CALIFORNIA STATE UNIVERSITY NORTHRIDGE
FULL DUPLEX TRANSMISSION OF RADIO OVER FIBER USING WDM AND
OADM TECHNOLOGY
A graduate project submitted in partial fulfillment of the requirements
For the degree of Masters of Science
In Electrical Engineering
By
Praveen Cheemaladinne
December 2015
The graduate project of Praveen Cheemaladinne is approved:
_____________________________________
Dr. Ali Amini
___________________
Date
_____________________________________
Dr. Xiyi Hang
___________________
Date
_____________________________________
Dr. Nagwa Bekir, Chair
___________________
Date
California State University, Northridge
ii
Acknowledgement
Working on this project on “Full duplex transmission of radio over fiber using WDM
and OADM technology” was a source of immense knowledge to me. With a profound
feeling of appreciation, I wish to express my sincere thanks to my supervisor, Dr. Nagwa
Bekir, for her encouragement and tremendous help in doing the project. Profound
knowledge and timely with came as a boon under the guidance of Dr. Nagwa Bekir. Her
valuable suggestions as final words during the course of work are greatly acknowledge.
I would also like to thank Dr. Ali Amini and Dr. Xiyi Hang for their extended
support, scholarly advice and inspiration in completion of my project successfully. I am
appreciative to the Department of Electrical and computer engineering for providing me
this opportunity and giving me the required information in finishing my project.
I would also like to thank my family and companions, who encouraged me to extend
my reach, for their affection, care and backing through all the extreme times during my
graduation.
iii
TABLE OF CONTENTS
Signature page
ii
Acknowledgement
iii
List of figures
vi
Abstract
ix
Chapter 1: Introduction
1
1.1 Foreword
1
1.2 Problem background
2
1.3 Objective
3
1.4 Scope of project
3
1.5 Methodology
4
1.6 Project outline
5
Chapter 2: RoF Principle & Modulation Formats
6
2.1 Overview
6
2.2 Principle
7
2.3 Benefits of radio over fiber system
8
2.4 Modulation formats
10
2.5 Performance difference between RZ and NRZ formats
12
2.6 Summary and discussion
15
Chapter 3: Introduction to components used in Full duplex RoF system
16
3.1 Laser diode
16
3.2 Photo detector
17
3.3 Optical fiber amplifier
18
3.4 Wavelength division multiplexing (WDM)
20
3.5 OADM
23
iv
3.6 Summary and discussion
25
Chapter 4: Sytem Design & Performance Measures
26
4.1 Design using NRZ modulation
26
4.2 Design using RZ modulation
28
4.3 Performance measures
29
4.4 Summary and discussion
34
Chapter 5: Simulation & Results
35
5.1 Simulation using NRZ encoder
35
5.2 Simulation using RZ encoder
47
5.3 Verification of performance difference between RZ and NRZ modulation 55
Chapter 6: Conclusion
58
References
59
v
LIST OF FIGURES
1.1
Layout of full duplex RoF system
2
2.1
Example for Radio over fiber system
6
2.2
General principle of fiber link
7
2.3
Basic layout of RoF systems
8
2.4
Attenuation profile of a Single mode Fiber cable
8
2.5
Diagram view of dead zones
9
2.6
NRZ pulse format
11
2.7
RZ pulse format
11
3.1
Structure of APD
18
3.2
Schematic of EDFA
19
3.3
Post amplifier
19
3.4
Line amplifier
20
3.5
Preamplifier
20
3.6
Illustration of Wavelength Division Multiplexing
20
3.7
Time division multiplexing
22
3.8
TDM vs WDM
22
3.9
Model of WDM
23
3.10
Optical add drop multiplexer
23
3.11
Basic model of OADM
24
4.1
Layout of transmitter at CS for downlink transmission
26
4.2
Illustration of OADM and Base station
27
4.3
Layout of full duplex RoF system using NRZ
28
4.4
Layout of full duplex RoF system using RZ
29
4.5
Relation between BER and Q factor
31
4.6
Eye pattern generation equipment layout
31
4.7
Possible NRZ combinations
32
4.8
Eye diagram interpretation
32
vi
4.9
Example of Down-link transmission using OFDM
33
5.1
Simulation layout of Transmitter at CS (using NRZ)
35
5.2
Simulation layout of transmission path of OADM and BS
36
5.3
Simulation setting for OADM
37
5.4
Simulation layout of receiver at CS (using NRZ)
37
5.5
Overall simulation layout of Full duplex transmission RoF using
38
NRZ modulation
5.6
Calculate dialog box
39
5.7
Eye diagram of downlink signal at BS (using NRZ)
40
5.8
Eye diagram of uplink signal at CS (using NRZ)
40
5.9
Eye diagram at CS_1 (using NRZ)
41
5.10
To make ‘Report’ in optisytem10
42
5.11
Iteration sweep settings for Optical fiber length
42
5.12
Q-factor vs Fiber length at BS (using NRZ)
43
5.13
Min. log of BER vs Fiber length at BS (using NRZ)
43
5.14
Q-factor vs Fiber length at CS (using NRZ)
44
5.15
Min. log of BER vs Fiber length at CS (using NRZ)
45
5.16
Q-factor vs Fiber length at CS_1 (using NRZ)
45
5.17
Min. log of BER vs Fiber length at CS_1 (using NRZ)
46
5.18
Simulation layout of transmitter at CS (using RZ)
47
5.19
Overall simulation layout of Full duplex transmission RoF using
48
RZ modulation
5.20
Eye diagram of downlink signal at BS (using RZ)
49
5.21
Eye diagram of uplink signal at CS (using RZ)
50
5.22
Eye diagram of uplink signal at CS_1 (using RZ)
50
5.23
Q-factor vs Fiber length at BS (using RZ)
51
5.24
Min. log of BER vs Fiber length at BS (using RZ)
52
5.25
Q-factor vs Fiber length at CS (using RZ)
52
5.26
Min. log of BER vs Fiber length at CS (using RZ)
53
5.27
Q-factor vs Fiber length at CS_1 (using RZ)
53
5.28
Min. log of BER vs Fiber length at CS_1 (using RZ)
54
vii
5.29
Q-factor vs Fiber length at BS (Comparison between RZ & NRZ)
55
5.30
Q-factor vs Fiber length at CS (Comparison between RZ & NRZ)
56
5.31
Q-factor vs Fiber length at CS_1(Comparison between RZ and
57
NRZ)
viii
Abstract
FULL DUPLEX TRANSMISSION OF RADIO OVER FIBER USING WDM AND
OADM TECHNOLOGY
By
Praveen Cheemaladinne
Master of Science in Electrical Engineering
Initially Communication was done via signals, voice and gradually started using devices.
Invention of optical fibers has revolutionized the telecommunications with the advantage of
transmitting large data in short period of time. To meet up the requirement of high bandwidth,
there is need of emerging technology such as Radio over Fiber (RoF) which facilitate the
wireless access by integrating both radio frequency (RF) electronic (wireless) and optical
technologies. This convergence of both wired and wireless system is a promising solution. The
main goal of this project is to design a Full duplex transmission of Radio over fiber (RoF) by
employing wavelength division multiplexing (WDM) and Optical Add-Drop multiplexer
(OADM) using two modulation techniques return to zero (RZ) and non-return to zero (NRZ).
OADM enables the two path transmission between central station (CS) to Base station (BS) and
vice versa. i.e, downlink and uplink. Optical system simulating tool optisystem10 is used to
analyze the performance difference between RZ and NRZ modulation technique. This analysis
is done based on the better performance measures in terms of Q-factor and BER in the design.
ix
CHAPTER 1: INTRODUCTION
1.1 FOREWORD:
From the start of human civilization to the point of 21st century, communication
technology has undergone lot of changes. Communication started with signals, voice and
gradually started using devices. Invention of optical fibers has revolutionized the
telecommunications with the advantage of transmitting large data in short period of time.
Optical fiber communication developed faster due to its advantages like large bandwidth
