IEEE - Aerospace and Electronic Systems - December 2021 - 17

Image licensed by Ingram Publishing
surface, which can be uncomfortable and impractical [6].
Thus, a non-contact unobtrusive continuous user authentication
scheme via non-contact respiration sensing would
be a significant improvement and have the potential to
reduce vulnerability to malicious attacks and increase
usability in practice [6].
ADVANTAGES OF RADAR-BASED RESPIRATION
SENSING FOR NON-CONTACT IDENTITY
AUTHENTICATION
What can radar-based respiration sensing technology contribute
to biometric identification systems? To answer this
question, the potential advantages of radar over other sensor
technologies will be described first followed by an
explanation of how respiration patterns can be a unique
biometric feature. Radar research in healthcare has primarily
focused on extracting breathing rate and heart rate
without attaching any sensors to the body [7], [8]. Before
describing the details of the basics of non-contact respiration
sensing, it is worth exploring why, in recent years,
radar rather than other technologies has received more
attention for physiological sensing. Radar is an attractive
approach as it is contactless and unobtrusive, so users do
not need to wear any sensors on the body and or to intentionally
engage themselves with the system [7], [8]. Radar
sensor performance does not depend on good lighting and
can even work through clothing or bedding as well as
other visual obstructions [7]. Most importantly, in terms
of perceived privacy, radar is far preferable to installing
video cameras in a house [8], as any type of camera can
be regarded as privacy-invasive. No intrusive video imaging
is required even to recognize people, as the level of
radar feature recognition provided is very rich and will be
discussed in a later section. A related topic of interest
involves addressing the cost and installation of radar in
the home environment as there is not yet mass-produced
equipment available for its use in healthcare. However,
there is a range of emerging mass-produced technology
for the autonomous vehicle market, and automotive radar
may provide the push to make healthcare radar accessible.
Thus, key is still research needed for the recognition of
breathing patterns or heart-based dynamics, which are
DECEMBER 2021
important and unique features needed for distinguishing
between different human beings.
Breathing is a complicated anatomical process that takes
place through the control ofthe central neural mechanism in
the coordination of lung, diaphragm, and respiratory
muscles [3], [6], [8], [9]. In general, the breathing mechanism
has three different stages: inhale, exhale, and a pause
in between [10]. Different clinical investigations proved
that, for awake adult human subjects at rest, there exists
diversity in breathing patterns not only in terms of tidal volume,
and inspiratory and expiratory duration, but also in airflow
profile [3]. Everyone selects for one pattern among the
number of infinite possible ventilatory variables and airflow
profiles. This variability is nonrandomand may be explained
by either a central neural mechanism or by the instability in
the chemical feedback loops [3]. In addition, heart-based
geometry is different between subjects and clinical investigation
also suggests that cardiac cycle variations occur
based on different shapes of the heart [6]. Radar can detect
respiratory motion and it is also possible to extract unique
features based on diversity in the breathing dynamics.
BASIC PRINCIPLE OF NON-CONTACT RADAR
RESPIRATION SENSING TECHNOLOGY
Radar stands for radio detection and ranging [1]. Radar
uses transceivers that have both transmitter and receiver
capability [1]. A radar transmits an electromagnetic signal
and collects the reflection of that signal from a target and
analyzes the waveform to extract the velocity, distance,
and position [4]. It can be compared with echolocation
commonly associated with animals such as bats or dolphins,
which transmit an ultrasound wave and get information
about prey from the reflected wave. A radar can have
two different antennas for transmission and reception or
may have a single antenna for both functions. Radar for
healthcare monitoring also works in different frequency
ranges from 2.4 to 24 GHz (within the industrial, the scientific,
and medical band). Radar also incorporates a digital
signal processing core where data can be filtered and
stored for further processing. Commonly used radar can
be classified into two different systems: a continuous
wave (CW) radar that sends an unmodulated (same carrier
frequency) signal; and a frequency modulated (different
IEEE A&E SYSTEMS MAGAZINE
17
IEEE - Aerospace and Electronic Systems - December 2021

Table of Contents for the Digital Edition of IEEE - Aerospace and Electronic Systems - December 2021
