![(a) Battery-free communication platform powered by outdoor ambient LTE signals [92]; (b) Wi-Fi backscatter tag utilizing ambient RF energy harvesting for signal modulation and backscattering [161].](https://www.researchgate.net/profile/Xiaoqiang-Gu/publication/356656004/figure/fig3/AS:1174493267529730@1657032424163/a-Battery-free-communication-platform-powered-by-outdoor-ambient-LTE-signals-92-b_Q320.jpg)
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Abstract—Continuous advances in ultra-low-power electronics
are a fundamental driving force of developing far-field wireless
power harvesting techniques, which generally harness
low/ambient energy in free space or dedicated wireless power from
a power base station. This paper reviews recent developments and
technology trends in far-field (radiative) wireless power
harvesting, including modeling of rectification process inside
nonlinear devices, insights into rectenna system design, and
demonstrations of emerging applications. The rectification process
inside nonlinear devices will be explored, upon which a device
selection guideline is provided to help quickly identify suitable
devices. Then, the optimization of rectenna design is discussed to
realize efficiency enhancement at a limited input power level. Since
far-field wireless power harvesting has become a critical enabling
technology for battery-free Internet of Things, a series of
promising applications are demonstrated in this article to
highlight the challenges and opportunities in the field.
Index Terms—Battery-free IoT, far-field RF energy harvesting,
far-field WPT, Internet of Things, rectenna, rectifier, Schottky
diodes, wireless power harvesting, wireless power transfer
I. INTRODUCTION
LECTROMAGNETICS-based wireless power transfer
(WPT) may be generally classified into two major
categories: near-field (non-radiative) and far-field (radiative)
transmission. Near-field WPT has attracted significant attention
since the innovation of resonant coupling WPT technology was
reported by a group of MIT researchers in 2007 [1]. Resonant
coupling WPT enables efficient short- and mid-range power
delivery, compared to the short-range two-coil proximity-
coupling WPT at a distance of a few centimeters [2]. Since
electromagnetic power decays at a rate of 60 dB/decade in the
near-field range [3], far-field or radiative WPT enabled by
electromagnetic wave propagation is highly desired [4-6] to
extend the distance of wireless power transmission further.
Far-field WPT dates back to more than one century ago when
Nikola Tesla conducted his experiments on radio waves-based
electric energy transmission at the turn of the 20th century [7].
Within the
research field remained dormant due to the limitations of solid-
state electronics, which could not even support efficient
This work was supported by the Natural Science and Engineering Research
Council (NSERC) of Canada and Mitacs through the Mitacs Accelerate
program.
Xiaoqiang Gu and Ke Wu are with the Poly-GRAMES Research Center,
Department of Electrical Engineering, École Polytechnique de Montréal,
communication and radar operation with sufficient power [8].
Then, roughly 60 years ago, far-field WPT re-gained
momentum and marched to a modern era as serious interest in
high-power WPT began, including the developments of the
microwave-powered helicopter [9], point-to-point long-
distance WPT [10], and solar-power satellite concept with
microwave power transmission to the Earth [11],[12]. Despite
those successful field experiments, high-power WPT saw
limited practical applications considering its high cost, bulky
size, system efficiency, and safety issues. Therefore, intensive
research efforts nowadays focus on low-power far-field
wireless power harvesting technology tailored to sensing and
communication [13],[14]. Far-field wireless power harvesting
has two fundamental application scenarios depending on how
radiofrequency (RF) power is transmitted to rectennas
(rectifying antennas). In the first application, rectennas harvest
pervasive RF power which is broadcasted by various sources in
the environment, such as digital TV (DTV) broadcasting
towers, Wi-Fi access points, and cellular base stations [15]. To
access both known and unknown users, such RF power is
transmitted omnidirectionally to ensure maximum coverage in
this case. In the second, wireless power delivery between power
sources and rectennas utilizes beam technology [16]. By
controlling the direction and width of beams, far-field wireless
power harvesting becomes more efficient at the cost of system
complexity. With the rollout of 5G, where beamforming
techniques are massively used, far-field wireless power
harvesting is believed to take advantage of them [17]. Due to
transmission loss and safety concerns, RF power density in free
space is normally limited, further resulting in the low rectifying
efficiency of rectennas, which has a direct impact on system
efficiency and applicability. Hence, the ultimate goal for
rectenna design is to maximize rectifying efficiency and dc
output in a certain power range.
Figure 1 offers a global view by summarizing the state-of-
the-art results of achieved high-efficiency far-field rectennas
operating in the wide frequency range from MHz to sub-THz.
The data summary are [18-24]
[20],[23],[25], 2.45 GHz [23], [26-32], 5.8 GHz [33-37], 2371
Montréal, QC H3T 1J4, Canada (e-mail: xiaoqiang.gu@polymtl.ca;
ke.wu@polymtl.ca).
Simon Hemour is with the IMS Laboratory, CNRS UMR 5218, Bordeaux
INP, University of Bordeaux, 33045 Talence, France (e-mail:
simon.hemour@u-bordeaux.fr).
Xiaoqiang Gu, Member, IEEE, Simon Hemour, Senior Member, IEEE, and Ke Wu, Fellow, IEEE
E
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GHz [38-47] 7 GHz [48-53], and beyond 97 GHz
[54],[55]. The RF nnas
operating under 2.45 GHz for a fair comparison. Such an RF
power level is comparable to what one can find in the
environment. At a higher frequency (< 71 GHz), the RF input
power is set at 10 dBm. Since fewer rectennas exist in the W
band and beyond,
marked. Typically, low frequency is beneficial for better
efficiency, which is good news for ambient RF power
harvesting. Higher frequency is more likely adopted by
beamforming-based wireless power delivery, which requires
more transmitting power due to more path loss. This paper reads
as follows. Section II discusses a city-wide ambient RF energy
density measurement conducted in Montreal to help understand
the distribution of ambient RF power and associated power
level in a typical metropolitan area. This vital step will guide
our rectenna design for practical applications. The power
conversion efficiency of far-field wireless power harvesting is
limited given its low incident power. Thus, nonlinear modeling
of the rectifying element of far-field wireless power harvesting
is crucial to further enhance its rectifying efficiency. The
Schottky diodes-based rectification mechanism is presented in
Section III, together with a discussion and outlook on other
promising diode candidates. Rectenna design is a systematic
work around the nonlinear rectifying element, consisting of
developing antennas, matching networks, and picking suitable
loading resistances. Based on the efficiency chain analysis of
rectenna, suggestions to achieve high-efficiency rectennas are
offered in Section IV. Finally, Section V discusses the outlook
of far-field wireless power harvesting to support our sustainable
and smart living through emerging applications.
