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ZANCO Journal of Pure and Applied Sciences
The official scientific journal of Salahaddin University-Erbil
ZJPAS (2017), 29 (s4); s217-s226
Investigating the Performance of Wavelength Division Multiplexing based RoF
Optical Network with Dispersion Compensation Fiber
Raghad Z.Yousif1, Nahlah Qader Mohammed2, Amanj Fransis3
1- Department of physics-Communication, College of Science, Salahaddin University, Erbil, Kurdistan Region, Iraq.
2- Department of Physics, College of Education, Salahaddin University -Erbil, Erbil, Kurdistan Region, Iraq.
3- Department of physics, College of Science, Salahaddin University, Erbil, Kurdistan Region, Iraq.
1. INTRODUCTION
Due to increased demand on high data
capacity of wireless communication systems
which is extremely expanded from voices and
simple messages to multimedia in order to
satisfy various demands of system users with
evolutionary future services. The fundamental
reason of Radio over Fiber (RoF) technology
is to improve the usage of spectrum resources
and reduce the cost of far off BS modules
(Turan Erdogan et al., 1997) (W.W.Hu et al.,
(2004). RoF systems is a promising
technique might urgently handle the huge
demands of the telecommunication networks,
as they could provide the sufficient bandwidth
for the transmission of broadband data to end-
users, other benefits are low attenuation loss,
and immunity to radio frequency interference
(H. Nasoha et al., 2007)(Hyun-Seung Kim et
al., 2009)(A Nirmalathas, et al., 2010)(Y. M.
Lin et al., 2010). The advantages of
communication system based on RoF
A R T I C L E I N F O
A B S T R A C T
Article History:
Received: 01/06/2017
Accepted: 05/08/2017
Published: 20/12/2017
A Radio-over-Fiber (RoF) make use of the advantages of wireless
communication system and optical networks. It uses optical fiber
as a core
technology due to the enormous advantages offered by it. Making use of mobility
available by wireless communication. Dispersion management and modulation
were investigated for two Channel-
WDM optical communication systems.
Dispersion compensatin
g fibers (DCF) are used to compensate the positive
dispersion accumulated over a protracted haul SMF between CS (central Site)
and BS (Base Station). The full duplex RoF system is implemented by means of
Wavelength Division Multiplexing (WDM) and Optical Add Drop Multiplexer
(OADM), where WDM enables transmission of many signals via a single mode
fiber for long distance while OADM allows data transmission of both down-link
and uplink a by single-
mode fiber (SMF). The modulation technique employed
by the proposed system is DPSK. which has some advantages over binary PSK,
like a low phase error rate and no need to know the absolute phase. The proposed
RoF system has been simulated using Opti System 13.0 simulator. The system
outcomes like the Q-factor, and BER with and without (DCF) at different fiber
length have been used to discuss system performance. It has been observed that
the proposed system improves the Q-factor by 6-units and the BER by 24 dB at
CS and 57dB at BS respectively.
Keywords:
RoF system, Sub-Carrier
Multiplexing, Wavelength
Division Multiplexing,
Wavelength Interleaving
*Corresponding Author:
Raghad Z.Yousif
Raghad.yousif@su.edu.krd
218 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
technology is the exquisite development space
in the 3G and 4G (Turan Erdogan et al., 1997)
(W.W.Hu et al., 2004). Current tendencies in
cellular networks are: reduction in cell size to
serve more users and Operation in
microwave/millimeter wave (mm-wave)
frequency bands to avoid spectral congestion
in lower frequency bands. It demands a large
number of Base Stations (BSs) make full
coverage for the served area, thus it’s
important to reduce the cost of BS. This
requirement has caused the improvement of
system architecture where functions such as
signal routing/processing, handover and
frequency allocation are carried out at a (CS),
in preference to on BS. Furthermore, such a
centralized configuration permits sensitive
devices to be located in safer and secure and
enables the cost of costly components to be
shared among several BSs. A RoF system
includes (CS) and a Remote Site (RS)
connected to an optical fiber link or network.
