<s>
In	O
telecommunication	O
,	O
especially	O
radio	O
communication	O
,	O
spread	B-Architecture
spectrum	I-Architecture
designates	O
techniques	O
by	O
which	O
a	O
signal	O
(	O
e.g.	O
,	O
an	O
electrical	O
,	O
electromagnetic	O
,	O
or	O
acoustic	O
)	O
generated	O
with	O
a	O
particular	O
bandwidth	B-Algorithm
is	O
deliberately	O
spread	O
in	O
the	O
frequency	O
domain	O
,	O
resulting	O
in	O
a	O
signal	O
with	O
a	O
wider	O
bandwidth	B-Algorithm
.	O
</s>
<s>
Spread-spectrum	B-Architecture
techniques	O
are	O
used	O
for	O
the	O
establishment	O
of	O
secure	O
communications	O
,	O
increasing	O
resistance	O
to	O
natural	O
interference	O
,	O
noise	O
,	O
and	O
jamming	O
,	O
to	O
prevent	O
detection	O
,	O
to	O
limit	O
power	O
flux	O
density	O
(	O
e.g.	O
,	O
in	O
satellite	B-Application
downlinks	O
)	O
,	O
and	O
to	O
enable	O
multiple-access	O
communications	O
.	O
</s>
<s>
Spread	B-Architecture
spectrum	I-Architecture
generally	O
makes	O
use	O
of	O
a	O
sequential	O
noise-like	O
signal	O
structure	O
to	O
spread	O
the	O
normally	O
narrowband	O
information	O
signal	O
over	O
a	O
relatively	O
wideband	O
(	O
radio	O
)	O
band	O
of	O
frequencies	O
.	O
</s>
<s>
Frequency-hopping	B-Protocol
spread	I-Protocol
spectrum	I-Protocol
(	O
FHSS	B-Protocol
)	O
,	O
direct-sequence	B-Device
spread	I-Device
spectrum	I-Device
(	O
DSSS	B-Device
)	O
,	O
time-hopping	O
spread	B-Architecture
spectrum	I-Architecture
(	O
THSS	O
)	O
,	O
chirp	B-Protocol
spread	I-Protocol
spectrum	I-Protocol
(	O
CSS	O
)	O
,	O
and	O
combinations	O
of	O
these	O
techniques	O
are	O
forms	O
of	O
spread	B-Architecture
spectrum	I-Architecture
.	O
</s>
<s>
The	O
first	O
two	O
of	O
these	O
techniques	O
employ	O
pseudorandom	O
number	O
sequencescreated	O
using	O
pseudorandom	O
number	O
generatorsto	O
determine	O
and	O
control	O
the	O
spreading	O
pattern	O
of	O
the	O
signal	O
across	O
the	O
allocated	O
bandwidth	B-Algorithm
.	O
</s>
<s>
Wireless	O
standard	O
IEEE	O
802.11	O
uses	O
either	O
FHSS	B-Protocol
or	O
DSSS	B-Device
in	O
its	O
radio	O
interface	O
.	O
</s>
<s>
The	O
core	O
principle	O
of	O
spread	B-Architecture
spectrum	I-Architecture
is	O
the	O
use	O
of	O
noise-like	O
carrier	O
waves	O
,	O
and	O
,	O
as	O
the	O
name	O
implies	O
,	O
bandwidths	B-Algorithm
much	O
wider	O
than	O
that	O
required	O
for	O
simple	O
point-to-point	O
communication	O
at	O
the	O
same	O
data	O
rate	O
.	O
</s>
<s>
Direct	O
sequence	O
(	O
DS	O
)	O
is	O
good	O
at	O
resisting	O
continuous-time	O
narrowband	O
jamming	O
,	O
while	O
frequency	B-Protocol
hopping	I-Protocol
(	O
FH	O
)	O
is	O
better	O
at	O
resisting	O
pulse	O
jamming	O
.	O
</s>
<s>
In	O
DS	O
systems	O
,	O
narrowband	O
jamming	O
affects	O
detection	O
performance	O
about	O
as	O
much	O
as	O
if	O
the	O
amount	O
of	O
jamming	O
power	O
is	O
spread	O
over	O
the	O
whole	O
signal	B-Algorithm
bandwidth	I-Algorithm
,	O
where	O
it	O
will	O
often	O
not	O
be	O
much	O
stronger	O
than	O
background	O
noise	O
.	O
</s>
<s>
By	O
contrast	O
,	O
in	O
narrowband	O
systems	O
where	O
the	O
signal	B-Algorithm
bandwidth	I-Algorithm
is	O
low	O
,	O
the	O
received	O
signal	O
quality	O
will	O
be	O
severely	O
lowered	O
if	O
the	O
jamming	O
power	O
happens	O
to	O
be	O
concentrated	O
on	O
the	O
signal	B-Algorithm
bandwidth	I-Algorithm
.	O
</s>
<s>
The	O
spreading	O
