i RADIO ACCESS POINT DESIGN FOR RADIO OVER FIBER ...
Transcript of i RADIO ACCESS POINT DESIGN FOR RADIO OVER FIBER ...
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RADIO ACCESS POINT DESIGN FOR RADIO OVER FIBER TECHNOLOGY
MOHAMMOUD MUNSOR MOHAMMOUD HADOW
A project report submitted in partial fulfillment of the requirements for the award of
the degree of Master of Engineering (Electrical-Electronics & Telecommunications)
Faculty of Electrical Engineering
Universiti Technologi Malaysia
APRIL 2008
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To my beloved late mother, may her soul rest in Paradise.
To my beloved father and brothers and sisters.
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ACKNOWLEDGEMENT
In the name of Allah s.w.t.
First of all I would like to say my gratitude to the one and only to our mighty
God, Allah S.W.T. for giving me the strength, good health, idea and talent to
complete this research project as one of the requirement for the conferment of the
Degree.
I would like to express sincere thanks to my supervisor Dr. Razali bin Ngah
for his invaluable guidance throughout the course of this project. His guidance, ideas,
encouragement, affable nature, kindness and support were greatly helpful. Even with
His busy schedule, he spent considerable amount of time helping me through the
different phases of this project
I wish to thank my parents, for their daily prayers, giving me the motivation
and strength, and encouraging me to accomplish and achieve my goals.
A special acknowledgment must be given to my brothers and sisters for their
Motivation help and support during my academic period at UTM. I am indebted to
Them and words will never express the gratitude I owe to them.
Last of all, a big appreciation is express to all parties that were directly or
indirectly involved in this project. May Allah SWT bless us.
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ABSTRAK
Rangkaian kawasan tempatan wayarles (WLAN) adalah digunakan pada
peningkatan bilangan tempat-tempat. Dalam bangunan-bangunan pejabat, hospital-
hospital, lapangan terbang melepak, dan sebagainya. dengan kenaikan gaji laris
untuk data berkelajuan lebih tinggi penghantaran, ini WLAN memerlukan untuk
menyediakan lebih tinggi keupayaan-keupayaan pemindahan data, yang memerlukan
frekuensi-frekuensi gelombang mikro tinggi. Oleh itu, jangkauan antena radio stesen
mendapat lebih kecil dan apa saja lagi titik akses radio (KETUKAN) adalah
diperlukan untuk meliputi satu kawasan tertentu. Untuk menyimpan stesen belanja
dapat dikawal, stesen-stesen antena sepatutnya seperti mudah sebagai mungkin dan
isyarat pemprosesan isyarat yang serupa sebagai mungkin dan banyak fungsi-fungsi
harus bertumpu di stesen hujung kepala kemudian untuk dibawa lutsinar antara titik
akses radio (KETUKAN) dan stesen hujung kepala. Satu mod gentian optikal,
sebagai digunakan dengan meluas dalam jarak jauh 50 kilometer dan rangkaian
metropolitan, menawarkan mencukupi lebar jalur untuk ini. Tetapi adalah mahal
untuk penggunaan tertutup. Tujuan kajian ini adalah mereka bentuk dan menyerupai
satu titik akses radio untuk radio atas teknologi gentian. Simulasi-simulasi telah
diusahakan menggunakan optisytem. Komponen-komponen bermakna adalah
electroabsoaption pemodulat (EAM) titik akses seperti radio daripada komponen
Power Amplifier PA dan Band-pass Filter BPF. Titik akses radio adalah dibuat-buat
di kekerapan 2.4 GHz.