The signal between CS and BS is transmitted
in the optical band via RoF network. This
architecture layout makes the design of BSs
quite simple. In the simplest case, the BS
includes particularly optical-to-electrical
(O/E) and electrical-to-optical (E/O)
converters, an antenna and some microwave
circuitry (two amplifiers and a diplexer). In
the event of ability region is in a GSM
network, and then CS might be the Mobile
Switching Centre (MSC) and RS the base
station (BS). As for narrowband
communication systems and wireless Local
Area Networks (WLANs), the CS will be the
head-end while the Radio Access Point (RAP)
would act as the RS. RoF systems span a wide
range (usually in the GHz region) and based
on the nature of the applications to distribute
the frequencies of the radio signals. Besides
transportation and mobility features, RoF
systems are also designed to carry out added
radio-system functionalities. These functions
include data modulation, signal processing,
and frequency conversion (up and down)
(Peng,et al., 2009)(Keiser, G 2011). As
depicted in Figure 1, RoF systems were in the
whole used to transport microwave signals
and to reap mobility functions in the CS. The
centralization of RF signal processing
functions has many advantages such as
permitting devices sharing, dynamic
allocation of resources, and simplifies system
operation and maintenance. These benefits
might be translated into major system
installation and operational savings, especially
in wide-coverage broadband wireless
communication systems, in which a high
density is necessary.
Figure (1) : The structure of Radio over fiber system
With the non-stopped development of studies
on RoF, in optical communication, (WDM)
technology has been very mature. WDM
support high bandwidth data transfer, it is able
to simplify the network structure that
multiplexing a multi-channel RoF signals and
transmitting in on a single fiber. The
integration between RoF and WDM is
abbreviated as WDM-RoF system (Turan
Erdogan et al., 1997) (W.W.Hu et al., 2004).
The management of dispersion and non-
linearity's are most important issue in WDM
systems (A. Mohan 2014), the dominant goal
of optical fiber communication system is to
increase the transmission distance (Y. Chaba et
al., 2010). The principles factors which impact
the WDM system are loss, dispersion, and non-
linear effects. In order to reduce the
impairments due to fiber non-linearity's optical
219 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
amplifiers such as Erbium Doped Fiber
Amplifier (EDFA) (Simranjit Singh et al.,
2014)has been used .The use of DCF is
powerful way to reduce the complete
dispersion in WDM network as they have
higher negative dispersion coefficient and can
be attached to the transmission fiber having
positive dispersion coefficient so that the
complete dispersion of the link becomes
zero.(Gurinder Singh et al., 2014).The
(Differential Phase Shift Keying )DPSK is a
quick and stable modulation format and
perfectly fits many optical applications. It has
some advantages to the binary PSK, like a
lower phase error rate and a no need to know
the absolute phase. In this paper Q-factor and
BER of 2 channel WDM systems are analyzed
over a distance less than 108km for channel
from CS to BS with and without DCF of 10km.
2. RELATED WORK
Husam Abduldaem Mohammed (Husam
Abdulaem mohammed 2013) investigated the
overall performance of DWDM system
utilizing EDFA and DCF for different
lengths of optical fiber and bitrates. He found
that the most effective factors causing
performance degradation are attenuation and
dispersion. EDFA is used in the system model
to combat the effects of attenuation and
scattering losses, while DCF is utilized to
mitigate the effects of dispersion. Bo-ning
HU, Wang Jing,Wang Wai, RuiMei
Zhao(Bo-ningHU et al., 2010) analyzed the
fiber optic dispersion and its effects on optical
transmission system. Most frequently used
dispersion compensation fiber technology.
Three schemes of dispersion compensation
with DCF are implemented using OptiSystem
and various results such as Q factor and BER
are analysed. X.YZou, M.Imran Hayee, S-M
Hwang and Alan E.Willner (Gerd Keiser et
al., 2000) analyzed the system limitations of
WDM transmission when using various types
of optical fiber to control dispersion and non-
linearity’s .n the system 2 to 8 10gbps WDM
channels are transmitted through a cascade of
EDFA’s experiencing dispersion, stimulated
Raman scattering. Sandeep Singh (Sandeep
singh., 2012) studied dispersion compensation
system in the WDM in this paper. Based on
optical transmission equation, considering the
various types of nonlinear effects and the
effect of EDFA, system simulation models are
constructed.
3. MATERIALS AND METHODS
This section will describe the main
optical and electrical components used in the
RoF proposed system implementation, with
proposed system diagram.