sequence	O
(	O
in	O
DS	O
systems	O
)	O
or	O
the	O
frequency-hopping	B-Protocol
pattern	O
(	O
in	O
FH	O
systems	O
)	O
is	O
often	O
unknown	O
by	O
anyone	O
for	O
whom	O
the	O
signal	O
is	O
unintended	O
,	O
in	O
which	O
case	O
it	O
obscures	O
the	O
signal	O
and	O
reduces	O
the	O
chance	O
of	O
an	O
adversary	O
making	O
sense	O
of	O
it	O
.	O
</s>
<s>
Moreover	O
,	O
for	O
a	O
given	O
noise	O
power	O
spectral	O
density	O
(	O
PSD	O
)	O
,	O
spread-spectrum	B-Architecture
systems	O
require	O
the	O
same	O
amount	O
of	O
energy	O
per	O
bit	O
before	O
spreading	O
as	O
narrowband	O
systems	O
and	O
therefore	O
the	O
same	O
amount	O
of	O
power	O
if	O
the	O
bitrate	O
before	O
spreading	O
is	O
the	O
same	O
,	O
but	O
since	O
the	O
signal	O
power	O
is	O
spread	O
over	O
a	O
large	O
bandwidth	B-Algorithm
,	O
the	O
signal	O
PSD	O
is	O
much	O
lower	O
often	O
significantly	O
lower	O
than	O
the	O
noise	O
PSD	O
so	O
that	O
the	O
adversary	O
may	O
be	O
unable	O
to	O
determine	O
whether	O
the	O
signal	O
exists	O
at	O
all	O
.	O
</s>
<s>
However	O
,	O
for	O
mission-critical	O
applications	O
,	O
particularly	O
those	O
employing	O
commercially	O
available	O
radios	O
,	O
spread-spectrum	B-Architecture
radios	O
do	O
not	O
provide	O
adequate	O
security	O
unless	O
,	O
at	O
a	O
minimum	O
,	O
long	O
nonlinear	O
spreading	O
sequences	O
are	O
used	O
and	O
the	O
messages	O
are	O
encrypted	O
.	O
</s>
<s>
The	O
high	O
bandwidth	B-Algorithm
occupied	O
by	O
spread-spectrum	B-Architecture
signals	O
offer	O
some	O
frequency	O
diversity	O
;	O
i.e.	O
,	O
it	O
is	O
unlikely	O
that	O
the	O
signal	O
will	O
encounter	O
severe	O
multipath	O
fading	O
over	O
its	O
whole	O
bandwidth	B-Algorithm
.	O
</s>
<s>
In	O
direct-sequence	B-Device
systems	O
,	O
the	O
signal	O
can	O
be	O
detected	O
by	O
using	O
a	O
rake	O
receiver	O
.	O
</s>
<s>
Multiple	O
access	O
capability	O
,	O
known	O
as	O
code-division	B-Protocol
multiple	I-Protocol
access	I-Protocol
(	O
CDMA	B-Protocol
)	O
or	O
code-division	B-Protocol
multiplexing	I-Protocol
(	O
CDM	O
)	O
.	O
</s>
<s>
The	O
concept	O
of	O
Frequency-hopping	B-Protocol
was	O
adopted	O
by	O
the	O
German	O
radio	O
company	O
Telefunken	O
and	O
also	O
described	O
in	O
part	O
of	O
a	O
1903	O
US	O
patent	O
by	O
Nikola	O
Tesla	O
.	O
</s>
<s>
During	O
World	O
War	O
II	O
,	O
Golden	O
Age	O
of	O
Hollywood	O
actress	O
Hedy	O
Lamarr	O
and	O
avant-garde	O
composer	B-Application
George	O
Antheil	O
developed	O
an	O
intended	O
jamming-resistant	O
radio	O
guidance	O
system	O
for	O
use	O
in	O
Allied	O
torpedoes	O
,	O
patenting	O
the	O
device	O
under	O
"	O
Secret	O
Communications	O
System	O
"	O
on	O
August	O
11	O
,	O
1942	O
.	O
</s>
<s>
Spread-spectrum	B-Architecture
clock	O
generation	O
(	O
SSCG	O
)	O
is	O
used	O
in	O
some	O
synchronous	B-Application
digital	I-Application
systems	I-Application
,	O
especially	O
those	O
containing	O
microprocessors	O
,	O
to	O
reduce	O
the	O
spectral	O
density	O
of	O
the	O
electromagnetic	O
interference	O
(	O
EMI	O
)	O
that	O
these	O
systems	O
generate	O
.	O
</s>
<s>
Practical	O
synchronous	B-Application
digital	I-Application
systems	I-Application
radiate	O
electromagnetic	O
energy	O
on	O
a	O
number	O
of	O
narrow	O
bands	O
spread	O
on	O
the	O
clock	O
frequency	O
and	O
its	O
harmonics	O
,	O
resulting	O
in	O
a	O
