<s>
In	O
electronics	O
and	O
telecommunications	O
,	O
modulation	B-Application
is	O
the	O
process	O
of	O
varying	O
one	O
or	O
more	O
properties	O
of	O
a	O
periodic	O
waveform	O
,	O
called	O
the	O
carrier	O
signal	O
,	O
with	O
a	O
separate	O
signal	O
called	O
the	O
modulation	B-Application
signal	O
that	O
typically	O
contains	O
information	O
to	O
be	O
transmitted	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
modulation	B-Application
signal	O
might	O
be	O
an	O
audio	O
signal	O
representing	O
sound	O
from	O
a	O
microphone	B-Application
,	O
a	O
video	B-Application
signal	I-Application
representing	O
moving	O
images	O
from	O
a	O
video	O
camera	O
,	O
or	O
a	O
digital	B-General_Concept
signal	O
representing	O
a	O
sequence	O
of	O
binary	O
digits	O
,	O
a	O
bitstream	O
from	O
a	O
computer	O
.	O
</s>
<s>
The	O
carrier	O
is	O
usually	O
higher	O
in	O
frequency	O
than	O
the	O
modulation	B-Application
signal	O
.	O
</s>
<s>
In	O
radio	O
communication	O
,	O
the	O
modulated	B-Application
carrier	O
is	O
transmitted	O
through	O
space	O
as	O
a	O
radio	O
wave	O
to	O
a	O
radio	O
receiver	O
.	O
</s>
<s>
Another	O
purpose	O
of	O
modulation	B-Application
is	O
to	O
transmit	O
multiple	O
channels	O
of	O
information	O
through	O
a	O
single	O
communication	O
medium	O
,	O
using	O
frequency-division	B-Protocol
multiplexing	I-Protocol
(	O
FDM	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
cable	O
television	O
(	O
which	O
uses	O
FDM	O
)	O
,	O
many	O
carrier	O
signals	O
,	O
each	O
modulated	B-Application
with	O
a	O
different	O
television	O
channel	O
,	O
are	O
transported	O
through	O
a	O
single	O
cable	O
to	O
customers	O
.	O
</s>
<s>
At	O
the	O
destination	O
end	O
,	O
the	O
carrier	O
signal	O
is	O
demodulated	B-Device
to	O
extract	O
the	O
information	O
bearing	O
modulation	B-Application
signal	O
.	O
</s>
<s>
A	O
modulator	B-Application
is	O
a	O
device	O
or	O
circuit	O
that	O
performs	O
modulation	B-Application
.	O
</s>
<s>
A	O
demodulator	B-Device
(	O
sometimes	O
detector	B-Algorithm
)	O
is	O
a	O
circuit	O
that	O
performs	O
demodulation	B-Device
,	O
the	O
inverse	O
of	O
modulation	B-Application
.	O
</s>
<s>
A	O
modem	B-Application
(	O
from	O
modulator	B-Application
–	O
demodulator	B-Device
)	O
,	O
used	O
in	O
bidirectional	O
communication	O
,	O
can	O
perform	O
both	O
operations	O
.	O
</s>
<s>
The	O
lower	O
frequency	O
band	O
occupied	O
by	O
the	O
modulation	B-Application
signal	O
is	O
called	O
the	O
baseband	B-Algorithm
,	O
while	O
the	O
higher	O
frequency	O
band	O
occupied	O
by	O
the	O
modulated	B-Application
carrier	O
is	O
called	O
the	O
passband	B-Algorithm
.	O
</s>
<s>
In	O
analog	B-Application
modulation	I-Application
,	O
an	O
analog	B-Application
modulation	I-Application
signal	O
is	O
"	O
impressed	O
"	O
on	O
the	O
carrier	O
.	O
</s>
<s>
