National Conference on Communication and Information Security (NCCIS 2012)
Daffodil International University, Dhaka, Bangladesh, 31 March 2012.
Effect of Modulation and Encoding
Techniques on BER in Optical Networks
Md. Anisul Islam Sarker, S.M Mominur Rahman,
Md. Sifatullah, Md. Mirza Goalm Rashed
Daffodil International University, Dhaka, Bangladesh
Email: mgarashed@daffodilvarsity.edu.bd
performance of AON [1]. In this work an optical
network project of LINK 3 ISP Company is taken
in consideration for the purpose of research. The
network layout is designed and simulated with the
help of Optisystem 9 software and the simulation
result gives the performance of the network in
terms of BER for different kinds of Encoding
techniques like RZ, NRZ encoding and modulation
techniques like Mach Zehnder and Eletro
Absorption.
Abstract— The Deployment of Wave Division
Multiplexing (WDM) technology has caused the
degradation of the of the quality of optical signal in the
high speed networks. Performance of an Optical
Network depends on a number of factors .Some of the
factors such as Modulation Technique, Encoding
Technique and Level of Power play a vital role on the
performance of network. This research work presents
the effects of different modulation techniques and
encoding techniques on the performance of an optical
network. The performance is measured in terms of
BER in all the cases.
II. MODULATION TECHNIQUES
Keywords: Optical Network, Modulation Technique,
Encoding Technique, BER
2.1 Electro-Absorption Modulation
Electro-absorption modulation is a modulation
which can be used for modulating the intensity of a
laser beam via an electric voltage. A change in the
absorption spectrum caused by an applied electric
field, which changes the band gap energy (thus the
photon energy of an absorption edge) but usually,
does not involve the excitation of carriers by the
electric
field.
For
modulators
in
telecommunications small size and modulation
voltages are desired. The EAM is candidate for use
in
external
modulation
links
in
telecommunications. These modulators can be
realized using either bulk semiconductor materials
or materials with multiple quantum dots or wells
[5].
2.2 Mach-Zehnder Modulation
A Mach-Zehnder modulation is an intensity
modulating of signal light, using a simple drive
circuit for the modulating voltage. The modulator
includes two waveguides with respective multiple
quantum well (MQW) structures. Well layers of
the MQW structures of the two optical waveguides
have different thicknesses or are made from
different materials so the phase of light propagating
through one waveguide advances and through the
other waveguide is delayed in response to the same
applied Voltage. The phase-changed light signals
are combined as an output light signal that is
intensity Modulated [6].
I. INTRODUCTION
In all-optical networks (AONs) the optical signal
degradation and distortion seriously increases due
to the deployment of wavelength-division
multiplexing. [1].So, it is very important to
evaluate the performance of the network to
understand whether it is capable to achieve the
minimum level of requirement or not. The bit error
rate (BER) is the most significant performance
parameter of any digital communications system. It
is a measure of the probability that any given bit
will have been received in error. A standard
maximum bit error rate specified for many systems
is 10-9. This means that the receiver is allowed to
generate a maximum of 1 error in every 109 bits of
information transmitted or, putting it another way,
the probability that any received bit is in error is
10-9.The BER depends primarily on the signal to
noise ratio (SNR) of the received signal which in
turn is determined by the transmitted signal power,
the attenuation of the link, the link dispersion and
the receiver noise [2]. BER of an optical network
also depends on so many other factors like Data
Rate, Number of users, Wavelength, Fiber Length,
Encoding Techniques etc [3] .Among all the
encoding techniques the NRZ(Non Return to Zero)
and RZ( Return to Zero)formats are two wellknown cost effective candidates for optimum
9
National Conference on Communication and Information Security (NCCIS 2012)
Daffodil International University, Dhaka, Bangladesh, 31 March 2012.
techniques (RZ-Return to Zero and NRZ Non
Return to Zero), modulation techniques (Mach
Zehnder and Electro Absorption) and different
levels of optical powers(20 dB and 15 dB) are used
in the following experiments.
III. DESCRIPTION OF THE
EXPERIMENTAL ARRANGEMENT
The total experimental simulation is based on an
optical network project conducted by the Link-3,
an ISP of Bangladesh. The layout of the network is
given in the Fig.1 and it is simulated by
OptiSystem 9.0 software. The objective of the
work is to observe the performance of the network
in terms of BER with the change of different
modulation and encoding techniques.
The results are represented in bellow:
Fig..2 BER Curve for RZ Encoding, Mach Zehnder Modulation
and Optical Power: 20 dB
Fig.1. Layout of the Optical Network
The components and parameters used in the
network are given below:
Table 1: Configuration and Parameters of the network
Input bit sequence.
4Bit and 8 Bit
Signal pulse generator:
Modulation Technique:
RZ pulse generator and NRZ
Pulse generator
Mach-Zehnder and ElectroAbsorption
Optical input power:
Optical Fiber cable length:
20 dB and 15 dB
50 km
A low pass filter
Quantity-1
BER analyzer
Quantity-1
Fig..3 BER Curve for RZ Encoding, Electro
Absorption Modulation and Optical Power: 20 dB
IV. ANALYSIS OF THE BER
The performance of a digital communication
system is commonly measured in terms of BER. It
is very important to maintain the BER within a
certain threshold to make sure that the quality of
the received message is up to the standard. In this
research work we have considered three parameters
for analyzing the performance of the netowork.The
parameters are Modulation Technique, Encoding
Technique and Input Optical power which has a
direct influence on the BER of the transmitted
signal in an optical network. In the simulation part
some experimental cases are considered which are
described in the following sections.
