<s>
Array	B-Algorithm
processing	I-Algorithm
is	O
a	O
wide	O
area	O
of	O
research	O
in	O
the	O
field	O
of	O
signal	O
processing	O
that	O
extends	O
from	O
the	O
simplest	O
form	O
of	O
1	O
dimensional	O
line	O
arrays	O
to	O
2	O
and	O
3	O
dimensional	O
array	O
geometries	O
.	O
</s>
<s>
The	O
sensors	O
used	O
for	O
a	O
specific	O
problem	O
may	O
vary	O
widely	O
,	O
for	O
example	O
microphones	B-Application
,	O
accelerometers	O
and	O
telescopes	O
.	O
</s>
<s>
Some	O
common	O
problem	O
that	O
are	O
solved	O
with	O
array	B-Algorithm
processing	I-Algorithm
techniques	O
are	O
:	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
metrics	O
are	O
often	O
assessed	O
noisy	O
environments	O
.	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
emerged	O
in	O
the	O
last	O
few	O
decades	O
as	O
an	O
active	O
area	O
and	O
was	O
centered	O
on	O
the	O
ability	O
of	O
using	O
and	O
combining	O
data	O
from	O
different	O
sensors	O
(	O
antennas	O
)	O
in	O
order	O
to	O
deal	O
with	O
specific	O
estimation	O
task	O
(	O
spatial	O
and	O
temporal	O
processing	O
)	O
.	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
is	O
used	O
in	O
radar	B-Application
,	O
sonar	B-Application
,	O
seismic	O
exploration	O
,	O
anti-jamming	O
and	O
wireless	O
communications	O
.	O
</s>
<s>
One	O
of	O
the	O
main	O
advantages	O
of	O
using	O
array	B-Algorithm
processing	I-Algorithm
along	O
with	O
an	O
array	O
of	O
sensors	O
is	O
a	O
smaller	O
foot-print	O
.	O
</s>
<s>
The	O
problems	O
associated	O
with	O
array	B-Algorithm
processing	I-Algorithm
include	O
the	O
number	O
of	O
sources	O
used	O
,	O
their	O
direction	O
of	O
arrivals	O
,	O
and	O
their	O
signal	O
waveforms	O
.	O
</s>
<s>
There	O
are	O
four	O
assumptions	O
in	O
array	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
The	O
second	O
assumption	O
is	O
that	O
for	O
far	O
field	O
array	B-Algorithm
processing	I-Algorithm
,	O
the	O
radius	O
of	O
propagation	O
is	O
much	O
greater	O
than	O
size	O
of	O
the	O
array	O
and	O
that	O
there	O
is	O
plane	O
wave	O
propagation	O
.	O
</s>
<s>
To	O
motivate	O
the	O
reader	O
,	O
some	O
of	O
the	O
most	O
important	O
applications	O
related	O
to	O
array	B-Algorithm
processing	I-Algorithm
will	O
be	O
discussed	O
.	O
</s>
<s>
Radar	B-Application
and	O
Sonar	B-Application
Systems	I-Application
:	O
</s>
<s>
array	B-Algorithm
processing	I-Algorithm
concept	O
was	O
closely	O
linked	O
to	O
radar	B-Application
and	O
sonar	B-Application
systems	I-Application
which	O
represent	O
the	O
classical	O
applications	O
of	O
array	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
Radar	B-Application
systems	I-Application
used	O
basically	O
to	O
detect	O
objects	O
by	O
using	O
radio	O
waves	O
.	O
</s>
<s>
Radar	B-Application
systems	I-Application
started	O
as	O
military	O
equipments	O
then	O
entered	O
the	O
civilian	O
world	O
.	O
</s>
<s>
In	O
radar	B-Application
applications	O
,	O
different	O
modes	O
can	O
be	O
used	O
,	O
one	O
of	O
these	O
modes	O
is	O
the	O
active	O
mode	O
.	O
</s>
<s>
By	O
using	O
the	O
returns	O
,	O
the	O
estimation	O
of	O
parameters	O
such	O
as	O
velocity	O
,	O
range	O
and	O
DOAs	O
(	O
direction	B-Algorithm
of	I-Algorithm
arrival	I-Algorithm
)	O
of	O
target	O
of	O
interest	O
become	O
possible	O
.	O
</s>
<s>
Sonar	B-Application
systems	I-Application
(	O
Sound	O
Navigation	O
and	O
Ranging	O
)	O
use	O
the	O
sound	O
waves	O
that	O
propagate	O
under	O
the	O
water	O
to	O
detect	O
objects	O
on	O
or	O
under	O
the	O
water	O
surface	O
.	O
</s>
<s>
Two	O
types	O
of	O
sonar	B-Application
systems	I-Application