II. AMBIENT RF POWER DENSITY MEASUREMENTS
Ambient RF energy is pervasive and abundant in cities and
urban areas , just like fresh air in free
space. Understanding how much ambient RF energy is
available in our environment should be the first step in rectenna
design. So far, multiple investigations in different cities
worldwide have been conducted to address this concern [56-
58]. For future city-wide Internet of Things (IoT) sensor
network planning, the dynamic measurements of ambient RF
energy density covering major streets, roads, and highways of
the city of Montreal were reported in [59]. Such a set of
measurements paints a global picture of ambient RF density in
a typical city, differing from previous works focusing on
limited discrete locations. The measurement frequencies of
interest cover from 400 to 2700 MHz, which deal with the
primary frequency bands in the outdoor environment before an
extensive implementation of the 5G infrastructure.
Ambient RF energy within the frequency spectrum of interest
mainly originates from three sources: DTV, communication,
and Wi-Fi bands. Since outdoor Wi-Fi signals are usually weak,
DTV and communication bands are the focus of this work. Fig.
2 shows the ambient RF power density of the GSM/LTE1900
and DTV bands in the core areas of Montreal. This area
involves the downtown (DT) and uptown (UT) regions
separated by Mount Royal, a small mountain. In Fig. 2(a),
triangle symbols mark the locations of cellular towers. The DT
area, on the east of Mount Royal, has a denser distribution of
cellular towers and thereby higher ambient RF power density.
The highest power density in DT 26.71 dBm/cm2. In
110 100
1
10
100 -20
-20
-20
10
10
10
check excel
check excel
-2.46
25.34
10
Rectifying efficiency (%)
Frequency (GHz)
95 GHz,
9.512 dBm
(2021) [52]
160 GHz,
dBm
(2015) [54]
96 GHz, 10 dBm
(2017) [49]
23 GHz (2021) [38]
35 GHz
(2020) [43]60 GHz
(2017) [46]*
1.9 GHz (2016)
[25]
0.9 GHz (2020) [18]
input power: 20 dBm GHz),
+10 dBm (5.8 GHz), marked GHz)
2.45 GHz (2012)
[31]
303 GHz,
25.34 dBm
(2018) [55]
5.8 GHz (2013) [36]
10 dBm
3 dBm#
11.8 dBm
Fig. 1. State-of-the-art rectifying efficiency results of high-efficiency far-field rectennas operating in a wide frequency range from MHz to sub-THz. The
references corresponding to the best performance so far in each frequency band have been highlighted. *The efficiency at 10 dBm in [46] is estimated based on
its efficiency performance at 8 dBm. #The efficiencies of two 94-GHz rectifiers overlap, with 32.3% at 3 dBm in [50] and 32.5% at 21 dBm in [53].
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contrast, on the other side of Mount Royal, the UT area sees
lower overall ambient RF power density due to fewer cellular
towers. The close relationship between the ambient RF power
density of the GSM/LTE1900 band and cellular tower locations
can be more easily captured in Fig. 2(a). The ambient RF power
density of the DTV band is presented in Fig. 2(b), together with
the locations of DTV broadcasting towers on Mount Royal. The
DT area still has a larger DTV power density than the UT area,
but their difference is smaller than that of the GSM/LTE1900
band.
The average ambient RF power density across the frequency
band from 400 to 2700 MHz in the DT and UT areas is
presented in Fig. 3. The DT ambient RF power density of the
entire band is larger than the UT, especially in the
communication bands. For example, all communication bands
in DT have an average RF power density at least 20 times (13
dBm/cm2) larger than in UT. While in the DTV band, there is a
relatively small difference between DT and UT Montreal of
about 2.5 dBm/cm2, which indicates that the level in DT is
about 78% larger than in UT. Moreover, the GSM/LTE1900
(a)
(b)
Fig. 2. Ambient RF power density in the core area of Montreal. (a) GSM/LTE1900 and (b) DTV bands. Triangle symbols in (a) indicate the locations of cellular
towers in Montreal. DTV broadcasting towers are marked in (b).
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band has the largest average power density in both DT and UT
areas 14 dBm/cm2 2,
respectively.
Similar to the RF surveys from outside of the 270 London
Underground stations at street level [15] and outside of 40
subway stations in Boston [60],
measurements also show that communication bands likely yield
higher power density than the DTV band. The average power
density of the GSM1800 band in the Greater London area is
around 2, slightly higher than the GSM900 band
2. While in Boston, the GSM850 band has the
highest power density in average. Based on these ambient RF
power density measurements, several remarks can be
highlighted regarding far-field wireless power harvesting:
(i) In DT areas, due to more cellular towers, the
communication bands should be given priority in terms of
recycling ambient RF energy wirelessly.
(ii) In UT areas, the DTV band, which is independent of
human activities and thus more stable, should be the first choice
for far-field RF energy harvesting.
(iii) Streets or buildings near metro stations or similarly
populated areas often have larger ambient RF power density
and are thereby suitable for implementing far-field wireless
energy harvesting.
(iv) For practical applications, designers should target a
2 or
lower.
III. LOW-POWER RECTIFICATION
A multitude of methods has been proposed to characterize
the nonlinear component of rectifiers and help predict the
rectifying efficiency. A closed-form analysis of the Schottky
diode-based rectifier was first reported in [61], which assumes
that rectifying efficiency only depends on diode electrical
parameters and the circuit losses at the fundamental frequency
and dc. Later, this method was expanded to take varying input
power levels into account [62]. A further update was made in
[63] to predict the rectifying efficiency of the class-F rectifier
by including the effect of diode breakdown voltage. Also, a
theoretical analysis of harmonically terminated high-efficiency
power rectifiers based on Fourier analysis of current and
voltage waveforms has been presented in [64]. A class-C single
Schottky-diode rectifier and a class-F1 GaN transistor rectifier
were selected for experimental validation, revealing the time
reversal duality between harmonically terminated power
amplifiers and rectifiers [64], [65].
Schottky barrier diodes are still the most widely adopted
nonlinear element for far-field wireless power harvesting
applications. Practical far-field wireless power harvesting often
features low-power rectification, so Schottky diodes cannot be
simplified using the ON-OFF model, which is common in high-
power rectification. Fig. 4 illustrates the diode junction
resistance variations due to applied voltage across it as a
Average ambient RF power density (dBm/cm2)
Average ambient RF power density (dBm/cm2)
DTV
LTE700
GSM/LTE850 GSM/LTE1900
LTE
1700/2100 LTE2600
Wi-Fi
DTV
LTE700
GSM/LTE850 GSM/LTE1900
LTE
1700/2100 LTE2600
Wi-Fi
downtown uptown
(a)
Average ambient RF power density (dBm/cm2)
Average ambient RF power density (dBm/cm2)
DTV
LTE700
GSM/LTE850 GSM/LTE1900
LTE
1700/2100 LTE2600
Wi-Fi
DTV
LTE700
GSM/LTE850 GSM/LTE1900
LTE
1700/2100 LTE2600
Wi-Fi
downtown uptown
(b)
Fig. 3. Average ambient RF power density along major roads in Montreal:
(a) downtown area and (b) uptown area.
upper envelope (largest Rj)
lower envelope (smallest Rj)
time-domain voltage signal
across the diode
-45 -40 -35 -30 -25
103
104
105
106
107
Diode junction resistance ()
Input power (dBm)
Fig. 4. Diode junction resistance variations due to applied voltage against
input power levels. Upper and lower envelopes are the largest and smallest
junction resistances at different power levels, respectively. Results are
obtained using diode SMS7630, and
the zero bias junction resistance . A series diode topology is used for
evaluation. The operating frequency is 2.45 GHz.
