
251Advanced Modulation Format Optical Transmitters
is
g
enerated
u
nder
s
oftware
p
rocessing,
a
s
f
ollows:
i
nput
Q
AM
s
ymbols
a
re
z
ero
p
added
t
o
ob
tain
i
nput
s
amples
f
or
i
nverse
f
ast
F
ourier
t
ransform
(
IFFT),
t
he
s
amples
a
re
i
nserted
t
o
c
reate
t
he
g
uard
b
and,
a
nd
t
he
O
FDM
s
ymbol
i
s
m
ultiplied
b
y
t
he
w
indow
f
unction
(
raised
c
osine
f
unction
c
an
b
e
u
sed).
T
he
p
urpose
o
f
c
yclic
ex
tension
i
s
t
o
p
reserve
t
he
o
rthogonality
a
mong
s
ubcarriers
e
ven
w
hen
t
he
n
eighboring
O
FDM
s
ymbols
p
artially
o
verlap as a result of dispersion.
A
s
ystem
a
rrangement
o
f
t
he
O
FDM
f
or
op
tical
t
ransmission
i
n
l
aboratory
d
emon-
stration
i
s
s
hown
i
n
F
igure
6
.34.
E
ach
i
ndividual ...