3.1 Dispersion Compensating Fiber (DCF)
The concept behind this compensation
approach is to make Single Mode Fibers (SMF)
followed or preceded by DCFs with negative
dispersion coefficient, with purpose to disable
the effect of positive dispersion of SMFs. The
dispersion compensating fiber for dispersion
compensation was proposed in 1980’s. The
components of DCF are not easily influenced
by temperature and bandwidth, due to the fact
that DCF is more stable. DCF in an efficient
way to mitigate the overall dispersion in WDM
network, because of its higher negative
dispersion coefficient therefore, it can be
connected to the transmission fiber having the
positive dispersion coefficient such that the
overall dispersion of the link is zero
Dsmf × Lsmf=- ( DDCF × LDCF) ……….
(1)
Where: D and L are the dispersion and
length respectively. As a result, among
various dispersion, compensating schemes,
DCF having high negative dispersion at 1550
nm is widely inserted at regular intervals
along the optical fiber link (Bryn J. Dixon et
al., 2001). In order to realize the high data
speed communication system, a specific DCF
220 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
having large negative dispersion for
cancelling the dispersion of a transmission
channel is currently installed in a repeater or a
transceiver.
3.2Wavelength Division Multiplexing (WDM)
The technique which allows the optical
fiber to carry multiple signals is called
wavelength division multiplexing. It is a
technique used in sending signals of several
different wavelengths of Light into the fiber
concurrently. In fiber optic communications
(WDM) is a technology which multiplexes
multiple optical carrier signals on a single
optical fiber by using different wavelengths of
Laser light to carry different signals. This
facilitates the improvement of system capacity
and also allows bi-directional transmission
over single fiber length for transmitter and
receiver. The basic operation of WDM is the
combination of multiple optical channels with
different wavelengths, received from different
optical sources into a single fiber using
multiplexers at the transmitter end, and de-
multiplexer in the receiver to split WDM
channels (Biswanath Mukherjee 2006). The
capacity of the Radio-over-Fiber (RoF)
systems can be increased by applying (WDM)
technology in the optical fiber. It is an
effective way to increase the usable
bandwidth of the fiber (Masuduzzaman
Bakaul 2006).
Figure (2) Wavelength division multiplexing.
WDM-RoF networks topologies are similar
to the network topology for other optical
networks, such as star network, ring network
and bus network. In this work, star network
has been used as shown in Figure (2) (David
Castleford 2002).
3.3 Differential Phase Shift Keying (DPSK)
An alternative to the BPSK can then be the
differential phase shift keying (DPSK)
method. The difference between the binary
and the differential PSK is that in the DPSK
the bits are represented by a change of phase.
When a binary one is sent the phase is
unchanged from the previous bit, and a binary
zero is represented by a change of the phase
(W. Stallings 2001). As shown in figure (3).
Figure (3) Differential Phase Shift Keying example
The DPSK technic has some advantages as
compared to PSK. Since the information is
stored in the phase change and not in the
phase itself it is miles good for systems where
the accurate phase is not known (M. Sundelin.
1995). For that reason, if the system is
affected by phase noise, the detection of the
signal is made easier with DPSK. But at the
same time, you lose about 3 dB of power by
using DPSK instead of PSK because of the
receiving technique.
3.4 Optical Add-Drop Multiplexing (OADM)
Optical add-drop multiplexer (OADM) is a
device used in wavelength division
multiplexing systems for multiplexing and
routing different channels of light (Sandeep
221 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
Singh et al., 2012). Add and drop here refer to
the capability of the device to add one or more
new wavelength channels to an existing multi-
wavelength WDM signal, and to drop one or
more channels, passing those signals to
another network path. An OADM may be
considered to be specific type of optical cross
connect. A traditional OADM consists of an
optical de- multiplexer, an optical multiplexer.
and between them a method of reconfiguring
the paths between de- multiplexer, multiplexer
and a set of ports for adding and dropping
signals. Physically, there are several ways to
realize an OADM. In proposed design, the
base station (BS) has been implemented using
OADM.
3.5 Proposed System Design
Simulation layout of proposed RoF system
to mitigate dispersion is shown in Figure 4.