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function of input power levels. This analysis is based on the
diode SMS7630, featuring a low junction potential (0.34 V) and
superior low-power rectifying performance. SMS7630 with
low package parasitics is suitable for applications under 24
GHz, according to its datasheet [66]. The load resistance is set
as As depicted in Fig. 4, diode junction resistance varies
more dramatically at higher input power levels. For example, at
the
In
contrast, when the the ratio
. Thus, the ON-OFF model for diode
SMS7630 can be considered valid at the input power of 25
Notably, each diode presents
different nonlinear behaviors, and thus the power level
corresponding to the valid ON-OFF model is different and
requires an individual investigation.
Since the ON-OFF model is often not valid in the low-power
rectification, a closed-form analysis method in the frequency
domain is preferred. Schottky diodes present two different
states in rectification, as shown in Fig. 5, which are the RF
power absorber and dc power provider. The Shockley diode
model, consisting of the series resistance , nonlinear junction
resistance , and nonlinear junction capacitance , is used for
characterizing the Schottky diodes [67]. When RF input power
flows into the diode, it passes both and . However, only
contributes to the power rectification process since does not
support a dc path [see Fig. 5(b)]. Thus, in Fig. 5(a), is shown
to absorb the input RF power at the fundamental frequency .
The fundamental power going through is finally dissipated
by . Hence, both and account for the parasitic loss of
the diode during power rectification. Since Joule heating is
inevitable during power conversion for , part of the power is
wasted in this form. In Fig. 5(a), the rectifier load is shorted by
the filtering capacitance , which runs parallel to it. Fig. 5(b)
illustrates the other state of the Schottky diode when the diode
junction becomes a dc power provider for the load resistance.
In this state, and can be ignored since they do not pass dc
power. It should be noted that Fig. 5 uses a single-series
topology. As for the single-shunt case, the only difference is
that the source resistance does not exist in the dc path.
Based on the above analysis, the diode junction resistance is
the most important element in power rectification. For Schottky
diodes, the current (I) voltage (V) relationship can be
characterized by an exponential expression:
(1)
where and are the current passing the junction
resistance and the voltage across it, respectively. contains
the input RF voltage and the generated dc voltage reversely
biasing the diode junction. , , and are the diode saturation
current, ideality factor and thermal voltage, respectively. This
exponential expression can be a good approximation and
replacement of the piecewise SPICE model of the Schottky
diodes [18]. Since contains all frequency components of
the diode current, it is essential to separate it in the frequency
domain to obtain the dc component. Both Bessel function and
Taylor series are suitable for dealing with the exponential
expression. The Bessel function offers better accuracy [68-70],
but the Taylor series provides an explicit and straightforward
relationship between the conversion efficiency of junction
resistance and input power [71]:
-400 -300 -200 -100 0 100
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.24
Junction capacitance (pF)
Voltage across the junction (mV)
Signal
waveforms
25 dBm
45 dBm
Time
Time
self-bias
voltage
Fig. 6. CV curve of diode SMS7630. Various input power levels cover
Rj
Cj
Rs
R0
RF
source Cl
@f0
(a)
Rj
Cj
Rs
R0
Rl
@dc
Idc
dc path
(b)
Fig. 5. Equivalent circuits corresponding to two states of Schottky diode-
based rectifier: (a) diode junction as an RF power absorber of input RF power;
and (b) diode junction as a dc power provider. Diode junction is characterized
by the Shockley model. generates dc current, which is denoted as Norton
current . In contrast, both and contribute to the parasitic loss. To
distinguish their different roles, and are marked in green, and and
are in orange. Matching network and diode parasitics are removed for
simplification purposes.
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(2)
where is the zero bias current responsivity
of the Schottky diodes [25], thus quantifying diode
nonlinearity. Note that is equal to half of the curvature of
diode detectors at zero bias operating conditions. As shown in
Fig. 5, inside the junction contributes to the parasitic loss
together with , as part of the RF power going through is
dissipated as Joule heating. Such parasitic loss can be quantified
as [71]:
(3)
Since (3) involves both and , it is also only valid when
the input power is low (). Fig. 6 shows that small
and large input power levels lead to significantly different diode
capacitance ranges. The capacitance (C) voltage (V) curve in
Fig. 6 is based on diode SMS7630. Signal waveforms of the
included for
comparison. Rectified dc voltage biases the diode reversely, so
the center line of the input signal waveform
location of higher reverse voltage.
dBm), the variations of are concentrated in a small range
close to its zero bias value . When the input power ramps up,
cannot be a satisfactory approximation as the change of
is dramatic, as shown in Fig. 6.
The power conversion efficiency of the Schottky diode-
based rectifier is defined as the ratio of generated dc power
at the load resistance over the input RF power :
(4)
Thus, a closed-form method to predict requires careful
modeling of both and . As shown in Fig. 5(a), the input
RF power comprises two parts, namely, the power absorbed by
and dissipated by :
(5)
where
. is the voltage magnitude of the
input power. is the current magnitude at the fundamental
frequency going through the junction resistance contained in
(1). The Bessel function is suitable to separate the diode current
in (1) into its dc component and other harmonics without
truncation errors like using the Taylor series. Thus, the
generated dc current and can be obtained as:
(6a)
(6b)
in which is the principal branch of the Lambert W function
[69], [72]; is defined as
; is the Bessel
function of the first kind of order and is the imaginary unit;
is the total generated dc voltage and is calculated by
multiplying and the total resistance in the circuit . Hence,
the dc power obtained by the load resistance is:
(7)
The power dissipated by can be obtained by:
(8)
where denotes the current going through . Note that and
have a 90° phase difference. represents the power
dissipated by the series resistance due to the current . To
precisely model , the following piecewise model should be
used:
(9)
in which and
. is the
forward-bias depletion capacitance coefficient; is the
junction potential; and is the grading coefficient. The current
of the voltage-controlled capacitance first needs to be
obtained:
(10)
Fig. 7. Comparison of power conversion efficiency results calculated by
different methods. The calculation errors are the result differences between this
proposed method and the harmonic balance simulator. Comparison is based on
diode SMS7630 at 900 MHz with a loa
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Then, can be expressed as:
(11)
Finally, the power conversion efficiency of the Schottky diode-
based rectifier is calculated by:
(12)
Through this closed-form method, the dynamic range of power
conversion efficiency has been extended. Fig. 7 demonstrates
the comparison of results obtained by three methods. As a
commercial simulator, the harmonic balance method features
numerical calculation in hybrid frequency and time domains.