frequency	O
spectrum	O
that	O
,	O
at	O
certain	O
frequencies	O
,	O
can	O
exceed	O
the	O
regulatory	O
limits	O
for	O
electromagnetic	O
interference	O
(	O
e.g.	O
</s>
<s>
Spread-spectrum	B-Architecture
clocking	O
avoids	O
this	O
problem	O
by	O
using	O
one	O
of	O
the	O
methods	O
previously	O
described	O
to	O
reduce	O
the	O
peak	O
radiated	O
energy	O
and	O
,	O
therefore	O
,	O
its	O
electromagnetic	O
emissions	O
and	O
so	O
comply	O
with	O
electromagnetic	O
compatibility	O
(	O
EMC	O
)	O
regulations	O
.	O
</s>
<s>
Active	O
EMI	O
reduction	O
techniques	O
such	O
as	O
spread-spectrum	B-Architecture
clocking	O
are	O
needed	O
in	O
these	O
cases	O
.	O
</s>
<s>
However	O
,	O
spread-spectrum	B-Architecture
clocking	O
,	O
like	O
other	O
kinds	O
of	O
dynamic	O
frequency	O
change	O
,	O
can	O
also	O
create	O
challenges	O
for	O
designers	O
.	O
</s>
<s>
Consequently	O
,	O
an	O
ability	O
to	O
disable	O
spread-spectrum	B-Architecture
clocking	O
in	O
computer	O
systems	O
is	O
considered	O
useful	O
.	O
</s>
<s>
Spreading	O
energy	O
over	O
a	O
larger	O
bandwidth	B-Algorithm
effectively	O
reduces	O
electrical	O
and	O
magnetic	O
readings	O
within	O
narrow	O
bandwidths	B-Algorithm
.	O
</s>
<s>
If	O
the	O
system	O
under	O
test	O
were	O
to	O
radiate	O
all	O
its	O
energy	O
in	O
a	O
narrow	O
bandwidth	B-Algorithm
,	O
it	O
would	O
register	O
a	O
large	O
peak	O
.	O
</s>
<s>
Distributing	O
this	O
same	O
energy	O
into	O
a	O
larger	O
bandwidth	B-Algorithm
prevents	O
systems	O
from	O
putting	O
enough	O
energy	O
into	O
any	O
one	O
narrowband	O
to	O
exceed	O
the	O
statutory	O
limits	O
.	O
</s>
<s>
The	O
usefulness	O
of	O
this	O
method	O
as	O
a	O
means	O
to	O
reduce	O
real-life	O
interference	O
problems	O
is	O
often	O
debated	O
,	O
as	O
it	O
is	O
perceived	O
that	O
spread-spectrum	B-Architecture
clocking	O
hides	O
rather	O
than	O
resolves	O
higher	O
radiated	O
energy	O
issues	O
by	O
simple	O
exploitation	O
of	O
loopholes	O
in	O
EMC	O
legislation	O
or	O
certification	O
procedures	O
.	O
</s>
<s>
This	O
situation	O
results	O
in	O
electronic	O
equipment	O
sensitive	O
to	O
narrow	O
bandwidth(s )	O
experiencing	O
much	O
less	O
interference	O
,	O
while	O
those	O
with	O
broadband	O
sensitivity	O
,	O
or	O
even	O
operated	O
at	O
other	O
higher	O
frequencies	O
(	O
such	O
as	O
a	O
radio	O
receiver	O
tuned	O
to	O
a	O
different	O
station	O
)	O
,	O
will	O
experience	O
more	O
interference	O
.	O
</s>
<s>
FCC	O
certification	O
testing	O
is	O
often	O
completed	O
with	O
the	O
spread-spectrum	B-Architecture
function	O
enabled	O
in	O
order	O
to	O
reduce	O
the	O
measured	O
emissions	O
to	O
within	O
acceptable	O
legal	O
limits	O
.	O
</s>
<s>
However	O
,	O
the	O
spread-spectrum	B-Architecture
functionality	O
may	O
be	O
disabled	O
by	O
the	O
user	O
in	O
some	O
cases	O
.	O
</s>
<s>
As	O
an	O
example	O
,	O
in	O
the	O
area	O
of	O
personal	O
computers	O
,	O
some	O
BIOS	B-Operating_System
writers	O
include	O
the	O
ability	O
to	O
disable	O
spread-spectrum	B-Architecture
clock	O
generation	O
as	O
a	O
user	O
setting	O
,	O
thereby	O
defeating	O
the	O
object	O
of	O
the	O
EMI	O
regulations	O
.	O
</s>
<s>
This	O
might	O
be	O
considered	O
a	O
loophole	O
,	O
but	O
is	O
generally	O
overlooked	O
as	O
long	O
as	O
spread-spectrum	B-Architecture
is	O
enabled	O
by	O
default	O
.	O
</s>
