Single Mode Optical Fiber in Rof System Using DWDM

K. Nishanth Rao et al. Int. Journal of Engineering Research and Applications www.ijera.com
ISSN: 2248-9622, Vol. 5, Issue 8, (Part - 5) August 2015, pp.141-148
www.ijera.com 141 | P a g e
Single Mode Optical Fiber in Rof System Using DWDM
K. Nishanth Rao, B. Kalpana, S.V.S Prasad
Assistant Professor, Dept of ECE, MLR Institute of Technology, Hyderabad
M. Tech Student, Dept of ECE, MLR Institute of Technology, Hyderabad
Associate Professor, HOD of ECE, MLR Institute of Technology, Hyderabad.
Abstract
Performance analysis was carried out to find the effect of crosstalk in a WDM system. Firstly, analysis of BER
was carried out without crosstalk. Then analysis of BER with crosstalk was done. Using equation for crosstalk,
number of channels was plotted using matlab. System parameters were optimized for a particular crosstalk.
Objective of the thesis work
Performance Analysis is carried out to find the effect of crosstalk due to optical cross connect in a DWDM
system considering a WDM based optical cross connect (OXC). An analysis is carried out to find the amount of
crosstalk due to OXC. The bit error rate performance degradation due to crosstalk is evaluated for OXC
parameter and number of wavelengths per fiber. The optimum parameters such as optimum number of channels
and hops are determined.
I. INTRODUCTION
OPTICAL wavelength division multiplexing
(WDM) networks are very promising due to their
large bandwidth, their large flexibility and the
possibility to upgrade the existing optical fiber
networks to WDM networks. WDM has already been
introduced in commercial systems. All-optical cross
connects (OXC), however, have not yet been used for
the routing of the signals in any of these commercial
systems. Several OXC topologies have been
introduced, but their use has so far been limited to
field trials, usually with a small number of input–
outputfibers and wavelength channels. The fact, that
in practical systems many signals and wavelength
channels could influence each other and cause
significant crosstalk in the optical cross connect, has
probably prevented the use of OXC‘s in commercial
systems. The crosstalk levels in OXC configurations
presented so far are generally so high that they give
rise to a significant signal degradation and to an
increased bit error probability. Because of the
complexity of an OXC, different sources of crosstalk
exist, which makes it difficult to optimize the
component parameters for minimum total crosstalk.
In this paper, the crosstalk with the bit error rate and
without bit error rate is calculated and compared with
each other, and the influence of the component
crosstalk on the total crosstalk is identified. We
present an analytical approximation for the total
crosstalk level in a WDM system, which makes the
component parameter optimization considerably
easier.
II. OPTICAL MULTIPLEXING
SCHEMES
A. Orthogonal frequency-division multiplexing
(OFDM):
Orthogonal frequency-division multiplexing
(OFDM) — essentially identical to Coded OFDM
(COFDM) and Discrete multi-tone modulation
(DMT) — is a frequency-division multiplexing
(FDM) scheme utilized as a digital multi-carrier
modulation method. A large number of closely-
spaced orthogonal sub-carriers are used to carry data.
The data is divided into several parallel data streams
or channels, one for each sub-carrier. Each sub-
carrier is modulated with a conventional modulation
scheme (such as quadrature amplitude modulation or
phase-shift keying) at a low symbol rate, maintaining
total data rates similar to conventional single-carrier
modulation schemes in the same bandwidth.OFDM
has developed into a popular scheme for wideband
digital communication, whether wireless or over
copper wires, used in applications such as digital
television and audio broadcasting, wireless
networking and broadband internet access.
The primary advantage of OFDM over single-
carrier schemes is its ability to cope with severe
channel conditions — for example, attenuation of
high frequencies in a long copper wire, narrowband
interference and frequency-selective fading due to
multipath — without complex equalization filters.
