IEEE Circuits and Systems Magazine - Q1 2022 - 62

Based on the UWB technologies, efficient wireless body area networks
(WBANs) with low-power consumption and robust performance can be
achieved under scenarios with multi-path effects.
For gait analysis, conventional methods are realized
based on camera, magnetic sensor, or ultrasound sensor.
However, they are usually composed of many discrete
components and are each with their limitations.
For example, the camera-based method will invade the
privacy of the human subject because the camera will
record the video of the whole scenario. Several essential
factors, including accuracy, reliability, cost, and power
consumption, need to be considered to achieve a reliable
and accurate monitoring system. UWB radar sensing
systems are promising to enable accurate gait analysis,
by which precise measurement, description, and
assessment on human gaits can be achieved [6], [18],
[71]. In [31], a UWB radar sensor in combination with
the short-time state-space signal processing method
(STSSM) is used to monitor subject walking. As demonstrated
in Fig. 32(a), the arm swing during walking can
be detected based on the high-resolution range profile
(HRRP). The Boulic model demonstrated in Fig. 32(b) is
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used to model the subject where human walking motion
is built by cycles with constant translational velocity
and factors such as human height and velocity considered
in the modeling. Fig. 32(c) and Fig. 32(d) show the
extracted walking patterns based on the spectrogram
using short-time Fourier transform (STFT). IR UWB radar
sensors demonstrate the capability to enable accurate
respiration/heartbeat monitoring for health status
monitoring. Moreover, through-a-wall monitoring can
be achieved based on a UWB sensing system [59]. The
principal component analysis (PCA) algorithm can be
leveraged to analyze the time-series data and obtain
the heartbeat information by processing the extracted
phase [59]. Based on the UWB technologies, efficient
wireless body area networks (WBANs) with low-power
consumption and robust performance can be achieved
under scenarios with multi-path effects [72]. A performance
summary of the state-of-the-art integrated UWB
transceivers for biomedical radar sensing and efficient
communications is demonstrated in Table III.
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Figure 31. The indoor and outdoor human walking trace
measured with the radar [32] © 2014 IEEE.
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H. Compact and Efficient Antenna Design for
Integrated Wideband RF Transceivers
To enable the high performance of wideband RF transceivers,
the antenna needs to be designed targeting
wide bandwidth, high gain, high efficiency, small size,
and easy integration [163], [165]. Notably, considering
the fabrication cost and application scenario requirements,
two design routes are followed: Off-chip highgain
antenna and on-chip integrated antenna, which is
demonstrated in Fig. 33. The characteristics, including
center frequency (CF) and fractional bandwidth (FBW)
of the antennas, are presented in Fig. 33. For the off-chip
wideband antenna, it has high freedom of design without
the restriction of space. The sinuous antenna could realize
ultra-wideband property while having a complicated
design structure [106]. The higher-order mode dielectric
resonator is introduced to enhance the gain without
additional chip area consumption, but the bandwidth
would be limited [120]. The Vivaldi antenna and its variants
are good candidates. As demonstrated in Fig. 33(a),
in [108], a full metallic structure of a tapered slot antenna
array is proposed for efficient beamforming. By loading
ultrathin microwave-absorbing materials at the side edges,
improved Vivaldi antenna structure could reduce the
radar cross-section (RCS) and improve the bandwidth
[107]. An E-plane double-ridged horn antenna could
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