Journal of Networks - Academy Publisher

162 JOURNAL OF NETWORKS, VOL. 5, NO. 2, FEBRUARY 2010
compensation of the nonlinear phase shift (as e.g.
proposed in [9]).
C. Summary
The performance of EDC in conjunction with coherent
reception for RZ-QPSK, RZ-8PSK and Star-RZ-16QAM
for the linear and nonlinear channel at 107Gb/s has been
investigated. For EDC a zero-forcing equalizer is applied,
using the MMSE criterion for deriving the coefficients.
The dispersion tolerance (at 2dB OSNR penalty) for the
investigated modulation formats using FFE equalizer
with various tap counts is shown in Table I.
TABLE I.
Dispersion tolerance (at 2dB OSNR penalty) in ps/nm (one-sided) for the
investigated modulation formats using FFE equalizers of various tap
counts for the linear and nonlinear fiber.
taps 0 9 15 9 15
QPSK 8 ps/nm
180
ps/nm
300
ps/nm
180
ps/nm
240
ps/nm
8-PSK 8 ps/nm
350
ps/nm
620
ps/nm
350
ps/nm
600
ps/nm
Star 16-
QAM
17 ps/nm
550
ps/nm
>800
ps/nm
350
ps/nm
380
ps/nm
IV. DIGITAL CARRIER RECOVERY IN COHERENT
PSK-RECEIVERS
In coherent receivers, a local laser (LO) at the receiver
front end is required, which down-converts the optical
signal into or close to the baseband. Here we assume an
intradyne receiver, where the nominal frequencies of
both, the received signal and the local laser are almost
equal, however there is no phase- or frequency locking of
the LO. Thus we have to compensate for (i) the carrier
frequency offset, (ii) the carrier phase, and (iii) the phase
noise of the lasers involved (transmitter and receiver). We
review carrier recovery schemes based on M-th power
approach in 100Gb/s applications with QPSK-modulation
and polarization multiplexing, where we can transmit
with a Baudrate of one quarter of the bitrate, making a
hardware implementation feasible in the near future. The
basic operation and fundamental limits of the recovery
scheme, as well as some optimization with respect to the
implementation of the filters involved, are shown in the
paper.
A. Simulation Setup
The receiver for a polarization multiplexing system is
shown in Fig. 9. The incoming signal is splitted with a
polarization beam splitter (PBS) into x- and ypolarization.
Each polarization is then mixed separately
with the local oscillator (LO). Due to some distortions on
the transmission channel the received x- and ypolarization
must not correspond to the two transmitted
signals. The separation of the two transmitted signals is
finally done during equalization in the digital signal
processing unit.
© 2010 ACADEMY PUBLISHER
Linear nonlinear
PBS
Pol X
LO
Pol Y
f T
f LO
f LO
f T
90º
Hybrid
I,X
LP
Q,X
LP
I,Y
LP
Q,Y
LP
A/D
A/D
A/D
A/D
~
f ADC
digital signal processing
The structure of the used carrier recovery is shown in
Fig. 10 which is a Viterbi-and-Viterbi approach [10] for
two polarisations. For each polarisation the incoming
complex signal is raised to the power of four in order to
eliminate the modulation Φmod. These two signals are
averaged and low pass filtered. The derived phase is
divided by four and subtracted from the phase of the
delayed incoming signal for each polarisation.
e
Figure 9. Coherent QPSK receiver with polarization multiplexing
and digital signal processing.
j( �
mod
( kT ) ���kT��noise( kT ))
Pol X
Pol Y
(.) 4
(.) 4
delay
D /2
�
i
i��D/2 LP
a
delay
arg(.)
arg(-(.))/4
arg(.)
clock recovery
equalization
carrier recovery
�
mod
( kT ) ���kT��noise( kT )
-
-
��kT�� �noise(
kT )
Figure 10. Structure of the carrier recovery for polarization
multiplexing.
Fig. 11 shows the impulse response of the three
investigated filter types. The first one is a rectangle (Fig.
11a)), the second one provides a hyperbolic shape (Fig.
11b)) and the third one is a Wiener filter where the
coefficients are dependent on the combined laser
linewidth and ASE noise (Fig.11 c) and d)) [11].
a) b)
c) d)
Figure 11. Impulse response of a) rectangular filter, b) hyperbolic
filter, c) Wiener filter for 30 dB OSNR and 20 MHz laser linewidth
and d) Wiener filter for 10 dB OSNR and 200 kHz laser linewidth.
B. Simulation Results
We investigate the influence of the filter type as well
as the influence of the filter order D on the performance
of the coherent QPSK receiver for different distortions,
i.e. in the presence of ASE noise, laser phase noise and
decoding