Feasibility of Ambient RF Energy Harvesting for Self-Sustainable M2M Communications Using Transparent and Flexible Graphene Antennas

Abstract

Lifetime is a critical parameter in ubiquitous, battery-operated sensors for machine-to-machine (M2M) communication systems, an emerging part of the future Internet of Things. In this practical article, the performance of radio frequency (RF) to DC energy converters using transparent and flexible rectennas based on graphene in an ambient RF energyharvesting scenario is evaluated. Full-wave EM simulations of a dipole antenna assuming the reported state-of-the-art sheet resistance for few-layer, transparent graphene yields an estimated ohmic efficiency of 5 %. In the power budget calculation, the low efficiency of transparent graphene antennas is an issue because of the relatively low amount of available ambient RF energy in the frequency bands of interest, which together sets an upper limit on the harvested energy available for the RF-powered device. Using a commercial diode rectifier and an off-the-shelf wireless system for sensor communication, the graphene-based solution provides only a limited battery lifetime extension. However, for ultra-low-power technologies currently at the research stage, more advantageous ambient energy levels, or other use cases with infrequent data transmission, graphene-based solutions may be more feasible.

Full text

Received August 12, 2016, accepted August 25, 2016, date of publication August 29, 2016, date of current version October 6, 2016.
Digital Object Identifier 10.1109/ACCESS.2016.2604078
Feasibility of Ambient RF Energy Harvesting for
Self-Sustainable M2M Communications Using
Transparent and Flexible Graphene Antennas
MICHAEL A. ANDERSSON1, (Student Member, IEEE), AYÇA ÖZÇELIKKALE2, (Member, IEEE),
MARTIN JOHANSSON3, (Senior Member, IEEE), ULRIKA ENGSTRÖM3, ANDREI VOROBIEV1,
AND JAN STAKE1, (Senior Member, IEEE)
1Terahertz and Millimetre Wave Laboratory, Department of Microtechnology and Nanoscience, Chalmers University of Technology, 412 96 Gothenburg, Sweden
2Department of Signals and Systems, Chalmers University of Technology, 412 96 Gothenburg, Sweden
3Ericsson Research, Ericsson AB, 417 56 Gothenburg, Sweden
Corresponding author: M. A. Andersson (andmic@chalmers.se)
This work was supported by VINNOVA through the SIO Grafen Program under Grant 2015-01439. The work of A. Özçelikkale was
supported by the EU Marie Sklodowska-Curie Fellowship.
ABSTRACT Lifetime is a critical parameter in ubiquitous, battery-operated sensors for machine-to-
machine (M2M) communication systems, an emerging part of the future Internet of Things. In this paper,
the performance of radio frequency (RF) to DC energy converters using transparent and flexible rectennas
based on graphene in an ambient RF energy-harvesting scenario is evaluated. Full-wave electromagnetic
(EM) simulations of a dipole antenna assuming the reported state-of-the-art sheet resistance for few-layer,
transparent graphene yields an estimated ohmic efficiency of 5%. In the power budget calculation, the low
efficiency of transparent graphene antennas is an issue because of the relatively low amount of available
ambient RF energy in the frequency bands of interest, which together sets an upper limit on the harvested
energy available for the RF-powered device. Using a commercial diode rectifier and an off-the-shelf wireless
system for sensor communication, the graphene-based solution provides only a limited battery lifetime
extension. However, for ultra-low-power technologies currently at the research stage, more advantageous
ambient energy levels, or other use cases with infrequent data transmission, graphene-based solutions may
be more feasible.
INDEX TERMS Energy harvesting, flexible electronics, graphene, machine-to-machine communications,
rectennas.
I. INTRODUCTION
In the future, it is envisaged that all devices that benefit from
an Internet connection will be interconnected. In such a net-
worked society, every citizen and company will be empow-
ered to reach their full potential. Internet of Things (IoT)
technology is a key enabler of this vision by delivering, for
example, machine-to-machine (M2M) communication on a
massive scale [1]. Information exchange between machines
is already an important component in automated production,
but in the future, areas such as smart energy distribution [2],
health care [3] and smart cities [4] will also rely on Internet
access. The market is now expanding toward both massive
IoT deployment and more advanced solutions that may be
categorized as critical IoT, as shown in Fig. 1. In massive
IoT applications, such as sensors that report wirelessly to
the cloud on a regular basis, the price of the communication
FIGURE 1. Application examples and requirements, massive vs. critical
IoT [1].
