Aerospace and Electronic Systems Magazine October 2016 - 2

In This Issue - Technically
RELEVANCE OF ELECTROMAGNETICS IN WIRELESS SYSTEMS DESIGN
The objective of this article is to illustrate that certain issues need to be factored
into the design of wireless radiofrequency systems (communication, radar, navigation, countermeasures, etc.). For example, the exclusive use of communication
principles in the design of the physical layer of such systems may not be sufficient. Three specific topics are treated: the different concepts of channel capacity
and their implications; the antenna and its relationships to the Maximum Power
Transfer Theorem and, ultrawideband (UWB) wireless transmission of signals
without any distortion. Examples are presented to illustrate these issues.

COGNITIVE RADARS IN SPECTRALLY DENSE ENVIRONMENTS

d at a later time come out the
w of conservation of energy ....
round the sun can be obtained
ations of the laws of conservamentum [7]. Moreover, the
p principles for they relate to
nstance, conservation of energy
are time-invariant, and vice
g and information theory, the
nt one. It appears as the energy
nformation, or one symbol of
mes in form of transmit power,
ust be supplied per unit of time
ng. Yet, it is interesting that
d, this very fact that is of such
ysics, apparently plays no role in
tion theory [9], signal processy [11] or signal theory [12]. The
ny research work in these areas
energy is conserved is explored
is strange absence of conservainformation theory and related
that inputs and outputs are
ach, instead of by a pair of conand momentum in Hamiltonian
current in circuit theory [13].

opment of the physical layer adms perform according to design
he principles of electromagnetantennas and maximum power
apacity and how it can be quantis and the radiation efficiency of
ximum power transfer theorem.

HAT ARE PARAMOUNT IN

hat must be understood and corntenna is a spatial filter. Unfortusiders this topic when developing
ng. An antenna is also a temporal

filter, as it can be tuned to a desired bandwidth, which the signal
processing community exploits to develop innovative algorithms
primarily based on the temporal properties of the signals. However,
an antenna is simultaneously both a temporal filter and a spatial filter
that are connected. This space-time relationship and its associated
properties can be characterized in an exact fashion with Maxwell's
equations. This set of equations is one of the few in physics that has
withstood the erosion and corrosion of progress; even the inclusion
of relativity has had no effect on Maxwell's equations, because it is
built into them. Recognition of this important space-time relationship can make it possible to address problems that may not be solved
by using temporal or spatial properties alone or independently.
Second, an antenna does not radiate power; it radiates power density, which is related to the electric (E(w)) and magnetic
(H(w)) fields-the exact relationship is given by the Poynting vector (E(w)H*(w)/2), where * denotes complex conjugation. When
a power density radiated by a transmitting antenna is incident on
a receiving antenna, the receiving antenna integrates the incident
electric field intensity (which has units of volt per meter) to generate a voltage at its terminals. In other words, an antenna is excited
by a voltage source or a current source and not by a power source.
Once the voltage applied to the antenna and the environment into
which it subsequently radiates (which dictates the input impedance
of the antenna) are known, the input power applied to the antenna
can be obtained. When comparing performances of systems containing radiating antennas, it is imperative that the input power to
the radiating antenna be kept fixed; otherwise, the comparison may
not be scientific. Hence, it is relevant to deal with antenna radiation
efficiency rather than use of the maximum power transfer theorem
that has no direct connection with the total amount of applied input
power. Note that in nature, the term power amplifier is a misnomer,
as no such device exists. The only physically realizable amplifier is
either a voltage amplifier or a current amplifier that converts the input direct current (DC) power into alternating current (AC) power;
hence, the terminology power amplifier leads to misunderstandings. Consequently, an antenna excitation needs to be implemented
and analytically modeled using one of these two sources-either
a voltage source or a current source, then focusing on the design
of the antenna, based on the radiation efficiency, which relates the
fraction of the input power that is radiated from it.
The last point addresses an important issue in electrical engineering in which two principles are of paramount importance:

IEEE A&E SYSTEMS MAGAZINE

Communication systems are proliferating at incredible rates, resulting in spectrally dense environments and fierce competition for frequency bands that traditionally have been exclusively allocated to radar systems as primary legal users.
To cope with the issue of spectrum crowding, future radar systems will need
to coexist with other radio frequency systems, anticipating their behavior and
properly reacting to avoid interference. To this end, they need critical and new
methodologies based upon cognition as an enabling technology. This work describes the main functions and the system architecture of a Cognitive Radar that
operates as a secondary user in a spectrally dense environment, trying to minimize the interference between the radar and the primary users.

CO-DESIGNED RADAR-COMMUNICATION USING LINEAR FREQUENCY
MODULATION WAVEFORM
As electromagnetic spectrum availability shrinks, there is growing interest in
combining multiple functions, such as radar and communications signals, into a
single multipurpose waveform. Historically mixed-modulation has used orthogonal separation of different message signals in different dimensions such as time
or frequency. This research explores an alternative approach of implementing
an in-band, mixed-modulated waveform that combines surveillance radar and
communication functions into a single signal. The contribution of this research
is the use of reduced phase angle binary phase shift keying (BPSK) along with
overlapped (channelized) spread-spectrum phase discretes based on pseudorandom noise sequences to encode multiple messages in a single pulse. The resulting mixed-modulated signal provides a low data rate communications message
while minimizing the effect on radar performance. For the purpose of this research, radar performance will be evaluated in terms of power spectral density,
matched filter auto-correlation for target detection, and the ambiguity function.

SIGNALING STRATEGIES FOR DUAL-FUNCTION RADAR-COMMUNICATIONS:
AN OVERVIEW
In this article, we consider dual-function radar communication systems which
are a special case of radar-communications co-existence. We present an overview of different strategies for radar-embedded communication signals. Such
strategies are key to establishing dual-functions systems that permit simultaneous execution of both radar and communication functions from a shared platform. We provide a balanced and complete account of existing methods and
discuss their respective advantages and disadvantages.

IEEE A&E SYSTEMS MAGAZINE

OCTOBER 2016

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine October 2016