International Journal of Science and Research (IJSR)
ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
Investigations on Spectral Efficiency of Optical
Communication System Using Wavelength Division
and Sub Carrier Multiplexing
Aparna R.1, Sheela Devi Aswathy Chandran2
1
P G Scholar, Optoelectronics and Communication Systems, Department of ECE, T K M Institute of Technology, Kollam, India
2
Assistant Professor, Department of ECE, T K M Institute of Technology, Kollam, India
Abstract: The growth of optical fiber communication had been tremendous over the past few decades. Multichannel optical systems
using Time Division Multiplexing (TDM), Wavelength Division Multiplexing (WDM), Sub Carrier Multiplexing (SCM) and their
combinations were developed in order to utilize the huge bandwidth provided by the optical fiber. In WDM, several optical carriers of
different wavelengths modulated by low bandwidth baseband data streams are transmitted. In SCM, several Gigahertz wide sub carriers
each carrying analog or digital data are multiplexed in radio frequency domain and this composite signal is subsequently modulated on
to a Terahertz optical carrier. SCM makes better use of available bandwidth when compared to WDM. The most significant advantage
of SCM in optical communication is its ability to place different optical carriers closely together. The bandwidth utilization is further
improved by a hybrid WDM/SCM system in which each WDM wavelength carries an SCM composite signal. The simulation and
performance comparison of a four channel WDM, a four channel SCM and a hybrid WDM/SCM system was done using Optisystem
12.0.
Keywords: Wavelength Division Multiplexing, Sub Carrier Multiplexing, Spectral Efficiency, Optisystem, Q Factor.
1. Introduction carriers have stable phase. Also because the bit rate
granularity is smaller which facilitates channel add/drop, the
An estimated one third of the world’s population is online combination of WDM and SCM can potentially make optical
now, a proportion that is sure to grow. More users imply networks more flexible and increase its spectral efficiency.
more services and more demand for bandwidth. Optical fiber
communication offers an enormous potential bandwidth for 2. System Design
data transmission. In order to utilize this huge bandwidth
offered by the optical fibers, new transmission technologies The simplified block diagram of WDM system is shown in
involving Time Division Multiplexing (TDM), Wavelength Figure 1. At the transmitting end there are several
Division multiplexing (WDM), Sub Carrier Multiplexing independently modulated light sources, each emitting signals
(SCM) and their combinations were developed. at a unique wavelength. The baseband data sequence is
provided by Pseudo Random Binary Sequence (PRBS)
The TDM strategy which was adopted in order to increase Generator which is then encoded by a Non Return to Zero
the bit rate carried by a single wavelength channel is highly (NRZ) encoder. This data will be used to modulate the light
sensitive to chromatic dispersion, non linear crosstalk and source via a Mach Zehnder Modulator (MZM). Here a
Polarization Mode Dispersion (PMD) because of the wide multiplexer is needed to combine these optical outputs into a
bandwidth of the signal. Later WDM technology was continuous spectrum of signals and couple them onto a single
developed which used low bit rates and power in a fiber.
wavelength channel by spreading the transmission capacity
into various wavelength channels. Hence problems in TDM At the receiving end a demultiplexer is required to separate
strategy like chromatic dispersion, non linear crosstalk and the optical signals into appropriate detection channels for
PMD was mitigated by the use of WDM system [1]. signal processing. The photo detectors will be used to detect
the baseband data, since they are usually sensitive over a
To further leverage the efficiency of bandwidth utilization, broad range of wavelengths. To prevent spurious signals
SCM was developed. SCM has a simple and low cost from entering a receiving channel, that is, to give good
implementation. In SCM, several sub carriers, each carrying channel isolation of the different wavelengths being used, the
analog or digital data are multiplexed in Radio Frequency demultiplexer must exhibit narrow spectral operation or very
(RF) domain and the composite signal is then transmitted stable optical filters with sharp wavelength cut offs must be
using a Terahertz optical carrier. SCM has better spectral used. The tolerable inter channel crosstalk levels can vary
efficiency when compared to WDM, since multiple signals widely depending on the application.
can be transmitted using a single wavelength and it is less
subjected to dispersion when compared to TDM, as the data
rate in each sub carrier is low [2]. One additional advantage
of SCM is that sub carriers can be modulated using various
modulation formats including phase modulations because RF
Volume 4 Issue 2, February 2015
www.ijsr.net
Paper ID: SUB151018 166
Licensed Under Creative Commons Attribution CC BY
International Journal of Science and Research (IJSR)
ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
(PSK) modulator that is subsequently modulated on the
optical carrier generated by a CW laser via a MZM. This way
each signal occupies a different portion of the optical
spectrum surrounding the centre frequency of the optical
carrier. At the receiving side, as normally happens in a
commercial radio service, the receiver is tuned to the correct
subcarrier frequency, filtering it out of other subcarriers and
is detected using a photo detector.
