1. technology and computing

Wearable Wireless Electronics
(Invited)
Victor Lubecke
Department of Electrical Engineering, University of Hawaii at Manoa
Abstract — A key component to wearable technology is
wireless functionality allowing untethered interaction between the
body and the outside world. Furthermore, wearables can use
wireless technology not only for communications, but also for
energy and sensing. An introductory overview is provided
highlighting applications and methods associated with research in
wearable wireless technology.
Index Terms — Wireless, wearables, body area networks
(BAN), radar, sensors.
I. INTRODUCTION
The recent renaissance in wearable technology transcends the
realm of the lab geek and brings devices right to the center of
mainstream technology adoption. Whether it’s a wristwatch or
eyewear providing one with smartphone technology, or body
sensors assessing one’s health, a key component is wireless
functionality which allows wearables provide the untethered
interaction between the body and the outside world.
Furthermore, an increasing number of applications use wireless
technology not only for communications, but also for energy
and sensing. An introductory overview is provided here to
highlight the applications and methods associated with research
in wearable wireless technology.
II. WEARABLE COMPUTERS AND BODY AREA NETWORKS
One path to wearable technology has come directly from the
scaling of familiar personal computer and audio/video systems
to the point where they are small enough to be worn in a
practical manner. Many examples currently being adopted are
made to be comfortably worn on the wrist or over the eyes, and
run their own mobile apps or operating systems, much like a
smart phone. The key difference is that these devices also
exploit the fact that as wearables they can more readily interact
with the user and collect information relating to activity, health
and environment. Another important difference is that these
devices have evolved significantly over smartphones in terms
of miniaturization. Figure 1 shows and example of an early
prototype for the Google Glass head-worn computer, compared
with a recent model in limited production. The core processing
for these devices come from System on Chip (SoC) devices
such as the OMAP from Texas instruments, typically integrated
with wireless connectivity support for standards such as Wi-Fi
or Bluetooth. They can also support or interact with mobile
telephone communications systems allowing extensive
mobility.
2011
2014
Fig. 1. Google Glass miniaturization [1].
Wearable computers can notably include features that
interact broadly and efficiently with the user, such as cameras,
accelerometers, thermometers, barometric sensors, GPS, touch
screens, and heads-up displays. These features not only
facilitate a natural user interface, but also allow the devices to
be used for fitness training and medical monitoring. While not
all of a user’s characteristics of interest can be found on the
wrist or face, sensors distributed around the body can also be
made to communicate with wearable computers through
specialized networks.
A wireless body area networks (WBAN) is a radio frequency
wireless network technology designed to facilitate
communications between body worn sensors. Advances in low
power electronics and MicroElectroMechanical Systems
(MEMS) have paved the way for ubiquitous biomedical
monitoring systems suitable for healthcare and safety
applications [1]. This technology leverages the IEEE 802.15.6
and IEEE 802.15.4j standards, emerging for medical
applications, and addresses the challenges associated with radio
wave propagation in the vicinity of the human body [survey]
[enabling]. WBAN’s can work in tiers, with sensors typically
communicating with a central device, such as a smartphone or
wearable computer which in turn provides broader wireless
network access as shown in figure 2 [2].
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III. ACTIVE RADIO SENSORS
Wearable sensors in WBAN’s can function conventionally,
independent of their wireless connectivity, or can leverage
wireless technology for sensing or energy functions. An
unusual example of a body worn sensor used to measure heart
rate and for a fish is illustrated in figure 4(a)(b). While simple
skin contact sensors can be used to measure human ECG,
similar measurements on fish require the surgical implantation
of electrodes. Here a Doppler radar is glued to the skin of the
fish to sense internal motion resulting from the pumping motion
of the heart [4][5]. Radar sensors worn by a human can be
similarly used for cardiopulmonary monitoring. A monitoring
system using radar worn on a seatbelt has been demonstrated
for driver health and fatigue monitoring. The system is
illustrated in figure 4(c). The system uses a nanosecond pulse
near-field sensing (NPNS) radar to collect cardiopulmonary
motion based data, which is in turn sent to a central computer
Fig. 2. Tiered Wireless Body Area Networks (WBANs)[2].