and ability to transmit longer distances
[1]
. The other communication technologies such as
wireless and satellite communications failed to provide high bandwidth. To meet up the
requirement of high bandwidth, there is need of emerging technology such as Radio over Fiber
(RoF) which facilitate the wireless access by integrating both radio frequency (RF) electronic
(wireless) and optical technologies. The main advantage of this technology is simplification of
remote accessing points which are termed as Base station (BS). RF signals are transmitted to
base station from central station (CS) and vice versa.
In this project, the main requirement of RoF link is to provide duplex operation (downlink
& uplink). This can be achieved by placing an Optical Add-Drop Multiplexer (OADM) in
between two end terminals. Number of OADM devices used in the link gives the number of
base stations available in RoF link [2]. Using time division multiplexing (TDM), data can be
transmitted only at 2.4 GB/sec over single mode fiber (SMF). But for higher bandwidth
applications TDM is not suitable. So wavelength division multiplexing (WDM) technique is
used to transmit different signals over SMF for longer distances.
RoF systems are used as alternative for cellular/ broadband networks because of its reduced
costs. This is due to simplified base stations which gives the opportunity of sharing expensive
equipment located at central station [3]. Central station performs all network operations and
optical fiber transmits the signals to the bases station after which radio antenna provides
wireless distribution.
1
1.2 PROBLEM BACKGROUND:
The essential part of any communication system is to cover the end user domain. In order
to offer the high data services like messages, audio and videos, system must be able to provide
high bandwidths. The combination of both wired and wireless system is a great solution for
increasing demand in communication systems.
Two possible modulation formats in this design are Return to zero (RZ) and Non return to
zero (NRZ). These techniques will be discussed in chapter 2 in detail.
Figure 1.1 Layout of full duplex RoF system
To provide full duplex transmission in RoF system OADM and WDM are used. This
project mainly focus on a link between two stations which were Central station (CS) and Base
station (BS). Two continuous wave lasers at 0.1 dBm emit light at frequencies 193.1 THz and
193.2 THz are used for downlink transmission. NRZ/RZ pulse generator generates an electrical
signal at bit rate 5Gb/s. Mach-Zehnder modulator[4] (MZM) was used to modulate the two
2
optical signals by the generated electrical signals and the modulated signals from both channels
are multiplexed by WDM. These multiplexed signals transmitted over fiber up to base station
through OADM. At the same time another uplink signal is added to OADM as uplink data which
transmitted towards Central station [5].
1.3 OBJECTIVE:
Main goal of this project is to design a full duplex transmission of Radio over fiber by
using WDM and OADM technology. Where OADM should be able to allow both uplink and
downlink data transmission. The performance of the ROF system is analyzed at base stations
with RZ and NRZ modulation techniques. These two modulation techniques are compared on
the basis of Q-factor & BER. Overall goal of this project is to achieve good Q-factor and low
bit error rate (BER). Total design illustrates both downlink and uplink data at central station by
means of eye diagrams.
1.4 SCOPE OF PROJECT:
The scope of this project is to study and analyses the performance of the transmission
system. Verification of its performance is done by using Optisystem10. Optisystem10 is an
optical simulating software. The system consists of single transmitter, receiver and optical link
with OADM device placed in between two end terminals. Optisystem10 enables us to study the
eye diagram, BER and Q factor.
3
1.5 METHODOLOGY:
Introduction
Overview of Radio over Fiber system using NRZ and RZ
modulation and performance difference in terms of BER and
Q factor
Theory and Model of WDM
Study and Model of OADM
Design of Full duplex RoF system using WDM and OADM
technology
Simulate and verify OADM in WDM-RoF
system using optisystem
Verify performance
difference between RZ and
NRZ modulation technique
based on BER and Q-factor
Conclusion
4
1.6 PROJECT OUTLINE:
CHAPTER 1: Provides the introduction to topic “Full duplex transmission of radio over fiber
using WDM and OADM technology”. Briefly discusses the project background, objective,
scope of project and outline of total project.
CHAPTER 2: Discusses Radio over fiber technology. It briefly gives principle and the
advantages. Discusses modulation formats RZ and NRZ in detail. Illustrates the performance
difference between RZ and NRZ modulations in terms of BER and Q-factor.
CHAPTER 3: Introduces the important components used in RoF system. Presents the
theoretical knowledge on WDM technology besides its advantages over TDM. Gives the model
of WDM used in RoF systems. Illustrates OADM technology and analysis how it enables two
way path transmission.
CHAPTER 4: Provides the approach & method to design full duplex RoF system using WDM
and OADM with the both RZ and NRZ modulation formats. Presents the block diagram of
whole system. Discusses the performance measures that is used analysing the total system.
CHAPTER 5: Simulates the OADM in WDM-Full duplex RoF system using Optisystem10. It
simulate the design of two models using both RZ an NRZ modulation techniques. Discusses the
simulation results and verification of the performance difference between RZ and NRZ
modulation techniques in terms of BER and Q-factor.
Finally chapter 6 provides conclusion of whole project. It deduces the performance and
best modulation technique based on results of bit rate and long distance applications.
5
CHAPTER 2: ROF PRINCIPLE & MODULATION FORMATS
2.1 OVERVIEW:
Today’s wireless communication has been advanced from sending simple messages to
sharing multimedia with upcoming revolutionary application services which occupies larger
data capacities. So to fulfil the future evolutionary services, Radio over fiber link could be
necessary solution as they have benefits of low attenuation loss, larger Bandwidth [5].
RoF is commonly a fiber link, which used to transmit modulated radio signals to get
wireless access. These radio signals can be data, modulated If or RF signal. This technique is
also utilized for cable TV networks and satellite stations. Modulation can be analog or digital
like PSK, QAM. After modulation these signals are transmitted over fiber.
Figure 2.1 Example for Radio over fiber system [6]
As shown in figure 2.1, In RoF system, RF signal is transmitted to and from central station
to base station and vice versa. Central station performs the most of the signal processing
operations. So heavy and costly equipment is located in this station which reduces the cost of
base stations. This is the reason why most of distributed antenna system uses Radio over fiber.
6
2.2 PRINCIPLE:
RoF systems can be divided into three types based on the frequency range of signal which
is transmitted over fiber. First one is base band over fiber. In this type low frequency signals
are transmitted over fiber and at base station converted to higher frequency, which leads this