Examples	O
are	O
amplitude	B-Device
modulation	I-Device
(	O
AM	O
)	O
in	O
which	O
the	O
amplitude	B-Application
(	O
strength	O
)	O
of	O
the	O
carrier	O
wave	O
is	O
varied	O
by	O
the	O
modulation	B-Application
signal	O
,	O
and	O
frequency	B-Application
modulation	I-Application
(	O
FM	B-Application
)	O
in	O
which	O
the	O
frequency	O
of	O
the	O
carrier	O
wave	O
is	O
varied	O
by	O
the	O
modulation	B-Application
signal	O
.	O
</s>
<s>
These	O
were	O
the	O
earliest	O
types	O
of	O
modulation	B-Application
,	O
and	O
are	O
used	O
to	O
transmit	O
an	O
audio	O
signal	O
representing	O
sound	O
in	O
AM	O
and	O
FM	B-Application
radio	O
broadcasting	O
.	O
</s>
<s>
More	O
recent	O
systems	O
use	O
digital	B-General_Concept
modulation	B-Application
,	O
which	O
impresses	O
a	O
digital	B-General_Concept
signal	O
consisting	O
of	O
a	O
sequence	O
of	O
binary	O
digits	O
(	O
bits	O
)	O
,	O
a	O
bitstream	O
,	O
on	O
the	O
carrier	O
,	O
by	O
means	O
of	O
mapping	O
bits	O
to	O
elements	O
from	O
a	O
discrete	O
alphabet	O
to	O
be	O
transmitted	O
.	O
</s>
<s>
This	O
alphabet	O
can	O
consist	O
of	O
a	O
set	O
of	O
real	O
or	O
complex	O
numbers	O
,	O
or	O
sequences	O
,	O
like	O
oscillations	O
of	O
different	O
frequencies	O
,	O
so-called	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
modulation	B-Application
.	O
</s>
<s>
A	O
more	O
complicated	O
digital	B-General_Concept
modulation	B-Application
method	O
that	O
employs	O
multiple	O
carriers	O
,	O
orthogonal	B-Protocol
frequency-division	I-Protocol
multiplexing	I-Protocol
(	O
OFDM	B-Protocol
)	O
,	O
is	O
used	O
in	O
WiFi	O
networks	O
,	O
digital	B-General_Concept
radio	O
stations	O
and	O
digital	B-General_Concept
cable	O
television	O
transmission	O
.	O
</s>
<s>
In	O
analog	B-Application
modulation	I-Application
,	O
the	O
modulation	B-Application
is	O
applied	O
continuously	O
in	O
response	O
to	O
the	O
analog	O
information	O
signal	O
.	O
</s>
<s>
Common	O
analog	B-Application
modulation	I-Application
techniques	O
include	O
:	O
</s>
<s>
Transpositional	O
Modulation	B-Application
(	O
TM	O
)	O
,	O
in	O
which	O
the	O
waveform	O
inflection	O
is	O
modified	O
resulting	O
in	O
a	O
signal	O
where	O
each	O
quarter	O
cycle	O
is	O
transposed	O
in	O
the	O
modulation	B-Application
process	O
.	O
</s>
<s>
In	O
digital	B-General_Concept
modulation	B-Application
,	O
an	O
analog	O
carrier	O
signal	O
is	O
modulated	B-Application
by	O
a	O
discrete	O
signal	O
.	O
</s>
<s>
Digital	B-General_Concept
modulation	B-Application
methods	O
can	O
be	O
considered	O
as	O
digital-to-analog	O
conversion	O
and	O
the	O
corresponding	O
demodulation	B-Device
or	O
detection	O
as	O
analog-to-digital	O
conversion	O
.	O
</s>
<s>
The	O
changes	O
in	O
the	O
carrier	O
signal	O
are	O
chosen	O
from	O
a	O
finite	O
number	O
of	O
M	O
alternative	O
