IEEE - Aerospace and Electronic Systems - August 2023 - 49

environment and is given as
SINRðf; u; fdÞ
¼ SNRoðf; uÞ Ls;1ðf; u; fdÞ Ls;2ðf; u; fdÞ
mid-band spectrum, from 3.45-3.55 GHz, between radar
and 5G [13].
While radar uses S-band spectrum for a number of
(2)
where Ls;1ðf; u; fdÞ is clairvoyant SINR loss and
Ls;2ðf; u; fdÞ is adaptive SINR loss [10], [11], f and u are
receive azimuth and elevation, and fd is Doppler frequency.
In (2), we let SNRo vary with angle due to
spatial deviation of GT and GR from their maximum
values. Both SINR loss terms abide by 0
Ls;1ðf; u; fdÞ;Ls;2ðf; u; fdÞ 1, where the upper bound is
the case of no loss yielding noise-limited performance,
viz., SINRðf; u; fdÞ¼ SNRoðf; uÞ. Ls;1ðf; u; fdÞ characterizes
performance loss due to the presence of clutter and
RFI. The radar design, selection of radar signal processing
algorithms, and nature of the clutter and interference environment
influence clairvoyant SINR loss. Ls;2ðf; u; fdÞ
represents deviation from the clairvoyant bound as a result
of parameter estimation error in the signal processing
implementation (specifically in adaptive filtering stages,
e.g., adaptive beamforming) [11]. The radar designer
strives to minimize SINR loss through system design and
implementation, including the selection of operating frequency
and bandwidth.
Radar application requires access to a breadth of operating
frequencies and favors increasing bandwidth, as
higher bandwidth implies better range resolution essential
to separating closely spaced objects and improving target
discrimination and identification capability. However,
radar's spectrum needs conflict with other spectrum
demands. This conflict is especially evident between radar
and wireless communication communities. The advent of
5G deployment exacerbates this challenge, as 5G communication
technology aims to support diverse use-case categories,
such as enhanced mobile broadband, ultra-reliable,
and low latency communications, and massive machinetype
communications with the corresponding requirement
for multiband frequency access [12]. The 5G control layer,
in particular, uses frequencies at S-band (between 2-4
GHz) due to its data capacity and range [12]. As a result
of the economic importance of 5G, the United States
recently developed an approach to share 100 MHz of
AUGUST 2023
applications, including air and missile defense, continued
and increasing pressure to share or vacate spectrum, not
just at S-band, will face the radar designer. In fact, this
trend commenced over a decade ago [14], and will likely
accelerate given consumer demand for wireless technology.
Radar must adapt to this reality.
This article is an overview of issues around radar spectrum
use. To this end, we discusses some of the key considerations
around frequency selection in radar in the
section " Frequency Selection and Use in Radar. " Next, we
describe radar degrees of freedom available to enhance
radar's access to spectrum in the section " Degrees of Freedom
for Radar Spectrum Utilization " and highlight radar
spectrum engagement strategies in the section " Spectrum
Engagement Strategies for Radar. " Last, we and provide a
brief " Summary " section.
FREQUENCY SELECTION AND USE IN RADAR
Frequency selection is an integral part of radar design.
The operational bandwidth (OBW) identifies the range
of frequencies over which the radar is capable of operating
and is usually described by the ratio of the highest
to lowest frequencies. For example, if the radar supports
frequencies from 2 to 4 GHz, the OBW is 2:1 (said,
" two-to-one " ). In contrast, the instantaneous bandwidth
(IBW) is the bandwidth of the transmit and receive signal
during a given dwell. The IBW is evenly distributed
around the operating center frequency and fits within
the operational frequency range of the system. Finally,
the fractional bandwidth is the ratio of the IBW to the
center frequency and characterizes narrowband versus
wideband operation. Wideband operation places additional
complexity on the radar antenna design and signal
processing.
Table 1 summarizes some of the more common considerations
in radar frequency selection. The first column gives
the design factor, while the second describes whether the
choice relates to operating frequency or IBW selection, and
the final column provides the design rationale.
IEEE A&E SYSTEMS MAGAZINE
49
IEEE - Aerospace and Electronic Systems - August 2023

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