type of system as expensive, second one is Intermediate Frequency (IF) over fiber, in which
signal are converted to IF frequency at central station and transmitted over fiber. Converted to
radio frequency again at base station.
Figure 2.2 General principle of fiber link [1]
The third type is Radio over Fiber which termed as (RoF). Radio signal is modulated by
light and directly transmitted over fiber. This type of system is simpler than the above two types
as no conversions are required at base station. This makes system cost effective. Central station
performs the all prominent operations.
In RoF system, radio/ Microwave signal is available at transmitter input which is
modulated by continuous wave light and transmitted to base station over fiber. RoF system
consists of single central station linked to different number of base stations
illustrates the basic idea of RoF systems.
7
[6]
. Figure 2.3
Figure 2.3 Basic layout of RoF systems [7]
2.3 BENEFITS OF RADIO OVER FIBER SYSTEM:
2.3.1 Low attenuation loss:
Transmission of high frequency RF/microwave signals through either copper
transmission lines or free space ends up with problems like losses, high cost. In copper
transmission lines, with increase in frequency impedance also increases which causes high
losses. Where as in free space absorption and reflection increases with frequency.
Figure 2.4 Attenuation profile of a Single mode Fiber cable [8]
As optical fiber has very low loss RoF technology can be used to achieve low loss
transmission mm waves. Standard single mode fiber have attenuation loss 0.2dB/km in 1.5 um
window and 0.5 dB/Km in 1.3 um window [8]. These losses are relatively much less compared
to transmission line or free space propagation.
8
2.3.2 Enormous bandwidth:
The prominent feature of optical fiber is large bandwidth. In optical fiber transmission
850nm. 1350nm and 1550nm wavelengths offer low attenuation. For these three windows, SMF
offers a bandwidth of 50THz. But it is operated normally at 1.6THz. These high bandwidths
helps to process signal at high speed. These features helps Radio over fiber system to implement
microwave functions like mixing, up-down conversion and filtering in optical domain.
Although optical fibers offer enormous bandwidth the limitations of BW in electronic
systems obstructs severely since the primary sources and receiving end users of transmission
data are electronic devices. This is also termed as ‘Electronic bottleneck” [1].
2.3.3 Can reach out dead zones:
DEAD ZONES are the places where it is almost impossible to transmit wireless signal
due to obstacles like mountains, Buildings, natural obstacles. Radio over fiber is able to cover
these dead zones.
Figure 2.5 diagram view of dead zones [8]
2.3.4 No electromagnetic interference:
Using Radio over fiber system there will be no electromagnetic interference. This is
attractive property of this technology. Since RF/Microwave signals are transmitted through
fiber cable. Even for shorter distances fiber cables are adopted to minimize the losses due to
interference.
9
2.3.5 Reliability& easy maintenance:
In RoF systems, main components & expensive equipment is placed at central station.
This eliminates need of a local oscillator & other important equipment at remote antenna unit
(RAU). So, RAU consists of photodetector, amplifier and antenna which makes it simpler. This
leads to less installation cost and easy to maintain. Even at bad weather conditions it is very
reliable.
2.3.6 Reduced power consumption:
As almost complex components are placed at central station, there will be reduced work
and simple RAU’s are available at base station. These RAU’s can be operated in passive mode
in most of applications. This leads to less power consumption. This is very important since these
are placed at remote locations, where it is difficult to feed from power grid.
2.3.7 Line of sight (LOS) OPERATION: There will be no multi path effects. These are
minimized.
2.4 MODULATION FORMATS:
In optical fiber communication, the main goal is to transmit as many number of bits as
possible for longer distance with less errors. Modulator converts the RF (electrical) signals in
to optical pulses. “1” represents as pulse of light where as “0” gives no light. Bit rate gives the
speed of link which is defined as number of “1s” & “0s” transmitted per second. Two generally
used modulation formats in optical fiber communications are
1. Return-to-Zero (RZ)
2. Non-Return-Zero (NRZ)
2.4.1 NON RETURN TO ZERO (NRZ):
In NRZ format, the power remains constant throughout the bit period. Amplitude never
drop to zero between two successive bits. NRZ uses low bandwidth for transmitter. This
requires simpler configuration which results less cost. Because of this NRZ modulation is
preferred in optical communication earlier days. It is not suitable in case of long distance and
10
high bit rate applications. But with number of channels NRZ may be better than RZ modulation
scheme [9].
Figure 2.6 NRZ pulse format [10]
The NRZ format, A pulse of amplitude ‘+V’ of the duration is used to represent a logic ‘’
and a pulse of amplitude ‘-V’ of same duration represents a logic “0” as shown in fig. 2.6
2.4.2 RETURN TO ZERO (RZ):
RZ format is used where pulse is on for shorter than bit slot. Its amplitude falls to zero
between two pulses. Generally RZ format is used for longer distances and higher bit rates which
is failed by NRZ to do so. RZ modulation signal is self-clocking, it doesn’t need separate clock
signal. At the times where NRZ modulation is employed only the problem is with fiber
dispersion. But with higher bit rate applications effect of non-linearity also considerable. RZ
has more advantages than NRZ format like broader frequency spectrum, high bit rate and
reduced dispersion tolerance [9].
Figure 2.7 RZ pulse format [10]
11
The RZ format, A pulse of amplitude “+V” represent a logic “1” with duration Tb/2 and a
pulse of amplitude “-V” of same duration represents a logic “0” as we can see that the pulse
return to zero after half the bit duration Tb/2 as shown in fig. 2.7
2.5 PERFORMANCE DIFFERENCE BETWEEN RZ AND NRZ FORMATS:
2.5.1 OPTICAL SPECTRUM:
In optical frequency spectrums, RZ has the broader signal than the NRZ spectrum. This
broadening of pulse increases as dispersion in the fiber increases. This is more beneficial for
RZ format since as pulse broadened the peak of the signal decreases. This is important
phenomena in case of nonlinearity effect as it is directly proportional to the intensity of pulse.
So RZ format reduces the nonlinearity than the NRZ modulation scheme.
2.5.2 RECEIVER SENSITIVITY:
Receiver sensitivity is defined as the required optical power to achieve certain bit error rate
(BER). As discussed in section 2.4.1 duty cycle selection will cause change in design
parameters. Receiver sensitivity is one of them. If the duty cycle of pulse in RZ format is
reduced there will be improvement in receiver sensitivity [11]. Because RZ pulse will have two
times the peak power of the NRZ pulse. This is due to saturation mode operation of optical
amplifiers.
The current passing through photo detector is given by eq (2.1)[11],
𝐼𝑝ℎ =
𝑞𝜂𝑃𝑜𝑝𝑡
(2.1)
ℎ𝑓
Where𝐼𝑝ℎ is the current, 𝑃𝑜𝑝𝑡 is the light power level,
f is the light's frequency,