link must be sufficiently low for the business case to make
sense. In particular, the cost of the power supply is a great
challenge in many wireless M2M applications, and there is
an increasing demand for systems that are self-sustainable
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M. A. Andersson et al.: Feasibility of Ambient RF Energy Harvesting for Self-Sustainable M2M Communications
or that offer extended battery lifetimes through the harvest-
ing of energy [5]. Radio frequency (RF) signals represent
an increasingly available and omnipresent energy source in
urban environments [6]. Notably, the antenna and circuitry for
recycling this ambient RF power can be made an integrated
part of the M2M communication electronics.
However, there is a need for low-cost and eco-friendly
antenna materials that are flexible and/or optically transparent
to fully embody the ubiquitous potential of IoT [7], [8].
Flexible wireless sensors could be integrated as wearables
in clothing or directly on the human body to monitor, for
instance, the temperature, blood pressure and heart rate of
patients on a continuous basis [3]. Arrays of RF energy har-
vesters that utilize transparent antennas, complemented with
solar cells and applied on window glass, allow for energy-
efficient day-and-night control of lighting and heating in
buildings [4].
Graphene has a clear cost benefit due to the accessibility of
carbon and is an eco-friendly alternative, particularly when
applied on substrates such as paper [9] and textiles [10]. The
potential use of recyclable substrates is important given the
vast number of M2M nodes. Moreover, few-layer graphene is
both flexible and offers a high degree of transparency given
its conductivity [11], [12]. Together, these properties make
graphene well suited given the requirements on the diversity
and cost of an RF energy-harvesting antenna for M2M sys-
tems. Conversely, the demands are currently inaccessible
using transparent antennas from indium tin oxide (ITO) [13],
AgHT [14] or metal grids [15], which are brittle and expen-
sive. Furthermore, graphene is a competitive option for pas-
sive and active electronics on flexible surfaces where oxide
semiconductors, compound semiconductors and silicon all
exhibit poor performance [16]. Rather, the main competitors
for flexible and opaque antennas are additively printed metal-
particle [17] and carbon-based inks [18]. Nevertheless, an
all-carbon system could benefit from the integration of the
antenna with graphene transistor rectifiers and the inherent
suitability of graphene for sensing [19] and energy stor-
age [20]. In addition, the field-effect allows tuning of the
sheet resistance if a back-gate is available [11]. Consequently,
a carbon-based system is a potentially low-cost and sustain-
able technology for diversified M2M systems.
In this practical article, we evaluate the feasibility of ambi-
ent RF energy harvesting in the common cellular communica-
tion frequency bands using a graphene antenna to improve the
battery lifetime of sensors in massive IoT applications. In our
envisioned scenario, invisible (to the human eye) graphene
antennas can be applied on both flexible and rigid surfaces,
e.g., paper [9], textiles [10] and plastics [21], for wear-
ables and window glass. We estimate the energy harvested
from ambient RF sources given a simulated state-of-the-art
graphene antenna, which is truly transparent in contrast to
reported simulations using copper ground planes [22], [23].
This is then converted to battery lifetime extensions, given
the power consumption for different wireless transmission
schemes, but excluding the details of the communication
channel [24]. In addition, the power management unit (PMU)
and the energy storage [25] in the harvester circuit are also not
addressed.
II. METHODS
This section describes the assumptions used in the power bud-
get calculations. State-of-the-art reported available ambient
RF power and graphene conductivity versus transparency are
reviewed. Furthermore, the antenna simulations are detailed.
Finally, the rectifier and transceiver parameters are discussed.
TABLE 1. Average and maximum ambient RF energy from base stations
and WiFi Hotspots (London underground stations, 2012) [26].
A. AMBIENT RF ENERGY SOURCES
The cellular communication frequency bands provide a
potentially omnipresent supply of ambient energy in urban
scenarios. Here, the bands for GSM, 3G, LTE, and WiFi with
frequencies between 900 MHz and 2.6 GHz are considered
because these bands have the greatest amount of ambient
energy. Table 1 summarizes the measured overall average and
maximum intensity per band, SBA, from base stations and
WiFi hotspots at all underground stations in both suburban
and urban areas in London, measured in the year 2012 using
an omnidirectional antenna [26]. For the first three commu-
nication standards, only the down-link transmission bands
are taken into account because these bands offer a relatively
steady supply of energy over time. Note the close to one
order of magnitude difference in the average and maximum
values acquired. Although this implies that better energy-
harvesting conditions could be obtained using dedicated
RF sources [27], the latter are not considered in this paper.