can	O
be	O
defined	O
the	O
active	O
one	O
and	O
the	O
passive	O
one	O
.	O
</s>
<s>
In	O
active	O
sonar	B-Application
,	O
the	O
system	O
emits	O
pulses	O
of	O
sound	O
and	O
listens	O
to	O
the	O
returns	O
that	O
will	O
be	O
used	O
to	O
estimate	O
parameters	O
.	O
</s>
<s>
In	O
the	O
passive	O
sonar	B-Application
,	O
the	O
system	O
is	O
essentially	O
listening	O
for	O
the	O
sounds	O
made	O
by	O
the	O
target	O
objects	O
.	O
</s>
<s>
It	O
is	O
very	O
important	O
to	O
note	O
the	O
difference	O
between	O
the	O
radar	B-Application
system	I-Application
that	O
uses	O
radio	O
waves	O
and	O
the	O
sonar	B-Application
system	O
that	O
uses	O
sound	O
waves	O
,	O
the	O
reason	O
why	O
the	O
sonar	B-Application
uses	O
the	O
sound	O
wave	O
is	O
because	O
sound	O
waves	O
travel	O
farther	O
in	O
the	O
water	O
than	O
do	O
radar	B-Application
and	O
light	O
waves	O
.	O
</s>
<s>
In	O
passive	O
sonar	B-Application
,	O
the	O
receiving	O
array	O
has	O
the	O
capability	O
of	O
detecting	O
distant	O
objects	O
and	O
their	O
locations	O
.	O
</s>
<s>
Deformable	O
array	O
are	O
usually	O
used	O
in	O
sonar	B-Application
systems	I-Application
where	O
the	O
antenna	O
is	O
typically	O
drawn	O
under	O
the	O
water	O
.	O
</s>
<s>
In	O
active	O
sonar	B-Application
,	O
the	O
sonar	B-Application
system	O
emits	O
sound	O
waves	O
(	O
acoustic	O
energy	O
)	O
then	O
listening	O
and	O
monitoring	O
any	O
existing	O
echo	O
(	O
the	O
reflected	O
waves	O
)	O
.	O
</s>
<s>
Difficulties	O
and	O
limitations	O
in	O
sonar	B-Application
systems	I-Application
comparing	O
to	O
radar	B-Application
systems	I-Application
emerged	O
from	O
the	O
fact	O
that	O
the	O
propagation	O
speed	O
of	O
sound	O
waves	O
under	O
the	O
water	O
is	O
slower	O
than	O
the	O
radio	O
waves	O
.	O
</s>
<s>
Despite	O
all	O
these	O
limitations	O
and	O
difficulties	O
,	O
sonar	B-Application
system	O
remains	O
a	O
reliable	O
technique	O
for	O
range	O
,	O
distance	O
,	O
position	O
and	O
other	O
parameters	O
estimation	O
for	O
underwater	O
applications	O
.	O
</s>
<s>
NORSAR	O
has	O
been	O
working	O
with	O
array	B-Algorithm
processing	I-Algorithm
ever	O
since	O
to	O
measure	O
seismic	O
activity	O
around	O
the	O
globe	O
.	O
</s>
<s>
NORSAR	O
has	O
ongoing	O
work	O
to	O
improve	O
array	B-Algorithm
processing	I-Algorithm
to	O
improve	O
monitoring	O
of	O
seismic	O
activity	O
not	O
only	O
in	O
Norway	O
but	O
around	O
the	O
globe	O
.	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
and	O
estimation	O
techniques	O
have	O
been	O
used	O
in	O
wireless	O
communication	O
.	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
techniques	O
got	O
on	O
much	O
attention	O
from	O
medical	O
and	O
industrial	O
applications	O
.	O
</s>
<s>
In	O
medical	O
applications	O
,	O
the	O
medical	O
image	O
processing	O
field	O
was	O
one	O
of	O
the	O
basic	O
fields	O
that	O
use	O
array	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
Other	O
medical	O
applications	O
that	O
use	O
array	B-Algorithm
processing	I-Algorithm
:	O
diseases	O
treatment	O
,	O
tracking	O
waveforms	O
that	O
have	O
information	O
about	O
the	O
condition	O
of	O
internal	O
organs	O
e.g.	O
</s>
<s>
Speech	O
enhancement	O
and	O
processing	O
represents	O
another	O
field	O
that	O
has	O
been	O
affected	O
by	O
the	O
new	O
era	O
of	O
array	B-Algorithm
processing	I-Algorithm
.	O
</s>
<s>
In	O
addition	O
to	O
these	O
external	O
sources	O
,	O
the	O
strength	O
of	O
the	O
desired	O
signal	O
is	O
reduced	O
due	O
to	O
the	O
relatively	O
distance	O
between	O
speaker	O
and	O
microphones	B-Application
