Chapter 3
COHERENT OPTICAL OFDM
3.1 INTRODUCTION
Coherent Optical OFDM (CO-OFDM) is considered a enabling technology of the
next generation optical communication system. As a coherent system, the CO-
OFDM system maintains both signal amplitude and phase, thus increasing
bandwidth utilization. The coherent optical communication system makes full
compensation of chromatic dispersion, after optical/electrical conversion,
possible. Optical frequency division multiplexing (OFDM) is an attractive
modulation scheme that recently received a lot of attention in the fiber optic
community. The CO - OFDM modulation scheme also leads to a high spectral
efficiency because of its partially overlapping subcarriers .Moreover, the cyclic
prefix code of the COOFDM system makes the system more resistant to inter-
symbol interference caused by Chromatic Dispersion and Polarization Mode
Dispersion .
The superior performance of CO-OFDM in terms of dispersion tolerance and
OSNR requirements makes it suitable for long haul networks. Coherent reception
requires that the polarization of the local oscillator should be matched with that
of the received original signal .Otherwise severe performance degradation results.
The state of polarization of light travelling through the conventional fiber varies
randomly. Hence polarization tracking device is necessary at the receiver side,
which is impractical in field application. Another way is to split the received
light into two orthogonal polarizations and process the received signals in
separate branches of the receiver. The performance of such a diversity receiver
would be independent of the state of polarization of the received signal. When
the modulation technique of OFDM combines with coherent detection, it brought
many benefits, the benefits of powerful techniques are multifold High spectral
efficiency, Robust to chromatic dispersion and polarization-mode dispersion
High receiver sensitivity, Dispersion Compensation Modules (DCM)-free
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Coherent Optical OFDM
operation , Less DSP complexity , Less oversampling factor , More flexibility in
spectral shaping and matched filtering [ Puneet mittal et al].
3.2 LITERATURE REVIEW
Veneetha in [Joseph et al] shows the simulations at 50 Gbps data rate and found
that the use of CO-OFDM allows the use of single standard mode fiber (SSMF)
in long haul networks without any dispersion compensation technique. The
dispersion tolerance of the OOFDM system is superior compared to conventional
modulation techniques. The use of polarization diversity schemes further improve
the performance of CO-OFDM. The receive diversity architecture exhibits best
BER performance compared to other transmit diversity schemes.
Sunaina N in [Sunaina N. et al] gives the Provision of broadband services for
downloading high definition data has become a difficult requirement for
dataproviders. The easy way to provide multipath, which are used in most
broadband systems between transmitters and receivers, is to use OFDM. From the
observations of this paper it is concluded that for long distance communication
Coherent detection OFDM method has more prominent performance than direct
detection method. In coherent detection method, as the modulating signal
mapping rate increases, it gives better performance in the constellation diagram.
Khaled in [Khaled et al] shows a coherent system integrated with WDM. The use
of WDM helps to increase the capacity of the system and increases data rate of
the system. The results shows that when fiber length is increased BER of the
system increases and larger OSNR is required to keep the BER less. Further
different modulation scope is provided for enhancement of the system.
Mohammad Jallal in [Dr.Sinan M. et al] simulated a CO-OFDM system with
Dense Wavelength Division Multiplexing (DWDM) to reach higher data rates
over 4500 km SSMF. Results shows good performance of BER, OSNR, and clear
constellation diagrams.
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Coherent Optical OFDM
3.3 SYSTEM DESIGN
Figure 3.1 Conceptual Diagram of CO-OFDM System [William et al]
Figure 3.1 shows the conceptual diagram of generic CO-OFDM
system that includes five blocks. In the RF OFDM transmitter, the
input digital data are first converted from serial to parallel into a
block of bits consisting of information symbols, each of which may
comprise of multiple bits for m-ary coding. This information
symbol is mapped into two dimensional complex signal. The time
domain OFDM signal is obtained through IFFT and a guard interval
is inserted to avoid channel dispersion. Signal is filtered through a
low pass filter to remove the alias signal. And RF to Optical up
converter transforms the baseband signal into optical domain using
an RF IQ modulation comprising a pair of Mach-Zehnder
Modulators (MZMs) with a 90 degree phase offset. Signal is up
converted into optical domain. Signals are then forwarded on
optical fiber link for further transmission towards receiver. The
optical signal is then fed into the Optical down-Converter, where
the optical OFDM signal is converted into an RF-OFDM signal.