Channel equalization is simplified because OFDM
may be viewed as using many slowly-modulated
narrowband signals rather than one rapidly-
modulated wideband signal. The low symbol rate
makes the use of a guard interval between symbols
affordable, making it possible to handle time-
spreading and eliminate intersymbol interference
RESEARCH ARTICLE OPEN ACCESS

(ISI). This mechanism also facilitates the design of
Single Frequency Networks (SFNs), where several
adjacent transmitters send the same signal
simultaneously at the same frequency, as the signals
from multiple distant transmitters may be combined
constructively, rather than interfering as would
typically occur in a traditional single-carrier system.
B. Wavelength-Division Multiplexing (WDM):
In fiber optic communications, wavelength-
division multiplexing (WDM) is a technology which
multiplexes multiple optical carrier signals on a
single optical fiber by using different wavelengths
(colours) of laser light to carry different signals. This
allows for a multiplication in capacity, in addition to
enabling bidirectional communications over one
strand of fiber. This is a form of frequency division
multiplexing (FDM) but is commonly called
wavelength division multiplexing.
The term wavelength-division multiplexing is
commonly applied to an optical carrier (which is
typically described by its wavelength), whereas
frequency-division multiplexing typically applies to a
radio carrier (which is more often described by
frequency). However, since wavelength and
frequency are inversely proportional, and since radio
are both forms of electromagnetic radiation, the two
terms are equivalent in this context.
A WDM system uses a multiplexer at the
transmitter to join the signals together, and a
demultiplexer at the receiver to split them apart. With
the right type of fiber it is possible to have a device
that does both simultaneously, and can function as an
optical add-drop multiplexer. The optical filtering
devices used have traditionally been etalons, stable
solid-state single-frequency Fabry-Perot
interferometers in the form of thin-film-coated
optical glass.
As explained before, WDM enables the
utilization of a significant portion of the available
fiber bandwidth by allowing many independent
signals to be transmitted simultaneously on one fiber,
with each signal located at a different wavelength.
Routing and detection of these signals can be
accomplished independently, with the wavelength
determining the communication path by acting as the
signature address of the origin, destination or routing.
Components are therefore required that are
wavelength selective, allowing for the transmission,
recovery, or routing of specific wavelengths.
In a simple WDM system each laser must emit
light at a different wavelength, with all the lasers
light multiplexed together onto a single optical fiber.
After being transmitted through a high-bandwidth
optical fiber, the combined optical signals must be
demultiplexed at the receiving end by distributing the
total optical power to each output port and then
requiring that each receiver selectively recover only
one wavelength by using a tunable optical filter. Each
laser is modulated at a given speed, and the total
aggregate capacity being transmitted along the high-
bandwidth fiber is the sum total of the bit rates of the
individual lasers. An example of the system capacity
enhancement is the situation in which ten 2.5-Gbps
signals can be transmitted on one fiber, producing a
system capacity of 25 Gbps. This wavelength-
parallelism circumvents the problem of typical
optoelectronic devices, which do not have
bandwidths exceeding a few gigahertz unless they are
exotic and expensive. The speed requirements for the
individual optoelectronic components are, therefore,
relaxed, even though a significant amount of total
fiber bandwidth is still being utilized.
Figure .1: Diagram Of A Simple WDM System
C. DWDM System
Dense wavelength division multiplexing, or
DWDM for short, refers originally to optical signals
multiplexed within the 1550 nm band so as to
leverage the capabilities (and cost) of erbium doped
fiber amplifiers (EDFAs), which are effective for
wavelengths between approximately 1525-1565 nm
(C band), or 1570-1610 nm (L band). EDFAs were
originally developed to replace SONET/SDH optical-
electrical-optical (OEO) regenerators, which they
have made practically obsolete. EDFAs can amplify
any optical signal in their operating range, regardless
of the modulated bit rate. In terms of multi-
wavelength signals, so long as the EDFA has enough
pump energy available to it, it can amplify as many
optical signals as can be multiplexed into its
amplification band (though signal densities are
limited by choice of modulation format). EDFAs
therefore allow a single-channel optical link to be
upgraded in bit rate by replacing only equipment at
the ends of the link, while retaining the existing
EDFA or series of EDFAs through a long haul route.
Furthermore, single-wavelength links using EDFAs
can similarly be upgraded to WDM links at
reasonable cost. The EDFAs cost is thus leveraged
across as many channels as can be multiplexed into
the 1550 nm band.