An example of a WBAN sensor systems for healthcare called
iCalm is shown if figure 3(a). The focus of this design includes
comfortable wearable sensors and low-power affordable
hardware implemented using the IEEE 802.15.4 wireless
standard. One sensor employed implements an exosomatic
measurement of galvanic skin response (EDA), using a small
voltage applied to the skin with the resulting potential drop
measured. A garment sewn implementation for infants is shown
in figure 3(b) [3]. The design of sensors which can operate and
be comfortably worn for long periods is a significant challenge
for wearable technology.
(a)
(b)
(c)
(a)
(b)
Fig. 3. iCalm autonomic activity monitoring WBAN (a) and
associated infant galvanic skin response sensor [3].
Fig. 4. Body contact radar sensors. A radar measured
ballistocardiograph for a fish (a), a fish sensor body jacket
(b), and a driver health management system (c) are
illustrated [4][5][6].
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and analyzed for signs of apnea or heart rate variability (HRV)
based indications of drowsiness [6].
Radar has also been applied for ambulatory gate monitoring.
A low-cost, low-complexity ambulatory human locomotion
tracking system suitable for clinical gait assessment has been
demonstrated using wearable ultra wideband (UWB)
transceivers. The proposed system is theoretically capable of
providing a ranging measurements within an error of 0.11 cm.
error based on anticipated inter-marker distances [7].
IV. PASSIVE, RF-POWERED, AND SELF-POWERED SENSORS
While body worn sensors typically require power to perform
sensing and communications functions, some can function as
passive scatter devices for sensing while others can derive
needed operation energy from wireless signals or even the
sensed parameters of the human body.
A harmonic radar system using passive scatter tags is shown
in figure 5. The tags are simple rf-ID circuits which modulate
the radar interrogation signal so that the return can be
distinguished from clutter. In this particular case, the 2.45 GHz
frequency of the interrogation signal is doubled. The
transceiver system and tag are shown in figure 5(a,b). Figure
5(c) shows a special case where moving clutter was tagged in
order to recognize and cancel it from the fundamental radar
return. The clutter was a mechanical mover oscillating steadily
at around 25 bpm. The frequency doubling tag on the cluttermover creates a clear return signal in the 4.9 GHz receive
channel. Together, reflections from the clutter-mover and a
respiring human subject create an erratic return signal in the
unprocessed 2.45 GHz receive channel (dark solid line). Using
information from the 4.9 GHz channel, an adaptive noise
cancellation (ANC) algorithm was applied to the 2.45 GHz
channel to filter out the reflected signal content related to the
tagged clutter-mover, leaving only content related to the
respiring human subject. The ANC processed channel (light
solid line) clearly tracks only the human respiration rate, as
indicated by comparison with a chest-belt reference signal (dark
dashed line) [8].
Some wearable sensors can not only sense the wearer’s
physiological information, but also derive enough energy from
the sensed parameters to power them and store or wirelessly
communicate data. For example, movement of the chest wall
during normal breathing can be monitored and harvested to
facilitate self-powered wearable respiratory sensors for
continuous remote monitoring. Tests with electromagnetic
respiratory effort harvester/sensor modules have been shown to
produce up to 6.44mW and harvest 30.4mJ during a 5-minute
exercise stage [9]. With any physiological sensor, it is
important that subject is not burdened by the harvesting to the
point where it affects the subject behavior. A statistical analysis
confirmed there is no significant change in the metabolic rate
(a)
(b)
(c)
Fig. 5. Harmonic radar system (a) used with frequency
doubling tag (b) to separate tagged subject from clutter,
and to remove contribution in fundamental radar due to
tagged clutter (c) [8].
of subjects wearing the electromagnetic harvester and
biosensor. A plot showing a comparison of energy expenditure
with and without the wearable sensor is shown in figure 6,
confirming this finding.
V. CONCLUSION
Wearable technology is rapidly gaining mainstream
acceptance. A key component is wireless functionality allowing
untethered interaction between the body and the outside world.
Wireless technology is incorporated not only for
978-1-4799-8275-2/15/$31.00 ©2015 IEEE
Fig. 6. Minimal difference in metabolic rate of a female
subject sampled every 30seconds (Dashed line: with the
harvester, Solid line: without the harvester) [9].
communications, but in many cases also for energy and sensing
functions.
ACKNOWLEDGEMENT
This work was supported in part by the National Science
Foundation (NSF) under grants CBET-1160326.
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978-1-4799-8275-2/15/$31.00 ©2015 IEEE