To be sure to cover the GSM1800 band with the highest
energy level, a center frequency of 2 GHz is selected for the
antenna design.
For comparison, a similar amount of power available
from incoherent blackbody radiation poses several difficulties
from a system perspective. In addition to the availability of
a>300 ◦C object [28], the radiation to be collected and
rectified is predominately emitted at frequencies above one
terahertz.
B. GRAPHENE CONDUCTIVITY VERSUS TRANSPARENCY
Assuming applications in which a transparency of >80% is
required, the number of graphene layers that can be used in
an antenna is limited to <10 [29]. Consequently, this sets
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M. A. Andersson et al.: Feasibility of Ambient RF Energy Harvesting for Self-Sustainable M2M Communications
FIGURE 2. The DC sheet resistance versus number of layers for few-layer
graphene grown by chemical vapor deposition on Cu and Cu-Ni with and
without surface treatments to enhance conductivity [30]–[32].
a limit on the achievable sheet resistance (which is sim-
ply the inverse of the conductivity for a two-dimensional
material). The state-of-the-art DC conductivities for large-
area graphene grown by chemical vapor deposition (CVD)
on a metal catalyst and transferred to insulating substrates
are summarized in Fig. 2. Post-transfer surface treatment is
applied to maximize the carrier concentration and minimize
the sheet resistance. The CVD graphene is selected because it
provides the best transparency for a certain sheet resistance,
compared to ink-jet printed graphene for example [33], and
it can be transferred to an arbitrary substrate. However, a
slightly higher sheet resistance in the case of flexible plas-
tics [30] and textile surfaces [10] have recently been reported.
Because the CVD growth is self-limited to few layers, layer-
by-layer transfer is required to reduce the sheet resistance.
The accumulation of defects is responsible for the saturating
improvement in the sheet resistance with the number of lay-
ers. The overall minimum value reported for transparent and
flexible CVD-grown graphene in the literature is 20 /sq for
a 4-layer, FeCl3-intercalated graphene film [31]. In contrast
to chemically surface-doped graphene, this intercalation has
proven to be stable over long periods of time [32]. This is
considerably better than the ultrathin (∼5 nm) gold films that
result in sheet resistance values ∼100 /sq combined with
a lower transmittance than graphene of 75% [34] (cf. at the
skin depth thickness of 2.5 µm, gold has only 0.01 /sq
at 2 GHz). However, micron-thick sputtered ITO films yield
<5/sq [13], and etched metal grids yield only <0.1 /sq,
both with 80% transmittance.
C. GRAPHENE DIPOLE ANTENNA SIMULATION
For simplicity, a linearly polarized planar on-wafer dipole
antenna [35], [36] is used to investigate the influence of
graphene conductivity on antenna performance. The antenna
is simulated in Ansys HFSS as an ohmic sheet with a variable
sheet resistance (i.e., not material specific to graphene) sup-
ported by a silicon wafer, as illustrated in Fig. 3. Theory [22]
FIGURE 3. The surface currents on dipole antenna elements with the
colored scale are the same in all three cases (from left to right) for a
perfect electrical conductor (PEC), Rsh =10 /sq and Rsh =50 /sq,
respectively.
and experiment [37]–[39] both agree that at microwave fre-
quencies the resistive part of the graphene impedance equals
the DC value and that the imaginary part is negligible. The
wafer is thin, and the antenna pattern (omnidirectional around
the antenna axis) is very close to that of a free space dipole.
Upon varying the sheet resistance, the radiation pattern does
not change its shape, only the absolute levels. It is illustrative
to inspect the surface currents in Fig. 3 for the antenna to
observe how drastically the behavior changes, with only a
moderate decrease in the conductivity. As a consequence of
the large ohmic loss, a higher gain or a more broadband
antenna (from a perfect electric conductor (PEC) perspective)
is not considered as the improvement in system efficiency
would be insignificant [35].