h is Planck's constant, q is the charge on an electron.
From equation (2.1)
𝐼𝑝ℎ ∝ 𝑃𝑜𝑝𝑡
(2.2)
12
Pelectrical ∝ Poptical2
(2.3)
(Pelectrical)RZ ∝ 2 * (Pelectrical) NRZ
(2.4)
In photo diode, photo current is directly proportional (Iph) to the optical power (Popt). Hence
the electrical power (Pelec) received at detector is proportional to square of optical power. As a
result power received in case of RZ pulse is two times that of NRZ pulse.
2.5.3 BIT RATE:
Higher bit-rate systems are limited by dispersion. The RZ format would be beneficial for
systems with few channels but would require NRZ as the number of channels increase.
Dispersion compensation based on chirped Fiber-Bragg gratings (FBG) to compensate for the
residual dispersion of dispersion compensation fibers (DCFs) is under development. The
effectiveness of FBG modules in mitigating residual dispersion effects at 40 Gb/s over the
multiple channels of the transmission spectrum is being explored.
2.5.4 SPAN LENGTH AND LINK LENGTH
The modulation format impacts the design of a given link; each stage of optical
amplification introduces noise due to the amplified spontaneous emission of optical amplifiers.
As a result, the optical signal to noise ratio (OSNR) degrades along the link. An empirical
expression for OSNR is given by the following equation [11]:
OSNR (dB) = 𝐴 − 10log(𝑁)– 𝑁𝐹–10log(𝐿)+Pout-10 log(𝑀) − 𝑘,
Where
A= 58 dB
N = number of amplifiers in the link,
𝑁𝐹 = noise figure of each amplifier,
𝐿 = loss
Pout = output power of amplifier,
13
(2.5)
𝑀 = number of channels
𝑘 = other factors.
Since the RZ scheme has a better baseline receiver sensitivity than the NRZ scheme, the
span length may be increased (received optical power decreased) for a given launch power and
receiver sensitivity. Several authors have shown experimentally that longer link lengths can be
achieved with the RZ format.
2.5.5 BER and Q-FACTOR:
The BER and Q-factor may be improved by choosing a proper line coding scheme which
includes various modulation formats like Non Return to Zero (NRZ), Return to Zero (RZ), OnOff Keying (OOK) and many more .The most commonly parameters used for measuring the
performance are Q-factor and Bit Error Rate (BER). The BER defines as the probability of
incorrect identification of a bit by the decision circuit in the receiver and Q-Factor measures the
quality of the transmission of signal in terms of its signal-to-noise ratio (SNR), higher the value
of Q-factor the better the SNR and therefore the lower the probability of bit errors. The BER
and Q-factor can be estimated by following expression [10] written in equation (2.6) and (2.7).
To achieve BER less than 10-9, Q-factor must be greater than and equal to 6 (Q ≥ 6).
1
𝑄
𝐵𝐸𝑅 = 2 𝑒𝑟𝑓𝑐 ( ) ≅
√2
−𝑄2
)
√2
𝑒𝑥𝑝(
(1.6)
𝑄√2𝜋
Parameter Q can be estimated as
𝑚 −𝑚
𝑄 = ( 𝜎1 − 𝜎 0 )
1
(1.7)
0
Where 𝑚1 , 𝑚0 the mean of the received signal at the sampling instant are when a bit 1 and 0 is
transmitted 𝜎1 , 𝜎0 are the standard deviations respectively
14
2.6 SUMMARY AND DISCUSSION:
In this chapter, the basic principle of Radio over fiber including different kinds of systems
like Base band over fiber, IF over fiber and Radio over fiber were discussed. The importance
of two modulation formats RZ and NRZ in Optical systems and performance difference between
two modulation schemes are discussed.
In next chapter, chapter 3, the important components used in full duplex RoF system will be
introduced. WDM model and its analysis along with OADM technology will also be discussed.
15
CHAPTER 3: INTRODUCTION TO COMPONENTS USED IN
FULL DUPLEX ROF SYSTEM
3.1 LASER DIODE:
A laser diode is a device whose active medium is a semiconductor similar to that found in
a Light Emitting Diode (LED). The most common type of laser diode is formed from an N-type
and P-type material as p-n junction which is powered by injected electric current. A laser diode
is formed by doping a very thin layer on the surface of a crystal wafer. Laser diodes form a
subset of the larger classification of semiconductor p-n junction diodes. Due to forward biasing
two type of charges are introduced in device. Holes are injected from the p-doped, and electrons
from the n-doped semiconductor [6].
Laser diodes has advantage of having high power, narrow spectral width and high speeds.
However, they are sensitive to temperature changes. Multimode diode lasers suffer from noise,
i.e., in different modes, laser power will be distributed randomly. When combined with
chromatic dispersion in the fiber, this leads to random fluctuations in intensity and changes in
transmitted pulses. In recent years, semiconductor technology has resulted in many
developments which leads to reliability of laser devices. The examples of such developments
are mentioned below [12]
Sources at 0.87μm: AlGaAs LEDs and AlGaAs/GaAs, Quantum well Laser devices
Sources at 1.3 μm: InGaAsP LEDs and InGaAsP/InP Laser with single mode fibers
Sources at 1.55 μm: DFB laser with narrow line width to minimise dispersion
3.1.1 REQUIREMENTS OF A SOURCE FOR OPTICAL TRANSMITTER:
The basic requirement of light source used in optical communication system depends on
type of application Long-haul communication, LAN (short haul) etc., the following are the main
requirements for the source [12]:
Power level: The main goal of communication is to reach the opposite end receiver and it should
be detectable. Source power must be sufficient so that it can transmit through the fiber.
16
Speed: The source should be able to modulate the source power at the required rate for
transmission of information.
Line width: The chromatic dispersion in the fiber must be minimized. So the narrow source
must have a narrow spectral width.
Noise: Random fluctuations in source power must be avoided. Most of noises are generated due
to the difference in the phases of two optical wave trains separated by time.
3.2 PHOTO DETECTOR:
There are two types of detectors in optical communication systems: the p-i-n photodiode
and the avalanche photodiode (APD). The APD has the advantage of providing gain before the
first electronic amplification stage in the receiver, thereby reducing the detrimental effects of
circuit noise. However, mechanism in the device introduces noise and has large response time,
which may decrease the bandwidth. Furthermore, APDs require a high-voltage supply and more
complicated circuitry to compensate for their sensitivity to temperature fluctuations [6].
When a photon with energy greater than the band gap Eg is incident on the
semiconductor, this energy is absorbed by the material and generates an electron-hole pair that
is an electron in the conduction band and a hole in the valence band. When the pair is created
within the space charge region, the electric field in the junction separates the charges and drifts
them to the neutral regions. The carrier drift generates a photocurrent in the external circuit that
provides an electrical signal. The photocurrent lasts the time needed for the electron and hole
to cross the depletion layer and reaches the neutral regions. When the drifting hole reaches the
p-type region it recombines with an electron entering the p side form the negative electrode that
is from the power supply [12].
Detectors at 0.87𝛍m:
Silicon p-i-n photodiodes and APDs are used at these wavelengths. In state of
