MIMO RADAR: AN IDEA WHOSE TIME
HAS
COME
Eran Fishlert, Alex Haimovicht, Rick
Blumt,
Dmitry Chizhik*, Len Cimini', Reinaldo Valeneuela*
t
New Jersey Institute
of
Technology, Newark, NJ 07 102, e-mail: eran.fishler@njit.edu, haimovic@njit.edu
$
Lehigh University, Bethlehem,
PA
18015-3084, e-mail: rblum@eecs.lehigh.edu
o
University
of
Delaware, Newark, DE 197 16, e-mail: cimini@ece.udel.edu
*
Bell Labs
-
Lucent Technologies, e-mail: chizhik,rav@lucent.com
ABSTRACT
It has been recently shown that multiple-input multiple-output
(MIMO) antenna systems have the potential to dramatically
improve the performance of comniunication systems over
single antenna systems. Unlike beamforming, which pre-
sumes a high correlation between signals either transmit-
ted or received by an array, the MIMO concept exploits
the independence between signals at the array elements.
In
conventional radar, target scintillations are regarded as a
nuisance parameter that degrades radar performance. The
novelty of MIMO radar is that it takes the opposite view,
namely, it capitalizes on target scintillations
to
improve the
radar's performance. In this paper, we introduce the MIMO
concept for radar. The MIMO radar system under consid-
eration consists of a transmit array with widely-spaced ele-
ments such that each views a different aspect of the target.
The array at the receiver is a conventional may used for
direction finding (DF). The system performance analysis is
carried out in terms of the Cramer-Rao bound of the mean-
square error in estimating the target direction. It is shown
that MIMO radar leads to significant perfomiance improve-
ment in
DF
accuracy.
I.
Introduction
The idea of active direction finding for radar or active sonar
is not new (see, for example
[
1,2]).
In radar
or
active sonar,
a known waveform is transmitted by an omnidirectional an-
tenna, and a target reflects some of the transmitted energy
toward an array of sensors that is used to estimate some un-
known parameters, e.g., bearing, range, or speed. There are
two common approaches for estimating the unknown pa-
rameters. In the first approach, high resolution techniques,
e.g., MUSIC or maximum likelihood (ML)
[3],
are used to
estimate parameters of the target of interest. In the second
approach, the array of sensors is used to steer a beam
to-
ward a certain direction in space and look for some energy,
WORK
BY
RICK BLUM AND ALEX HAlMOVlCH WAS SUP-
PORTED
IN
PART
BY
THE AIR FORCE OFFICE OF SCIENTIFIC
RE-
SEARCH.
0-7803-8234-x/04/$17.00
0
2004 IEEE
essentially the same way as a conventional radar with a
di-
rectional antenna. It is well known that an array of receivers
can steer a beam toward any direction in space by using a
process known as
beantfor-nzirzg
[4].
Unlike high resolution
techniques, beamforming is based on a fixed transforma-
tion.
The advantages of using an array of closely spaced sen-
sors at the receiver are well known (see, for example
[S,
4,3,
6,7].
Among these advantages are: the lack of any mechan-
ical elements in the system, the ability to use advanced sig-
nal processing techniques for improving performance, and
the ability to steer multiple beanis at once. In this paper
we are concerned with radars employing multiple antennas
both at the transmitter and at the receiver.
Transmit arrays have been proposed in the form
of
elec-
tronic steered mays (ESA). With an ESA, phase shifts at the
transmit antennas form and steer the transmit beam similar
to a directional antenna, except that the steering is electronic
rather than mechanical. Before introducing a new concept
for radar with multiple transmit antennas, a fair question
to
ask is whether an ESA has any processing gain (in addition
to its mechanical advantages). The ESA essentially mimics
the scanning operation of a directional antenna. However,
as we show next, ESA's have no advantage over systems
that use a single omnidirectional antenna at the transmit-
ter.
To
that end, we note that the error in that angle of
ar-
rival estimation is a function of the total received energy.
Assume that the total transmitted power is independent of
the number of transnlit antennas. For the single transnlit
antennas case, say that the average transmitted power is
P
and the duration
of
the transmitted waveform is
T
sec-
onds. The energy received from the target is
uPT,
where
U
represents the target's'radio cross section
(RCS).
Now, as-
sume
an
ESA that creates a beam with beamwidth
+.
With
the beamwidth
4,
the transmitter can realize a gain of
y
(in linear scale). However, since the transmitter needs to
scan the whole space in
T
seconds, it can illuminate the
target for only
2
seconds. The total received energy is
UP?%
=
OPT.
This demonstrates that the amount of
energy received by the radar is independent of the exact
number of elements of the ESA. Therefore, the ESA has
71