We assume that the antenna can be matched to a diode
rectifier with an impedance in the low krange for all
considered sheet resistances given that Rant <1k. The
radiation efficiency (due solely to ohmic losses in the sheet)
is used as a benchmark parameter, and it is found that a
sheet resistance <40 /sq is required for an efficiency >1%.
The full dependence is shown in Fig. 4. The state-of-the-
art graphene provides 5% efficiency for the dipole antenna,
which is comparable to the number in [22] for a graphene
patch antenna. To further confirm the trend from the simula-
tion, an experimental point (black cross) is inserted in Fig. 4
from a printed non-transparent graphite dipole antenna [18].
FIGURE 4. Antenna radiation efficiency at 2 GHz versus graphene sheet
resistance. The black cross shows an experimental confirmation of the
simulated trend from a non-transparent graphite dipole antenna of [18].
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D. PERFORMANCE OF RF-TO-DC DIODE RECTIFIERS
The estimated RF power received at the antenna terminals is
the product of the assumed input ambient RF energy and the
simulated antenna gain or effective area as [35]
Prec =Aeff ·SBA =G·λ2
4π·SBA,(1)
where λis the wavelength and Gis the antenna gain. Because
the antenna orientation is arbitrary relative to the incoming
radiation, the average gain over all pointing directions must
be used. This essentially corresponds to an omnidirectional
antenna with the same ohmic efficiency, which is required
to match the antenna used in collecting the ambient inten-
sity data [26]. To account for polarization in a fading urban
environment, two perpendicularly polarized dipoles can be
employed [40].
The resulting output DC power is given proportionally
from the input received RF power by the rectifier effi-
ciency, ηr, from PDC =ηr·Prec. Considering the low
graphene sheet resistance required and the wide metal-
graphene contact periphery to the antenna, we regard the
influence of the metal-graphene interface to be negligible.
The rectifier efficiency is linearly dependent on the RF input
power. Considering a single diode (or diode connected field
effect transistor), it is [41]
ηr=R2
I
Precη2
mRj
41+ω2C2
jRjRs'R2
I
Precη2
mRj
4
=R2
V
Precη2
m
4Rj
.(2)
In the above expression, RIand RVare the (short circuit)
current and (open circuit) voltage responsivity, respectively.
Furthermore, ηmis the antenna to rectifier impedance match-
ing coefficient, Rjis the junction or differential channel
resistance (assumed to be equal to the load resistance RL),
ω=2πfis the frequency, and Rsis the parasitic series
resistance. This expression is valid under the condition
that RsRj. A comparison of the performance between a
graphene transistor, which would allow for an all-graphene
integrated system, and commercial Schottky diodes is pre-
sented in Fig. 5 based on (2). Schottky diodes are funda-
mentally limited to RI=19.4 A/W (which is closely
achieved in practice). The small nonlinearity of the current
in a graphene FET (GFET), however, makes them practically
limited to RI∼1 A/W [42], [43]. State-of-the-art for all
solid-state detectors in Fig. 5 are the Sb backward diodes with
RI'23.5 A/W [44], [45]. Deviations from (2) occur at
higher input power levels where the small-signal approxima-
tion is not valid, as indicated by the solid line in Fig. 5.
In the following, we consider diode-based rectifiers
because their efficiency is up to two orders of magnitude
higher than that for current CVD GFETs. Actual reported
diode rectifiers realized with RF energy harvesting in mind
provide efficiencies of 40%, 20% and 5% at 100 µW,
10 µW and 1 µW of received power at the matched antenna
FIGURE 5. Rectifier efficiency, Schottky diodes are limited to
RI=19.4 A/W, Sb diodes to RI=23.5 A/W and reported
GFETs have RI∼1 A/W.
terminals [46], respectively, corresponding well with the high
input power in Fig. 5.
E. POWER CONSUMPTION FOR THE
TRANSCEIVER AT THE SENSOR
The above subsections provided one end of the power-budget
calculation. Now the other end is considered: the DC power
consumption by the sensor operations for which the harvested
power will be used. Due to the high energy cost of operating
a transceiver circuit, we focus on the energy expenditure
for communications. We consider the M2M communication
scenarios and associated power consumption model in [47]:
the transceiver is operated in discontinuous reception (DRX)
mode, periodically listening for control signals. At regular
intervals, the transceiver is turned on to communicate the
sensor reading, but it is off for a vast majority of the time to
save power. Short bursts of data are sent with reporting time
intervals of hours or days. Each packet is assumed to be on the
order of 1 kbyte or less in size, consistent with [47] and the
nature of the M2M sensor communications considered here.
