<s>
Quadrature	B-Device
amplitude	I-Device
modulation	I-Device
(	O
QAM	B-Device
)	O
is	O
the	O
name	O
of	O
a	O
family	O
of	O
digital	O
modulation	B-Application
methods	O
and	O
a	O
related	O
family	O
of	O
analog	B-Application
modulation	I-Application
methods	O
widely	O
used	O
in	O
modern	O
telecommunications	O
to	O
transmit	O
information	O
.	O
</s>
<s>
It	O
conveys	O
two	O
analog	O
message	O
signals	O
,	O
or	O
two	O
digital	O
bit	O
streams	O
,	O
by	O
changing	O
(	O
modulating	O
)	O
the	O
amplitudes	B-Application
of	O
two	O
carrier	O
waves	O
,	O
using	O
the	O
amplitude-shift	O
keying	O
(	O
ASK	O
)	O
digital	O
modulation	B-Application
scheme	O
or	O
amplitude	B-Device
modulation	I-Device
(	O
AM	O
)	O
analog	B-Application
modulation	I-Application
scheme	O
.	O
</s>
<s>
The	O
two	O
carrier	O
waves	O
are	O
of	O
the	O
same	O
frequency	O
and	O
are	O
out	O
of	O
phase	O
with	O
each	O
other	O
by	O
90°	O
,	O
a	O
condition	O
known	O
as	O
orthogonality	B-Application
or	O
quadrature	O
.	O
</s>
<s>
At	O
the	O
receiver	O
,	O
the	O
two	O
waves	O
can	O
be	O
coherently	O
separated	O
(	O
demodulated	O
)	O
because	O
of	O
their	O
orthogonality	B-Application
property	O
.	O
</s>
<s>
Another	O
key	O
property	O
is	O
that	O
the	O
modulations	B-Application
are	O
low-frequency/low	O
-bandwidth	O
waveforms	O
compared	O
to	O
the	O
carrier	O
frequency	O
,	O
which	O
is	O
known	O
as	O
the	O
narrowband	O
assumption	O
.	O
</s>
<s>
Phase	B-Device
modulation	I-Device
(	O
analog	O
PM	O
)	O
and	O
phase-shift	B-General_Concept
keying	I-General_Concept
(	O
digital	O
PSK	B-General_Concept
)	O
can	O
be	O
regarded	O
as	O
a	O
special	O
case	O
of	O
QAM	B-Device
,	O
where	O
the	O
amplitude	B-Application
of	O
the	O
transmitted	O
signal	O
is	O
a	O
constant	O
,	O
but	O
its	O
phase	O
varies	O
.	O
</s>
<s>
This	O
can	O
also	O
be	O
extended	O
to	O
frequency	B-Application
modulation	I-Application
(	O
FM	B-Application
)	O
and	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
,	O
for	O
these	O
can	O
be	O
regarded	O
as	O
a	O
special	O
case	O
of	O
phase	B-Device
modulation	I-Device
.	O
</s>
<s>
QAM	B-Device
is	O
used	O
extensively	O
as	O
a	O
modulation	B-Application
scheme	O
for	O
digital	O
telecommunication	O
systems	O
,	O
such	O
as	O
in	O
802.11	O
Wi-Fi	O
standards	O
.	O
</s>
<s>
Arbitrarily	O
high	O
spectral	B-Protocol
efficiencies	I-Protocol
can	O
be	O
achieved	O
with	O
QAM	B-Device
by	O
setting	O
a	O
suitable	O
constellation	B-Application
size	O
,	O
limited	O
only	O
by	O
the	O
noise	O
level	O
and	O
linearity	O
of	O
the	O
communications	O
channel	O
.	O
</s>
<s>
QAM	B-Device
is	O
being	O
used	O
in	O
optical	O
fiber	O
systems	O
as	O
bit	O
rates	O
increase	O
;	O
QAM16	O