Figure.2: Multichannel DWDM transmission system
D. Transponder
Transponders receive optical signals and send
them out carrying digital information at predefined
wavelengths in accordance with the ITU-T guidelines
(see reference table on pages 75 to 79) . A single
channel transmitter typically consists of a high power
distributed feedback (DFB) laser followed by a
modulator and power amplifier (also referred to as a
post-amplifier or booster). Direct modulation of the
laser is only possible up to 2.5 Gbps. For higher
transmission rates as a result of laser chirp, an
external modulator must be used. DFB lasers offer
greater precision than Fabry-Perot (FP) lasers, the
latter of which emits harmonics close to the main
peak rendering them unsuitable for DWDM systems.
In DWDM systems both fixed and uneable laser
sources can be utilized. In networks with dense
channel spacing, transponder temperature must be
stabilized. This can be enabled with the use of
thermo-electric coolers.
E. Multiplexer (MUX)
MUX are deployed in DWDM systems to
combine the signals at different wavelengths onto a
single fiber through which they then travel
simultaneously. Each wavelength carries its own
information and represents a channel. An ideal MUX
requires uniformly high transmission across the
passband with a very high drop at the edge.
F. Fiber
The fiber is one of the most critical components
of a DWDM system as it provides the physical
transportation medium. Optical fibers consist of both
core and cladding. The core is the inner, light-guiding
section and is surrounded by the cladding. As the
refractive index of the core is higher than that of the
cladding, light entering it at an angle – or numerical
aperture – is fully reflected (almost 100 percent) off
the core/cladding boundary and propagates down the
length of the fiber. Optical fibers can be divided into
multimode and singlemode fibers, each
approximately the size of a human hair, with an outer
diameter of 125 μm. Core size however differs. The
diameter of multimode fibers range from between 50
μm and 62.5 μm, whilst for singlemode fibers it is
between 7 and 10 μm. Light propagates down the
fiber core in a stable path known as a mode. In
multimode fibers, multiple paths arise making them
unsuitable for use in long haul DWDM transmission.
In DWDM systems the fibers can be used either
unidirectionally (signals transmitted in one direction
only per fiber) or bi-directionally (signals traveling in
both directions).
G. Amplifier
Amplifiers boost signals traveling down a fiber
so they can cover longer spans. In the early stages of
fiber optic telecommunications, lasers emitted
relatively low power which led to the signal having to
be frequently electrically regenerated (figure 7).
These amplifiers receive the optical signal and
convert it into an electrical signal (O/E conversion)
which is then reshaped, retimed and amplified again.
This is the so called 3R regenerator. Finally, the
signal is converted back to an optical signal (E/O
conversion). Optical fiber amplifiers (OFAs) can be
used to provide a more economical solution. These
can work solely in the optical domain, performing a
1R (optical reamplification only) regeneration. OFAs
simultaneously amplify each wavelength of the
DWDM signal without the need for demultiplexing
and remultiplexing. One major advantage of OFAs is
their transparency to signal speed and data type.
Three types of OFAs are deployed in DWDM
systems: erbium doped fiber amplifier (EDFA),
semiconductor optical amplifiers (SOA) and Raman
fiber amplifiers (RFA). In DWDM systems, the
multiplexed signal has to be demultiplexed before
each channel is regenerated, emitted by a laser and
then multiplexed again. This is a process which is
both complex and expensive.
H. Demultiplexer (DEMUX)
DEMUXs unscramble multiplexed channels
before they are fed into their corresponding receivers.
They work similarly to MUXs but operate in the
reverse direction. It is common to preamplify optical
signals before they are separated by the optical filters
of the demultiplexer. The performance of a MUX or
DEMUX is related to its capability to filter each
incoming signal. The Bragg grating is currently the
most popular technique used in DWDM systems.
I. Receiver
Receivers are used to convert optical signals into
electrical signals. The light pulses transmitted over
the optical fiber are received by a light sensitive
device known as a photo diode which is made of
semi-conductor material.