symbols	O
(	O
the	O
modulation	B-Application
alphabet	O
)	O
.	O
</s>
<s>
A	O
simple	O
example	O
:	O
A	O
telephone	O
line	O
is	O
designed	O
for	O
transferring	O
audible	O
sounds	O
,	O
for	O
example	O
,	O
tones	O
,	O
and	O
not	O
digital	B-General_Concept
bits	O
(	O
zeros	O
and	O
ones	O
)	O
.	O
</s>
<s>
Computers	O
may	O
,	O
however	O
,	O
communicate	O
over	O
a	O
telephone	O
line	O
by	O
means	O
of	O
modems	B-Application
,	O
which	O
are	O
representing	O
the	O
digital	B-General_Concept
bits	O
by	O
tones	O
,	O
called	O
symbols	O
.	O
</s>
<s>
If	O
the	O
modem	B-Application
plays	O
a	O
melody	O
consisting	O
of	O
1000	O
tones	O
per	O
second	O
,	O
the	O
symbol	O
rate	O
is	O
1000	O
symbols/second	O
,	O
or	O
1000	O
baud	O
.	O
</s>
<s>
Since	O
each	O
tone	O
(	O
i.e.	O
,	O
symbol	O
)	O
represents	O
a	O
message	O
consisting	O
of	O
two	O
digital	B-General_Concept
bits	O
in	O
this	O
example	O
,	O
the	O
bit	O
rate	O
is	O
twice	O
the	O
symbol	O
rate	O
,	O
i.e.	O
</s>
<s>
According	O
to	O
one	O
definition	O
of	O
digital	B-General_Concept
signal	O
,	O
the	O
modulated	B-Application
signal	O
is	O
a	O
digital	B-General_Concept
signal	O
.	O
</s>
<s>
According	O
to	O
another	O
definition	O
,	O
the	O
modulation	B-Application
is	O
a	O
form	O
of	O
digital-to-analog	O
conversion	O
.	O
</s>
<s>
Most	O
textbooks	O
would	O
consider	O
digital	B-General_Concept
modulation	B-Application
schemes	O
as	O
a	O
form	O
of	O
digital	B-General_Concept
transmission	O
,	O
synonymous	O
to	O
data	O
transmission	O
;	O
very	O
few	O
would	O
consider	O
it	O
as	O
analog	B-Protocol
transmission	I-Protocol
.	O
</s>
<s>
The	O
most	O
fundamental	O
digital	B-General_Concept
modulation	B-Application
techniques	O
are	O
based	O
on	O
keying	O
:	O
</s>
<s>
PSK	O
(	O
phase-shift	B-General_Concept
keying	I-General_Concept
)	O
:	O
a	O
finite	O
number	O
of	O
phases	O
are	O
used	O
.	O
</s>
<s>
ASK	O
(	O
amplitude-shift	O
keying	O
)	O
:	O
a	O
finite	O
number	O
of	O
amplitudes	B-Application
are	O
used	O
.	O
</s>
<s>
QAM	B-Device
(	O
quadrature	B-Device
amplitude	I-Device
modulation	I-Device
)	O
:	O
a	O
finite	O
number	O
of	O
at	O
least	O
two	O
phases	O
and	O
at	O
least	O
two	O
amplitudes	B-Application
are	O
used	O
.	O
</s>
<s>
In	O
QAM	B-Device
,	O
an	O
in-phase	O
signal	O
(	O
or	O
I	O
,	O
with	O
one	O
example	O
being	O
a	O
cosine	O
waveform	O
)	O
and	O
a	O
quadrature	O
phase	O
signal	O
(	O
or	O
Q	O
,	O
with	O
an	O
example	O
being	O
a	O
sine	O
wave	O
)	O
are	O
amplitude	B-Device
modulated	I-Device
with	O
a	O
finite	O
number	O
of	O
amplitudes	B-Application
and	O
then	O
summed	O
.	O
</s>
<s>
In	O
all	O
of	O
the	O
above	O
methods	O
,	O
each	O
of	O
these	O
phases	O
,	O
frequencies	O
or	O
amplitudes	B-Application
are	O
assigned	O
a	O
unique	O
pattern	O
of	O
binary	O
bits	O
.	O
</s>
<s>
Usually	O
,	O