.	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
techniques	O
have	O
opened	O
new	O
opportunities	O
in	O
speech	O
processing	O
to	O
attenuate	O
noise	O
and	O
echo	O
without	O
degrading	O
the	O
quality	O
of	O
and	O
affecting	O
adversely	O
the	O
speech	O
signal	O
.	O
</s>
<s>
In	O
general	O
array	B-Algorithm
processing	I-Algorithm
techniques	O
can	O
be	O
used	O
in	O
speech	O
processing	O
to	O
reduce	O
the	O
computing	O
power	O
(	O
number	O
of	O
computations	O
)	O
and	O
enhance	O
the	O
quality	O
of	O
the	O
system	O
(	O
the	O
performance	O
)	O
.	O
</s>
<s>
In	O
addition	O
to	O
these	O
applications	O
,	O
many	O
applications	O
have	O
been	O
developed	O
based	O
on	O
array	B-Algorithm
processing	I-Algorithm
techniques	O
:	O
Acoustic	O
Beamforming	O
for	O
Hearing	O
Aid	O
Applications	O
,	O
Under-determined	O
Blind	O
Source	O
Separation	O
Using	O
Acoustic	O
Arrays	O
,	O
Digital	O
3D/4D	O
Ultrasound	O
Imaging	O
Array	O
,	O
Smart	O
Antennas	O
,	O
Synthetic	O
aperture	O
radar	B-Application
,	O
underwater	O
acoustic	O
imaging	O
,	O
and	O
Chemical	O
sensor	O
arrays	O
...	O
etc	O
.	O
</s>
<s>
An	O
alternative	O
is	O
to	O
more	O
fully	O
exploit	O
the	O
underlying	O
data	O
model	O
,	O
leading	O
to	O
so-called	O
parametric	O
array	B-Algorithm
processing	I-Algorithm
methods	O
.	O
</s>
<s>
When	O
applying	O
the	O
ML	O
technique	O
to	O
the	O
array	B-Algorithm
processing	I-Algorithm
problem	O
,	O
two	O
main	O
methods	O
have	O
been	O
considered	O
depending	O
on	O
the	O
signal	O
data	O
model	O
assumption	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
in	O
the	O
Deterministic	O
ML	O
the	O
signals	O
are	O
considered	O
as	O
unknown	O
,	O
deterministic	O
quantities	O
that	O
need	O
to	O
be	O
estimated	O
in	O
conjunction	O
with	O
the	O
direction	B-Algorithm
of	I-Algorithm
arrival	I-Algorithm
.	O
</s>
<s>
According	O
to	O
the	O
Deterministic	O
ML	O
the	O
signals	O
are	O
considered	O
as	O
unknown	O
,	O
deterministic	O
quantities	O
that	O
need	O
to	O
be	O
estimated	O
in	O
conjunction	O
with	O
the	O
direction	B-Algorithm
of	I-Algorithm
arrival	I-Algorithm
.	O
</s>
<s>
By	O
using	O
Discrete	B-Algorithm
Fourier	I-Algorithm
Transform	I-Algorithm
(	O
DFT	O
)	O
it	O
is	O
possible	O
to	O
analyze	O
signals	O
in	O
the	O
time	O
domain	O
as	O
well	O
as	O
in	O
the	O
spectral	O
domain	O
.	O
</s>
<s>
Array	B-Algorithm
processing	I-Algorithm
technique	O
represents	O
a	O
breakthrough	O
in	O
signal	O
processing	O
.	O
</s>
<s>
Many	O
applications	O
and	O
problems	O
which	O
are	O
solvable	O
using	O
array	B-Algorithm
processing	I-Algorithm
techniques	O
are	O
introduced	O
.	O
</s>
<s>
It	O
is	O
highly	O
expected	O
that	O
the	O
importance	O
of	O
array	B-Algorithm
processing	I-Algorithm
will	O
grow	O
as	O
the	O
automation	O
becomes	O
more	O
common	O
in	O
industrial	O
environment	O
and	O
applications	O
,	O
further	O
advances	O
in	O
digital	O
signal	O
processing	O
and	O
digital	O
signal	O
processing	O
systems	O
will	O
also	O
support	O
the	O
high	O
computation	O
requirements	O
demanded	O
by	O
some	O
of	O
the	O
estimation	O
techniques	O
.	O
</s>
<s>
In	O
this	O
article	O
we	O
emphasized	O
the	O
importance	O
of	O
array	B-Algorithm
processing	I-Algorithm
by	O
listing	O
the	O
most	O
important	O
applications	O
that	O
include	O
a	O
form	O
of	O
array	B-Algorithm
processing	I-Algorithm
techniques	O
.	O
</s>
<s>
We	O
briefly	O
describe	O
the	O
different	O
classifications	O
of	O
array	B-Algorithm
processing	I-Algorithm
,	O
spectral	O
and	O
parametric	O
based	O
approaches	O
.	O
</s>