The low-power model is reported in [71], which is a
combination of different efficiencies containing the nonlinear
conversion efficiency of junction resistance in (2), parasitic
efficiency in (3), and dc source-to-load transfer efficiency in
(18). This model is a good approximation at low power levels
and represents a linear relationship (see Fig. 7) since it includes
only zero bias junction resistance and capacitance. The
comparison highlights the advantage of this proposed closed-
form method which enhances its efficiency prediction power
range. The errors in Fig. 7 are below 5% throughout the entire
power range until the breakdown point of the diode SMS7630.
Result errors of this method mainly come from the assumption
that only contains the fundamental signal and generated dc
component. Such a claim is valid in a low power range, as
higher harmonics are negligible. However, when input power
increases, higher harmonics start to play an increasingly
important role [61]. This will affect the accuracy of the majority
of closed-form methods in a relatively higher power range.
Apart from Schottky diodes, Fig. 8 includes the development
of diode rectification capabilities over the years, as the latest
update of our previous work [73]. A general way to obtain zero
bias current responsivity is through the manipulation of
diode IV curve:
(13)
of Schottky diodes has a simple form of ,
indicating that is restrained by its physical mechanism. The
limitation of is about 19.4 A/W at 300 K for Schottky
diodes, as shown in Fig. 8. However, Schottky diodes are still
the most reliable and cheapest option for rectifier design at this
moment, although their manufacturers reached this a
1900 1920 1940 1960 1980 2000 2020
10-3
10-2
10-1
100
101
102
10-3
10-2
10-1
100
101
102
Milestone : MgO Barrier
U Maryland
HSMS285X & 286X
Osaka U/AIST
AIST/JST/Osaka U/
Canon Anleva
EPM/Everspin/UofM
ETL/TIT
U Collorado (MIIM)
ARL/CSM/NREL
Motorola Labs/Phiar Corp (MIIM)
Bell labs
CV7180
Corborundum
Perikon
Galena-iron
Molybdenite-copper
Bell labs
Bell labs
U Penn
BAT63
Esaki VX6507
UND/HRL
Audion
Milestone : junction grown process
Milestone : Silicon purity
Milestone : point contact
Schottky diodes
Spin-diodes
MIM Diodes
Tunnel Diodes
Zero bias current responsivity (A/W)
Year
Thermal Voltage limitation
Planar epitaxial Schottky diodes
Point contact Schottky diodes
Tunnel diodes (Esaki diodes)
Metal-Insulator-Metal (MIM) diodes
Spin diodes (Magnetic tunnel junction)
Spin-torque effect diodes
(Magnetic tunnel junction at ferromagnetic resonance)
Thermal voltage limitation in Schottky diodes = 19.4 A/W
1MW RF broadcasted resonant cavity magnetron 1 computation per µJ
MIM diode operating at 148THz
Milestone : Molecular Beam Epitaxy technique
BAT15
DMK2790
SMS2876
MA4E1317
U Notre Dame
Fujitsu
(1)
(2)
(3) (4)
(3) HUST & Nantong U
(4) Martin Luther U & Max Planck Ints.
(2) Tohoku U
(1) NSRDEC/Mines/ARL/Brown U/NREL
1900 1920 1940 1960 1980 2000 2020
Fig. 8. Development of diode rectification capabilities to date.
load
receiving
antenna
diode
matching
network
RF
power
energy flow
antenna lossinsertion
loss
conversion
loss
parasitic
loss
harmonic loss
dc
transfer
loss
RF
input
power
dc
output
power
Fig. 9. Schematic diagram of a typical rectenna. Its loss mechanism is
presented below, corresponding to the diagram.
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long time ago. Schottky diodes can be engineered at the metal-
semiconductor junction to achieve a low forward turn-on
voltage [74]. Furthermore, their unipolar operation involves
only majority carriers, indicating that negligible storage of
excess minority carriers occurs. Such a feature is crucial to
obtain small capacitances to boost their operating frequency
further [75]. For example, zinc oxide Schottky diodes
exhibiting a maximum intrinsic cutoff frequency above 100
GHz are reported to be suitable for rectifier design [75]. Also,
subthreshold Schottky barrier-based thin-film transistors show
ultralow-power and high-gain features [76], [77]. Different
from Schottky diodes relying on thermionic emission, tunnel
diodes take advantage of quantum mechanical tunneling.
Hence, they do not have any limitations regarding . A
decade ago, of tunnel diodes exceeded 19.4 A/W and
recently reached 29 A/W, as presented in Fig. 8 [78-80]. This
slow-increasing trend will continue, as high-sensitivity tunnel
diodes are also of great interest for terahertz (THz) sensing and
imaging applications [81]. Another diode choice for low-power
rectification is the metal-insulator-metal (MIM) diode, widely
used in RF and THz applications. MIM diode is a combination
of two medals with one layer or a stack of thin insulators in
between. Due to their femtosecond switching time enabled by a
tunneling-based conduction mechanism, MIM diodes are often
used in THz rectifier design [82]. The resonant tunneling effect
is projected to offer a high and can be designed at the zero
bias point theoretically [83]. However, the development path of
MIM diodes in Fig. 8 indicates that they still face multiple
challenges in low-power rectification, such as structure design,
fabrication, and characterization [84-86]. For example, the
resonant tunneling of MIM diodes at room temperature and near
zero bias is still difficult to achieve at this moment, as this
sensitive effect requires material engineering and careful
property control [82]. It is expected that of MIM diodes
will increase continuously and still be favored for high
frequency rectifier design, in which the use of Schottky diodes
is limited. Spin diodes for low-power rectification are not
studied intensively, but their operation principle also anticipates
a high . A rectifier based on the spin-orbit torque and Hall
effect is theoretically projected to possess an extremely high
[87]. Based on the evolution trendof in Fig. 8 [88, 89],
spin diodes have the hope to break the thermal voltage of
Schottky diodes if they can be well-engineered to enhance their
zero bias nonlinearity. It is also noted that a sophisticated design
for an enhanced of nonlinear diodes leads to higher
sensitivity for ambient RF energy rectification but may
compromise its power handling capacity. For example,
backward tunnel diodes have outstanding performance for
rectifying weak RF signals, but they reach their breakdown
[90].
IV. RECTENNA DESIGN PRINCIPLES
Apart from a nonlinear element, the rectenna contains three
other parts, including a receiving antenna, a matching network,
and a load, as shown in Fig. 9. The efficiencies of the receiving
antenna, matching network, and dc source-to-load transfer
process are denoted as , , and , respectively. The
total efficiency can be seen as an efficiency chain of
each component in Fig. 9. Therefore, together with the
nonlinear conversion efficiency and parasitic efficiency ,
the total rectenna efficiency can be expressed as:
(14)
Corresponding to the efficiency chain in (14), the loss
mechanism of the rectenna is expressed as shown in Fig. 9. Note
that the harmonic loss or efficiency is not considered as it is
often negligible for low-power rectification. Furthermore, the
efficiency of each part is fairly orthogonalized except for those
dependent on the diode junction resistance. Hence, maximizing
the efficiency of each part can be an effective strategy to obtain
an enhanced total efficiency .