each	O
phase	O
,	O
frequency	O
or	O
amplitude	B-Application
encodes	O
an	O
equal	O
number	O
of	O
bits	O
.	O
</s>
<s>
This	O
number	O
of	O
bits	O
comprises	O
the	O
symbol	O
that	O
is	O
represented	O
by	O
the	O
particular	O
phase	O
,	O
frequency	O
or	O
amplitude	B-Application
.	O
</s>
<s>
In	O
the	O
case	O
of	O
PSK	O
,	O
ASK	O
or	O
QAM	B-Device
,	O
where	O
the	O
carrier	O
frequency	O
of	O
the	O
modulated	B-Application
signal	O
is	O
constant	O
,	O
the	O
modulation	B-Application
alphabet	O
is	O
often	O
conveniently	O
represented	O
on	O
a	O
constellation	B-Application
diagram	I-Application
,	O
showing	O
the	O
amplitude	B-Application
of	O
the	O
I	O
signal	O
at	O
the	O
x-axis	O
,	O
and	O
the	O
amplitude	B-Application
of	O
the	O
Q	O
signal	O
at	O
the	O
y-axis	O
,	O
for	O
each	O
symbol	O
.	O
</s>
<s>
PSK	O
and	O
ASK	O
,	O
and	O
sometimes	O
also	O
FSK	O
,	O
are	O
often	O
generated	O
and	O
detected	O
using	O
the	O
principle	O
of	O
QAM	B-Device
.	O
</s>
<s>
The	O
resulting	O
so	O
called	O
equivalent	B-Algorithm
lowpass	I-Algorithm
signal	I-Algorithm
or	O
equivalent	B-Algorithm
baseband	I-Algorithm
signal	I-Algorithm
is	O
a	O
complex-valued	O
representation	O
of	O
the	O
real-valued	O
modulated	B-Application
physical	O
signal	O
(	O
the	O
so-called	O
passband	B-Algorithm
signal	I-Algorithm
or	O
RF	O
signal	O
)	O
.	O
</s>
<s>
These	O
are	O
the	O
general	O
steps	O
used	O
by	O
the	O
modulator	B-Application
to	O
transmit	O
data	O
:	O
</s>
<s>
Map	O
the	O
codewords	O
to	O
attributes	O
,	O
for	O
example	O
,	O
amplitudes	B-Application
of	O
the	O
I	O
and	O
Q	O
signals	O
(	O
the	O
equivalent	O
low	O
pass	O
signal	O
)	O
,	O
or	O
frequency	O
or	O
phase	O
values	O
.	O
</s>
<s>
Adapt	O
pulse	B-Algorithm
shaping	I-Algorithm
or	O
some	O
other	O
filtering	O
to	O
limit	O
the	O
bandwidth	O
and	O
form	O
the	O
spectrum	O
of	O
the	O
equivalent	O
low	O
pass	O
signal	O
,	O
typically	O
using	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
.	O
</s>
<s>
Perform	O
digital	B-General_Concept
to	O
analog	O
conversion	O
(	O
DAC	O
)	O
of	O
the	O
I	O
and	O
Q	O
signals	O
(	O
since	O
today	O
all	O
of	O
the	O
above	O
is	O
normally	O
achieved	O
using	O
digital	B-General_Concept
signal	I-General_Concept
processing	I-General_Concept
,	O
DSP	O
)	O
.	O
</s>
<s>
Carry	O
out	O
the	O
modulation	B-Application
,	O
for	O
example	O
by	O
multiplying	O
the	O
sine	O
and	O
cosine	O
waveform	O
with	O
the	O
I	O
and	O
Q	O
signals	O
,	O
resulting	O
in	O
the	O
equivalent	O
low	O
pass	O
signal	O
being	O
frequency	O
shifted	O
to	O
the	O
modulated	B-Application
passband	B-Algorithm
signal	I-Algorithm
or	O
RF	O
signal	O
.	O
</s>
<s>
Sometimes	O
this	O
is	O
achieved	O
using	O
DSP	O
technology	O
,	O
for	O
example	O
direct	O
digital	B-General_Concept
synthesis	O
using	O
a	O
waveform	O
table	O
