IEEE Circuits and Systems Magazine - Q1 2021 - 43

Besides that, it is highly possible that radars and
communication devices coexist in the same location.
Such coexistence may result in both applications sharing the same spectrum and lead to interference. To
facilitate coexistence in the radio spectrum, all radar
sensors must comply with regulations of unlicensed
operation. The Federal Communication Commission
(FCC) in the US allows unlicensed UWB transmission
in the 3.1 to 10.6 GHz range with an average transmitted power of less than −41.3 dBm/MHz [4]. Radars in
the unlicensed frequency band are increasingly being considered for indoor scanning and localization in
coexistence with 5G and the Internet of Things (IoT).
In other situations, radars and communication devices may utilize the same hardware to reduce cost and
complexity. It is also foreseen in the near future that a
growing number of communication devices and detection radars coexist and share the spectrum in a heterogenous way. Thus, advancements in techniques to
mitigate such coexistence are one of the main issues
currently being investigated.
This survey provides a review of the state-of-the-art
in this growing research area from the different aspects
of processing platforms, detection algorithms, operating frequencies and wireless communication hardware.
Such review is the first of its kind, to the best of the
authors' knowledge. Some of these researches may
not be necessarily applied for vital sign detection but
can potentially be used in such application. The rest
of this review is organized as follows. The next section
will describe and summarize the technical background
of radar principles, classifying the types of radar and
the processing platforms used, with a specific focus on
FPGAs, the signal processing algorithms, the operating
frequency spectrum utilized, and the communication of
data. Finally, this review ends with a future perspective
of potential radar architectures and features that are
most suited for applications in vital sign detection. This
work intends to highlight the main challenges in vital
sign detection using radar techniques and concentrate
on its real-time detection aspect to depart from existing
reviews available in literature. This is due to the need
for alternative solutions and considerations for realtime radar detection, which include innovative parallel
processing paradigms on reconfigurable processing devices such as the FPGA.

erate the wave to be transmitted via an antenna. Once
the transmitted signal hits the target, a portion of the
signal is reflected to the radar while the rest is reflected
in other directions or absorbed by the body.
The type and shape of the transmitted signal depends on the radar type. There are four widely used radar types for vital sign detection: continuous wave radar
(CW), ultra-wide band impulse radar (UWB-IR), linearly
frequency-modulated continuous wave radar (LFMCW)
or (FMCW) and step frequency continues wave radar
(SFCW). The CW radar transmits unmodulated continuous wave single tone signals, which can be written as
follows [5]:
T ^ t h = cos ^2rft + z ^ t hh

where f is the oscillation frequency, t is the
elapsed time, and z ^ t h is the phase noise caused by
the oscillator.
The UWB-IR radars, on the other hand, transmits
pulses which are wide in bandwidth. The rate at which
these pulses are transmitted per second is called the
pulse repetition frequency (PRF). The interval between
transmitted pulses is usually used to listen for incoming
reflections from objects. The pulse signal is modulated
before being amplified and emitted. The transmission
signal for the ith frame, can be written as:
S i ^ t h = p ^t - iTf h cos ^2rf0 ^t - iTf hh

FIRST QUARTER 2021

(2)

where p ^t - iTf h is the pulse signal, t is elapsed time, Tf
is the duration of the frame (Tf = (1/fp)), fp is the pulse
repetition frequency, and cos (2rf0 ^t - iTf h is the carrier
with the carrier frequency f0 [6].
The FMCW radar transmits chirps of sinusoidal signals, which frequency is linearly swept from f0 to f1 . The
complex chirp signal can be modeled as follows [7]:
S ^ t h = A t exp ^ j ^2rf0 t + rKt 2 hh, 0 1 t 1 Ts

(3)

where A t is the magnitude associated with the power transmitted, f0 is the start frequency, t is the time
elapsed, and K is the slope of the sweep of frequencies
from f0 to f1 during the duration Ts . The sweeping bandwidth can then be written as:
B = f1 - f0 = KTs

II. Radar Background and Principles
Radars use electromagnetic (EM) waves to detect and
monitor remote targets. They transmit and capture reflected radio frequency (RF) waves from one or several
targets and process them to obtain information about
the targets. In general, a signal source is needed to gen-

(1)

(4)

The transmitted FMCW waveform is illustrated in Figure 1.
Next is the SFCW radar, which transmits series of discrete tones in a stepwise manner. The waveform of the
SFCW consists of N coherent pulses, which frequencies
monotonically increases by a fixed increment, Tf. If the
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