<s>
Multiple-input	O
multiple-output	O
(	O
MIMO	O
)	O
radar	O
is	O
an	O
advanced	O
type	O
of	O
phased	B-Algorithm
array	I-Algorithm
radar	I-Algorithm
employing	O
digital	O
receivers	O
and	O
waveform	O
generators	O
distributed	O
across	O
the	O
aperture	O
.	O
</s>
<s>
MIMO	B-Algorithm
radar	I-Algorithm
signals	O
propagate	O
in	O
a	O
fashion	O
similar	O
to	O
multistatic	B-Algorithm
radar	I-Algorithm
.	O
</s>
<s>
MIMO	B-Algorithm
radar	I-Algorithm
may	O
also	O
be	O
used	O
to	O
obtain	O
low-probability-of-intercept	O
radar	O
properties	O
.	O
</s>
<s>
MIMO	B-Algorithm
radar	I-Algorithm
systems	O
transmit	O
mutually	O
orthogonal	O
signals	O
from	O
multiple	O
transmit	O
antennas	O
,	O
and	O
these	O
waveforms	O
can	O
be	O
extracted	O
from	O
each	O
of	O
the	O
receive	O
antennas	O
by	O
a	O
set	O
of	O
matched	O
filters	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
MIMO	B-Algorithm
radar	I-Algorithm
system	O
has	O
3	O
transmit	O
antennas	O
and	O
4	O
receive	O
antennas	O
,	O
12	O
signals	O
can	O
be	O
extracted	O
from	O
the	O
receiver	O
because	O
of	O
the	O
orthogonality	O
of	O
the	O
transmitted	O
signals	O
.	O
</s>
<s>
That	O
is	O
,	O
a	O
12-element	O
virtual	O
antenna	O
array	O
is	O
created	O
using	O
only	O
7	O
antennas	O
by	O
conducting	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
on	O
the	O
received	O
signals	O
,	O
thereby	O
obtaining	O
a	O
finer	O
spatial	O
resolution	O
compared	O
with	O
its	O
phased	O
array	O
counterpart	O
.	O
</s>
<s>
The	O
picture	O
shows	O
a	O
M-by-N	O
MIMO	B-Algorithm
radar	I-Algorithm
system	O
.	O
</s>
<s>
In	O
order	O
to	O
make	O
a	O
comparison	O
between	O
phased	B-Algorithm
array	I-Algorithm
radars	I-Algorithm
and	O
MIMO	B-Algorithm
radars	I-Algorithm
,	O
the	O
relationship	O
between	O
transmit/receive	O
antenna	O
arrays	O
and	O
virtual	O
arrays	O
are	O
discussed	O
in	O
several	O
sources	O
.	O
</s>
<s>
To	O
estimate	O
the	O
direction	B-Algorithm
of	I-Algorithm
arrival	I-Algorithm
of	O
the	O
targets	O
according	O
to	O
the	O
N*M	O
signals	O
,	O
methods	O
like	O
MUSIC	O
(	O
algorithm	O
)	O
and	O
maximum	O
likelihood	O
estimation	O
are	O
commonly	O
used	O
with	O
good	O
results	O
.	O
</s>
<s>
There	O
are	O
a	O
variety	O
of	O
orthogonal	O
signal	O
sets	O
used	O
in	O
the	O
field	O
of	O
MIMO	B-Algorithm
radar	I-Algorithm
.	O
</s>
<s>
One	O
of	O
the	O
proposed	O
signal	O
sets	O
is	O
the	O
spectrally	O
interleaved	O
multi-carrier	O
signal	O
,	O
which	O
is	O
a	O
modified	O
version	O
of	O
orthogonal	B-Protocol
frequency-division	I-Protocol
multiplexing	I-Protocol
signal	O
.	O
</s>
<s>
Another	O
proposed	O
signal	O
set	O
is	O
orthogonal	O
chirp	B-Application
signal	O
,	O
which	O
can	O
be	O
expressed	O
as	O
:	O
</s>
<s>
By	O
choosing	O
different	O
initial	O
frequencies	O
,	O
these	O
chirp	B-Application
waveforms	O
can	O
be	O
made	O
orthogonal	O
.	O
</s>