A. Antenna Efficiency
An antenna is a component used to collect propagating RF
energy in free space and forward them to a rectifying circuit. It
serves as an impedance transformer to overcome the impedance
discontinuity between free space and a
transmission line, which directly impacts the amount of RF
energy received by the rectenna. Maximizing antenna
efficiency means harnessing as much RF energy in free space
(a)
Antenna
arrays
Rectifiers Butler matrix
Motor
rotation
H plane
030
60
90
120
150
180
-150
-120
-90
-60
-30 A1
A2
A3
A4
A5
(b)
Fig. 10. (a) RF energy harvesting T-shirt with several printed rectenna arrays
[101]; (b) Rectenna able to collect RF power from all directions in free space
[110].
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as possible. The following measures can be taken in the antenna
design for rectennas:
• Multi-band and broadband antennas
As shown in the results of the dynamic RF energy
measurements (Fig. 3), RF energy exists across a wide
frequency band. Its relatively low power spectral density in
single bands makes multi-band or broadband antennas desirable
for ambient RF energy harvesting. The straightforward choice
to implement a multi-band antenna is to combine several single-
band antennas, featuring suppressed mutual coupling effect
[91]. Furthermore, typical multi-band antenna designs include
coplanar-waveguide monopoles [92], slot antennas [93] and
patch antennas [94]. For example, a triple-band multi-port
ambient RF energy harvester has been presented recently [95].
This high-gain multi-beam ambient RF energy harvester covers
the GSM1800, UMTS2100, and Wi-Fi bands, achieving
rectifying efficiencies of 25.3%, 27.9%, and 19.3% at an input
Another multi-band
ambient RF energy harvester based on a bow-tie antenna
covering four frequency bands at 840 MHz, 1.86, 2.1, and 2.45
GHz is reported in [96]. Four single ring-loop rectifiers are
To realize a broadband operation, spiral structures are
popular choices in rectenna design [97], [98]. A broadband
rectenna with dual circular polarization is reported to operate in
six bands from 0.55 to 2.45 GHz [99]. The receiving antenna is
a self-complementary log-periodic cross dipole that evolves
from a bow-tie-shaped cross dipole.
• Rectenna array technique
The receiving power of an antenna is directly associated
with its physical size. Thus, the array technique is an efficient
solution to acquire more RF power by increasing the effective
antenna aperture. A narrowband patch antenna array with dual
polarization is designed to harvest and monitor the operation of
a 1.96-GHz cellular base station in [100]. Each antenna element
of this 4 5 array collects RF energy separately, and the
rectified dc power is finally combined at the load. Recently,
quasi-self-complementary tightly coupled bow-tie arrays with
16 and 81 elements are -2
power densities [101]. They have been screen-printed on a T-
shirt for RF energy harvesting, as shown in Fig. 10(a). They
achieve a 5% to 10% efficiency for low incident power densities
and obtain 32% efficiency W/cm2, operating in a
frequency range of 2 to 5 GHz. Recent works also include a
low-profile lightweight rectenna array based on coplanar
waveguide [102], a rectenna array coupled with resonant micro-
converters [103], and a hybrid power combining rectenna array
[104]. Since a rectenna array inevitably occupies more space, it
is challenging to keep a highly compact design in a low-
frequency band. So, the array solution is often adopted for
rectenna design in millimeter-wave frequency and beyond
[105], [106].
• Optimal angular coverage
For ambient RF energy harvesting where the locations of
energy sources are unknown, omnidirectional antennas are
preferred to enhance angular coverage to maximize energy
reception in free space. For example, omnidirectional dipole
rectennas featuring quasi full spatial coverage are often used
[107]. While for beamforming directive wireless energy
delivery, high-gain antennas can be a perfect fit to enhance
rectifying efficiency. However, the broad angular coverage and
the high gain of a single-element antenna contradict each other.
Fig. 11. Photo of the 44 cross-dipole array for energy harvesting [125].
Direct matching between antenna elements and the rectifying circuit is
realized at the plane of the dipoles.
(a)
(b)
Fig. 12. (a) Backward tunnel diode-based rectifier showing superior
efficiency in low-power harvesting [90]; (b) Hybrid RF and solar energy
harvesting system through a transparent multiport antenna for indoor
applications [130].
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To overcome the low-efficiency issues due to omnidirectional
dipole antennas, a well-organized combination of multiple
high-gain antennas to target RF energy in each direction can be
an attractive solution. Such implementations have been
intensively reported recently [108-110]. Fig. 10(b) presents a
rectenna array that can harvest RF energy from all directions
[110]. 2, the state-of-the-
art result reports a rectifying efficiency of 42.5%, covering 66%
of all possible arrival angles of the incoming radiation. In
comparison, the proposed rectenna array can cover 97% of the
possible arrival angles at a rectifying efficiency of 45%.
• Multi-polarization
Antenna polarization mismatch between RF sources and
rectennas results in reduced RF power reception and thus less
output of rectennas. The best possible way to target ambient RF
sources whose polarization is unknown is to employ an all-
polarization technique to achieve better overall reception and
maximum portability [111]. For example, a rectenna that is able
to collect RF waves in all polarization senses simultaneously is
reported in [112]. It contains a dual circularly polarized patch
antenna and is fed by orthogonally polarized waves. By
applying multi-polarization, the complexity of the rectenna
often becomes higher. As for the beamforming directive
wireless power harvesting, where multiple moving rectennas
may exist, circular polarization is often introduced due to its
rotation independence between transmitter and receiver. Hence,
circular polarization is considered a better choice for
beamforming directive wireless power harvesting due to the
enhanced efficiency regardless of the antenna polarization
angles [111].
B. Matching Network Efficiency
A matching network placed between a receiving antenna and
a rectifying device accomplishes impedance transform to
maximize power transfer. Matching network design in
rectennas becomes challenging due to the wide variations of
diode input impedance, which is sensitive to both frequency and
input power. If the Q factor is introduced, the insertion loss of
the matching network () can be written as:
(15)
where is the required quality factor of the diode together
with the load. is the quality factor of the designed matching
network. can be obtained by:
(16)
in which and are reactance and resistance of the diode
together with the load, respectively. The diode presents a
dominant capacitance component [23], making large. In
other words, high-Q matching network design is required to
minimize its insertion loss, as is revealed by (15).
• High-Q matching network
Technical solutions for obtaining a high-Q matching network
include using high-Q lumped components for low-frequency
applications and substrate integrated waveguides for high-
frequency scenarios. Also, Π- and T-matching networks possess
a higher Q factor than conventional L-matching networks [113].