and	O
QAM64	O
can	O
be	O
optically	O
emulated	O
with	O
a	O
3-path	O
interferometer	O
.	O
</s>
<s>
In	O
a	O
QAM	B-Device
signal	O
,	O
one	O
carrier	O
lags	O
the	O
other	O
by	O
90°	O
,	O
and	O
its	O
amplitude	B-Device
modulation	I-Device
is	O
customarily	O
referred	O
to	O
as	O
the	O
in-phase	O
component	O
,	O
denoted	O
by	O
The	O
other	O
modulating	O
function	O
is	O
the	O
quadrature	B-Algorithm
component	I-Algorithm
,	O
So	O
the	O
composite	O
waveform	O
is	O
mathematically	O
modeled	O
as	O
:	O
</s>
<s>
At	O
the	O
receiver	O
,	O
a	O
coherent	O
demodulator	B-Device
multiplies	O
the	O
received	O
signal	O
separately	O
with	O
both	O
a	O
cosine	O
and	O
sine	O
signal	O
to	O
produce	O
the	O
received	O
estimates	O
of	O
and	O
.	O
</s>
<s>
Low-pass	B-Algorithm
filtering	I-Algorithm
removes	O
the	O
high	O
frequency	O
terms	O
(	O
containing	O
)	O
,	O
leaving	O
only	O
the	O
term	O
.	O
</s>
<s>
This	O
filtered	O
signal	O
is	O
unaffected	O
by	O
showing	O
that	O
the	O
in-phase	O
component	O
can	O
be	O
received	O
independently	O
of	O
the	O
quadrature	B-Algorithm
component	I-Algorithm
.	O
</s>
<s>
This	O
comes	O
at	O
the	O
expense	O
of	O
demodulation	B-Device
complexity	O
.	O
</s>
<s>
But	O
the	O
sender	O
and	O
receiver	O
of	O
a	O
quadrature-modulated	O
signal	O
must	O
share	O
a	O
clock	O
or	O
otherwise	O
send	O
a	O
clock	O
signal	O
.	O
</s>
<s>
If	O
the	O
clock	O
phases	O
drift	O
apart	O
,	O
the	O
demodulated	O
I	B-Algorithm
and	I-Algorithm
Q	I-Algorithm
signals	O
bleed	O
into	O
each	O
other	O
,	O
yielding	O
crosstalk	O
.	O
</s>
<s>
Analog	O
QAM	B-Device
is	O
used	O
in	O
:	O
</s>
<s>
NTSC	O
and	O
PAL	B-Language
analog	O
color	O
television	O
systems	O
,	O
where	O
the	O
I	O
-	O
and	O
Q-signals	O
carry	O
the	O
components	O
of	O
chroma	O
(	O
colour	O
)	O
information	O
.	O
</s>
<s>
The	O
QAM	B-Device
carrier	O
phase	O
is	O
recovered	O
from	O
a	O
special	O
colorburst	O
transmitted	O
at	O
the	O
beginning	O
of	O
each	O
scan	O
line	O
.	O
</s>
<s>
C-QUAM	O
(	O
"	O
Compatible	O
QAM	B-Device
"	O
)	O
is	O
used	O
in	O
AM	O
stereo	O
radio	O
to	O
carry	O
the	O
stereo	O
difference	O
information	O
.	O
</s>
<s>
In	O
the	O
frequency	O
domain	O
,	O
QAM	B-Device
has	O
a	O
similar	O
spectral	O
pattern	O
to	O
DSB-SC	B-Device
modulation	B-Application
.	O
</s>
<s>
where	O
denotes	O
the	O
Fourier	O
transform	O
,	O
and	O
and	O
are	O
the	O
transforms	O
of	O
and	O
This	O
result	O
represents	O
the	O
sum	O
of	O
two	O
DSB-SC	B-Device
signals	O
with	O
the	O
same	O
center	O
frequency	O
.	O
</s>
<s>
The	O
factor	O
of	O
represents	O
the	O
90°	O
phase	O
shift	O
that	O
enables	O
their	O
individual	O
demodulations	B-Device
.	O
</s>
<s>
As	O
in	O
many	O
digital	O