Figure 1: Simplified block diagram of a WDM system
A four channel WDM system is simulated in optisystem
software, the simulation layout of which is shown in Figure 2.
Here four PRBS generators generate four different data
signals at a bit rate of 1 Gbps which are encoded using a
NRZ pulse generator. These signals are used to modulate
optical carriers generated by CW lasers of frequencies 193.1
THz, 193.2 THz, 193.3 THz, and 193.4 THz with a 10 dBm
power, using MZM. These signals are then multiplexed using
a 4×1 WDM multiplexer (MUX). This composite signal is Figure 3: Simplified block diagram of SCM system
then transmitted through the single mode fiber of length 50
km. A four channel SCM system is simulated in optisystem
software. The simulation layout of an SCM system for four
At receiver end, a 1×4 WDM demultiplexer (DEMUX) is users is shown in Figure 4. Four PRBS generator, generate
used to split this optical signal into four signals. These four different data signals at a bit rate of 1 Gbps. These data
demultiplexed signals are then applied to photo detectors signals are used to modulate four different electrical carriers
centered at frequencies 193.1 THz, 193.2 THz, 193.3 THz, having frequencies 10 GHz, 15 GHz, 20 GHz and 25 GHz
193.4 THz respectively, which will convert these optical respectively. This modulation is done by electrical PSK
signals directly into baseband signal. Low Pass Filters (LPFs) modulator. To remove unwanted frequencies these signals
are used to filter out the higher frequency components. And are passed through band pass Bessel filters (frequencies same
at the output of LPFs we will get the data signals which were as carrier frequencies and bandwidth= 1.5×Bit Rate). These
initially transmitted. Then an eye diagram analyzer can be signals are then combined using electrical 4×1 power
used to generate the eye diagram, which can be used for the combiner and this combined signal is used to modulate an
performance analysis of the system. optical carrier generated by a CW laser of frequency 193.1
THz with a power of 10 dBm by a MZM. This modulated
signal is then transmitted through the single mode fiber of
length 50 km.
At receiver end an optical 1×4 power splitter is used to split
this optical signal into four signals. These optical signals are
then passed through optical band pass filters (frequencies
193.110 THz, 193.115 THz, 193.120, 193.125 THz
respectively and band width= 1.5×Bit Rate). These filtered
signals are then applied to photo detectors which will convert
these optical signals directly into baseband signals. LPFs are
used to filter out the higher frequency components. And at
the outputs of LPFs we will get data which was initially
transmitted. Then an eye diagram analyzer can be used to
generate the eye diagram, which can be used for the
performance analysis of the system.
Figure 2: Simulation layout of a 4 channel WDM system
SCM follows a different approach compared to WDM and is
spectrally efficient when compared to WDM. In an SCM
infrastructure, the baseband data generated by a PRBS
generator is first modulated on a Gigahertz wide sub carrier
as shown in Figure 3 via an electrical Phase Shift Keying
Volume 4 Issue 2, February 2015
www.ijsr.net
Paper ID: SUB151018 167
Licensed Under Creative Commons Attribution CC BY
International Journal of Science and Research (IJSR)
ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
demultiplexed by a 1×2 demultiplexer, detected by photo
detector and then two SCM channels in each wavelength
channel is detected by means of optical band pass filters
which filters out the two sub carrier frequencies. The
baseband data from each RF sub carriers is recovered by low
pass filters.
Figure 4: Simulation layout of a 4 channel SCM system
The basic configuration of a WDM/SCM optical system is
shown in Figure 5. Here, n independent high speed digital
signals are mixed by N different microwave carrier
frequencies fi. These are combined and optically modulated
onto an optical carrier. Then m wavelengths are multiplexed
together in an optical WDM configuration. At the receiver,
Figure 6: Simulation layout of a hybrid WDM/SCM system
an optical demultiplexer separates the wavelengths for
individual optical detectors. Then, RF coherent detection is
used at the SCM level to separate the digital signal channels. 3. Results and Discussions
