IEEE - Aerospace and Electronic Systems - August 2023 - 56

An Overview of Radar Operation in the Presence of Diminishing Spectrum
performance, embedded computing; and, decision support
algorithms for resource management.
In the following, we summarize plausible approaches
of varying complexity and utility to cope with spectrum
allocation demands on radar.
RADAR OPERATING BAND RELOCATION
Relocating the radar operating band to accommodate wireless
communication systems is a limited option. Relocating
the radar operating band requires, as a minimum,
redesign of the radar front-end (viz., antenna subsystem
and receiver-exciter). The merit of this approach varies
by radar system. In the case of older systems, designing a
completely new radar is preferable given the vast changes
in technology over the past two decades and an increasingly
challenging and dynamic operating environment
favoring configurability. It seems reasonable that a redesign
of the RF front-end of recently fielded, lower cost
radar systems is feasible and cost-effective relative to
other options. On the other hand, advanced, multifunction
radars on military aircraft and naval vessels represent significant
investment due to high complexity and optimization
for their various, important, mission sets; moving the
operating bands of such radars is generally a difficult,
costly, and time-consuming endeavor. For this reason,
spectrum sharing may prove a better option.
SPECTRUM OPEN SHARING
In the open sharing model, all users are given peer status.
Contrary to current approaches to frequency allocation,
spectrum access is unregulated, and spectral coexistence
becomes a shared responsibility. Each peer system must
effectively exploit space, time, and polarization to execute
its core functions. The open sharing model forces all
potential users to determine coexistence strategies, since
the alternative suggests highly suboptimal performance
for all users. Given sensitive aspects of military and air
traffic control radar systems, a peer-to-peer relationship
with consumer electronic systems is unreasonable, making
this option undesirable for all but the most restrictive
scenarios.
DYNAMIC SPECTRUM ALLOCATION (DSA)
As discussed in [30] and [31], the idea behind DSA is to
exploit spatial and temporal variability in spectrum
usage, enabling dynamic spectrum assignment over relatively
coarse operating windows. This approach does
not remove white space in the spectrum. Rather, spectrum
is given exclusively to a designated user at a specific
time and within a particular region. Use of
56
SPECTRUM UNDERLAY
Spectrum underlay involves hierarchical access, with a
designation of primary and secondary users [30]. In the
underlay model, secondary users constrain their transmit
power to avoid interfering with the primary user. As previously
mentioned, traditional approaches to radar implementation
strive to achieve performance as close as
possible to noise limited. In this case, a number of factors
influence operation and resulting performance, such as the
geometry of primary and secondary users, as well as factors
in system design (e.g., use of multiaperture antenna
systems and adaptive nulling or polarization channels).
SPECTRUM OVERLAY
Overlay is also an approach based on hierarchical access,
restricting where and when secondary users can transmit
without the power constraints of the underlay strategy
[30]. Hierarchical access overlay attempts to optimize
spectrum usage by taking advantage of open spectrum
over space and time. In this approach, secondary users
must sense and probabilistically model spectrum usage,
subsequently determining a policy for spectrum action
that nominally minimizes collisions with primary users.
This approach especially acknowledges those situations
where the primary user does not require spectrum at that
point in space and time.
Moreover, this approach can leverage use scenarios
for the primary user. In the case of radar, statistically
bounded collisions are likely to only have a marginal
impact on mission performance during overlay conditions.
Extraordinary operating conditions (e.g., due to anomalous
propagation, intentional denial of spectrum access,
etc.) will likely not be properly modeled by the secondary
user, resulting in an increase in collisions. Through
detailed, operational scenario modeling, it is possible to
manage such occurrences; however, system security considerations
will generally prevent open access to such
information.
COGNITIVE RADAR
The aforementioned methods of radar spectrum access
require frequency sensing and decision support. A key
question following our prior discussion is: if given
IEEE A&E SYSTEMS MAGAZINE
AUGUST 2023
monitoring mechanisms, such as microwave radiometers,
and beaconing to shepherd spectrum to waiting
users, is one possible strategy; naturally, this approach
raises security concerns for radar. DSA is feasible for a
number of applications where the radar's spatio-temporal
use of spectrum is relatively low, as might be the
case for mobile radar platforms.
IEEE - Aerospace and Electronic Systems - August 2023

Table of Contents for the Digital Edition of IEEE - Aerospace and Electronic Systems - August 2023