preamplifiers, silicon APDs has a 10-15dB sensitivity which is an advantage over silicon p-i-n
photodiodes because their internal gain makes the noise of the preamplifier relatively less
important. The sensitivity of Si APDs has several hundred Mb/s corresponds to about one
hundred photons/bit.
17
Detectors at 1.3𝛍m and 1.55𝛍m:
Silicon is not usable in this region because its bandgap is greater than the photon energy.
Germanium and InGaAs p-i-n photodiodes are both used. InGaAs is preferred because it has
greater thermal stability and lower dark noise. Typical InGaAs p-i-n photodiodes have quantum
efficiencies ranging from 0.5 to 0.9, responsivities = 1A/W, and response times that are in the
tens of ps. Low noises APDs and InGaAs APDs etc.
Figure 3.1: Structure of APD [12]
An optical detector can be either a semiconductor positive-intrinsic-negative (PIN)
diode or an avalanche photodiode (APD). A PIN diode depends on intensity and operating
wavelength, which accordingly changes its electrical conductivity. The PIN diode has an
intrinsic region between p-type and n-type semiconductor material.
3.3 OPTICAL FIBER AMPLIFIER:
Erbium-doped silica fibers which are used as laser amplifiers, are becoming increasingly
important components of 1.55um fiber-optic systems. These devices offer high-gain
amplification around 30 to 45 dB. These fibers also have low noise, near the wavelength of
lowest loss in silica glass. They are pumped by InGaAsP lasers usually in the range of 1.48 um,
exhibit low insertion loss which is almost < 0.5 dB and polarization insensitivity. These kind of
devices usually have minimum crosstalk between signals since they operated for longer time
[13]
.
18
Figure 3.2: Schematic of EDFA [13]
An Er 3+-doped fiber amplifier may be used as an optical-power amplifier placed directly
at the output of the source laser, or as an optical preamplifier at the photodetector input (or
both). It can also serve as an all-optical repeater, replacing the electronic repeaters that provide
reshaping, retiming, and regeneration of the bits (e.g., those used in current long-haul undersea
fiber-optic systems). All-optical repeaters are advantageous in that they offer increased gain and
bandwidth, insensitivity to bit rate, and the ability to simultaneous amplify multiple optical
channels.
Based on the position of the amplifier we can represent the fiber amplifier as shown in
figures 3.3, 3.4 , 3.5 post amplifier, line amplifier, Preamplifier.
Figure 3.3: Post amplifier
Figure 3.4: Line amplifier
19
Figure 3.5: Preamplifier
3.4 WAVELENGTH DIVISION MULTIPLEXING (WDM):
The WDM is an important milestone in the development of optical communications.
Usually WDM is a passive device that can converge optical signals of different wavelengths
coming from different fibers on to a single fiber. Because of this, networks can be simpler and
flexible. Mainly in RoF systems number of base stations can be increased which are linked to
single central station.
Figure 3.6: Illustration of Wavelength Division Multiplexing [14]
In fiber-optic communications, wavelength-division multiplexing (WDM) is a
technology which multiplexes a number of optical carrier signals onto a single optical fiber by
using different wavelengths of laser light. This technique enables bidirectional communications
over one strand of fiber, as well as multiplication of capacity [14].
Since wavelength and frequency are closely related to each other, this form of multiplexing
is often called frequency division multiplexing (FDM). Each WDM fiber has a certain
bandwidth within the range of frequencies it can carry. The term wavelength-division
20
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).
3.4.1 ADVANTAGES OF WDM:
Capacity: Most optical telecommunications are prominently using WDM systems they allow
them to increase the capacity without adding the fiber lines.
Scalability: Additional demand can be fulfilled by upgrading additional multiplexers and
demultiplexers at each end of fiber terminals.
Transparency: Any transmission can be transmitted through each optical channel. The format
can be digital or analog.
Wavelength routing and switching: Since most of the optical devices depend on wavelength,
Wavelength can be used as another dimension to time and space in designing communication
networks and switches.
3.4.2 TDM VS WDM:
Time division multiplexing is one of the multiplexing technique which is less efficient one.
In TDM, multiple signals are transmitted over single channel. These incoming signals are
equally divided into fixed length time slots. [4]
Figure 3.7: Time division multiplexing [15]
Due to division of time as the name suggests (TDM), 2/3 of each signal is lost. This
uneconomical as most of signal from source is lost. WDM can be alternative multiplexing
21
technique to overcome this. Using TDM, we can transmit only 2.4 GB/S over single mode fiber
(SMF). But for for higher bandwidth application TDM is not suitable. So WDM is better
technique to transmit the different signals over SMF for longer distances. This comparison
between TDM & WDM is illustrated in figure 3.8
Figure 3.8: TDM vs WDM [15]
3.4.3 MODEL OF WDM:
Figure 3.9: Model of WDM[11]
22
In the above figure 3.9, each wavelength of frequency is transmitted from transmitter to
different receiver. The advantage of this multiplexing is there will be no interference. Using
WDM network, the systems are operated at bit rates of 40Gbps and above.
3.5 OADM:
An optical add-drop multiplexer (OADM) is a device used in wavelength-division
multiplexing systems for multiplexing and routing different channels of light into or out of a
single mode fiber (SMF). This is a type of optical node, which is generally used for the
construction of optical telecommunications networks. "Add" and "drop" here refer to the
capability of the device to add one or more new wavelength channels to an existing multiwavelength WDM signal, and/or to drop (remove) one or more channels, passing those signals
to another network path. An OADM may be considered to be a specific type of optical crossconnect [5].
Figure 3.10: Optical add drop multiplexer [16]
A traditional OADM consists of three stages: an optical demultiplexer, an optical
multiplexer, and between them a method of reconfiguring the paths between the demultiplexer,
the multiplexer and a set of ports for adding and dropping signals. The demultiplexer separates
wavelengths in an input fiber onto ports. The reconfiguration can be achieved by a fiber patch
panel or by optical switches which direct the wavelengths to the multiplexer or to drop ports.
The multiplexer multiplexes the wavelength channels that are to continue on from demultiplexer
ports with those from the add ports, onto a single output fiber. The basic model of OADM is
shown in figure 3.11.
23
Figure 3.11: Basic model of OADM [7]
All the light paths that directly pass an OADM are termed cut-through light paths, while
those that are added or dropped at the OADM node are termed as added/dropped light paths.
An OADM with remotely reconfigurable optical switches (for example 1×2) in the middle stage
is called a reconfigurable OADM (ROADM). Ones without this feature are known as fixed
OADMs. But the term OADM can used to both types, it is often used interchangeably with
ROADM.
3.5.1 ADVANTAGES OF OADM:

