electrical efficiency in computer technology. This explains why relying on wireless power
to perform meaningful computation tasks at reasonable distances became feasible only over the last few
years and so now justifies the recent interest in wireless power.
The emergence of RF identification (RFID) technology over the last decade is the first sign of this interest
in far-field wireless power. RFID tags use the RF signal of an RFID reader (transmitter) for power and communication. They rely on backscattering modulation
to reflect and modulate the incoming RF signal. The
interest in RFID is motivated by numerous industrial
applications and has led to the development of a physical layer, standards, and a body of research focused on
the design of novel tags, protocols, and readers.
Recent work proposes that, in the future, wireless
networking go beyond conventional communicationcentric transmission. In the same way that wireless
(via RF) has disrupted mobile communications for the
last 40 years, wireless (via RF) will now disrupt the
delivery of mobile power. However, current wireless
networks are designed for communication purposes
only. While mobile communication has become a relatively mature technology as it evolves toward 5G, the
development of mobile power is in its infancy and has
yet to reach even its first generation (1G): not a single
standard exists for mobile power and far-field WPT.
Despite being subject to regulations regarding
exposure to electromagnetic fields (as is wireless communication), wireless power brings numerous new

opportunities. It enables proactive and controllable
energy replenishment so that devices no longer depend
on centralized power sources, which allows for genuine mobility: no wires, no contacts, and no (or, at least,
reduced-size) batteries, leading to lighter, more compact devices. It is also an ecologically sound solution,
eliminating the production, maintenance, and disposal of trillions of batteries, in addition to prolonging
device lifetimes and creating a long-term, predictable, a nd reliable energ y supply (as opposed to
ambient energy-harvesting technologies such as
solar, thermal, and vibration). This is particularly
important for future networks with ubiquitous and
autonomous low-power and energy-limited devices,
device-to-device communications, and the IoT with its
massive connectivity.
Radio waves simultaneously carry both energy and
information. Traditionally, energy and information
have been treated separately and have evolved as two
independent fields of work in academia and industry,
i.e., wireless power and wireless communication. This
separation has several consequences:
* Current wireless networks pump RF energy into
the free space (for communication purposes) but
do not make use of it for energizing devices.
* Providing ubiquitous mobile power would require
deploying a separate network of dedicated energy transmitters.
Imagine, instead, a wireless network in which information and energy flow together through the wireless
medium. Wireless communication (or wireless information transfer) and WPT would refer to two extreme
strategies targeting, respectively, communications
only and power only. A unified wireless information
and power transfer (WIPT) design could evolve unobtrusively in between those two extremes, making the
best use of the RF spectrum/radiations and of network
infrastructure to communicate, energize, and, hence,
outperform traditional systems that rely on a separation of communications and power.
This article reviews some promising recent approaches that could move this vision closer to reality. Most
work published within the microwave and communication/signal processing communities emphasizes either
RF, circuit, and antenna solutions for WPT on the one
hand or communications, signal, and system designs
for WPT on the other. This review article, in contrast,
uniquely bridges RF, signal, and system designs to bring
those areas closer to one another and so provide a better
understanding of the fundamental building blocks for
an efficient WPT network architecture. We start by reviewing the engineering requirements and design challenges involved in making mobile power a reality. We
then summarize the state of the art in a wide range of
areas spanning sensors and devices, RF design for wireless power, and wireless communications. We identify
the limitations of each and make critical observations,

September/October 2018

Table of Contents for the Digital Edition of IEEE Microwave Magazine - September/October 2018