,	O
instead	O
of	O
analog	O
signal	O
processing	O
.	O
</s>
<s>
At	O
the	O
receiver	O
side	O
,	O
the	O
demodulator	B-Device
typically	O
performs	O
:	O
</s>
<s>
Frequency	B-Algorithm
shifting	I-Algorithm
of	O
the	O
RF	O
signal	O
to	O
the	O
equivalent	O
baseband	B-Algorithm
I	O
and	O
Q	O
signals	O
,	O
or	O
to	O
an	O
intermediate	O
frequency	O
(	O
IF	O
)	O
signal	O
,	O
by	O
multiplying	O
the	O
RF	O
signal	O
with	O
a	O
local	O
oscillator	O
sine	O
wave	O
and	O
cosine	O
wave	O
frequency	O
(	O
see	O
the	O
superheterodyne	O
receiver	O
principle	O
)	O
.	O
</s>
<s>
Sampling	O
and	O
analog-to-digital	O
conversion	O
(	O
ADC	O
)	O
(	O
sometimes	O
before	O
or	O
instead	O
of	O
the	O
above	O
point	O
,	O
for	O
example	O
by	O
means	O
of	O
undersampling	B-Algorithm
)	O
.	O
</s>
<s>
Detection	O
of	O
the	O
amplitudes	B-Application
of	O
the	O
I	O
and	O
Q	O
signals	O
,	O
or	O
the	O
frequency	O
or	O
phase	O
of	O
the	O
IF	O
signal	O
.	O
</s>
<s>
Quantization	B-Algorithm
of	O
the	O
amplitudes	B-Application
,	O
frequencies	O
or	O
phases	O
to	O
the	O
nearest	O
allowed	O
symbol	O
values	O
.	O
</s>
<s>
Mapping	O
of	O
the	O
quantized	B-Algorithm
amplitudes	B-Application
,	O
frequencies	O
or	O
phases	O
to	O
codewords	O
(	O
bit	O
groups	O
)	O
.	O
</s>
<s>
As	O
is	O
common	O
to	O
all	O
digital	B-General_Concept
communication	O
systems	O
,	O
the	O
design	O
of	O
both	O
the	O
modulator	B-Application
and	O
demodulator	B-Device
must	O
be	O
done	O
simultaneously	O
.	O
</s>
<s>
Digital	B-General_Concept
modulation	B-Application
schemes	O
are	O
possible	O
because	O
the	O
transmitter-receiver	O
pair	O
has	O
prior	O
knowledge	O
of	O
how	O
data	O
is	O
encoded	O
and	O
represented	O
in	O
the	O
communications	O
system	O
.	O
</s>
<s>
In	O
all	O
digital	B-General_Concept
communication	O
systems	O
,	O
both	O
the	O
modulator	B-Application
at	O
the	O
transmitter	O
and	O
the	O
demodulator	B-Device
at	O
the	O
receiver	O
are	O
structured	O
so	O
that	O
they	O
perform	O
inverse	O
operations	O
.	O
</s>
<s>
In	O
this	O
case	O
,	O
modulation	B-Application
symbols	O
(	O
rather	O
than	O
bits	O
,	O
characters	O
,	O
or	O
data	O
packets	O
)	O
are	O
asynchronously	O
transferred	O
.	O
</s>
<s>
The	O
opposite	O
is	O
synchronous	B-Protocol
modulation	I-Protocol
.	O
</s>
<s>
The	O
most	O
common	O
digital	B-General_Concept
modulation	B-Application
techniques	O
are	O
:	O
</s>
<s>
MSK	B-Device
and	O
GMSK	O
are	O
particular	O
cases	O
of	O
continuous	B-Device
phase	I-Device
modulation	I-Device
.	O
</s>
<s>
Indeed	O
,	O
MSK	B-Device
is	O
a	O
particular	O
case	O
of	O
the	O
sub-family	O
of	O
CPM	O
known	O
as	O
continuous-phase	O
frequency-shift	O
keying	O
(	O
CPFSK	O
)	O
which	O
is	O
defined	O
by	O
a	O
rectangular	O
frequency	O
pulse	O
(	O
i.e.	O
</s>
<s>
OFDM	B-Protocol
is	O
based	O
on	O
the	O
idea	O