The planning of entire bandwidth assignment need not be carried out during initial
deployment of a system. The configuration can be done as and when required without
affecting traffic already passing the ROADM.

ROADM allows for remote configuration and reconfiguration.

In ROADM, as it is not clear beforehand where a signal can be potentially routed, there is
a necessity of power balancing of these signals. ROADMs allow for automatic power
balancing.
24
3.6 SUMMARY AND DISCUSSION:
In this chapter, the important components used in full duplex Radio over fiber system were
introduced. The requirements of optical source (Laser diode), different photo detectors and
EDFA were discussed. The model of WDM and its advantages were presented. The two way
path transmission of OADM device was illustrated.
In next chapter, chapter 3, the approach and method to implement the full duplex RoF
system using both RZ and NRZ modulation formats will be presented.
25
CHAPTER 4: SYTEM DESIGN & PERFORMANCE MEASURES
In this project, full duplex transmission of radio over fiber proposed taking into account
Wavelength division multiplexing (WDM) and Optical Add-Drop multiplexing (OADM).
These two techniques are useful for both downlink and uplink data transmission over fiber.
Below the design of full duplex transmission of radio over fiber is discussed using both NRZ
and RZ modulation techniques.
4.1 DESIGN USING NRZ MODULATION:
For downlink transmission, the central station (CS) is designed of two signal generators,
two continuous wave (CW) lasers followed by two Mach-Zehnder modulators (MZM). For
multiplexing these signals WDM is used. The central frequencies of CW laser diodes are 193.1
THz and 193.2 THz respectively.
Figure 4.1 layout of transmitter at CS for downlink transmission
As shown in figure 4.1, at transmitter side two laser diodes emit light at frequencies 193.1
THz and 193.2 THz which are provided to MZMs. Two incoming signals from signal generators
at 5 GB/s is mixed with NRZ signal. The resulted microwave signals are transmitted MZMs
along with optical carrier from the laser diodes. The modulated signals from both channels are
multiplexed using WDM multiplexer.
26
Then these multiplexed signals are amplified by an EDFA and transmitted over single mode
fiber to base station (BS). After transmitting the signals up to base station, the appropriate
frequency (193.1 THz) is dropped by OADM. As discussed in section 3.5 the OADM, with 40
dB add-drop isolation, consists of one fiber bragg grating (FBG) located between two optical
circulators. The OADM is used to drop the down-link optical carriers to the dedicated BS. This
downlink data signal is passed through a PIN detector of responsivity 1 A/W and a Bessel low
pass filter [7].
Figure 4.2 Illustration of OADM and Base station
The resulting electrical signal is then given to a BER tester for analysing the downlink
signal. At the same time as shown in figure 4.2, another optical carrier of same frequency 193.1
THz modulated by baseband signal of data rate 5 GB/s is added from BS to fiber backbone, as
uplink data, by OADM. Now the multiplexed signal which contains carrier frequency (193.2
THz), other than that dropped at the BS, along with the uplink data of frequency 193.1 THz
added from base station is transmitted towards CS.
Finally, when the multiplexed uplink data is transported up to the central station over singlemode fiber, it is splitted using optical splitter and passed through optical tuneable band pass
filter to separate two different signals from two base stations, which are modulated using
different optical carrier frequency.
27
Figure 4.3 Layout of full duplex RoF system using NRZ
After separating the two signals, they are passed through optical receiver and electrical
demodulator for getting back the electrical signal. This signal is passed through the multiplot
and BER for signal analysis. The entire block diagram of design using NRZ modulation is
shown in figure 4.3.
4.2 DESIGN USING RZ MODULATION:
Figure 4.4 shows the design of full duplex transmission using RZ modulation. The optical
carriers of frequencies 193.1 THz and 193.2 THz are emitted by two CW lasers. These carriers
are modulated by electrical signal of data rate 2.5 GB/s generated by signal generator and RZ
encoder. RZ encoder is replaced by NRZ encoder and same procedure is followed as discussed
in previous section.
28
Figure 4.4 Layout of full duplex RoF system using RZ
4.3 PERFORMANCE MEASURES:
4.3.1 BER and Q FACTOR:
The performance of an optical communication system is often characterized using the bit
error rate (BER). Q-factor and BER are the most important factors that limiting the transmission
distance in optical communication systems. In order to transmit signals over long distances, it
is necessary to have a low BER and high Q-factor within the fiber.
Q factor measures the quality of an analog transmission signal in terms of its signal-tonoise ratio (SNR). As such, it takes into account physical impairments to the signal. For
example, noise, chromatic dispersion and any polarization or non-linear effects – which can
29
degrade the signal and ultimately cause bit errors. In other words, the higher the value of Q
factor the better the SNR and therefore the lower the probability of bit errors [17].
The bit error rate (BER) is the percentage of bits that have errors relative to the total
number of bits received in a transmission. For example, a transmission might have a BER 10−6
, it means out of 1,000,000 bits transmitted, one bit was in error. 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 present.
However the BER measurement for high performance transmission link can be extremely
difficult. For example, when requiring a BER of 1 × 10−15 a minimum measurement time of
27 hours is required at 10 GB/s data rate [18]. In these circumstances, Q factor measurement has
become the new quality evaluation parameter. System Q factor adopts the concept of S/N ratio
in a digital signal and is an evaluation method that assumes a normal noise distribution. It is
defined as [10]:
𝑚1 − 𝑚0
𝑄=(
𝜎1 − 𝜎0
)
(4.1)
Where 𝑚1 , 𝑚0 correspond to the levels of the transmitted data ‘1’s and ‘0’s, and 𝜎1 , 𝜎0
correspond to the standard deviation of the noise on ‘1’s and ‘0’s respectively. The BER is
related to Q as follows [10]:
1
𝑄
𝐵𝐸𝑅 = 2 𝑒𝑟𝑓𝑐 ( ) ≅
√2
−𝑄2
)
√2
𝑒𝑥𝑝(
(Where Q ≥ 6)
𝑄√2𝜋
(4.2)
This relationship between BER and Q is further illustrated in figure 4.5. As can be seen from
the figure, the minimum value of Q that will make BER less than 10-9 is 6. Usually much higher
values of Q would be expected of a practical system.
30
Figure 4.5 Relation between BER and Q factor [18]
4.3.2 EYE DIAGRAM:
The eye diagram technique is simple but powerful measurement method to find the
performance of the transmission system. The eye diagram measurements are in time domain
and allow the effects of waveform distortion to be shown immediately on an oscilloscope [8].
Figure 4.6 shows the general eye pattern measurement using basic equipment.
Figure 4.6 eye pattern generation equipment layout [8]
The output of Pseudo random bit sequence (PRBS) generator is applied to vertical input
of an oscilloscope after has been transmitted through an optical cable. Also generator trigger
signal is used to trigger the horizontal sweep of oscilloscope. This results in the type pattern
shown in figure 4.8. This display pattern is formed because of 8 possible 3 bit long NRZ
31
combinations shown in figure 4.7. When these 8 combinations are superimposed
simultaneously eye pattern shown in figure 4.8 is formed.
Figure 4.7 possible NRZ combinations [8]
Figure 4.8 eye pattern diagram [8]
To understand the eye diagram further consider the simplified drawing shown in figure 4.9.
From this figure the following information can be deduced [8].
1. Width of eye opening defines the time interval over which received signal can be sampled
without error. The best time to sample the received wave form is when height of eye opening
is maximum.
32
2. The maximum distortion is given by the vertical distance between the top of the eye opening
and the maximum signal level. The greater the eye closer becomes, the more difficult it is
to detect the signal.
3. The height of eye opening at specified sampling time shows the noise margin or immunity
to noise. Noise margin can be defined as the ratio of peak signal voltage V1 for an alternating
bit sequence to maximum signal voltage V2.
𝑉
𝑁𝑜𝑖𝑠𝑒 𝑚𝑎𝑟𝑔𝑖𝑛 = (𝑉1 ) × 100
2
(4.3)
4. The rate at which eye closes as sampling time is varied determines the sensitivity of the
system to timing errors.
5. The amount of distortion at ∆𝑇 at the threshold level indicates the amount of jitter and it is
given by
∆𝑇
𝑇𝑖𝑚𝑖𝑛𝑔 𝑗𝑖𝑡𝑡𝑒𝑟 = (𝑇 ) × 100
𝑏
Where Tb is one bit interval
Figure 4.9 eye diagram interpretation [8]
33
(4. 4)
4.4 SUMMARY AND CONCLUSION:
In this chapter, the methodology of designing the full duplex system is presented. The entire
block diagrams of both RZ modulation and NRZ modulation techniques are shown. The
performance measures BER, Q factor and eye diagrams were discussed in detail.
Based on this design methodology, the next chapter, chapter 5, will present the simulation
of the full duplex transmission of ROF system using Optisystem10. Discusses the simulation
results and verification of the performance difference between RZ and NRZ techniques in terms
of BER and Q factor.
34
CHAPTER 5: SIMULATION & RESULTS
In this chapter, simulation of “Full duplex transmission of radio over fiber using WDM
and OADM” is carried out using optical system simulator Optisystem10 by Optiwave. The
simulation of both designs mentioned in chapter 4 are shown below.
5.1 SIMULATION USING NRZ ENCODER:
Figure 5.1 shows the simulation layout of transmitter at central station (CS). Pseudo
random bit sequence generator at 5 GB/s is mixed with NRZ encoder is given as input signal to
the Mach-Zehnder modulator (MZM). Set the frequencies of two continuous wave (CW) lasers
at 193.1 THz and 193.2 and feed to MZMs which is modulated by the given input signal. These
signals are connected to WDM (MUX) as shown in figure 5.1.
Figure 5.1 Simulation layout of Transmitter at CS (using NRZ)
35
Figure 5.2 shows layout of base station and transmission path of Optical Add-Drop
multiplexer (OADM). The length of optical fiber is set to 70 km up to base station. OADM is
placed at this position to drop the appropriate frequency (193.1) to the base station (BS). This
appropriate frequency is transmitted as downlink data signal to PIN photodetector and passed
through low pass Bessel filter of responsivity 1A/W. The corresponding electrical signals are
given to “BER analyser B_S” to analyse the downlink data at BS as shown in figure 5.2.
Figure 5.2 Simulation layout of transmission path of OADM and Base station (BS)
Another same optical carrier at frequency 193.1 THz is added back to OADM from BS.
The simulation setting for OADM is shown in figure 5.3. OADM will drop the carrier of
frequency 193.1 GHz at BS and allow carrier at 193.2 GHz along with added optical carrier
frequency of 193.1 GHz. This added signal from BS acts as uplink data and it transmitted
towards CS.
36
Figure 5.3 Simulation setting for OADM
Then these signals are demultiplexed by WDM (Demux) and given to corresponding
detectors. The resulted electrical signals are transmitted through low pass Bessel filter of cutoff frequency 0.75xBit rate. As discussed in section 4.3.2 terminals of trigger and data out are
connected to “BER analyzer_CS” to get the performance results like eye diagram and BER at
CS. Figure 5.4 shows ‘receiver layout at central station’ used in simulation.
Figur 5.4 Simulation layout of receiver at Central station (CS) (using NRZ)
37
38
Figure 5.5 Overall simulation layout of Full duplex transmission RoF using NRZ modulation
The whole simulation layout of “full duplex transmission of radio over fiber (RoF) using
NRZ modulation” is shown below in figure 5.5. the followng gives the parameters of
components consisdered for simulation using optisystem10.