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ABSTRACT
Wireless local area network (WLAN) is being used at increasing number of
places. In office buildings, hospitals, airport lounges, etc. with the raise in demand
for higher speed data delivery, these WLAN need to provide higher data transfer
capacities, which requires high microwave frequencies. Thus, the reach of the radio
antenna station gets smaller and ever more radio access point(RAP) is needed to
cover a certain area. To keep station’s cost under control, the antenna stations should
be as simple as possible and as much as possible signal processing signal functions
should be centralized at the head end station. The modulated microwave signals need
then to be carried transparently between the radio access point (RAP) and head end
station. Single mode optical fiber, as extensively used in long distance 50 km and
metropolitan network, offer adequate bandwidth for this. The purpose of this study
is to design and simulate a radio access point for radio over fiber technology. The
simulations were performed using Optisytem. The main components were
electroabsoaption modulator (EAM) as radio access point instead of the component
Power Amplifier (PA) and Band-pass Filter (BPF). The radio access point is
simulated at frequency of 2.4 GHz.
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EAM
RF
GSM
UMTS
RAP
BS
CATV
MSC
RS
WLAN
LAN
IF
POFs
SMFs
EAT
MZM
SSB
CW
DC
NRZ
RAU
NLS
PSK
RZ
PMD
AM
CD
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Electro-absorption Modulator
Radio Frequency
Global System Mobile
Universal Mobile Telecommunication System
Radio Access Point
Base Station
Cable Television
Mobile Switching Center
Remote Site
Wireless Local Area Network
Local Area Network
Intermediate Frequency
Polymer Optical Fibers
Single Mode Fibers
Electro-Absorption Transceiver
Mach- Zenhder- Modulator
Single Side-band
Carrier Wave
Direct- Current
Non-Return-to Zero
Radio Access Unit
Nonlinear Schrödinger
Phase Shift Keying
Return to Zero
Polarization mode Dispersion
Amplitude Modulation
Chromatic Dispersion
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xi
LIST OF FIGURES xii
LIST OF ABBREVIATIONS
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1 INTRODUCTION 1
1.1 Introduction
1.2 Objective of Project
1.3 Scope of Project
1.4 Problem Statement
1.5 Thesis outlines
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2 LITERATURE REVIEW
RADIO OVER FIBER(ROF)
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2.1 Introduction
2.2 Radio over Fiber Technology
2.3 Why RF-over fiber
2.4 Benefits of Radio over fiber system
2.4.1 Low attenuation loss
2.4.2 Large bandwidth
2.4.3 Immunity to radio frequency interference
2.4.4 Easy installation and maintenance
2.4.5 Operational flexibility
2.4.6 Reduced power consumption
2.4.7 Millimeter waves
2.4.7.1 Advantages of mm-waves
2.4.7.2 Disadvantages of mm-waves
2.4.8 Radio system functionality
2.5 Applications of Radio over fiber technology
2.5.1 Cellular networks
2.5.2 Satellite communications
2.5.3 Video distribution systems
2.5.4 Mobile broadband service
2.5.5 Wireless LANs
2.5.6 Vehicle communication and control
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3 PROJECT BACKGROUND RADIO ACCESS POINT
(RAP)
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3.1 Introduction
3.2 Optical transmission links
3.2.1 Optical fiber
3.2.1.1 Optical transmission in fiber
3.2.1.2 Multimode versus single mode fiber
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3.2.1.3 Attenuation in fiber
3.2.1.4 Dispersion in fiber
3.2.1.5 Nonlinearities in fiber
3.2.2 Optical transmitters
3.2.2.1 How a laser works
3.2.2.2 Semi-conductor diode laser
3.2.2.3 Optical modulation
3.2.3 Optical receivers
3.2.3.1 Photodetectors
3.2.4 Optical amplifiers
3.2.4.1 Doped fiber amplifier
3.3 Radio over fiber optical links
3.3.1 Introduction to ROF analog optical links
3.3.2 Basic radio signal generation and transportation
methods
3.3.3 ROF link configurations
3.3.4 State –of-the Art Millimeter waves generation
and transport technology
3.3.4.1 Optical heterodyning
3.3.4.2 External modulation
3.3.4.3 Up- and –down conversion
3.3.4.4 Optical transceiver
3.3.4.5 Comparison of mm-wave generation and
transport techniques
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4 METHODOLOGY 42
4.1 Introduction
4.2 Simulation using optisystem software
4.3 Simulation model
4.4 Simulation of the RAU
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SIMULATION RESULTN AND DISCUSSION
5.1
5.2
Introduction
Optical Transmitters
5.3 Parameter values of components
5.4 External Mach-Zehnder modulator(MZM) with carriers wav
5.5 Optical modulation converter and method for converting the
modulation format of an optical signal
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REFERENCES
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusion
6.2 Recommendation and future work
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LIST OF TABLES
TABLE
TITLES PAGE
3.1 Comparison of millimeter-wave generation & transport
techniques.
