IEEE Signal Processing - July 2018 - 123

(dB)

Of course, the frequency use and emissions by radars and
attempts to add cognition to classical radar systems have been
other transmitting devices are all regulated. Many countries,
made both theoretically and practically, as presented in this
but not all, have adopted the ITU emission standard [34]. Figarticle and proven by the many conference and journal papers
ure 15 shows a typical emission mask that
published over the last decade. (A sample of
might be applied to radar systems. There is
them are in the list of references.) However,
What is really difficult to
a band over which the radar is designed to
many problems are still open and require
implement in a machine
transmit. It is fixed in frequency and goes
additional effort to make cognitive radars a
such as a radar is the
down −40 dB from the peak. Outside, at
full reality.
capability of learning from lower power levels, OOB emissions are
In active radars, cognition requires wavethe mistakes that occurred permitted, with a rolloff of generally
forms and circuits to be reconfigurable and
as a result of poor
−20 dB/decade (−40 dB/decade is under
optimizable in real time. Initial progress has
consideration). The radar transmissions
been made in the two separate fields [6], but
decisions in the past.
should not exceed the limits imposed by
a fully optimized solution that includes all
the mask, but unfortunately unwanted emissions often occur
of the important aspects of radar circuitry has not yet been prebecause of nonlinearity in the transmitter and the steep rise
sented [7], even though some attempts to consider the radar
and fall times of the radar pulses [26].
as a holistic system (hardware in the loop) have been preAn intermediate step toward arbitrary waveform generasented, as in [35].
tion is the selection of waveforms or waveform parameters
The dynamic reconfiguration of the spectrum portion to
from a prespecified set. Many modern radars already have this
be used for transmitting, as described in previous sections, is
capability, and a first step toward making cognitive radars a
not always easily implementable. The main reason is that quite
reality could be implementing cognitive processing to choose
often, because of the nonlinear operational regime of the highamong the set of allowable waveforms [9].
power radar radio-frequency circuitry (particularly for vacuum
In passive multisensory radar systems, the cost must be
tube amplifiers), there is a nonnegligible spectral spreading outkept low, because this is one of the main reasons that justify
side the assigned radar band (spectral regrowth). This makes
their use, despite their poorer performance compared to active
the coexistence of communications and radar systems in close
systems. Cognitive algorithms implemented in passive systems
bands with narrow guard bands difficult [26]. Magnetron tubes,
should then be easy to implement and not be very demandquite often used in legacy radar systems because they are inexing in terms of energy and memory use. Fortunately, the rapid
pensive, have serious drawbacks in terms of spectral purity. To
increase in the performance of digital signal processors, fieldreduce the out-of-band (OOB) emissions, bandpass filters are
programmable gate arrays, and application-specific integrated
often used, though the cost of this improvement in spectral puricircuits has made the signal processing more compact and low
ty means a significant loss in the effective transmitted power.
power [33].
Solid-state-based amplifiers are much easier to control in terms
There are numerous exciting future research directions to
of OOB, but unfortunately they cannot provide the high peak
be explored to make cognitive radars a reality. Some general
power of tubes, and anyway they represent only a small minorareas include the extension of the basic concepts to multiuser
ity of current operational systems.
and multiobjective systems and to systems with large degrees
of freedom available for adaptation (e.g., frequency, antennas,
waveforms/codes, polarization, power resources, and transmitter/receiver selection) and the expansion of the role of learning
0
and knowledge storage/recovery over longer time horizons.
In-Band or
Necessary
What is really difficult to implement in a machine such
Ultimate
Emissions
Suppression
as a radar is the capability of learning from the mistakes that
Level (Here
occurred as a result of poor decisions in the past, and hence
60 dB)
the ability to make a very informed decision in the future, as
envisioned in [28].
-40
20 dB/Decade Rolloff
-60
Spurious
OOB
Emissions Emissions
0
Frequency Relative to Fundamental

FiguRe 15. A graph of a generic ITU spectral mask, showing the required
suppressions relative to power at fundamental (decibel).

Authors
Maria S. Greco (m.greco@iet.unipi.it) received her M.S.
degree in electronic engineering in 1993 and her Ph.D. degree
in information engineering in 1998. She is a full professor in
the Department of Information Engineering, Pisa University,
Italy. She is a corecipient of the 2001 and 2012 IEEE
Aerospace and Electronic Systems Society's (AESS's) Barry
Carlton Award for Best Paper and a recipient of the IEEE
AESS 2008 Fred Nathanson Young Engineer of the Year
award. She has been general chair, technical program chair,

IEEE Signal Processing Magazine

July 2018

123

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