The implemented model is illustrated in Fig. 6. Descrip-
tions of the power consumptions and durations for the differ-
ent periods (or phases) of the DRX scenario are listed below.
•Active, receiving data from a data center (Pact ,tact )
•Nonactive, transceiver turned OFF (sleeping mode) to
conserve energy (Psleep,tDRX )
FIGURE 6. Model for the transceiver to be powered by RF energy
harvester [47].
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TABLE 2. Parameters used to calculate the DC power consumption for the off-the-shelf [47] and ultra-low-power [48] transceivers.
•Transmission, sensor data reading sent (Ptx,ttx )
•Sync, establishing contact with data center before active
or transmission periods (Pclock ,tsync)
We make the most conservative assumption that the
transceiver is turned ON to receive instructions at the same
interval as the transmission is occurring, i.e., the reporting
time t=tDRX .
Hereafter, two cases are considered in the power-budget
calculations. The first is an off-the-shelf transceiver exam-
ple with estimated performance as of year 2020 [47]. The
second is an ultra-low-power transceiver demonstrated at a
research institution, with a power-optimized design in terms
of both transmitter power and in particular a very low par-
asitic sleep power [48]. The powers and durations used for
the two scenarios are compared in Table 2. Note that the
reported sleep power in [48] includes the base power, which
is considered as a separate item in [47]. In the limit of a long
reporting interval, the effects of sleep and base powers in
the consumption model converge. Thus, as a consequence of
the high base power, the off-the-shelf device will still have a
higher power consumption than the ultra-low-power design
at rest.
III. RESULTS
In this section, we present to what extent the RF energy
harvester could prolong the battery lifetime of the sensor
and whether it could make it completely self-sustainable.
In addition to the power consumption, the outcome relies
both directly and indirectly on the assumed available ambient
intensity, SBA (cf. the average and maximum values for each
frequency band in Table 1). This is because the harvested
power depends on the input as PDC =ηr·Aeff ·SBA and
because a higher received RF power significantly increases
the rectifier efficiency compared to low input power levels.
In all the cases presented below, the summed received power
across all the communication bands in Table 1 is used. The
received power is calculated separately in each band taking
into consideration the slightly non-constant antenna gain. The
rectifier efficiency is assumed to be ηr=50%, which is
to a certain extent optimistic for RF input powers from the
antenna at the lower end of the scale. Nevertheless, integrated
rectifier circuits [49] with, for instance, special PMUs [50]
have been reported to achieve high efficiency also in the
low-µW power range. The three main situations investigated
are listed below.
•Case #1 - Average SBA and off-the-shelf transceiver
(most conservative parameter set)
•Case #2 - Average SBA and ultra-low-power transceiver
(relaxed assumption with respect to energy
consumption)
•Case #3 - Maximum SBA and off-the-shelf transceiver
(optimistic assumption with respect to energy
availability)
A. CASE #1 - CONSERVATIVE PARAMETER SET
First, we consider both the average available power for har-
vesting [26] and the power consumption of an off-the-shelf
transceiver, as shown in Table 2 [47]. In this case even a
skin depth limited metal dipole antenna results in a minimal
increase in the battery lifetime and still provides a high net
power consumption even with very long reporting intervals,
as shown in Fig. 7. Accordingly, neither the sheet resis-
tance of state-of-the-art transparent but non-flexible ITO [13]
and metal grids [15] nor state-of-the-art flexible but non-
transparent printed silver nanoparticle thin-films [17] are
good enough. If a transparent graphene antenna is employed,
even with a state-of-the-art sheet resistance, the enhancement
in battery lifetime is negligible, as shown in Fig. 8.
FIGURE 7. Net power consumption (µW) for a predicted off-the-shelf
transceiver as of year 2020 [47] and the average ambient
RF intensity [26].
B. CASE #2 - RELAXED CONSUMPTION ASSUMPTION
Second, we consider the average available power for harvest-
ing [26], but using a transceiver with an ultra-low power con-
sumption, as shown in Table 2 [48]. Now, the best envisioned
graphene antenna could also be used to fully power the system
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FIGURE 8. Relative battery lifetime increase as a percentage for an
off-the-shelf transceiver as of year 2020 [47] and the average ambient
RF intensity [26].