modulation	B-Application
schemes	O
,	O
the	O
constellation	B-Application
diagram	I-Application
is	O
useful	O
for	O
QAM	B-Device
.	O
</s>
<s>
In	O
QAM	B-Device
,	O
the	O
constellation	B-Application
points	O
are	O
usually	O
arranged	O
in	O
a	O
square	O
grid	O
with	O
equal	O
vertical	O
and	O
horizontal	O
spacing	O
,	O
although	O
other	O
configurations	O
are	O
possible	O
(	O
e.g.	O
</s>
<s>
The	O
simplest	O
and	O
most	O
commonly	O
used	O
QAM	B-Device
constellations	O
consist	O
of	O
points	O
arranged	O
in	O
a	O
square	O
,	O
i.e.	O
</s>
<s>
16-QAM	B-Device
,	O
64-QAM	B-Device
and	O
256-QAM	B-Device
(	O
even	O
powers	O
of	O
two	O
)	O
.	O
</s>
<s>
Non-square	O
constellations	O
,	O
such	O
as	O
Cross-QAM	O
,	O
can	O
offer	O
greater	O
efficiency	O
but	O
are	O
rarely	O
used	O
because	O
of	O
the	O
cost	O
of	O
increased	O
modem	O
complexity	O
.	O
</s>
<s>
By	O
moving	O
to	O
a	O
higher-order	O
constellation	B-Application
,	O
it	O
is	O
possible	O
to	O
transmit	O
more	O
bits	O
per	O
symbol	O
.	O
</s>
<s>
However	O
,	O
if	O
the	O
mean	O
energy	O
of	O
the	O
constellation	B-Application
is	O
to	O
remain	O
the	O
same	O
(	O
by	O
way	O
of	O
making	O
a	O
fair	O
comparison	O
)	O
,	O
the	O
points	O
must	O
be	O
closer	O
together	O
and	O
are	O
thus	O
more	O
susceptible	O
to	O
noise	O
and	O
other	O
corruption	O
;	O
this	O
results	O
in	O
a	O
higher	O
bit	O
error	O
rate	O
and	O
so	O
higher-order	O
QAM	B-Device
can	O
deliver	O
more	O
data	O
less	O
reliably	O
than	O
lower-order	O
QAM	B-Device
,	O
for	O
constant	O
mean	O
constellation	B-Application
energy	O
.	O
</s>
<s>
Using	O
higher-order	O
QAM	B-Device
without	O
increasing	O
the	O
bit	O
error	O
rate	O
requires	O
a	O
higher	O
signal-to-noise	O
ratio	O
(	O
SNR	O
)	O
by	O
increasing	O
signal	O
energy	O
,	O
reducing	O
noise	O
,	O
or	O
both	O
.	O
</s>
<s>
If	O
data-rates	O
beyond	O
those	O
offered	O
by	O
8-PSK	O
are	O
required	O
,	O
it	O
is	O
more	O
usual	O
to	O
move	O
to	O
QAM	B-Device
since	O
it	O
achieves	O
a	O
greater	O
distance	O
between	O
adjacent	O
points	O
in	O
the	O
I-Q	O
plane	O
by	O
distributing	O
the	O
points	O
more	O
evenly	O
.	O
</s>
<s>
The	O
complicating	O
factor	O
is	O
that	O
the	O
points	O
are	O
no	O
longer	O
all	O
the	O
same	O
amplitude	B-Application
and	O
so	O
the	O
demodulator	B-Device
must	O
now	O
correctly	O
detect	O
both	O
phase	O
and	O
amplitude	B-Application
,	O
rather	O
than	O
just	O
phase	O
.	O
</s>
<s>
64-QAM	B-Device
and	O
256-QAM	B-Device
are	O
often	O
used	O
in	O
digital	B-Device
cable	I-Device
television	I-Device
and	O
cable	B-Device
modem	I-Device
applications	O
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
64-QAM	B-Device
and	O
256-QAM	B-Device
are	O
the	O
mandated	O