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3.2 Millimeter wave – band ROF experiments 41
5.1 Pseudo Random Bit Sequence generator 50
5.2 Electrical PSK modulator 50
5.3 Transimpendence amplifier 51
5.4 CW laser prosperities 52
5.5 Mach-Zehnder Modulator 54
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LIST OF FIGURE
TITLE
PAGE
2.1 The radio over fiber system concept ( Braodband radio hand) 6
3.1 General radio over fiber 18
3.2 Optical transmission link 19
3.3 Multimode (a) and single mode (b) optical fiber 21
3.4 Light traveling via total internal reflection within an optical fiber 22
3.5 The general structure of a laser 26
3.6 Structure of semiconductor laser diode 27
3.7 Intensity-modulation direct-detection (IMDD)analog optical link 33
3.8 Representative ROF link configurations (a) EOM, RF modulated
Signal .(b) EOM,RF modulated signal
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3.9 Representative ROF link configurations .(c)EOM,IF baseband
Modulated signal (d) Direct modulation.
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3.10 Optical heterodyning 38
3.11 Electro-absorption transceiver (EAT) 40
4.1 Flow chart of the methodology of the project 43
4.2 Direct modulation 44
4.3 Externally modulated 45
4.4 Simulation model with external modulated signal 45
4.5 Simulation model without external modulated signal 46
5.1 The basic model used to simulate the ROF system 48
5.2 Transmitter components 49
5.3 Laser intensity carrier wave 53
5.4 Simulation diagram for radio over fiber using (EAM) 55
5.5 Output of the RF spectrum analyzer for (a) pseudo-Random bit 56
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sequence (b) Electrical PSK Modulator
5.6 Output of the optical fiber and MZM modulator 57
5.7 Output of the signal ( pseudo random bit sequence ) 57
5.8 Output of the signal using PSK modulation 57
5.9 Output of EAM for different spectrums 58
5.10 Bit error analyzer of simulation diagram (a) Q factor (b) Min
BER
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5.11 Bit error analyzer of the simulation diagram (c) threshold (d)
height
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LIST OF ABBREVIATIONS
LD
EDFA
OTDM
DWDM
MZI
SCM
IMDD
RHD
MVDS
MBS
ITS
RVC
IVC
LED
WDM
SDH
BB
PSTN
EOM
PD
ROF
SC
GIPOF
O/E
E/O
EAM
RF
GSM
UMTS
RAP
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Laser Diode
Erbium Doped Fiber Amplifier
Optical Time Division Multiplex
Dense Wave Length Division Multiplex
Mach Zehnder Interferometer
Subcarrier Multiplexing
Intensity Modulation and Direct Detection
Remote Heterodyning and Detection
Multipoint Video Services
Mobile broadband System
Intelligent Transport Systems
Road-to-Vehicle Communication
Inter-Vehicle Communication
Light Emitting Diode
Wave Division Multiplexing
Synchronous Digital Hierarchy
Base Band
Public Switched Telephone Network
External Optical Modulator
Photo - detector
Radio Over Fiber
Switching Centers
Graded Index Polymer Optical Fiber
Optical-to-Electrical
Electrical-to-Optical
Electro-absorption Modulator
Radio Frequency
Global System Mobile
Universal Mobile Telecommunication System
Radio Access Point
BS
CATV
MSC
RS
WLAN
LAN
IF
POFs
SMFs
EAT
MZM
SSB
CW
DC
NRZ
RAU
NLS
PSK
RZ
PMD
AM
CD
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Base Station
Cable Television
Mobile Switching Center
Remote Site
Wireless Local Area Network
Local Area Network
Intermediate Frequency
Polymer Optical Fibers
Single Mode Fibers
Electro-Absorption Transceiver
Mach- Zenhder- Modulator
Single Side-band
Carrier Wave
Direct- Current
Non-Return-to Zero
Radio Access Unit
Nonlinear Schrödinger
Phase Shift Keying
Return to Zero
Polarization mode Dispersion
Amplitude Modulation
Chromatic Dispersion
CHAPTER 1 1.1 Introduction
Wireless Communication is becoming an integral part of today’s society. The
proliferation of mobile and other wireless devices coupled with increased demand for
broadband services are putting pressure on wireless systems to increase capacity. To
achieve this, wireless systems must have increased feeder network capacity, operate
at higher carrier frequencies, and cope with increased user population densities.