Furthermore, multi-stage matching networks can mitigate
design difficulties for achieving a higher Q factor. For example,
if a two-stage L-matching is employed, the required Q factor
reduces approximately by a square root factor to . Finally,
if stages are used, theoretically speaking, decreases to:
(17)
Since more stages of the matching network inevitably
introduce more conductor loss and dielectric loss, a low-loss
substrate is desired for achieving low insertion loss. Such a
multi-stage matching network also broadens the operating
bandwidth and makes the fabrication process less susceptible,
though at the cost of a larger form factor.
• Self-tunable matching networks
To overcome the impedance variations due to the change of
diode operation conditions, a tunable matching network can be
employed [114], [115]. Hence, it is possible to maintain a low
in matching networks in a wide dynamic range of operating
frequency and input power. For example, an ultra-wideband
rectenna achieves an operation frequency range from 0.9 to 3
GHz through a complementary matching stub [116]. The
(a)
(b)
Fig. 13. (a) Battery-free communication platform powered by outdoor
ambient LTE signals [92]; (b) Wi-Fi backscatter tag utilizing ambient RF
energy harvesting for signal modulation and backscattering [161].
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tunable feature is realized by an LC series-parallel resonant
circuit embedded into a traditional L-shaped matching stub.
Since such a matching stub can exhibit open and short circuits
as a function of frequency, the nonlinear input impedance of the
rectifier can be tuned to conjugately match the antenna over a
wide frequency range.
• Resistance compression networks
Resistance compression networks were first proposed to
substantially decrease the variation in effective resistance of a
tuned RF inverter with changing load conditions [117]. This
technique is also suitable for tackling the sensitivity and
nonlinearity of rectifiers, reported in [118],[119]. Efforts to
further extend its operation bandwidth have been made using
the hybrid resistance compression technique [120].
• Antenna and matching network co-design
Compared to the multi-band or broadband antenna design,
bandwidth enhancement of matching networks is more
challenging. The typical way to achieve multi-band or
broadband rectenna design is to use multiple matching
networks, each covering a certain bandwidth. In this case, the
antenna design can be a single broadband antenna [121] or
multiple mutually coupled ones [122]. Another solution is to
design the receiving antenna to conjugately match the diode
directly, which can remove the matching network [123],[124].
As the diode mainly shows the capacitive component, designing
an inductive antenna can match
conjugately. In [125], a 44 cross dipole array is developed to
directly match the rectifying circuit at the dipole plane, as
shown in Fig. 11. Experiments show that this design has
enhanced the available power for each diode and thereby
increased the efficiency.
C. Diode Conversion Efficiency
Based on the discussion in the last section, especially the
explicit expression in (2), multiple measures can be introduced
to leverage diode conversion efficiency.
• Devices with stronger nonlinearity
The current responsivity is proportional to the diode
conversion efficiency in the low-power rectification. Thus,
building rectennas on a device with stronger nonlinearity can
effectively leverage . As shown in Fig. 8, recent progress in
semiconductor technology to enhance diode rectification
capabilities is encouraging. For example, Fig. 12(a)
demonstrates one ambient RF power harvester based on a
heterojunction backward tunnel diode, which breaks the
efficiency barrier of the Schottky diodes relying on thermionic
emission [90]. Recently, a nanoscale rectifier concept,
simultaneously utilizing the Hall effect and the spin-orbit
torque, shows great potential in extremely low RF power
harvesting, thus outperforming tunnel diodes [87].
Furthermore, devices based on graphene have shown great
potential to offer strong nonlinearity and high sensitivity across
a wide frequency range [126-128]. However, such nonlinear
devices usually require sophisticated fabrication and sometimes
only exist as experimental prototypes in the laboratory.
• Hybrid power harvesting
The advantage of increasing input power is evident, which
can simply lead to larger dc output power. However, another
implicit benefit revealed by (2) is that higher RF input power
also leverages . Thus, a linear increase of RF input power
brings enhanced dc output by a square factor [129]. To find
more available power, one solution is to add other ambient
energy sources in our environment, such as solar [130-133],
thermal [124, 134, 135], vibration energy [136], [137], etc. One
example showing an ambient power harvester with a single
diode is developed to scavenge both RF and thermal energy
cooperatively [124]. When both RF and thermal power levels
of about
33.4%. A hybrid RF and solar energy harvester is proposed in
[130], as shown in Fig. 12(b). It utilizes a transparent micro
meshed antenna on the top surface of a solar cell with rectifiers
positioned underneath. With the light illumination of 360 lux,
the solar cell obtains 1.68-mW power, adding an extra
of power to the rectenna when RF input power
density varies from 13.30 to 52.96 mW/m2. Furthermore, one
Front Back
(a)
Office
card-type sensor wearable antenna
real time
high eff. RF-dc conversion
control circuit of
2nd battery
BLE wireless comm.
Tx ant.
(b)
Fig. 14. (a) Battery-free cellphone prototype powered by directive RF
power harvesting from a base station 9.4 meters away [165]; (b) Wireless-
powered wearable vital sensor with a size similar to a credit card and its
experiment in a town hall [166].
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work capable of collecting three weak power sources of solar,
RF, and vibration energy is presented to achieve an efficiency
> 80% over a wide input power range [138]. Different power
combination techniques need to be carefully selected to
maximize the dc output [139]. Hybrid RF and vibration energy
harvesting is recommended to combine ac power into diodes for
exacerbating nonlinear rectification. Such a solution is similar
to RF combining strategy based on multiple antennas, which
gains a higher efficiency due to more power fed to one single
rectifier [30]. In contrast, a dc combining strategy is preferred
for hybrid RF and thermal/solar harvesting since thermal/solar
energy conversion already outputs dc energy. One of the key
components for this design is dc combining circuits that can add
up dc outputs from both RF and thermal/solar harvesting units
[140], [141]. Special attention needs to be given to the optimum
load [100], connection scheme of unequal rectenna elements
[142], and loading effects of thermal/solar units on the
matching condition of dc combining circuits [132], which may
degrade the conversion efficiency of hybrid harvesting.
• Optimum operating temperature
For Schottky diodes that are dependent on thermionic
emission, a lower operating temperature boosts and further
helps to leverage , as revealed by (2). For example, the
maximum of Schottky diodes increases from 19.4 A/W at
the of heterojunction backward tunnel diode in [90].
However, temperature variations have an influence on diode
behaviors, mainly nonlinear junction resistance, capacitance,
and potential voltage [143]. Under a low-temperature operation
condition, increases, but and decreases. Therefore, an
optimum operating temperature exits at each input power level
[18]. Thus, the thermal factor should be taken into consideration
for maintaining good performance in a practical situation.
