
123Overview of Modeling Techniques for Optical Transmission Systems
offer
exible
a
nd
f
ast-processing
m
ethods
t
o
ob
tain
t
he
B
ER
p
erformance.
S
everal
m
ain
c
haracteristics on these methods are as follows:
•
T
he
rst
m
ethod
i
mplements
t
he
E
xpected
M
aximization
(
EM)
t
heorem
i
n
w
hich
t
he
p
robability
d
istribution
f
unction
(
pdf)
o
f
r
eceived
e
lectrical
s
ignals
i
s
e
stimated
a
s
a m
ixture
o
f
M
ultiple
G
aussian
D
istributions
(
MGD).
A
lthough
t
he
a
pplication
o
f
t
his
m
ethod
i
n
o
ptical
c
ommunications
h
as
r
ecently
b
een
r
eported
[
40,41],
t
he
g
uidelines
f
or
o
ptimizing
t
he
a
ccuracy
o
f
t
his
m
ethod
h
ave
y
et
t
o
b
e
p
resented.
•
T
he
s
econd
m
ethod,
w
hich
i
s