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Coherent Optical OFDM
Different kinds of modulations can be used on transmitter and
receiver side for better results..
3.4 SIMULATIVE DESIGN
CO-OFDM system is designed and investigated with QAM for long haul
transmission. The System of CO-OFDM is fully designed and implemented using
OptiSystem v.13 and v.14.
OptiSystem is a comprehensive software design tool that enables users to plan,
test, and simulate various optical links in the transmission layer of modern optical
networks, and also it can be used by telecommunications companies all over the
world for planning and implementing a fully designed optical network, which is a
low cost and time saving approach, and the researchers can use it to work in
highly effective and efficient manner.
Figure 3.2 CO-OFDM Transmitter
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Coherent Optical OFDM
In the system simulated the Figure 3.2 represents RF OFDM
transmitter block, The transmitter of CO-OFDM is built with a
Pseudo Random Binary Sequence (PRBS), to generate a bit
sequence to approximate the random data characteristics. The 4-
QAM encoder generates the signal and signal is connected to the
OFDM modulator with a 512 subcarrier and 1024 FFT points.
Signal is filtered using low pass filters to remove alias signal. The
in-phase (I) and Quadrature components(Q) of the resulting signal
from the OFDM modulator is transmitted to the direct I/Q optical
modulator. The direct I/Q modulator consists of two Mach-Zehnder
modulators (MZM) and will modulate the electrical signal from the
OFDM modulator to the optical carrier with a laser source of 193.05
THz. The power of the laser source is varied .
Figure 3.3 Optical Fiber link
Figure 3.3 represents an optical fiber link, The resulting signal
from the two MZMs is then transmitted through the single mode
dispersion compensation fiber in the system. The DCF is used to
compensate for the fiber dispersion in system. The attenuation is
0.2 dB/km and the DCF attenuation is 0.4 dB/km. The SMF
dispersion is 16 ps/nm/km for 100 km. The SMF will produce a
dispersion of 16 × 100 = 1600 ps/nm. Therefore, to compensate the
dispersion of the 100 km SMF, a 20 km long DCF is required with
dispersion of -80 ps/nm/km. This will produce a dispersion of
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Coherent Optical OFDM
−1600 ps/nm, which will be negative to cancel the positive
dispersion of the SMF. An Erbium Doped Fiber Amplifier (EDFA)
is used with to amplify the signal and to compensate for the loss.
Figure 3.4 Coherent Optical OFDM Receiver
At the receiver side, the incoming optical signal is detected by two identical pairs
of balanced coherent detectors with a local oscillator (LO) to perform the I/Q
optical to electrical conversion and to cancel the noise. Each detector consists of
two couplers and two PIN photo detectors. Each PIN photo detector has a dark
current of 10 nA, a responsivity of 1 A/W, thermal noise of 100e-24 W/Hz, and a
center frequency of 193.05 THz. After detecting the signal by the balanced
detectors, the signal is sent to the OFDM demodulator. The OFDM demodulator
should have the similar parameters to that of OFDM modulator The guard
interval is then removed. Finally, the resulting signal is fed into a 4-QAM
decoder to create a binary signal and is checked through electrical constellation
Visualizer.
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Coherent Optical OFDM
3.4.1 RESULTS AND DISCUSSIONS
In order to check the performance of CO-OFDM systems, system is simulated
using Optisystem tool with v.14.
Table 3.1 Global Parameters
Parameters Values
Sequence length 16384
Sample per bit 8
No. of symbols 131072
Table 3.2 SMF Parameters
Parameters Values
Dispersion 16 ps/nm/km
Dispersion Slope 0.08 ps/nm^2/km
Effective Area 80 um^2
PMD Coefficient 0.2 ps/km
Attenuation 0.2 dB/Km
Table 3.2 DCF Parameters
Parameters Values
Dispersion -80 ps/nm/km
Dispersion slope -0.45 ps/nm^2/km
Effective Area 30 um^2
PMD coefficient 0.2 ps/km
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Coherent Optical OFDM
Attenuation 0.4 dB/km
3.4.1.1 CO-OFDM SYSTEM WITH POWER VARIATIONS
System given in figure 3.1 is simulated with dense wavelength division
multiplexing with the starting frequency of CW laser with 193.05 THz. System is
simulated with 8 users wavelength division multiplexing.