FIGURE 9. Net power production (µW) for an ultra-low-power
transceiver [48] and the average ambient RF intensity [26].
even without a battery and having a reporting frequency of
several times per day. This is shown in Fig. 9, which presents
the net power production. For a self-sustainable system under
these conditions, the requirement is that the graphene sheet
resistance only needs to reach a value <100 /sq. This
conductivity can be satisfied for transparent graphene even
under bent operating conditions [30].
C. CASE #3 - RELAXED PRODUCTION ASSUMPTION
Third, we consider the deployment of the sensor at a location
where the maximum ambient power for harvesting is avail-
able [26]. Now, both a transparent metal grid antenna [15]
and a flexible printed silver dipole antenna [17] enable an
off-the-shelf transceiver [47] to be self-sustainable with the
year 2020 consumption and a reporting interval of several
times per day. However, in a scenario in which only flexibility
is required, printed carbon could instead be the material of
FIGURE 10. Net power consumption (µW) for a predicted off-the-shelf
transceiver as of year 2020 [47] and the maximum ambient
RF intensity [26].
choice for the antenna [18]. Furthermore, a state-of-the-art
transparent graphene antenna provides a decent 30% bat-
tery lifetime increase, as shown in Fig. 10. The consump-
tion of the transceiver with the employed reporting scheme
is ∼10 µW on average.
D. ESTIMATED BATTERY LIFETIMES FOR TRANSCEIVER
Considering the three main cases above, it is also of interest
to compare the absolute numbers for the battery lifetimes,
particularly for the current off-the-shelf transceiver. This
requires ∼10 µW DC power on average, with the reporting
model used in case #1 and case #3. Let us consider the case
in which the battery capacity is 6500 J ≈2 Wh (equiva-
lent to one AA battery), on the assumption that there is no
battery self-discharge and that the best possible envisioned
transparent graphene antenna is used. Then, with a t=24 h
reporting interval, the average power consumption provides
a very long battery lifetime even without employing any RF
energy harvesting. The actual numbers for all the different
cases are presented in Table 3, also including case #2.
TABLE 3. Battery lifetime comparison with 2 Wh capacity for the three
listed scenarios using a state-of-the-art graphene antenna.
IV. CONCLUSIONS
This study has examined the viability of using graphene
rectennas in optically transparent RF-to-DC energy convert-
ers on rigid and flexible surfaces for ambient RF energy har-
vesting to power, entirely or partially, sensor nodes in wireless
Massive M2M communication networks. Full-wave EM sim-
ulations using the state-of-the-art sheet resistance for trans-
parent graphene provides a dipole antenna ohmic efficiency
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of 5%. Based on the low graphene antenna efficiency, the har-
vested power in the power budget example was in the range
of only 1 to 50 µW from the conservative average value to the
more optimistic maximum value on the reported ambient RF
intensity in urban and suburban environments. In the calcu-
lations, a diode-based rectifier was assumed, as the graphene
transistor rectifier efficiency is currently far inferior.
Altogether, the sensor reporting interval in a DRX scheme
then needs to be on the order of one day to offer a significant
improvement in battery lifetime. This is a relevant reporting
time scale for various envisioned sensors in future massive
M2M systems. Nevertheless, under conservative conditions
for ambient RF energy levels and an off-the-shelf transceiver,
the current graphene rectenna usefulness would be minimal.
However, a graphene antenna could be a possible option
under favorable conditions on available power or in a future
scenario in which current ultra-low-power, proof-of-concept
transceivers have been commercialized. Specifically, at long
reporting intervals, the transceiver base power consumption
is a key parameter for a self-sustainable sensor to be feasible
with the currently best graphene dipole antenna. Provided
that a simultaneous hundred-fold decrease in the transmission
power is possible, the base power consumption needs to be
reduced by at least three orders of magnitude from current off-
the-shelf transceivers. However, obtaining an order of mag-
nitude lower sheet resistance for transparent CVD graphene,
i.e., an order of magnitude higher antenna efficiency, remains
unreachable. To make graphene compete with state-of-the-art
transparent metal grid antennas is thus extremely challenging.
In applications that require flexibility but not transparency,
however, ink-jet-printed carbon antennas constitute a
favorable alternative.