4.1 BER Analysis for 4-bit Input
The figures[Fig.2-Fig.9] give the BER curves for 4
bit inputs (1010).Different types of encoding
Fig.4. BER Curve for NRZ Encoding, Mach
Zehnder Modulation and Optical Power: 20 dB.
10
National Conference on Communication and Information Security (NCCIS 2012)
Daffodil International University, Dhaka, Bangladesh, 31 March 2012.
Fig..8 BER Curve for NRZ Encoding, Mach
Zehnder Modulation and Optical Power: 15 Db
Fig..5 BER Curve for NRZ Encoding, Electro
Absorption Modulation and Optical Power: 20 dB
Fig..9 BER Curve for NRZ Encoding,
Electro Absorption Modulation, Optical Power: 15 dB
Fig..6 BER Curve for RZ Encoding, Mach
Zehnder Modulation and Optical Power: 15 dB
Table 2: BER Analysis for 4-Bit Input
Fig.7. BER Curve for RZ Encoding, Electro
Absorption Modulation and Optical Power: 15 dB.
11
National Conference on Communication and Information Security (NCCIS 2012)
Daffodil International University, Dhaka, Bangladesh, 31 March 2012.
From Table- 2, it can be said that Mach Zehnder
Modulation gives some Bit Error for both the RZ
and NRZ encoding technique and Bit error rate
found little bit smaller when 15 dB power is
applied instead of 20dB power with NRZ signal
generator. It is also found that there is no bit error
rate for Electro Absorption modulation technique
in case of both Encoding Schemes.
4.2 BER Analysis for 8-bit Input
The figures[Fig.10-Fig.16] give the BER curves
for 8 bit inputs (10101100).Different types of
Encoding Techniques (RZ- Return to Zero and
NRZ Non Return to Zero ), Modulation
Techniques (Mach Zehnder and Electro
Absorption) and different levels of optical
powers(20 dB and 15 dB) are used in the following
experiments.
Fig.12 BER Curve for NRZ Encoding,
Mach Zehnder Modulation and Optical Power: 20 dB
Fig.10 BER Curve for RZ Encoding,
Mach Zehnder Modulation, Optical Power: 20 dB.
Fig.13 BER Curve for NRZ Encoding,
Electro Absorption Modulation and Optical Power: 20 dB
Fig.11 BER Curve for RZ Encoding, Electro Absorption
Modulation and Optical Power: 20 dB.
Fig. 14 BER Curve for RZ Encoding, Mach
Zehnder Modulation and Optical Power: 15 dB
12
National Conference on Communication and Information Security (NCCIS 2012)
Daffodil International University, Dhaka, Bangladesh, 31 March 2012.
5.3.4 Results and Analysis for 8-bit input
Fig.15 BER Curve for RZ Encoding, Electro Absorption
Modulation and Optical Power: 15 dB
Table 3: 8-bits result from BER analysis
The results of Table- 3 show that only Electro
Absorption modulation technique with RZ
encoding gives no Bit Error whereas the all other
combinations are giving some Bit Errors. Among
all other options Bit Error Rate is found to decrease
when power level is reduced from 20 dB to 15 dB.
V. CONCLUSION
A real life Optical Network project is simulated in
this research work. The performance of the
network is tested by using different types of
modulation and encoding techniques and measured
in terms of BER. For 4 Bit Input , considering the
BER and Utilized power it is found that RZ- Mach
Zehnder, RZ-Electro Absorption and NRZ-Electro
Absorptions provides optimum performance and
for 8- Bit Input RZ-Electro Absorption provides
excellent performance both in the case of high
power and low power utilization. The experimental
results suggests that the combination of RZ
Encoding and Electro Absorption modulation
provides a optimized state of the network both in
the cases small and big size data packets.
In this research work, only the effect of Modulation
techniques and encoding techniques on BER is
tested. There are a lot of parameters which have
effect on the BER are not considered in this work.
More simulations can be conducted in future taking
more parameters in consideration and this may
result more perfect outcomes.
Fig.16 BER Curve for NRZ Encoding,
Mach Zehnder Modulation, Optical Power: 15
Fig.17 BER Curve for NRZ Encoding,
Electro Absorption Modulation and Optical Power: 15 dB
13
National Conference on Communication and Information Security (NCCIS 2012)
Daffodil International University, Dhaka, Bangladesh, 31 March 2012.
REFERENCES
1.
2.
Optimization of All-Optical Network Testbed
Ragarding
NRZ
and
RZ
Modulation,
http://ieeexplore.ieee.org/Xplore/ defdeny.jsp
Practical Bit Error Rate Measurements on Fibre
Optic Communications Links in Student Teaching
Laboratories,
http://spie.org/etop/ETOP2005_021.pdf (Retrieved
on January 13, 2012).
3.
Monte Carlo Analysis of Broadband Optical Passive
Networks,
http://www.idosi.org/wasj/wasj12%288%29/5.pdf
4.
Comprehensive Assessment of New Modulation
Techniques,
5.I. B. Akca, “Electro-optic and electro-absorption
characterization of InAsquantum dot waveguides”,
2008, 3439, PP 16
6.
14
Mach-Zehnder Modulator, “United States Patent
No-US6, 1, 98,854B1”, 2001