
323Digital Coherent Optical Receivers
8.3.1.2 PSK
C
oherent
S
ystem
Under the phase shift keying modulation format, the detection is similar to that of
Figure 8.4
for
heterodyne
detection,
but
after
the
BPF,
an
electrical
mixer
is
used
to
track
the
p
hase
o
f
t
he
d
etected
s
ignal
(
Figure
8
.6).
T
he
r
eceived
s
ignal
i
s
g
iven
b
y
r t P P t t n t n t
s LO IF x IF y IF
( ) cos[( ) ( )] cos( ) sin( )= ℜ + + +2 ω ϕ ω ω (8.18)
The information is contained in the time-dependent phase term ϕ(t).
When
the
phase
and
frequency
of
the
voltage
control
oscillator
(VCO)
are
matched
with
those
o
f
t
he
s
ignal
c
arrier,
t
hen
t
he
r
eceived
e
lectrical
s
ignal
c
an
b
e