<s>
Carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
(	O
CFO	O
)	O
is	O
one	O
of	O
many	O
non-ideal	O
conditions	O
that	O
may	O
affect	O
in	O
baseband	B-Algorithm
receiver	O
design	O
.	O
</s>
<s>
In	O
designing	O
a	O
baseband	B-Algorithm
receiver	O
,	O
we	O
should	O
notice	O
not	O
only	O
the	O
degradation	O
invoked	O
by	O
non-ideal	O
channel	O
and	O
noise	O
,	O
we	O
should	O
also	O
regard	O
RF	B-Protocol
and	O
analog	O
parts	O
as	O
the	O
main	O
consideration	O
.	O
</s>
<s>
Those	O
non-idealities	O
include	O
sampling	O
clock	O
offset	O
,	O
IQ	O
imbalance	O
,	O
power	O
amplifier	O
,	O
phase	O
noise	O
and	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
nonlinearity	O
.	O
</s>
<s>
Carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
often	O
occurs	O
when	O
the	O
local	O
oscillator	O
signal	O
for	O
down-conversion	O
in	O
the	O
receiver	O
does	O
not	O
synchronize	O
with	O
the	O
carrier	O
signal	O
contained	O
in	O
the	O
received	O
signal	O
.	O
</s>
<s>
For	O
an	O
OFDM	B-Protocol
system	O
,	O
the	O
orthogonality	B-Application
among	O
sub	O
carriers	O
is	O
maintained	O
only	O
if	O
the	O
receiver	O
uses	O
a	O
local	O
oscillation	O
signal	O
that	O
is	O
synchronous	O
with	O
the	O
carrier	O
signal	O
contained	O
in	O
the	O
received	O
signal	O
.	O
</s>
<s>
Hence	O
,	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
always	O
exists	O
even	O
if	O
there	O
is	O
no	O
Doppler	O
effect	O
.	O
</s>
<s>
In	O
a	O
standard-compliant	O
communication	O
system	O
,	O
such	O
as	O
the	O
IEEE	O
802.11	O
WLAN	B-Architecture
the	O
oscillator	O
precision	O
tolerance	O
is	O
specified	O
to	O
be	O
less	O
than	O
±20	O
ppm	O
,	O
so	O
that	O
CFO	O
is	O
in	O
the	O
range	O
from	O
-	O
40	O
ppm	O
to	O
+40	O
ppm	O
.	O
</s>
<s>
If	O
the	O
TX	O
oscillator	O
runs	O
at	O
a	O
frequency	O
that	O
is	O
20	O
ppm	O
above	O
the	O
nominal	O
frequency	O
and	O
if	O
the	O
RX	O
oscillator	O
is	O
running	O
at	O
20	O
ppm	O
below	O
,	O
then	O
the	O
received	O
baseband	B-Algorithm
signal	I-Algorithm
will	O
have	O
a	O
CFO	O
of	O
40	O
ppm	O
.	O
</s>
<s>
The	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
can	O
first	O
be	O
normalized	O
with	O
respect	O
to	O
the	O
sub	O
carrier	O
spacing	O
(	O
and	O
then	O
decomposed	O
into	O
the	O
integral	O
component	O
and	O
fractional	O
component	O
,	O
that	O
is	O
,	O
and	O
.	O
</s>
<s>
The	O
fractional	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
,	O
,	O
results	O
in	O
attenuation	O
in	O
magnitude	O
,	O
phase	O
shift	O
,	O
and	O
ICI	O
,	O
while	O
the	O
integer	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
,	O
,	O
causes	O
index	O
shift	O
as	O
well	O
as	O
phase	O
shift	O
in	O
the	O
received	O
frequency-domain	O
signals	O
.	O
</s>
<s>
The	O
range	O
of	O
estimated	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
is	O
.	O
</s>
<s>
In	O
the	O
IEEE	O
802.16e	O
OFDM	B-Protocol
mode	O
standard	O
,	O
the	O
oscillator	O
deviation	O
is	O
within	O
±8	O
ppm	O
.	O
</s>
<s>
In	O
the	O
following	O
,	O
algorithms	O
for	O
resolving	O
such	O
frequency	O
ambiguity	O
in	O
the	O
estimated	O
carrier	B-Algorithm
frequency	I-Algorithm
offset	I-Algorithm
will	O
be	O
presented	O
.	O
</s>
<s>
In	O
the	O
802.16e	O
OFDM	B-Protocol
mode	O
,	O
the	O
initial	O
estimated	O
CFO	O
is	O
within	O
.	O
</s>
<s>
In	O
MIMO-OFDM	O
systems	O
,	O
the	O
transmit	O
antennas	O
are	O
often	O
co-located	O
,	O
so	O
are	O
the	O
receive	O
antennas	O
.	O
</s>
<s>
The	O
ML	O
estimation	O
for	O
the	O
fractional	O
CFO	O
is	O
quite	O
popular	O
in	O
MIMO-OFDM	O
systems	O
.	O
</s>
<s>
In	O
framebased	O
OFDM	B-Protocol
systems	O
,	O
both	O
the	O
residual	O
CFO	O
tracking	O
and	O
the	O
SCO	O
tracking	O
are	O
inevitable	O
,	O
because	O
the	O
receiver	O
may	O
operate	O
for	O
a	O
long	O
period	O
of	O
time	O
.	O
</s>
<s>
In	O
packet-based	O
OFDM	B-Protocol
systems	O
,	O
however	O
,	O
the	O
influences	O
of	O
these	O
two	O
offsets	O
depend	O
on	O
the	O
packet	O
length	O
and	O
the	O
magnitude	O
of	O
the	O
offsets	O
.	O
</s>
<s>
In	O
many	O
OFDM	B-Protocol
wireless	O
communication	O
standards	O
,	O
for	O
example	O
,	O
DVB-T	O
,	O
IEEE	O
802.11	O
a/g/n	O
,	O
and	O
IEEE	O
802.16e	O
OFDM	B-Protocol
mode	O
,	O
dedicated	O
pilot	O
subcarriers	O
are	O
allocated	O
to	O
facilitate	O
receiver	O
synchronization	O
.	O
</s>