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MICHAEL A. ANDERSSON (S’12) was born
in Varberg, Sweden, in 1988. He received the
B.Sc. degree in electrical engineering and the
M.Sc. degree in wireless and photonics engineer-
ing from the Chalmers University of Technology
in 2010 and 2012, respectively. He is currently
pursuing the Ph.D. degree with the Terahertz
and Millimetre Wave Laboratory, Department
of Microtechnology and Nanoscience, Chalmers
University of Technology. His research focuses on
the development of fabrication and modeling techniques for graphene-based
transistors and circuits at microwave and terahertz frequencies.
AYÇA ÖZÇELIKKALE (M’10) received the B.Sc.
degree in electrical engineering and a double major
B.A. degree in philosophy from the Middle East
Technical University, Ankara, Turkey, and the
M.S. and Ph.D. degrees in electrical engineer-
ing from Bilkent University, Ankara. She spent
part of her doctoral studies with the Department
of Mathematics and Statistics, Queens University,
Kingston, ON, Canada. She is currently a Post-
Doctoral Researcher with Chalmers University of
Technology, Gothenburg, Sweden. Her research interests are in the areas
of energy harvesting, statistical signal processing, communications, and
optimization.
MARTIN JOHANSSON (M’93–SM’06) received
the M.S. degree in engineering physics and
the Ph.D. degree in electromagnetics from the
Chalmers University of Technology in
1986 and 1997, respectively. He joined Ericsson
Research, Ericsson AB, Gothenburg, Sweden,
in 1997, where he currently serves as an Expert in
antenna technology. His current research interests
include antenna technology for mobile communi-
cations, antenna system modeling, and determin-
istic channel modeling.
ULRIKA ENGSTRÖM received the M.S. degree
in physics and engineering physics and the
Ph.D. degree in physics from the Chalmers Univer-
sity of Technology in 1994 and 1999, respectively.
She joined Ericsson Research, Ericsson AB,
Gothenburg, Sweden, in 1999, where she is cur-
rently a Team Leader as part of Ericssons’ 5G
Research. Her current research interests include
5G systems with a focus on antenna systems for
mobile communications and new use cases for 5G.
ANDREI VOROBIEV received the M.Sc. degree
in physics of semiconductors and dielectrics from
Gorky State University, Gorky, Russia, in 1986,
and the Ph.D. degree in physics and mathematics
from the Institute for Physics of Microstructures,
Russian Academy of Sciences, Nizhny Novgorod,
Russia, in 2000. In 2008, he was an Associate Pro-
fessor in physical electronics from the Chalmers
University of Technology, Gothenburg, Sweden.
He currently holds a Senior Research Chair with
the Chalmers University of Technology. His main research interests are
in development and application of emerging functional materials and phe-
nomena in microwave devices. His current activities focus on materials,
technology, and design of graphene-based field-effect transistors and com-
ponents/systems for microwave/terahertz applications.
JAN STAKE (S’95–M’00–SM’06) was born
in Uddevalla, Sweden, in 1971. He received
the M.Sc. degree in electrical engineering and
the Ph.D. degree in microwave electronics
from the Chalmers University of Technology,
Göteborg, Sweden, in 1994 and 1999, respectively.
In 1997, he was a Research Assistant with the
University of Virginia, Charlottesville, USA. From
1999 to 2001, he was a Research Fellow with
the Millimetre Wave Group, Rutherford Appleton
Laboratory, Didcot, U.K. He then joined Saab Combitech Systems AB as
a Senior RF/microwave Engineer, until 2003. From 2000 to 2006, he held
different academic positions with the Chalmers University of Technology,
and from 2003 to 2006, he was also the Head of the Nanofabrication
Laboratory, Department of Microtechnology and Nanoscience. In 2007, he
was a Visiting Professor with the Submillimeter Wave Advanced Technology
Group, Caltech/JPL, Pasadena, USA. He is currently a Professor and the
Head of the Terahertz and Millimetre Wave Laboratory, Chalmers University
of Technology, Sweden. He is also the Co-Founder of Wasa Millimeter
Wave AB, Göteborg. His research involves graphene electronics, high-
frequency semiconductor devices, terahertz electronics, submillimeter wave
measurement techniques (terahertz metrology), and terahertz in biology and
medicine. He serves as the Editor-in-Chief of the IEEE Transactions on
Terahertz Science and Technology.
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