J. Limitations of WDM
Crosstalk will be one of the major limitations for
the introduction of OXC in all optical networks. In

this paper the influence of the components on the
total OXC crosstalk is investigated.
K. Crosstalk
Crosstalk occurs in devices that filter and
separate wavelengths. A small proportion of the
optical power that should have ended up in a
particular channel(on a particular filter output)
actually ends up in an adjacent (or another)
channel.Crosstalk is critically important in WDM
systems. When signals from one channel arrive in
another they become noise in the other channel. This
can have serious effects on the signal-to-noise ratio
and hence on the error rate of the system. Crosstalk is
usually quoted as the ―worst case‖ condition. This is
where the signal in one channel is right at the edge of
its allowed band. Crosstalk is quoted as the
loss in dB between the input level of the signal and
its (unwanted) signal strength in the adjacent channel.
A figure of 30 dB is widely considered to be an
acceptable level for most systems.
Figure. 3:Defination of Cross talk
L. Types of crosstalk
Different kinds of crosstalk exist, depending on
their source. First one has to make a distinction
between interband crosstalk and intraband crosstalk.
Interband crosstalk: Interband crosstalk is the
crosstalk situated in wavelengths outside the channel
slot (wavelengths outside the optical bandwidth).
This crosstalk can be removed with narrow-band
filters and it produces no beating during detection, so
it is less harmful. In a WDM networks, interband
crosstalk appears from channels of different
wavelengths.
Intraband Crosstalk: The crosstalk within the same
wavelength slot is called intraband crosstalk.(fig 1.2).
It cannot be removed by an optical filter and
therefore accumulates through the network. Since it
cannot be removed, one has to prevent the crosstalk.
In this paper intraband crosstalk is studied since the
network performance will be limited by this kind of
crosstalk. Intraband crosstalk occurs when the signal
and the interferer has the closely-valued wavelengths.
Intraband however, can be coherent or incoherent
crosstalk. If the signal crosstalk mixing takes place
within the laser coherence length, then intraband
crosstalk is defined as coherent. Otherwise incoherent
crosstalk will appear.
Figure.5: Interband and Intraband Crosstalk
Moreover, within the intraband crosstalk, a
distinction between incoherent and coherent crosstalk
has to be made. These types of crosstalk are not well
defined in literature and therefore a definition is
given here. To make a distinction between both types
of intraband crosstalk one has to look at the
consequences. The interference of the signal channel
and the crosstalk channel at the detector results in a
beat term.
III. ANALYSIS OF CROSSTALK IN
WDM SYSTEM
Firstly the block diagrams representing the
system block diagram and WDM system with Hops.
Secondly, analysis of Bit Error Rate without
Crosstalk, which is in ideal case, is given. Then
analysis of Bit Error Rate with Cross talk using
equations in both the cases is given.
A. WDM system block diagram
Figure.6: Sample WDM System
The above block diagram shows a simple WDM
system. Here L numbers of signals are multiplexed in
a channel in a multiplexer from transmitters. N
number of channels are going through Optical Cross
connect and each channel are demultiplexed using a
demultiplexer and the receiver receives the desired
signal.

IV. SYSTEM BLOCK DIAGRAM WITH
HOPS
The above diagram shows a WDM system block
diagram with hops. Here three hops are shown. L
number of signals are multiplexed and passed
through a optical fiber. Which is demultiplexed to get
the desired signal and a new signal is multiplexed by
the transmitter. The signal passes through optical
fiber again. This way hops are used.
A. Block diagram of optical cross connect
Figure.7: Schematic illustration of conventional OXC
node
The above diagram shows an optical cross
connect where M input fibers are coming. The cross
connect switches the signal to the desired location to
pass on the other side to be demultiplexed.
B. Block diagram of oxc crosstalk
Figure.8: Schematic illustration of conventional OXC
node
The above diagram shows the crosstalk in the
Optical switch. In the switch, signal from input 1 is
connected with output 3. And signal from input 3 is
connected to output 1. But it can be seen that in
output 3, a little portion of input 3 has entered along
with the signal 1. Similarly at output 1 a little portion
of input 1 has entered along with the signal 3. this
unwanted portion of signal that enters in the output of
the Optical Cross connect is the crosstalk due to
OXC.