Figure 3.5 Constellation diagram of CO-OFDM with power -6 dBm
Figure 3.5 shows the result of the system with power of -6 dBm. It is found that
at lower power level signal gets distorted and noise level is more.
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Coherent Optical OFDM
Figure 3.6 Constellation diagram of system with power of -4dbm
Figure 3.6 represents an improvement in the recovered signal, distortion
decreased as power level is increased.
Figure 3.7 Constellation diagram of System with power of -2 dBm
In figure 3.7 gives improved results with a power of -2 dBm. Signal gets better
with the increase in power level of the system.
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Coherent Optical OFDM
Figure 3.8 Constellation diagram of the System with power of 0dbm
Figure 3.8 gives the best results with decreased distortion and decreased noise
level.
Results shown above shows that as the power level of the system increases
system performance gets improved and noise reduces with increase in power.
3.4.1.2 RELATION OF POWER AND Q-FACTOR
Figure 3.9 Relation between Power and Q-factor
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Coherent Optical OFDM
In figure 3.9 relation of power with Q-factor is shown. Graph represents the
variation of power, it shows as power goes on increasing, Q-factor of the system
keeps on increasing. So to have a high Q-factor, power of the system should be
more.
3.4.1.3 PERFORMANCE OF FILTERS IN CO-OFDM SYSTEMS
Digital filters combined with coherent detection is a powerful combination,
enabling uncompensated transmission of 80Gbit/s data over high PMD fiber [Seb
J. Savory]. Different filters like Gaussian filter, Butterworth filter, Bessel filter
are used here to find the effect on coherent OFDM.
Figure 3.10 Spectrum of transmitted signal
Figure 3.10 shows the spectrum of transmitted signal that is going to be
filtered on receiver side.
Further results based on filters are given. Filters are basically used in
communication for wavelength selection, which can select desired wavelength to
pass and reject the others. It is widely used in DWDM systems for wavelength
selection, optical performance monitoring, optical noise filtering, and optical
amplifier noise suppression etc. Optical multiplexers makes different coupling
into an optical fiber and different wavelength carries different information. At the
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Coherent Optical OFDM
receiving end if the signal need to be separated with desired wavelength from
optical fiber than it is necessary to use optical filter.
Figure 3.11 Constellation of Bessel filter
Figure 3.11 shows the constellation of Bessel filter, it is an analog linear filter
with a maximally flat group and phase delay that preserves the wave shape of the
signal. Results shows that received signal is corrupted with noise and signal is
scattered.
Figure 3.12 Received Constellation of Butterworth filter
The Butterworth filter is the best compromise between attenuation and phase
response . It has no ripple in the pass band or the stop band, and because of which
it is sometimes called a maximally flat filter. The Butterworth filter achieves its
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Coherent Optical OFDM
flatness at the expense of a relatively wide transition region from pass band to
stop band, with average transient characteristics.
Figure 3.12 represents the constellation diagram of Butterworth filter which
shows that it is some what better performing than Bessel filter but it needs phase
shift.
Figure 3.13 Constellation of Gaussian filter
Figure 3.13 represents constellation of Gaussian filter which is having improved
signal than Bessel and Butterworth filter. Gaussian filter is the filter with impulse
response of Gaussian function. It is ideally considered as a time domain filter.
Figure 3.14 Constellation of Rectangular Filter
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Coherent Optical OFDM
Figure 3.14 represents best results when rectangular filter is used. It only requires
some phase shift.
Table 3.1 Comparison of all filters performance in tabular form.
Filters In phase amplitude Quadrature amplitude
Bessel 0.341 0.345
Butterworth 0.305 0.309
Gaussian 0.423 0.426
Rectangular 1.262 1.361
Figure 3.15 Performance comparison of all filters
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