However, raising the carrier frequency and thus reducing the radio cell size leads to
costly radio systems while the high installation and maintenance costs associated
with high-bandwidth silica fiber render it economically impractical for in-home and
office environments.
Radio-over-fiber (RoF) technology has emerged as a cost effective approach
for reducing radio system costs because it simplifies the remote antenna sites and
enhances the sharing of expensive radio equipment located at appropriately sited
(e.g. centrally located) Switching Centers (SC) or otherwise known as Central
Sites/Stations (CS). On the other hand, Graded Index Polymer Optical Fiber (GIPOF)
is promising higher capacity than copper cables, and lower installation and
maintenance costs than conventional silica fiber.
Wireless access – fixed or mobile is regarded as an excellent way to achieve
broadband services. Of course, it is the only possibility for mobile access (in
particular if global mobility is required), however wide application of fixed wireless
broadband access is also foreseen. It is well known that both due to unavailability
of lower microwave frequencies and to the insufficient bandwidth of lower frequency
ranges, next generation wireless access systems – both mobile and fixed – will
operate in the upper microwave/millimeter wave frequency band. As in a cellular
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system both increased traffic and propagation properties of millimeter-waves require
small cells, further as millimeter-wave circuits are rather expensive, the cost of base
stations (BSs) will be of determining role.
One emerging technology applicable in high capacity, broadband millimeter-
wave access systems is Radio over Fiber called also (Fiber the Air). In this system in
order to decrease the costs of BSs, most of signal processing (including coding,
multiplexing, RF generation & modulation etc) is made in central stations (CSs)
rather than in the BSs. The signal to and from these is transmitted in the optical band,
via a fiber optic network. This architecture makes design of BS-s really simple, in
the simplest case a BS doesn’t comprise else than optical-to-electrical (O/E) and
electrical-to-optical (E/O) converters, an antenna and some microwave circuitry (two
amplifiers and a diplexer). Or, as it will be mentioned, in principle even the
amplifiers can be omitted. In the last decade or so significant research work was done
in this field with significant results; the number of publications is abundant. The
most important results are summarized in a recent monograph [1]. While,
architecture, techniques, benefits, as well as problems to be solved are extensively
discussed in [1] and papers referred to in [1], not too much has been told about
special problems of resource management and channel allocation. The aim of this
report is, after presenting basic design and fields of application of the RoF concept,
to give an, as far as known by the author, first short outline on these questions.
1.2 Objective of project
The objective of project is to simulate a low power radio access point (RAP)
for transmission using optisystem simulation software. The main part in the radio
access point is electro absorption modulator (EAM) as radio unit.
1.3 Scope of project
In this project, simulation model will be developed that integrates both radio
frequency (RF) wireless and optical fiber systems that would be transparent to
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different systems such as GSM, UMTS.etc. In this simulation models might consist
of Pico cell base station and central end optical fiber link model that may use
commercially available parameters and power control subsystem modeled in the
optisystem are used to contact the whole heretical models.
First of all, the principle of radio over fiber (ROF) system characterized by
fiber optic link and free space radio discussed. An electroabsorption modulator
transceiver will be used for (RAP) designed. Thereby the (RAP) cost will be reduced.