• Waveform manipulation
The nonlinear junction resistance of Schottky diodes can be
characterized by an exponential I-V relationship in (1). Thus,
some specially designed signal waveforms can result in higher
. Waveform manipulation, which helps increase rectifying
efficiency, has been reported in multiple studies [144-146].
Early work in 2004 shows that the simultaneous incidence of
two independent RF sources combined nonlinearly would
produce more rectified power than the sum of single power
incidence [97]. Later, experiments prove that QPSK modulated
signals can achieve higher rectifying efficiency than single tone
signals [147]. Chaotic waveforms also improve wireless power
transmission efficiency [148]. In [149], a theoretical model
shows that high peak-to-average power ratio (PAPR) signals
increase the output power at low input powers but decrease the
maximum rectifying efficiency at high input powers. Similar
findings regarding the high PAPR waveform improving low-
are reported in [148], [150].
A further design and optimization method of multisine
waveforms for WPT is reported in [151]. With the popular
trend of developing simultaneous wireless information and
power transfer (SWIPT) systems, the significance of
accounting for the nonlinearity of rectifiers is highlighted for
waveform design and optimization in [152].
D. dc Source-to-Load Transfer Efficiency
(18)
For quantifying the ratio of dc power reaching the load over
the RF input power absorbed by junction resistance , (2) can
be combined with (18). Thus, one can write it as:
(19)
The optimal load leading to the maximum value of
in (19) can be calculated by setting the derivation of
(19) to be 0. After some mathematical treatment, can be
obtained as [25]:
It should be noted that this optimal load value is only valid
The
Maximum Power Point Tracking (MPPT) technique is widely
used to ensure that an optimal load is connected to the rectennas
[15]. For instance, a novel fractional open-circuit voltage
approximation method is proposed to obtain the optimal load in
[153]. The reported rectenna shows measured MPPT accuracy
Another work with a larger
using an MPPT method is presented in [154]. A measured peak
rectifying efficiency reaches 48.19% with respect to the input
power of 0 dBm at 900 MHz.
V. EMERGING APPLICATIONS
The far-field wireless power harvesting technique offers new
opportunities for battery-free devices and systems. According
to the RF energy sources and available dc output power, far-
field wireless power harvesting can mainly be divided into two
groups: ambient RF power harvesting and directive RF power
transfer. Ambient RF power in free space is pervasive but weak
and often unpredictable. Thus, ambient RF power harvesting is
suitable for energizing devices and systems featuring low-
power and low-duty-cycle operations. Fig. 13(a) shows a
battery-free communications setup with TI eZ430-RF2500
sensors powered by outdoor LTE signals [60]. Such a
demonstration was conducted on multiple locations in Boston
with an efficiency of up to 45% in LTE700, GSM850, and
GSM900 bands.
to operate in active, standby, and low power modes at an RF
input Similar
ambient RF-powered sensors are reported extensively in the
literature [155-158]. Sensors consume different amounts of
current while carrying out various tasks. Hence, they can be
seen as a dynamic load. So, an energy-aware interface between
sensors and the power management module of rectennas is
highly recommended for the efficient operation of ambient RF-
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powered sensors. Such an energy-aware interface ensures
maximum power transfer between energy demanded (sensors)
and energy harvested (storage capacitor and rectenna) under
different loading conditions through adaptive approaches [159],
[160]. Using energy storage elements (a supercapacitor or a
battery) in self-powered sensors is to achieve long-term and
reliable operation. Another popular application is ambient RF
energy harvesting enabled backscattering. Fig. 13(b)
demonstrates a Wi-Fi backscatter, which is an RF-powered
device able to communicate with a Wi-Fi-enabled mobile
device [161]. The Wi-Fi backscatter tag communicates by
modulating the signals from the Wi-Fi helper. And the Wi-Fi
reader decodes the signals through channel changes that are
created on the received Wi-Fi packets. The necessary energy
consumption of Wi-Fi backscatter tags comes from its RF
energy harvesting module. Fig. 13(b) has presented one
prototype antenna that works across the 2.4-GHz Wi-Fi
channels for both signal modulation and energy harvesting.
Later, such ambient RF energy harvesting enabled
backscattering applications have been developed for low-power
intelligent agriculture [162], LoRa communication [163], and
smart fabrics [164].
Directive RF power transfer is introduced to offer relatively
higher output power with more flexibility. For example, Fig.
14(a) shows a battery-free cellphone prototype operated on the
power that is harvested from RF signals transmitted by a base
station 9.4 m away [165]. This cellphone prototype has been
optimized with a power consumption of only a few micro-watts.
It can support the real-time sense of speech, actuation of
earphones, and switch between uplink and downlink
communications. Another application of a wirelessly powered
wearable vital sensor system is presented in Fig. 14(b) [166].
The transmitter operates at 927 MHz and outputs 1-W power in
consideration with commercial application. Under local
regulation and in a permitted area for scientific studies, the
same measurement was conducted with 5-W transmitting
power to drive various vital sensors. The card-type sensor
contains a wearable antenna, an energy harvester, a control
circuit, and Bluetooth communication modules. The directive
RF power transfer technique keeps the battery voltage of
sensors the same, and theoretically, sensors can work forever.
Furthermore, a commercial product offering 5-W remote
charging for smartphones is about to roll out soon [167]. The
charger possesses space positioning and energy transmission
features, which means it can track the location of smartphones
and deliver energy precisely. The transmitter and receiver
(smartphone) have 144- and 14-element antenna arrays,
respectively. Localization communication and directive energy
transmission are realized in the millimeter-wave band, with a
working distance of several meters.
VI. CONCLUSION
This article begins with the discussion of dynamic ambient
RF power density measurements in Montreal to better
understand how much ambient RF energy exists in free space.
It has been shown that rectenna design is systematic work, of
which nonlinear diode modeling is at the core since frequency
conversion relies solely on it. Although other diode candidates
are not limited by the thermal voltage limitation like Schottky
diodes, they will not shortly replace Schottky diodes in practical
applications, considering the low cost, high reliability, and
mature manufacturing technology of Schottky diodes.
Efficiency chain analysis enables us to break down the rectenna
design and find efficient solutions to enhance the efficiency of
each part. Furthermore, overall optimization of rectenna still
requires the coordinated design of receiving antenna and
matching network according to the diode selection and loading
condition. Currently, far-field wireless power harvesting
supports two different real-world application scenarios.
Ambient RF energy harvesting is suitable for low-power and
low-duty-cycle IoT sensing and communication applications.
Such battery-free and maintenance-free applications are
effective supplements for sustainable smart cities and similar
applications. Directive RF energy transfer based on
beamforming technology offers more flexibility and
opportunities to support emerging wearable electronics, smart
homes, and remote healthcare monitoring applications.