of	O
frequency-division	B-Protocol
multiplexing	I-Protocol
(	O
FDM	O
)	O
,	O
but	O
the	O
multiplexed	O
streams	O
are	O
all	O
parts	O
of	O
a	O
single	O
original	O
stream	O
.	O
</s>
<s>
The	O
bit	O
stream	O
is	O
split	O
into	O
several	O
parallel	O
data	O
streams	O
,	O
each	O
transferred	O
over	O
its	O
own	O
sub-carrier	O
using	O
some	O
conventional	O
digital	B-General_Concept
modulation	B-Application
scheme	O
.	O
</s>
<s>
The	O
modulated	B-Application
sub-carriers	O
are	O
summed	O
to	O
form	O
an	O
OFDM	B-Protocol
signal	O
.	O
</s>
<s>
OFDM	B-Protocol
is	O
considered	O
as	O
a	O
modulation	B-Application
technique	O
rather	O
than	O
a	O
multiplex	O
technique	O
since	O
it	O
transfers	O
one	O
bit	O
stream	O
over	O
one	O
communication	O
channel	O
using	O
one	O
sequence	O
of	O
so-called	O
OFDM	B-Protocol
symbols	O
.	O
</s>
<s>
OFDM	B-Protocol
can	O
be	O
extended	O
to	O
multi-user	O
channel	O
access	O
method	O
in	O
the	O
orthogonal	O
frequency-division	O
multiple	O
access	O
(	O
OFDMA	O
)	O
and	O
multi-carrier	O
code-division	B-Protocol
multiple	I-Protocol
access	I-Protocol
(	O
MC-CDMA	O
)	O
schemes	O
,	O
allowing	O
several	O
users	O
to	O
share	O
the	O
same	O
physical	O
medium	O
by	O
giving	O
different	O
sub-carriers	O
or	O
spreading	O
codes	O
to	O
different	O
users	O
.	O
</s>
<s>
However	O
,	O
they	O
only	O
work	O
with	O
relatively	O
constant-amplitude-modulation	O
signals	O
such	O
as	O
angle	B-Device
modulation	I-Device
(	O
FSK	O
or	O
PSK	O
)	O
and	O
CDMA	B-Protocol
,	O
but	O
not	O
with	O
QAM	B-Device
and	O
OFDM	B-Protocol
.	O
</s>
<s>
Nevertheless	O
,	O
even	O
though	O
switching	O
amplifiers	O
are	O
completely	O
unsuitable	O
for	O
normal	O
QAM	B-Device
constellations	O
,	O
often	O
the	O
QAM	B-Device
modulation	B-Application
principle	O
are	O
used	O
to	O
drive	O
switching	O
amplifiers	O
with	O
these	O
FM	B-Application
and	O
other	O
waveforms	O
,	O
and	O
sometimes	O
QAM	B-Device
demodulators	B-Device
are	O
used	O
to	O
receive	O
the	O
signals	O
put	O
out	O
by	O
these	O
switching	O
amplifiers	O
.	O
</s>
<s>
Automatic	O
digital	B-General_Concept
modulation	B-Application
recognition	O
in	O
intelligent	O
communication	O
systems	O
is	O
one	O
of	O
the	O
most	O
important	O
issues	O
in	O
software-defined	B-Architecture
radio	I-Architecture
and	O
cognitive	B-Protocol
radio	I-Protocol
.	O
</s>
<s>
According	O
to	O
incremental	O
expanse	O
of	O
intelligent	O
receivers	O
,	O
automatic	O
modulation	B-Application
recognition	O
becomes	O
a	O
challenging	O
topic	O
in	O
telecommunication	O
systems	O
and	O
computer	O
engineering	O
.	O
</s>
<s>
Moreover	O
,	O
blind	O
recognition	O
of	O
modulation	B-Application
type	O
is	O
an	O
important	O
problem	O
in	O
commercial	O
systems	O
,	O
especially	O
in	O
software-defined	B-Architecture
radio	I-Architecture