Once familiar with principle and the environment of the optisystem software low
power (RAP), at the last the behavior of the system will be anal sized.
1.4 Problem statement
The difficulty which faces radio communication is limited available
frequency spectrum. Also numerous reflection stationary objects such as wells,
furniture and movable object such as people, animals cause hard environment for –
high speed radio transmission. In addition using many components in radio access
point make the system less reliability and cost, lastly poor signal coverage.
1.5 Thesis outlines
This is written to bring the reader step by step going in the main core of the
content Chapter 1 Provides the introduction to this project where brief background
of the study problem and to the statement of the problem. Followed by the Objective,
and the scope of the study.
Chapter 2 reviews the literature, which includes introduction to the RoF, the
benefits, and applications of the Radio over Fiber Technology in both satellite and
mobile radio communications. In addition various types of RoF BS or radio access
unit have also been covered.
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Chapter 3 covers the basic optical fiber communication link and surveys the
state of the art on RoF technologies with a special emphasis devoted to RoF system
operating at mm-wave bands and provides information about the fiber characteristics,
and structure of electroabsorption modulator which presented the main component in
radio access point (RAP).
Chapter 4 describes the methodological processes by showing detailed
diagram of the methods implemented as well as highlighting briefly the steps those
have been followed to achieve the objective of this project.
Chapter 5 presents the results derived from the methods explained where
some analyses and simulations were done based on the EAM effects. Finally the
conclusions of the study, as well as some suggestions for future work were summed
up in Chapter6.
CHAPTER 2
LITERATURE REVIEW
RADIO OVER FIBER (ROF)
2.1 Introduction
This chapter highlights the literature cited on the radio-over-fiber (RoF)
technology has emerged as a cost effective approach for reducing radio system costs
because it simplifies the remote antenna sites and enhances the sharing of expensive
radio equipment located at appropriately sited (e.g. centrally located) Switching
Centers (SC) or otherwise known as Central Sites/Stations (CS). On the other hand,
Graded Index Polymer Optical Fiber (GIPOF) is promising higher capacity than
copper cables, and lower installation and maintenance costs than conventional silica
fiber.
2.2 Radio-over-Fiber Technology
Radio over Fiber (ROF) is refers to a technology where, light is modulated in
radio frequency and transmitted over optical fiber to facilitate wireless access.
Although RF transmission over fiber is done in many occasions such as in Cable TV
(CATV) networks and in Satellite base stations, the term ROF is usually applied
when this is done for wireless access.
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RoF technology is a technology by which microwave (electrical) signals are
distributed by means of optical components and techniques. A RoF system consists
of a Central Site (CS) and a Remote Site (RS) connected by an optical fiber link or
network. If the application area is in a GSM network, then the CS could be the
Mobile Switching Centre (MSC) and the RS the base station (BS) as in shown in
Figure 2.1 for wireless Local Area Networks (WLANs), the CS would be the headed
while the Radio Access Point (RAP) would act as the RS.
Figure 2.1 The Radio over Fiber System Concept (Broadband radio hand)
Pioneer RoF systems such as the one depicted in Figure 2.1 were primarily
used to transport microwave signals, and to achieve mobility functions in the CS.