Input power at transmitter: 0.1 dBm

Frequencies at CW laser: 193.1 THz and 193.2THz

Bit rate : 5 GB/s

Extinction ratio of Mach-Zehnder modulator: 30 dB

Optical Fibre length: 140 km

Attenuation Coefficient : 0.2dB/km
Figure 5.6 Calculate dialog box
To calculate the whole simulation click the ‘calculate’ button on top of layout. As a result
the above dialog box will be appeared as shown in figure 5.6. Then run the simulation until it
shows ‘calculation finished’. Figure 5.7 shows the BER analyser eye diagram at BS while BER
analysers eye diagram at CS are shown in figure 5.8 and figure 5.9 respectively.
39
Figure 5.7 Eye diagram of downlink signal at BS (using NRZ)
Figure 5.8 Eye diagram of uplink signal at CS (using NRZ)
40
5.9 Eye diagram at CS_1 (using NRZ)
As discussed in section 4.3, Q-factor is more than 6 and BER is also less than 10-9, so
satisfactory performance is achieved by using NRZ modulation. Q-factor is 15.77 at BS and
15.59 at CS. Similarly BER is almost 2.4×10-56 at BS while BER at CS is 3.8 ×10-55. At CS_1,
BER is very high and equal to ‘1’ as the length of fiber is 140 Km compared with fiber of 70
km for BER of CS. For that reason eye diagram is more distorted at CS_1 and the length of 140
Km shouldn’t be used in the design.
5.1.1 EFFECT OF FIBER LENGTH ON SYSTEM PERFORMANCE:
To observe the variation of Q-factor and BER along the length of fiber, go to ‘Report’ at
bottom of window and insert an ‘Opti2Dgraph’ as shown in figure 5.10 below. Then from
‘project browser’ plot the graphs of Q-factor vs fiber length and min. log of BER vs fiber length.
41
Figure 5.10 to make ‘Report’ in optisytem10
Figure 5.11 shows the iteration sweep settings of both optical fibers. Total number of
iterations are taken as 7.
Figure 5.11 Iteration sweep settings for Optical fiber length
42
Figure 5.12 Q-factor vs Fiber length at BS (using NRZ)
Figure 5.13 Min. log of BER vs Fiber length at BS (using NRZ)
43
The variation of Q-factor along the fiber length at BS is shown in figure 5.12. The Q-factor
of both stations BS, CS (CS, and CS_1) decreases as length of fiber increases. The Q-factor of
downlink signal at BS (almost 15 at 70 km from figure 5.12) is slightly more than the Q-factor
of uplink signal at CS (almost 15 at 70 km from figure 5.14) as the fiber length increases. Most
probably because of additive losses in OADM. Q-factor becomes zero as fiber length is reached
to 140 km at CS_1 as shown in figure 5.16.
Figure 5.14 Q-factor vs Fiber length at CS (using NRZ)
Similarly BER increases as length increases. Figure 5.13, figure 5.15, and figure 5.17 shows
the variation of min. log of BER along the fiber length at BS, CS and CS_1 respectively. BER
is less for downlink signal at BS than the uplink signal at CS due to additive losses in OADM
as mentioned above. Min. log of BER gradually increased to ‘0’ at CS_1 as fiber length varied
from 1-140 km.
44
Figure 5.15 Min. log of BER vs Fiber length at CS (using NRZ)
Figure 5.16 Q-factor vs Fiber length at cs_1 (using NRZ)
45
Figure 5.17 Min. log of BER vs Fiber length at CS_1 (using NRZ)
46
5.2 SIMULATION USING RZ ENCODER:
Now NRZ encoder is replaced by RZ encoder and same procedure is followed as discussed
in previous section. The optical carriers of frequencies 193.1 THz and 193.2 THz are emitted
by two CW lasers. These carriers are modulated by electrical signal of data rate 5 GB/s
generated pseudo random bit sequence generator and RZ encoder. Figure 5.18 shows the
simulation layout of transmitter at CS using RZ modulation. The overall simulation of system
is shown in figure 5.19.
Figure 5.18 simulation layout of transmitter at CS (using RZ)