C. Analysis of Bit Error Rate without Crosstalk
Bit Error Rate can be calculated with and
without Crosstalk using some equations. In this
section the ideal case is shown. So Crosstalk is taken
to be zero. Equation for crosstalk is given is the next
section. Bit Error Rate: The number of bit errors that
occur within the space of one second. This
measurement is one of the prime considerations in
determining signal quality. The higher the data
transmission rate the greater the standard.
The BER is an indication of how often data has
to be retransmitted because of an error. Too high a
BER may indicate that a slower data rate would
actually improve overall transmission time for a
given amount of transmitted data since the BER
might be reduced, lowering the number of packets
that had to be resent.
For most practical WDM networks, this
requirement of BER is 10
-12
(~ 10
-9
to 10
-12
), which
means that a maximum one out of every 10
12
bits can
be corrupted during transmission. Therefore, BER is
considered an important figure of merit for WDM
networks; all designs are based to adhere to that
quality.
BER in WDM system is calculated by the equation:
BER =. 5 erfc (Q/√2) (eq. 1) Here Q is a function
proportional to the receiver signal-to-noise ratio
(SNR).
D. Analysis of Bit Error Rate with Crosstalk
In practical case zero crosstalk is not possible. So
BER is calculated with equation 1 taking in the value
of σc
BER =. 5 erfc (Q/√2) (eq. 1)
Here Q is a function proportional to the receiver
signal-to-noise ratio (SNR).
It is expressed as:
Q = (Rd * Ps) ^2/√(σase^2 + σc^2)
V. RESULTS AND DISCUSSION
BER or Bit Error Rate is plotted as a function of
input power (Pin) in dbm in Figure: 1 Here the value
of Rb(bit rate) is 10GHz. The Pin is taken from range
-8 dbm to 100 dbm. The resulting graph of BER VS
Pin is plotted bellow. It is seen that without crosstalk
the BER increase with increase in Pin.
Figure 9
This is the plot of BER vs power in dbm for
different bandwidths. Here for without crosstalk
different values of bandwidth(B) is taken and has
been shown in graph. The input power is taken from
range -8 dbm to -1 dbm. Then the corresponding

values of BER are plotted against Pin for the
corresponding different values of B.
Figure.10: BER vs input power for different
Bandwidth.
The resulting graph of BER VS Pin is plotted
above. We have plotted this graph for three different
bandwidths. It is seen that the BER increases with
increase in Pin. It is also shown that to use more
bandwidth we need more input power. For example,
when we use 4*Rb as our bandwidth, we need -
1.2dbm power where we need 0.1 dbm input power
to use 6*Rb as bandwidth. At the same time BER
also increase with increase in bandwidth.
A. BER with Crosstalk
BER or bit error rate is plotted as a function of input
power (Pin) in dbm in Figure: 3.
Figure.11: BER vs Input power in dbm
In this graph we have plotted BER against Input
power in dbm when crosstalk is available. Now we
have plotted the graph of BER against input power in
dbm for different crosstalk. Different values of
crosstalk have been taken here for a fixed bandwidth
and analysis the graphs for different crosstalk. The
input power is taken from range -5dbm to 20dbm.
Figure 12: BER vs input power in dbm for different
crosstalk.
Here we show that BER increase for increasing
crosstalk. For using more input power we get more
crosstalk. For example, when 1.8 dbm is used as
input power, the crosstalk is and for 9.1dbm input
power, crosstalk is. At the same time BER is also
increasing.
Figure below shows Crosstalk plotted against
number of channel using in WDM system. The input
power is taken from the range -8dbm to 20 dbm.
Effective adjacent and non adjacent both are taken
0.5. We have plotted this graph for 2 hoops and 10
channels.
Figure 13: crosstalk vs number of channel
From this graph it can be said that crosstalk
increase if we use more channels. The graph of
crosstalk vs number of channel is plotted for different
number of hops. In this graph we have changed the
value of M for the same input power, which is -8dbm
to 20 dbm and for 10 channels.