Channel add/drop is also possible at both the wavelength and
SCM levels. The Figure 7 shows the spectrum of wavelength division
multiplexed signal for four data streams. As shown in figure
the four data streams of bit rate 1 Gbps are modulated into
four optical carriers of frequencies 193.1 THz, 193.2 THz,
193.3 THz and 193.4 THz.
Figure 5: Basic configuration of a WDM/SCM optical
system
A hybrid WDM/SCM system with two WDM channels, each
of which carries two SCM channels simulated in Optisystem
is shown in Figure 6. Each WDM channel have two SCM Figure 7: Spectrum of 4 channel wavelength division
channels of bit rate 1 Gbps each which are electrically multiplexed signal
modulated into two RF sub carriers of frequency 2 and 5
GHz and is then combined by a 2×1 power combiner. This The Figure 8 shows the spectrum of sub carrier multiplexed
composite SCM signals are then modulated into two optical signal for four data streams. As shown above the four data
carriers of frequency 193.1 and 193.2 THz which is then streams of bit rate 1 Gbps are modulated into four RF
multiplexed by a 2×1 WDM multiplexer and is transmitted subcarriers of frequencies 10 GHz, 15 GHz, 20 GHz and 25
through an optical fiber. The received signal is then GHz which are multiplexed in RF domain. This SCM
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Paper ID: SUB151018 168
Licensed Under Creative Commons Attribution CC BY
International Journal of Science and Research (IJSR)
ISSN (Online): 2319-7064
Index Copernicus Value (2013): 6.14 | Impact Factor (2013): 4.438
composite signal is then modulated on to a single optical signal provides more efficient utilization of optical
carrier of frequency 193.1 THz. bandwidth.
References
[1] Ashutosh Jha, Shubham Agarwal, Vinay Paul, Prof
Sugumaran S, “Performance Analysis of WDM and
SCM using Available Modulation Techniques”,
International Journal for Scientific Research and
Development, Vol. 2, Issue 01, 2014.
[2] S.Revathi , G.Aarthi, “Performance analysis of Wave
Length Division and Sub Carrier Multiplexing using
different modulation techniques” , International Journal
of Engineering Research and Applications (IJERA),
Vol. 1, Issue 2, 2014.
[3] Jitender Kumar, Manisha Bharti, Yogendra Singh,
“Effect of Signal Direct Detection on Sub-Carrier
Multiplexed Radio over Fiber System”, International
Journal of Advanced Research in Computer and
Figure 8: Spectrum of 4 channel sub carrier multiplexed Communication Engineering, Vol. 3, Issue 1, January
signal. 2014.
[4] Jitender Kumar, Manisha Bharti, Yogendra Singh, “Sub-
The Figure 9 shows the spectrum of a hybrid WDM/SCM Carriers Multiplexing at Various Data Rates on Radio
signal in two WDM channels each of which carries two SCM Over Fiber Systems”, International Journal of Advanced
channels. As shown above there are two WDM channels Research in Electronics and Communication Engineering
centered at frequency of 193.1 and 193.2 THz, each of which (IJARECE), Volume 2, Issue 10, October 2013.
carries two RF sub carriers of frequencies 2 and 5 GHz [5] Satbir Singh, Amarpal Singh, “Simulative Analysis of
modulated with two data streams of bit rate 1 Gbps each. influence of RF sub carriers on performance of 10 GHz-
band bidirectional radio over fiber (RoF) system” ,
Journal of Scientific and industrial Research,vol.69,
January 2010.
[6] Rongqing Hui, Benyuan Zhu, Renxiang Huang,
Christopher T. Allen, Kenneth R. Demarest, Douglas
Richards, “Subcarrier Multiplexing for High-Speed
Optical Transmission” , Journal Of Lightwave
Technology,Vol. 20, No. 3, March 2002.
[7] Paul D. Sargis, Ronald E. Haigh, and Kent O,
McCammon, “Dispersion-reduction technique using
subcarrier multiplexing”, Lawrence Livermore National
Laboratory, October 1995.
Figure 8: Spectrum of a hybrid WDM/SCM signal.
4. Conclusion
A WDM system allows multiple connections over a single
fiber by assigning different wavelength channels for different
connections, each which can operate at arbitrary data rates. In
an SCM infrastructure, the baseband data is first modulated
on a Gigahertz wide sub carrier that is subsequently
modulated in the Terahertz optical carrier. SCM makes better
use of available bandwidth and increases the spectral
efficiency compared to WDM. Hence SCM is spectrally
more efficient than WDM. But a hybrid WDM/SCM system
in which each WDM channel carries an SCM composite
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Paper ID: SUB151018 169
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