IEEE Circuits and Systems Magazine - Q1 2021 - 46

human targets and/or human target localization and
tracking. On the other hand, the detection distance in
radars is dependent on the type of application. In the
case that a long-range detection is needed in the target
application, signals with high power and very directive
antennas can be introduced. Despite that, for the indoor
vital sign detection, the transmit power is limited to
0 dBm/50MHz, which then also restricts the maximum
detection distance. A FMCW, a UWB impulse, or a SFCW
radars can be used in vital sign detection for multiple
targets. However, UWB impulse radars are more complex and costlier compared to the CW radars. On the
other hand, both FMCW and SFCW radars are capable
of vital sign detection for multiple subjects, localization
and tracking, features high SNR, are less complex and
more cost efficient compared to UWB impulse radars.
The preference of SFCW over FMCW signal types in vital
sign detection tends to be influenced by factors such as
availability of stepped frequency in the radar's transmit
signals. Due to this feature, compressive sensing can be
applied to this radar type, resulting in improved detection speed [2]. Since CW radars are less immune to jammers, they typically achieve less SNR compared to the
other radar types. Depending on the coherence integration and design, UWB-IR, FMCW and SFCW enable higher process gains and consequently improve SNR relative
to CW radars. The sign (x) in Table 2 indicate that the
radar does not possess the specified feature, or it exists
with significant limitations.
The detection of vital signs of a human using radar
systems involves the use of electromagnetic frequency
spectrum, which also varies depending on the type of
Table 2.
Features of different potential radar types for use in
vital sign detection.

Feature

FMCW UWB-IR SFCW

Long distance
detection



High detection
accuracy





Target speed detection 



Multiple subjects'
detection



Through wall detection 



Localization



Simplicity

 





Low cost

 





Target range



Tiny motion detection



High SNR





IEEE CIRCUITS AND SYSTEMS MAGAZINE

radar and their application. One of the main frequency
bands used is the industrial, scientific and medical
(ISM), a band of radio and microwave frequencies reserved and designated for industrial, scientific and medical equipment that use RF. Besides that, UWB radars
are increasing being chosen due to the regulation by the
Federal Communication Commission (FCC). This regulation allows unlicensed wireless operation of radars in
the UWB band of 3.1 to 10.6 GHz [11]-[13], with not more
than −41.3 dB/MHz of average power transmission. On
the other hand, the European Telecommunication Standards Institute (ETSI) also permits the unlicensed operation of communication technologies in the spectrum
between 6 and 8.5 GHz, whereas the Korean Communication Commission (KCC) permits operation in the frequency band of 7.2 to 10.2 GHz [4], [14], [15].
Besides the radar type and the operating frequency
band, several other important considerations in designing radar-based vital sign detection systems include the
type of vital signal to be detected and the power consumption level. A comparison of the power consumption levels of several recently reported radars operating
between 2 and 15 GHz is presented in Table 3 [16]. Notice
that the highest DC peak power consumption of 148 mW
is observed in [17], whereas the lowest of 19 mW is presented in [16]. During active detection, the design in [18]
consumed the highest DC power of 695 mW, while [16]
again featured a remarkably low DC power consumption
of 0.68 mW. Such low power consumption levels will enable the effective implementation of battery-powered
radar-based sensors. The state-of-the-art literature for
radar-based vital sign detection is summarized based
on the type of radar in Table 4. The following subsections will present more details of the different radar
types and their respective state-of-the-art literature.
A. CW Radar
The authors in [1] used a CW Doppler and a pulse radar
to wirelessly detect heart signal and breathing signal.
Several sources of signal distortion were also introduced into these signals to be evaluated. One of the distortion sources studied in this work is channel imbalance in quadrature receivers. This work also proposed
an innovative hardware design using packet radar and
low pulse IF receiver architecture to overcome these
issues. Next, the work in [3] developed a CMOS direct
conversion CW radar. This radar sensor contains a voltage-controlled oscillator to generate the CW signal, and
other necessary components such as frequency divider, power amplifier and quasi circulator (QC). This design also includes a clutter canceller block consisting of
variable gain amplifier and 360° phase shifter. This clutter canceller performs cancellation for the transmitted
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