IEEE - Aerospace and Electronic Systems - April 2022 - 43

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radar waveform. Most previous DFRC works by embedding
one communication symbol within one or multiple
radar pulses. The symbol rate is hence limited by radar
pulse repetition frequency (PRF).
In some recent research [10]-[15], frequency-hopping
MIMO (FH-MIMO) radar is introduced to DFRC,
which breaks the above limit and substantially
increases the communication symbol rate to multiples
of (e.g., 15 times) PRF. Pioneering in conceiving the
novel DFRC architecture based on FH-MIMO radar
(FH-MIMO DFRC), the research reported in [10]-[15]
focuses on analyzing the impact of information modulation
on the radar ranging performance. In these
works, little attention is paid to effective implementation
of data communications. Some later research
reported in [16]-[18] develops methods to address the
practical issues, e.g., channel estimation and synchronization,
in FH-MIMO DFRC.
Motivated by the rapidly increasing interest in
DFRC and the lack of awareness and general understanding
of FH-MIMO DFRC, this article aims to provide
a timely introduction of the novel DFRC
architecture. We start by discussing the potential applications
and channel scenarios of FH-MIMO DFRC and
also the commonly used signal model in the literature.
Then, we survey the existing signaling strategies for
FH-MIMO DFRC, illustrate their modulation and
demodulation methods, and also compare the signaling
strategies from various aspects. We further discuss the
issue of channel estimation in FH-MIMO DFRC and
report recently developed solutions. Finally, we highlight
the major unsolved issues in FH-MIMO DFRC
and suggest potential solutions to shed light on future
research directions. It is noteworthy that this article
provides a better coverage on the communication
aspects of FH-MIMO DFRC, which are rarely dealt
with in prior articles.
SCENARIOS AND SIGNAL MODEL FOR FH-MIMO DFRC
FH-MIMO radar is pulse based and hence is likely to be
employed in applications requiring a large range coverage,
APRIL 2022
such as long-range air-surveillance. Besides, such radar is
generally placed at a certain height, e.g., several hundred
meters, above the sea level, looking beyond clutter areas,
so as to reduce ground clutters and cofrequency interference
from terrestrial wireless systems. Therefore, FHMIMO
DFRC is promising for providing ground-to-air
(G2A) communications for different types of aircraft, as
depicted in Figure 1. The aircraft can be an UAV, a CAP
or a high-altitude platform (HAP).
In addition to target detection/tracking, FH-MIMO
DFRC allows a radar to perform secure, long range, low
latency, and high-speed wireless broadband connections
with aircraft or warships over very wide areas. (Note that
if the radar is performing wide-area search with a directional
beam, the high-speed communication link can be
intermittent.) DFRC benefits radar by, for instance, allowing
the sensing results, e.g., target information or radar
imaging, to be shared with other (airborne or groundbased)
radar nodes. More benefits of DFRC to both radar
and communications are comprehensively summarized in
several recent overview/survey papers [3]-[5]. We underline
that, as not captured in the previous work, the G2A
link provided by FH-MIMO DFRC may help support
seamless wireless coverage, as expected to be realized by
future 6G networks [1]. In particular, FH-MIMO DFRC
can contribute to building the integrated space and terrestrial
network (ISTN), which is envisioned to be at the core
of 6G communication systems [19]. Employing satellites,
in combination with FH-MIMO DFRC, may provide a
more cost-effective solution to providing wireless connectivity
for people and vehicles in remote rural areas and in
the air, as well as at sea, as compared with the solutions
solely relying on satellites. Against the above potential
scenarios, we depict below the channel models suitable
for FH-MIMO DFRC.
CHANNEL MODELS SUITABLE FOR FH-MIMO DFRC
The channel distribution for FH-MIMO DFRC can vary
with the altitude of an airborne user equipment (UE). The
typical altitudes of UAVs, CAPs, and HAPs are 103 m,
10 103
m, and 20 103
IEEE A&E SYSTEMS MAGAZINE
m above the sea level,
43
IEEE - Aerospace and Electronic Systems - April 2022

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