IEEE Circuits and Systems Magazine - Q1 2022 - 49
respiration [81]. Moreover, specific
motions like falls or gaits
can be detected. The time-domain
motion of the subject can
be represented as
TRR Rttt mM()=+ (8)
()
where the term ()tRm
() ,
represents
the micro motions resulting from
heartbeat and respiration, and the
term ()R tM
stands for the macro
motions such as gaits or falls. The
heartbeat can be modelled as a
sinusoid signal with a frequency
between 1 Hz to 3 Hz, while respiration
is represented as a sinusoidal
signal with a frequency
between 0.1 Hz to 0.5 Hz [6], [12],
[13]. When detecting motions such
as gaits or falls, the term ()tRm
negligible and R ()tM
is
represents
the range variations caused by
the macro movements. To extract
the information related to
TRt
(),
specific algorithms are required.
Since the extracted phase from
the coherent radar sensor is highly
sensitive and indicative of the
micro-movements of the subjects,
it is used to monitor respiration
rate and heartbeat rate. As the
beam is steered to the subject, the
transmitted power is pinpointed
to the target to sense the human
subject's vital signs; thus, the efficiency
and system SNR are enhanced
[81]. The de-chirped IF
signal is acquired and processed
to extract the information relative
to multimodal vital signs through
the interferometric phase processing
algorithm demonstrated
in Fig. 13(a). The system configuration
is illustrated in Fig. 13(b),
which contains the chip-based
phased-array radar TRX, the
broadband Vivaldi antenna array,
and the SPI interface for controlling
[81], [94].
Fig. 14 demonstrates the architecture
of an integrated chirp-based
FIRST QUARTER 2022
IEEE CIRCUITS AND SYSTEMS MAGAZINE
49
Digital-Tunable Mixed-Signal-Mode Chirp Synthesizer
Saturated DA-PA
BWmod, Tmod
Adaption Words
FM[0:19]
FM
Gen.
fref
f
Sawtooth
1-Z-1
fref
t
10 bit ADC
fref
B0~B10
ADC
TRX Chip
PGA
Fine
A1~A4
Coarse
S3 S2 S1
6 Biquads
Vcm
12th Order BPF
ILPS LNA
Echos
F[0:10]
fref
Fractional
TDC
fref
fref
Digital
LPF
F[11:18]
Preset
Register
D[0:14]
P-IDAC
fVCON
DA
Σ
1/8
Chirp
R-IDAC
Iref
VCO
fVCOP
DA
Figure 9. Schematic of the Ku-band FMCW radar for SAR imaging [84] © 2016 IEEE.
fVCON
fVCOP
IEEE Circuits and Systems Magazine - Q1 2022
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