That is, modulated microwave signals had to be available at the input end of the RoF
system, which subsequently transported them over a distance to the RS in the form of
optical signals. At the RS the microwave signals are re-generated and radiated by
antennas. The system shown in Figure 2.1 was used to distribute GSM 900 network
traffic. The added value in using such a system lay in the capability to dynamically
allocate capacity based on traffic demands. RoF systems of nowadays, are designed
to perform added radio-system functionalities besides transportation and mobility
functions. These functions include data modulation, signal processing, and frequency
conversion (up and down) [12] [13] for a multifunctional RoF system, the required
electrical signal at the input of the RoF system depends on the RoF technology and
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the functionality desired. The electrical signal may be baseband data, modulated IF,
or the actual modulated RF signal to be distributed. The electrical signal is used to
modulate the optical source. The resulting optical signal is then carried over the
optical fibre link to the remote station. Here, the data is converted back into electrical
form by the photodetector. The generated electrical signal must meet the
specifications required by the wireless application be it GSM, UMTS, wireless LAN
or other. By delivering the radio signals directly, the optical fibre link avoids the
necessity to generate high frequency radio carriers at the antenna site. Since antenna
sites are usually remote from easy access, there is a lot to gain from such an
arrangement. However, the main advantage of RoF systems is the ability to
concentrate most of the expensive, high frequency equipment at a centralized
location, thereby making it possible to use simpler remote sites. Furthermore, RoF
technology enables the centralizing of mobility functions such as macro-diversity.
2.3 Why RF-over fiber
Optical fiber is small in size, flexible. very low loss and is very well
established technology .it offer a number of inherent advantages over the use of
coaxial cable or free transmission, including :
i. Very low loss (less than 0.5 dB signal loss per km.
ii. Nonconductive medium ,resulting in electrical isolation for safety and
interference
iii. Immunity to electrical interference.
iv. Easy to deployment due to small size and flexibility of rugged cable.
RF-over fiber links offer unbeatable advantages for the transmission of
analog ,RF and digital data signals over the use optical fiber , and are used for
the replacement of conventional coaxial cable in many different applications.
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2.4 Benefits of Radio-over-Fibre Systems
Advantages and benefits of the RoF technology include the following:
2.4.1 Low Attenuation Loss
Electrical distribution of high frequency microwave signals either in free
space or through transmission lines is problematic and costly. In free space, losses
due to absorption and reflection increase with frequency. In transmission lines,
impedance rises with frequency as well. Therefore, distributing high frequency radio
signals electrically over long distances requires expensive regenerating equipment.
As for mm-waves, their distribution via the use of transmission lines is not feasible
even for short distances. The alternative solution to this problem is to distribute
baseband signals or low intermediate frequencies (IF) from the Switching Centre
(SC) to the Base Stations (BS) [1]. The baseband or IF signals are then up converted
to the required microwave or mm-wave frequency at each base station, amplified and
then radiated. Such a system places stringent requirements (such as linearity) on
repeater amplifiers and equalizers. In addition, high performance local oscillators
would be required for up conversion at each base station. This arrangement leads to
complex base stations with tight performance requirements. An alternative solution
is to use optical fibres, which offer much lower losses.
Commercially available standard Single Mode Fibres (SMFs) made from
glass (silica) have attenuation losses below 0.2 dB/km and 0.5 dB/km in the 1.5 µm
and the 1.3 µm windows, respectively. Polymer Optical Fibres (POFs), a more recent
kind of optical fibres exhibit higher attenuation ranging from 10 – 40 dB/km in the
500 -1300 nm regions [11] . These losses are much lower than those encountered in
free space propagation and copper wire transmission of high frequency microwaves.
Therefore, by transmitting microwaves in the optical form, transmission distances are
increased several folds and the required transmission powers reduced greatly.
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2.4.2 Large Bandwidth
Optical fibres offer enormous bandwidth. There are three main transmission
windows, which offer low attenuation, namely the 850nm, 1310nm and 1550nm
wavelengths. For a single SMF optical fibre, the combined bandwidth of the three
windows is in the excess of 50THz. However, today’s state-of-the-art commercial
systems utilize only a fraction of this capacity (1.6 THz). But developments to
exploit more optical capacity per single fibre are still continuing. The main driving
factors towards unlocking more and more bandwidth out of the optical fibre include
the availability of low dispersion (or dispersion shifted) fibre, the Erbium Doped
Fibre Amplifier (EDFA) for the 1550nm window, and the use of advanced multiplex
techniques namely Optical Time Division Multiplexing (OTDM) in combination
with Dense Wavelength Division Multiplex (DWDM) techniques.