Figure 14: crosstalk vs number of channels

It can be said that if we increase the number of
hop then crosstalk also increases. Another way it can
be analyzed that for a fixed number of crosstalk if we
use more hops, the number of channel decrease and
in another way we can use more channels for less
hops. Now from the figure 4, for one bit error rate we
can find the power penalty from the input power.
Power penalty is the difference between two powers.
So for calculating power penalty we need to calculate
the difference of input power with crosstalk from the
power without crosstalk.Power penalty vs crosstalk is
plotted here.
Figure 15: power penalty vs crosstalk
From this graph we can be able to find out the
crosstalk for different power penalty.
Figure 16: power penalty vs crosstalk.
For example, it can be said that for 8.1 power
penalty relative crosstalk is -40 db, which is 0.0004.
in this way we can find crosstalk for a given power
penalty. After finding the amount of crosstalk we can
find the number of channels and number of hops can
be used for that particular crosstalk from figure 6
which is crosstalk vs number of channels.
For this graph we are able to find out the number
of channels and hops for related crosstalk. For
example for power penalty 8.1db we got the
corresponding crosstalk to be -40db which is 10^(-4).
From the above graph, we can find combinations of
hops and channels. Here, the combinations are :
Hoops Channels
6 9.6 = 10(approx.)
10 8.7 = 9 (approx.)
14 8.2 = 8 (approx.)
18 7.8 = 8 (approx.)
22 7.4 = 7 (approx.)
26 7.2 = 7 (approx.)
Numbers of channel are plotted against number
of hops.
This graph can be plotted manually too. Which is
given below. This graph shows different
combinations of hops and channels for a crosstalk 10
^ (-4) which we got for power penalty 8.1 db. From
this graph any combination can be used for this
power penalty. Similarly, we can plot a graph for
hops and channels for any power penalty and
corresponding crosstalk. Therefore a relationship
between hops and channels is established using this
graph.
VI. CONCLUSION
In this thesis paper, we have used some basic
equations to optimize the relation between hoops and
channels. No new equation were derived or used to
form this relationship. The graph was plotted
manually using the graphs plotted with the basic

equations of BER and Crosstalk in matlab software.
At first a graph for BER vs. Pin was plotted using
BER equation. Then a graph of crosstalk vs. number
of channels was plotted for different number of hops.
From this graph power penalty was found out. This
power penalty was used to plot a graph of power
penalty vs. crosstalk. From this graph we have taken
a particular crosstalk. And from the graph of
crosstalk vs. channel, the combination of hoops and
channels were found out. Which were plot manually
to get the final graph. The graphs are shown in the
Result section.
In this final graph, we have shown that for a
particular Power Penalty, combination of hoops and
channels can be plotted. The user can use any
combination as required by the system. Hence, we
can conclude that by this process we can find number
of hoops and channels for a given Power penalty.
FUTURE WORK
Further research can be carried out to evaluate
the performance of a WDM network with OXC using
different topologies of the OXC and to find a
topology with optimum system performance. Work
can be carried out to evaluate the performance of a
WDM system with bi-directional OXC and find the
limitations due to crosstalk and optimum system
parameters. Work can be carried out with precoding
techniques to minimize the effect of Bit noise due to
crosstalk and signal in a WDM system.
REFERENCES
[1]. ‗Performance Limitations of Optical Cross
Connect without Wavelength Converter due
to Crosstalk‘ by M. S, Islam, S.P.
Majumder, Ngee Thiam Sim
[2]. ‗Crosstalk in WDM Communication‘ by
Idelfonso Tafur Monry, EdwardbTangdiong
ga
[3]. ‗Analysis of Signal Distortion and Crosstalk
Penalties Induced By Optical Filters in
Optical Networks.‘ By John D. Downie, A.
Boh Ruffin
[4]. ‗Optical Crosstalk in Fiber Radio WDM
Networks‘ by David Castleford,
Ampalavanapillai Nirmalathas, Dalama
Nobak, Rodney S. Tucke.

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