
333Digital Coherent Optical Receivers
DSP,
w
hich
h
as
b
een
w
idely
u
sed
i
n
w
ireless
c
ommunications,
h
as
a
ke
y
r
ole
t
o
p
lay
i
n
t
he
i
mplementation
o
f
c
oherent
o
ptical
c
ommunication
s
ystems.
D
SP
t
echniques
a
re
a
pplied
t
o
c
oherent
o
ptical
c
ommunication
s
ystems
t
o
o
vercome
t
he
d
ifculties
o
f
O
PLL
a
nd
a
lso
t
o
i
mprove
t
he
p
erformance
o
f
t
he
t
ransmission
s
ystems
i
n
t
he
p
resence
o
f
ber
d
egrading effects such as chromatic dispersion, PMD, and ber nonlinearities.
Coherent
o
ptical
r
eceivers
h
ave
t
he
f
ollowing
a
dvantages:
(
1)
t
he
s
hot-noise-limited
r
eceiver
s
ensitivity
c
an
b
e
a
chieved
w
ith
s
ufcient
L
O
p
ower;
(
2)
c
losely
s
paced
W
DM
c
hannels
c
an
b
e
s
eparat ...