The enormous bandwidth offered by optical fibres has other benefits apart
from the high capacity for transmitting microwave signals. The high optical
bandwidth enables high speed signal processing that may be more difficult or
impossible to do in electronic systems. In other words, some of the demanding
microwave functions such as filtering, mixing, up- and down-conversion, can be
implemented in the optical domain. For
instance, mm-wave filtering can be achieved by first converting the electrical signal
to be filtered into an optical signal, then performing the filtering by using optical
components such as the Mach Zehnder Interferometer MZI or Bragg gratings), and
then converting the filtered signal back into an electrical signal [14]. Furthermore,
processing in the optical domain makes it possible to use cheaper low bandwidth
optical components such as Laser Diodes (LD) and modulators, and still be able to
handle high bandwidth signals [12] - [4].
The utilization of the enormous bandwidth offered by optical fibers is
severely hampered by the limitation in bandwidth of electronic systems, which are
the primary sources and end users of transmission data. This problem is referred to as
the “electronic bottleneck”. The solution around the electronic bottleneck lies in
effective multiplexing. OTDM and DWDM techniques mentioned above are used in
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digital optical systems. In analogue optical systems including RoF technology, Sub-
Carrier Multiplexing (SCM) is used to increase optical fibre bandwidth utilization.
In SCM, several microwave subcarriers, which are modulated with digital or
analogue data, are combined and used to modulate the optical signal, which is then
carried on a single fibre. This makes the RoF system cost effective
2.4.3 Immunity to Radio Frequency Interference
Immunity to electromagnetic interference is a very attractive property of
optical fibre communications, especially for microwave transmission. This is so
because signals are transmitted in the form of light through the fibre. Because of this
immunity, fibre cables are preferred even for short connections at mm-waves.
Related to RFI immunity is the immunity to eavesdropping, which is an important
characteristic of optical fibre communications, as it provides privacy and security.
2.4.4 Easy Installation and Maintenance
In RoF systems, complex and expensive equipment is kept at the SCs,
thereby making remote base stations simpler. For instance, most RoF techniques
eliminate the need for a local oscillator and related equipment at the Remote Station
(RS). In such cases a photodetector, an RF amplifier, and an antenna make up the
RS equipment. Modulation and switching equipment are kept in the SC at the head
end and shared by several RS. This arrangement results in smaller and lighter RS,
effectively reducing system installation and maintenance costs. Easy installation and
low maintenance costs of RS are very important requirements for mm-wave systems,
because of the large numbers of the required antenna sites. Having expensive RS
would render the system costs prohibitive. The numerous antennas are needed to
offset the small size of radio cells (micro- and Pico-cells), which is a consequence of
limited propagation distances of mm-wave microwaves. In applications where RSs
are not easily accessible, the reduction in maintenance requirements has many
positive implications.
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2.4.5 Operational Flexibility
RoF does offer operational benefits in terms of operational flexibility.
Firstly, depending on the microwave generation technique, a RoF distribution system
can be made signal format transparent. For instance the Intensity Modulation and
Direct Detection (IMDD) technique can be made to operate as a linear system and
therefore as a transparent system. This can be achieved by using low dispersion fibre
(SMF) in combination with pre-modulated RF subcarriers (SCM). When this
happens, then, the same RoF network can be used to distribute multi-operator and
multi-service traffic, resulting in huge economic savings. Secondly, with the
switching, modulation, and other functions performed at a centralized SC, it is
possible to allocate capacity dynamically. For instance in a RoF based distribution
system for GSM traffic, more capacity can be allocated to an area (e.g. shopping
mall) during peak times and then re-allocated to other areas when off-peak (e.g. to
populated residential areas in the evenings). This can be achieved by allocating
optical wavelengths as need arises [49]. Allocating capacity dynamically as need for
it arises obviates the requirement for allocating permanent capacity, which would be
a waste of resources in cases where traffic loads vary frequently and by large
margins. Furthermore, having a SC facilitates the consolidation of other signal
processing functions such as mobility functions.