The PRBS represents the information or data
that is to be transmitted. From the pseudo-
random bit sequence generator which
generates electrical signal with data rate of 5
Gbps. Then the data signal is introduced to a
DPSK modulator with 1 bit/symbol, which
has a carrier of frequency 5 GHz. The DPSK
modulated electrical signal is filtered by a low
pass Bessel filter, Bessel LPF is used with
cut-off frequency of 1.5 x bit rate of the
signal. The two channels RF electrically
modulated signals then introduced to two
Mach-Zehnder modulators with optical
carriers of frequencies 193.1 THz and 193.2
THz (the frequency spacing between the
channels is 10 GHz) emitted by two CW
lasers of 0.1 dBm power. The modulated
optical carriers from two (CS) channels are
multiplexed by (2 x1) WDM multiplexer up to
(BS). Many wavelengths of light from
different transmitters are combined together
by the WDM multiplexer. Then the output of
the MUX is fed to a single mode fiber (SMF)
via a booster. The (SMF) used has a
dispersion of 17 ps/nm/km, with dispersion
slop of 0.075 ps/nm2/km, effective area of
80µm2, nonlinearity coefficient of 2.6*10-20,
PMC Coefficients of 0.2 ps/km and a loss of
0.2 dB/km. After a length of 50km in the
direction of BS the multiplexed optical signal
is fed to the EDFA with a gain of 10dB which
is used in the proposed system to compensate
for the attenuation losses. Multiplexed optical
channel consists of 50 km of SMF and 10 km
of DCF. The most fundamental reason that
222 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
limits the transmission of high-speed signals
on the 1550nm optical fiber is the linear
dispersion, the dispersion of SMF is
17 ps/nm/km, and therefore the DCF might be
used for compensating their dispersion
performance. DCF’s chromatic dispersion is
negative (dispersion coefficient is
–85ps/nm/km), DCF’s dispersion slop of -0.3
ps/nm2/km, effective area of 30µm2,
nonlinearity coefficient of 2.6*10-20, PMC
Coefficients of 0.2 ps/km and a loss of 0.4
dB/km. Its dispersion characteristics are
coinciding contrary with the SMF’s, if the
length of DCF is the SMF’s 1/5, then the total
transmission line dispersion value close to
zero. But, the DCF attenuation is larger, to
solve this problem, EDFA was brought to
compensate linear loss after the DCF and near
to the BS. At the BS, OADM is employed
such that a signal with frequency 193.1 THz is
simultaneously added and dropped as uplink
and downlink data respectively. The dropped
signal is first passed through an optical Bessel
filter of frequency 193.1 THz and bandwidth
10 GHz, then the signal is detected using a
PIN detector, filtered using a low pass Bessel
filter and fed to a BER analyzer for the
analysis of Q factor and BER of downlink.
The multiplexed signal having frequency
193.2 THz along with uplink data of 193.1
THz, added from BS, is transmitted to CS via
SMF of attenuation 0.2 dB/Km and length of
50km before introduce it to the EDFA
and DCF of 10km before feed the optical
signal to destination CS. At CS, the signal is
de-multiplexed; the role of optical receiver is
to convert the optical signal into electrical
form. Thus, the optical signal optically filtered
using an optical Bessel filter and detected
using a PIN detector of responsivity 1 A/W
and dark current of 10 nA. The recovered
electrical signal is filtered using a low pass
Bessel filter with cut-off frequency of
0.75*symbol rate .and given to a BER
analyzer for the analysis of uplink data. To
analyze the performance of RoF system with
different SMF the 50km fiber is replaced by
different distances from (12-108) km.
4. RESULTS AND DISCUSSION
The performance of DPSK modulated
WDM RoF system with and without DCF and
EDFA is investigated in terms of the Q-factor,
Bit Error Rate (BER). The bit rates are
considered constant and equal to 5 Gb/s. The
measurement component used is BER
analyzer to measure Q-factor & BER. The
spectral power spectrum with respect of
frequency at the output of the WDM in the
Central site is depicted in figure (5). While the
power spectrum at the output of the Base
station OADM is depicted in figure (6). The
chromatic dispersion which results in signal
broadening over large bandwidth in an optical
signal transmitted form CS to BS has been
mitigated by using DCF. The EDFA of a gain
of 10dB has been added before the DCF to
compensate for the SMF linear losses.
Figure (5) Optical spectrum of the WDM signal at CS
(channel spacing 10GHz).