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Xiaoqiang Gu (Member, IEEE) was born in
Changzhou, China, in 1990. He received the B.Eng.
degree in Electrical Engineering from the University
of Electronic Science and Technology of China,
Chengdu, China, in 2012, the M.Eng. degree in
Optical Engineering from Zhejiang University,
Hangzhou, China, in 2015, and the Ph.D. degree in
Electrical Engineering from the École
Polytechnique de Montréal, Montréal, QC, Canada,
in 2020. He is currently a Mitacs Accelerate
Industrial Postdoc Fellow with the École Polytechnique de Montréal.
His current research interests include self-sustainable IoT circuits and
systems, wireless power harvesting, backscattering, and nonlinear circuit
analysis and design.
Dr. Gu was a recipient of the FRQNT postdoctoral research fellowship in
2021, the URSI GASS Young Scientist in 2020, the IEEE MTT-S Graduate
Fellowship Award in 2019, the STARaCom Scholarship in 2020, and the Best
Student Paper Award of the IEEE MTT-S Wireless Power Transfer Conference
in 2017. He also received the Student Grant of the European Microwave Week
in 2019, the Travel Grant Award of the IEEE MTT-S International Microwave
Workshop Series on 5G Hardware and System Technologies in 2018. He was
among the Best Student Paper Finalists at the IEEE MTT-S Wireless Power
Transfer Conference in 2018.
Simon Hemour (Senior Member, IEEE) received
the B.S. degree in electrical engineering from the
University of Grenoble Alpes, Grenoble, France, in
2004, and the M.S. and Ph.D. degrees in optics,
optoelectronics, and microwave engineering from the
Grenoble Institute of Technology, Grenoble, in 2006
and 2010, respectively.
In 2003, he was with the European Organization
for Nuclear Research, Geneva, Switzerland, as a
member of the Instrumentation Department, where
he was involved in ATLAS experiment on the Large Hadron Collider. From
2006 to 2007, he was a Research Assistant with the Pidstryhach Institute of
Applied Problems of Mechanics and Mathematics, National Academy of
Science of Ukraine, Lviv, Ukraine. In 2007, he joined the IMEP-LAHC
MINATEC Laboratory, Grenoble. From 2011 to 2015, he was with the Poly-
Grames Research Center, École Polytechnique de Montréal, Montreal, QC,
Canada, where he was coordinating the Wireless Power Transmission and
Harvesting Research Group. He joined the Université de Bordeaux, Bordeaux,
France, in 2015, where he is currently an Associate Professor, and he leads
research in wireless micro energy solutions for IoT and biomedical
applications. His current research interests include wireless power transfer and
hybrid energy harvesting, backscattering, nonlinear and negative resistance
devices, innovative RF measurements, RF interferometry, low power
microwave, and millimeter wave conversion circuits, development of RF
transponders and sensors for wireless systems and biomedical applications.
Dr. Hemour is a member of the IEEE MTT-25 Wireless Energy Transfer
and Conversion Technical Committee and the IEEE MTT-28 Biological Effect
and Medical Application Technical Committee. He has been invited to give
many invited talks and plenary speeches at various international meetings,
conferences and forums, and his pioneering work on low power RF energy
harvesting has been highly cited. He was the TPC Chair of the 2018 Wireless
Power Transfer Conference (WPTC) and will be the General Chair of the 2022
WPTC. He is part of the Editorial Board of the Wireless Power Transfer Journal
(Cambridge University Press). He has served as Guest Editor for the IEEE
TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES and
the IEEE Journal of Electromagnetics, RF and Microwaves in Medicine and
Biology.
Ke Wu (Fellow, IEEE) received the B.Sc. (Hons.)
degree in radio engineering from Southeast
University, Nanjing, China, in 1982, the D.E.A.
(Hons.) and Ph.D. (Hons.) degrees in optics,
optoelectronics, and microwave engineering from the
Institut National Polytechnique de Grenoble (INPG)
and the University of Grenoble, Grenoble, France in
1984 and 1987, respectively.
He was the Founding Director of the Center for
Radiofrequency Electronics Research of Quebec
(Regroupement stratgique of FRQNT), Montreal, QC, Canada. He has held
guest, visiting, and honorary professorships with many universities around the
world. He is currently a professor of electrical engineering with Polytechnique
Montréal (University of Montreal), Montreal, QC, where he is the Director of
the Poly-Grames Research Center. He has authored or coauthored more than
1300 referred articles and a number of books/book chapters. He has filed more
than 50 patents. His current research interests include substrate integrated
circuits and systems, antenna arrays, field theory and joint field/circuit
modeling, ultra-fast interconnects, wireless power transmission and harvesting,
MHz-through-THz technologies and transceivers for wireless sensors and
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systems as well as biomedical applications, and the modeling and design of
microwave and terahertz photonic circuits and systems.
Dr. Wu is a Fellow of the Canadian Academy of Engineering (CAE) and the
Royal Society of Canada (The Canadian Academy of the Sciences and
Humanities). He is a member of the Electromagnetics Academy, Sigma Xi,
URSI, and IEEE-Eta Kappa Nu (IEEE-HKN). He was a recipient of many
awards and prizes, including the First IEEE MTT-S Outstanding Young
Engineer Award, the 2004 Fessenden Medal of the IEEE Canada, the 2009
Thomas W. Eadie Medal of the Royal Society of Canada, Queen Elizabeth II
Diamond Jubilee Medal in 2013, the 2013 FCCP Education Foundation Award
of Merit, the 2014 IEEE MTT-S Microwave Application Award, the 2014
Marie-Victorin Prize (Prix du Québec - the highest distinction of Quebec in the
natural sc
Innovation of Polytechnique Montreal, the 2015 IEEE Montreal Section Gold
Medal of Achievement, and the 2019 IEEE MTT-S Microwave Prize. He was
the Tier-I Canada Research Chair of RF and millimeter-wave engineering and
the Industrial Research Chair in Future Wireless Technologies with the
Polytechnique Montréal (University of Montreal), Montreal, QC. He has held
key positions in and has served on various panels and international committees,
including the Chair of technical program committees, international steering
committees, and international conferences/symposia. In particular, he was the
General Chair of the 2012 IEEE Microwave Theory and Techniques Society
(IEEE MTT-S) International Microwave Symposium (IMS). He was on the
Editorial/Review Boards for many technical journals, transactions,
proceedings, and letters as well as scientific encyclopedia, including Editor and
Guest Editor. He was the Chair of the joint IEEE Montreal chapters of MTT-
S/APS/LEOS and then the restructured IEEE MTT-S Montreal Chapter,
Canada. He was with the IEEE MTT-S and Administrative Committee
(AdCom) as the Chair for the IEEE MTT-S Transnational Committee, the
Member and Geographic Activities (MGA) Committee, the Technical
Coordinating Committee (TCC), and the 2016 IEEE MTT-S President among
many other AdCom functions. He is currently the Chair of the IEEE MTT-S
Inter-Society Committee. He was a Distinguished Microwave Lecturer of IEEE
MTT-S from 2009 to 2011. He was the Inaugural Representative of North
America as a member of the European Microwave Association (EuMA)
General Assembly.
