IEEE Circuits and Systems Magazine - Q1 2021 - 66

Meanwhile, in [89], a ubiquitous off-the-shelf WLANenabled device was used to detect breathing using the received signal strength (RSS). This can be performed due
to the introduction of a dominant periodic component in
the standard WLAN received signal. The proposed system can help to reliably extract the hidden breathing signal from a noisy WLAN RSS. The system handles many
challenges, including noise elimination, interfering human, sudden movements as well as abnormal breathing
situations. The functionality of remote monitoring may be
restricted when using the available wireless infrastructure. Once there is a wireless terminal with an RF frontend transceiver and network connection, the detection of
vital signs and the communication of the collected data
to a remote monitoring facility will take place. Information on human respiration and heart rate only requires
low bandwidth transmission capability. In [90], Victor et
al. used an add-on module to an existing wireless terminal to detect human heart and breathing activities. The
module included an antenna and mixing element to receive the transmission from the wireless terminal, which
then produced a Doppler-based signal proportional to
the heart and chest motion. This produced signal can be
used for detection of heart and breathing activities and
can potentially be relayed by the wireless terminal to a
remote heath monitoring facility via the existing telecommunication network and infrastructure.
B. Communication
This section illustrates the different approaches and
techniques used for combined sensing and communication functionality. The main drive for the integration of
radar sensing with communication is to arrive at a compact hardware solution. Components such as transceiver and antenna can serve dual functions-in sensing and
in communication. These systems operate in two modes:
the detection mode measures range, velocity angle, etc.,
whereas the communication mode receives and demodulates the spread spectrum and returns connection with
a remote station. Other solutions include system solution where the frequency band of the communication
transceiver is smaller compared to the pulse spectrum
of the radar. This is so that both bands overlap, and the
same RF front end could be used for both purposes, thus
decreasing the cost of the system. This subsection also
presents several designs where signals from sensor networks are transmitted wirelessly to base stations.
The research presented in [91] studied the approach
of using the same UWB transceiver for both sensing and
communication. This system is focused specifically on
heart rate variability (HRV) and its link as an indicator
for the cardiovascular nerve system. Off the shelf commercial transceivers were used with minor modifica66

IEEE CIRCUITS AND SYSTEMS MAGAZINE

tions. The higher resolution in UWB systems offers more
accurate sensing, whereas its resistance to multipath is
used for high speed communication. From the bio-signal
types that can be measured using this approach, heartrate was selected due to its importance. In this study,
UWB radar principles were used to measure to heartbeat and the UWB communication standards were used
to wirelessly transmit these measurement results. Such
approach with dual purpose-sensing/detection and
communication, makes these devices ideal nodes for
wearable computing and in body area networks.
Next, Bharat et al. in [92] highlighted the many advantages of using UWB as both a sensing and a communications standard for biomedical applications. These
include its low radiated power (−41.3 dB/MHz), low
power consumption, ability to coexist well with other
wireless technologies and robustness to interference
and multipath. This work integrated the sensing and
communication functionalities into a single device using FM-UWB, enabling it to be used in two operational
modes for heart rate monitoring. It is able to collect vital
signs from its sensors and transmit to other sensors or
to repositories in real time. While a data rate of 240 kbps
is generally sufficient in biomedical applications, heart
monitoring requires less than 100 bps. This can be easily implemented in FM-UWB technology, enabling the
health data to be transmitted to a remote medical server
frequently for better diagnosis or for better responsiveness to emergencies. Next, the integrated transceiver
proposed in [92] includes several purpose-dedicated
components. These include the FM modulator for sensing, and the FSK demodulator for communication. Common components for both purposes used are the low
pass filter and low noise amplifier. Sensing in the transceiver was performed using the simultaneous multiple
frequency transmission method. This involved a slight
increase in the complexity of the transmitter hardware
compared to a conventional transmitter, where two FM
modulators are needed instead of one.
In [93], a biomedical wireless radar sensor network
(BWRSN) for vital signs monitoring and fall detection
was proposed. This is to overcome the limitations of using a single radar in real situations. The BWRSN consists
of four radar-based sensor nodes and a base station.
Each node consists of a microwave radar, a Zigbee module, and a microcontroller [93], [94]. The radar block
generates and sends a CW signal at 5.8 GHz to the target
and receives the reflected signal. The digitized baseband information is then transmitted wirelessly to the
base station for remote data processing to determine
the vital signs rate and fall detection incidents. The proposed BWRSN was tested in a lab with two nodes fixed
to the ceiling, and the other two were on the wall as
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IEEE Circuits and Systems Magazine - Q1 2021

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