The impact of the changing the SMF length (the
fiber length is changed form 12-108 km) on system
performance is studied with and without (DCF-
EDFA)
223 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
Figure (6) Optical spectrum of the signal dropped
form OADM at BS.
components in the optical channel, between
CS and BS. Figure (7) shows a graph of
Q-factor in the CS and BS without (DCF-
EDFA) components
10 20 30 40 50 60 70 80 90 100
2
4
6
8
10
12
14
16
18
20
distance in km
Q-Factor
Distance vs Q-Factor at Central Station and Base Station without DCF
Q-Factor BS without DCF
Q-Factor CS without DCF
Figure (7) Q-Factor vs. optical fiber length without
(DCF-EDFA)
Basically, the Q-factor is decreased with fiber
length increase. But this degradation in Q-
factor is mitigated by using (DCF-EDFA)
components in the channel, which is illustrated
by Figure (8).
Basically, the Q-factor is decreased
with fiber length increase. But this degradation
in Q-factor is mitigated by using (DCF-EDFA)
components in the channel, which is illustrated
by Figure (8).
10 20 30 40 50 60 70 80 90 100 110
0
5
10
15
20
25
30
35
40
45
distanc e in km
Q-Factor
Distanc e vs Q-Factor at Cent ral Station and Base Station with DCF
Q-Factor BS DCF
Q-Factor CS DCF
Figure (8) Q-Factor vs. optical fiber length with
(DCF-EDFA)
As an example, at fiber length of 50km the
Q-factor at CS is about 9 while it's equal to 12 at
BS without using (DCF-EDFA) components. But
when using (DCF-EDFA) components the
optical signal is restored and hence the Q-factor
is enhanced to 14.5 at CS and 18 at BS with
calculated gain about 6. Figure (9) and figure
(10) depicts the effect of (DCF-EDFA)
components in improving the proposed RoF
system BER.
10 20 30 40 50 60 70 80 90 100
-80
-70
-60
-50
-40
-30
-20
-10
0
distanc e in km
log(BER)
Distanc e vs Log10(BER) at Central Station and Base Station without DCF
BER BS without DCF
BER CS without DCF
Figure (9) BER vs. optical fiber length without (DCF-
EDFA)
Basically, BER worsen with the increase of fiber
length and as observed form the figures (7) and
(8) that the BER for received signal by the BS is
224 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
better than BER at CS, but at a distance of
more than 70 km the BER is comparable
between CS and BS in case without (DCF-
EDFA) components, while this distance is
increased to about 95km with (DCF-EDFA)
components.
10 20 30 40 50 60 70 80 90 100 110
-350
-300
-250
-200
-150
-100
-50
0
distance in km
log(BER)
Distance vs Log10(BER) at Central Station and Base Station with DCF
BER BS DCF
BER CS DCF
Figure (10) BER vs. optical fiber length with (DCF-
EDFA)
proposed diagram. It’s clear that after 0.5 sec
time period there would be a real degradation
in the proposed system performance due the
increase in transmitted signal; dispersion
which increased with increased transmitted
signal duration. Again, if we take as an
example a fiber length of 50km the log (BER)
is -18dB, and -33dB at the CS and the BS
respectively with (DCF-EDFA) components
while in other case these values become
(-42dB) at CS and -90dB at BS providing a
BER gain of 24 dB at CS and 57dB at BS
respectively. The BER is further improved if
the length of fiber decreased, it could reach
-320dB at fiber length of about 22km.
Figure (11) a,b,c gives graphical illustration
for the BER with respect to the Q-factor as
indicated by the
(a)
(b)
(C)
Figure (11): (a ,b ,c): BER and Q-Factor vs. bit period
with (DCF-EDFA)
225 Raghad Z. et al. /ZJPAS: 2017, 29 (s4): s217-s226
5. CONCLUSIONS
The performance Investigated through
OptiSystem13, 2-channels WDM RoF
communication system using DPSK modulation
with optical channel based on EDFA and DCF is
designed and presented. Here externally
modulated transmitter is used to achieve
stability and reduced non-linear effects. The
EDFA with dispersion compensation technique
provides better Q-factor and minimum BER in
both CS and BS. The results obtained indicated
that the Q-factor is very much enhanced by the
proposed system while BER is very much
reduced. After using dispersion compensation,
the signal is restored hence the Q-factor is
increased.
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ZJPAS (2017), 29 (s4); s217-s226