.	O
</s>
<s>
Obviously	O
,	O
with	O
no	O
knowledge	O
of	O
the	O
transmitted	O
data	O
and	O
many	O
unknown	O
parameters	O
at	O
the	O
receiver	O
,	O
such	O
as	O
the	O
signal	O
power	O
,	O
carrier	O
frequency	O
and	O
phase	O
offsets	O
,	O
timing	O
information	O
,	O
etc.	O
,	O
blind	O
identification	O
of	O
the	O
modulation	B-Application
is	O
made	O
fairly	O
difficult	O
.	O
</s>
<s>
There	O
are	O
two	O
main	O
approaches	O
to	O
automatic	O
modulation	B-Application
recognition	O
.	O
</s>
<s>
Digital	B-Protocol
baseband	I-Protocol
modulation	I-Protocol
changes	O
the	O
characteristics	O
of	O
a	O
baseband	B-Algorithm
signal	I-Algorithm
,	O
i.e.	O
,	O
one	O
without	O
a	O
carrier	O
at	O
a	O
higher	O
frequency	O
.	O
</s>
<s>
This	O
can	O
be	O
used	O
as	O
equivalent	O
signal	O
to	O
be	O
later	O
frequency-converted	O
to	O
a	O
carrier	O
frequency	O
,	O
or	O
for	O
direct	O
communication	O
in	O
baseband	B-Algorithm
.	O
</s>
<s>
The	O
latter	O
methods	O
both	O
involve	O
relatively	O
simple	O
line	B-Protocol
codes	I-Protocol
,	O
as	O
often	O
used	O
in	O
local	O
buses	O
,	O
and	O
complicated	O
baseband	B-Algorithm
signalling	O
schemes	O
such	O
as	O
used	O
in	O
DSL	O
.	O
</s>
<s>
Pulse	O
modulation	B-Application
schemes	O
aim	O
at	O
transferring	O
a	O
narrowband	O
analog	O
signal	O
over	O
an	O
analog	O
baseband	B-Algorithm
channel	I-Algorithm
as	O
a	O
two-level	O
signal	O
by	O
modulating	O
a	O
pulse	O
wave	O
.	O
</s>
<s>
Some	O
pulse	O
modulation	B-Application
schemes	O
also	O
allow	O
the	O
narrowband	O
analog	O
signal	O
to	O
be	O
transferred	O
as	O
a	O
digital	B-General_Concept
signal	O
(	O
i.e.	O
,	O
as	O
a	O
quantized	B-Algorithm
discrete-time	O
signal	O
)	O
with	O
a	O
fixed	O
bit	O
rate	O
,	O
which	O
can	O
be	O
transferred	O
over	O
an	O
underlying	O
digital	B-General_Concept
transmission	O
system	O
,	O
for	O
example	O
,	O
some	O
line	B-Protocol
code	I-Protocol
.	O
</s>
<s>
These	O
are	O
not	O
modulation	B-Application
schemes	O
in	O
the	O
conventional	O
sense	O
since	O
they	O
are	O
not	O
channel	O
coding	O
schemes	O
,	O
but	O
should	O
be	O
considered	O
as	O
source	B-General_Concept
coding	I-General_Concept
schemes	O
,	O
and	O
in	O
some	O
cases	O
analog-to-digital	O
conversion	O
techniques	O
.	O
</s>
<s>
The	O
use	O
of	O
on-off	B-Device
keying	I-Device
to	O
transmit	O
Morse	O
code	O
at	O
radio	O
frequencies	O
is	O
known	O
as	O
continuous	O
wave	O
(	O
CW	O
)	O
operation	O
.	O
</s>
<s>
Space	O
modulation	B-Application
is	O
a	O
method	O
whereby	O
signals	O
are	O
modulated	B-Application
within	O
airspace	O
such	O
as	O
that	O
used	O
in	O
instrument	O
landing	O
systems	O
.	O
</s>
<s>
The	O
microwave	O
auditory	O
effect	O
has	O
been	O
pulse	O
modulated	B-Application
with	O
audio	O
waveforms	O
to	O
evoke	O
understandable	O
spoken	O
numbers	O
.	O
</s>