modulation	B-Application
schemes	O
for	O
digital	B-Device
cable	I-Device
(	O
see	O
QAM	B-Device
tuner	I-Device
)	O
as	O
standardised	O
by	O
the	O
SCTE	O
in	O
the	O
standard	O
.	O
</s>
<s>
Note	O
that	O
many	O
marketing	O
people	O
will	O
refer	O
to	O
these	O
as	O
QAM-64	O
and	O
QAM-256	O
.	O
</s>
<s>
In	O
the	O
UK	O
,	O
64-QAM	B-Device
is	O
used	O
for	O
digital	B-Application
terrestrial	I-Application
television	I-Application
(	O
Freeview	B-Device
)	O
whilst	O
256-QAM	B-Device
is	O
used	O
for	O
Freeview-HD	O
.	O
</s>
<s>
Communication	O
systems	O
designed	O
to	O
achieve	O
very	O
high	O
levels	O
of	O
spectral	B-Protocol
efficiency	I-Protocol
usually	O
employ	O
very	O
dense	O
QAM	B-Device
constellations	O
.	O
</s>
<s>
For	O
example	O
,	O
current	O
Homeplug	O
AV2	O
500-Mbit/s	O
powerline	B-Device
Ethernet	I-Device
devices	O
use	O
1024-QAM	B-Device
and	O
4096-QAM	B-Device
,	O
as	O
well	O
as	O
future	O
devices	O
using	O
ITU-T	B-General_Concept
G.hn	I-General_Concept
standard	O
for	O
networking	O
over	O
existing	O
home	O
wiring	O
(	O
coaxial	B-Protocol
cable	I-Protocol
,	O
phone	O
lines	O
and	O
power	B-Device
lines	I-Device
)	O
;	O
4096-QAM	B-Device
provides	O
12	O
bits/symbol	O
.	O
</s>
<s>
Another	O
example	O
is	O
ADSL	O
technology	O
for	O
copper	O
twisted	O
pairs	O
,	O
whose	O
constellation	B-Application
size	O
goes	O
up	O
to	O
32768-QAM	O
(	O
in	O
ADSL	O
terminology	O
this	O
is	O
referred	O
to	O
as	O
bit-loading	O
,	O
or	O
bit	O
per	O
tone	O
,	O
32768-QAM	O
being	O
equivalent	O
to	O
15	O
bits	O
per	O
tone	O
)	O
.	O
</s>
<s>
Ultra-high	O
capacity	O
Microwave	O
Backhaul	O
Systems	O
also	O
use	O
1024-QAM	B-Device
.	O
</s>
<s>
With	O
1024-QAM	B-Device
,	O
adaptive	B-Protocol
coding	I-Protocol
and	I-Protocol
modulation	I-Protocol
(	O
ACM	O
)	O
and	O
XPIC	O
,	O
vendors	O
can	O
obtain	O
gigabit	O
capacity	O
in	O
a	O
single	O
56MHz	O
channel	O
.	O
</s>
<s>
In	O
moving	O
to	O
a	O
higher	O
order	O
QAM	B-Device
constellation	B-Application
(	O
higher	O
data	O
rate	O
and	O
mode	O
)	O
in	O
hostile	O
RF/microwave	O
QAM	B-Device
application	O
environments	O
,	O
such	O
as	O
in	O
broadcasting	B-Operating_System
or	O
telecommunications	O
,	O
multipath	O
interference	O
typically	O
increases	O
.	O
</s>
<s>
There	O
is	O
a	O
spreading	O
of	O
the	O
spots	O
in	O
the	O
constellation	B-Application
,	O
decreasing	O
the	O
separation	O
between	O
adjacent	O
states	O
,	O
making	O
it	O
difficult	O
for	O
the	O
receiver	O
to	O
decode	O
the	O
signal	O
appropriately	O
.	O
</s>
<s>
There	O
are	O
several	O
test	O
parameter	O
measurements	O
which	O
help	O
determine	O
an	O
optimal	O
QAM	B-Device
mode	O
for	O
a	O
specific	O
operating	O
environment	O
.	O
</s>