Input power at transmitter: 0.1 dBm

Frequencies at CW laser: 193.1 THz and 193.2THz

Bit rate : 5 GB/s

Extinction ratio of Mach-Zehnder modulator: 30 dB

Optical Fibre length: 140 km

Attenuation Coefficient : 0.2dB/km
47
48
Figure 5.19 Overall simulation layout of Full duplex transmission RoF using RZ modulation
Figure 5.20 Eye diagram of downlink signal at BS (using RZ)
After running ‘Calculate dialog box’, the BER analyser eye diagrams at BS, CS and CS_1
are shown in figure 5.20, figure 5.21 and figure 5.22 respectively. Q-factor is more than 6 and
BER is less than 10-9 satisfactory performance is even with RZ modulation. Q-factor at BS is
19.24, while Q-factor at CS is 19.63. This slight difference is due to add-drop multiplexing at
OADM. Q-factor is zero at CS_1 as fiber length is 140 Km. Similarly BER at BS is 7.7×10-83
where it is almost same as BER at CS 3.4×10-86. At CS_1 it is ‘1’ as the length of fiber is 140
Km compared with fiber of 70 km for BER of CS. The eye diagram at CS_1 is more distorted
since signal is transmitted over fiber length of 140 km. As mentioned before in NRZ format,
this shouldn’t be acceptable in actual design.
49
Figure 5.21 Eye diagram of uplink signal at CS (using RZ)
Figure 5.22 Eye diagram of uplink signal at CS_1 (using RZ)
50
5.2.1 EFFECT OF FIBER LENGTH ON SYSTEM PERFORMANCE:
To analyse the variation of Q-factor and min. log of BER along the fiber length ‘Report’
tool is used. The variation of Q-factor along the fiber length at BS is shown in figure 5.23. The
Q-factor of both stations BS, CS (CS, and CS_1) decreases as length of fiber increases. The Qfactor of downlink signal at BS (almost 280 at 10 km from figure 5.23) and it is reduced to 20
as length reached to 70 km. At CS, Q factor of uplink signal is around 200 at 10 km and it is
reduced to 20 at 70 Km as that of a BS as shown in figure 5.25. Most probably this slight
difference is because of add-drop multiplexing of signals in OADM. Q-factor becomes zero as
fiber length is reached to 140 km at CS_1 as shown in figure 5.27.
Figure 5.23 Q-factor vs Fiber length at BS (using RZ)
51
Figure 5.24 Min. log of BER vs Fiber length at BS (using RZ)
Figure 5.25 Q-factor vs Fiber length at CS (using RZ)
52
Figure 5.28 Min. log of BER vs Fiber length at CS (using RZ)
Figure 5.27 Q-factor vs Fiber length at CS_1 (using RZ)
53
Similarly BER increases as length increases. Figure 5.24, figure 5.26, and figure 5.28 shows
the variation of min. log of BER along the fiber length at BS, CS and CS_1 respectively. BER
is almost same for downlink signal at BS and the uplink signal at CS. But Min. log of BER
gradually increased to a very high value of zero at CS_1 as signal is transmitted over fiber length
of 140 Km. This length seems very large for acceptable communication system.
Figure 5.28 Min. log of BER vs Fiber length at CS_1 (using RZ)
54
5.3 VERIFICATION OF PERFORMANCE DIFFERENCE BETWEEN RZ AND NRZ MODULATION:
By superimposing the above results in Optisystem10, we can compare the performance
difference between two modulation techniques RZ and NRZ. The variation of Q-factor at
different stations are shown in figure 5.29, figure 5.30 and figure 5.31.
Variation of Q-factor at BS by using two modulation techniques RZ and NRZ is shown in
figure 5.29. At fiber length 10 Km, RZ system has better Q factor (280) than NRZ system (208)
for same bit rate 5 GB/s. As length increases up to 70 Km, Q-factor is reduced to 20 for RZ
system and reduced to 15 for NRZ. So Q-factor for RZ is superior over NRZ for all base station
lengths.
Figure 5.29 Q-factor vs Fiber length at BS (Comparison between RZ and NRZ)
55
Similarly at CS, Figure 5.30, initially Q-factor is almost same at fiber length 10 Km but as
length is varied RZ system has better performance than NRZ system in terms of Q-factor.
Figure 5.30 Q-factor vs Fiber length at CS (Comparison between RZ and NRZ)
Figure 5.31 shows variation of Q-factor with length at second channel of CS (CS_1).
Clearly RZ system has the better performance over NRZ system. It falls down to zero in both
cases as signal transmitted over 140 Km.
56
Figure 5.31 Q-factor vs Fiber length at CS_1 (Comparison between RZ and NRZ)
Similar results can be expected in case of ‘min. log of BER vs Fiber length’ as BER is
directly proportional to the Q-factor as discussed in section 4.3.1.
As mentioned in section 4.3, the system with high Q-factor and hence with less BER will
have better performance. From the above simulation results, system has better Q factor and less
BER when RZ modulation is used than the NRZ modulation. Both NRZ and RZ have their
drawbacks as NRZ is affected by nonlinearity, RZ is affected by dispersion. But by analysing
the simulation graphs and eye diagrams in this project, RZ modulation provides the better
performance for longer distances and high bit rate applications.
57
CHAPTER 6: CONCLUSION
The simulation of full duplex transmission of radio over fiber by employing WDM and
OADM is done using Optisystem10. WDM has given the advantage of transmitting multiple
signals through single fiber, OADM enabled two path transmission between BS and CS and
vice versa. RZ and NRZ modulation techniques are simulated and analyzed by using
optisytem10. RZ has the edge over NRZ modulation, in terms of better Q-factor and low BER
even with increase in distance. This makes RZ modulation superior over NRZ modulation for
high bit rate and long distance applications.
58
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