2.4.6 Reduced Power Consumption
Reduced power consumption is consequence of having simpler RSs with
reduced equipment .most the complex equipment is kept at the central SC. In some
applications, the antenna sites are operated in passive mode .For instance , 5 GHz
fiber- radio system employing picocells (small radio cells) can have the RSs (BSs)
operate in passive mode .reduced power at the RSs is significant considering that
RSs are sometimes placed in remote location not fed by the power grid.
12
2.4.7 Millimeter Waves
Millimeter waves offer several benefits. However, mm-waves cannot be
distributed electrically due to high RF propagation losses. In addition, generating
mm-wave frequencies using electrical devices is challenging. These issues describe
the electronic bottleneck already discussed above. The most promising solution to
the problem is to use optical means. Low attenuation loss and large bandwidth make
the distribution of mm-waves cost effective. Furthermore, some optical based
techniques have the ability to generate unlimited frequencies. For instance,
microwave frequencies that can be generated by Remote Heterodyning and Detection
(RHD) methods are limited only by the bandwidth of photodetectors. Advantages
and disadvantages of mm-waves are listed below
2.4.7.1 Advantages of mm-waves
They provide high bandwidth due to the high frequency carriers. Secondly,
due to high RF propagation losses in free space, the propagation distances of mm-
waves are severely limited. This allows for well-defined small radio sizes (micro-
and Pico-cells), where considerable frequency re-use becomes possible so that
services can be delivered simultaneously to a larger number of subscribers.
2.4.7.2 Disadvantages of mm-waves
The negative side of mm-waves is the need for numerous BSs, which is a
consequence of high RF propagation losses. Unless the BSs are simple enough,
installing and maintaining the mm-wave system can be economically prohibitive
owing to the numerous required BSs
13
2.4.8 Radio System Functionalities
As stated earlier, RoF technology is not only used for distributing RF signals
but for radio system functionalities as well. Among these, modulation and frequency
conversion have been mentioned above. However, application of RoF technology for
radio system functionalities goes beyond modulation and frequency conversion to
encompass signal processing at very high frequencies. These functions include
filtering, attenuation control and signal processing in high frequency phased array
antenna systems, just to name but a few. These functions are also referred to as
microwave functions. Many of these functions are difficult to achieve in the
microwave (electrical) domain due to limited bandwidth and other electromagnetic
wave propagation limitations. However, if the processing is done in the optical
domain, unlimited signal processing bandwidth becomes available. As a result,
many microwave functions can be performed by optical components without needing
E/O conversion for processing by microwave components and vice versa [14].
2.5 Applications of Radio-over-Fibre Technology
Some of the applications of RoF technology include satellite
communications, mobile radio communications, broadband access radio, Multipoint
Video Distribution Services (MVDS), Mobile Broadband System (MBS), vehicle
communications and control, and wireless LANs over optical networks. The main
application areas are briefly discussed below.
2.5.1 Cellular Networks
The field of mobile networks is an important application area of RoF
technology. The ever-rising number of mobile subscribers coupled with the
increasing demand for broadband services have kept sustained pressure on mobile
networks to offer increased capacity. Therefore, mobile traffic (GSM or UMTS) can
be relayed cost effectively between the SCs and the BSs by exploiting the benefits of
14
SMF technology. Other RoF functionalities such as dynamic capacity allocation offer
significant operational benefits to cellular networks
2.5.2 Satellite Communications
Satellite communications was one of the first practical uses of RoF
technology. One of the applications involves the remoting of antennas to suitable
locations at satellite earth stations. In this case, small optical fibre links of less than
1km and operating at frequencies between 1GHz and 15GHz are used. By so doing,
high frequency equipment the second application involves the remoting of earth
stations themselves. With the use of RoF technology the antennae need not be within
the control area (e.g. Switching Centre). They can be sited many kilometres away for
the purpose of, for instance improved satellite visibility or reduction in interference
from other terrestrial systems. Switching equipment may also be appropriately sited,
for say environmental or accessibility reasons or reasons.
2.5.3 Video Distribution Systems
One of the major promising application areas of RoF systems is video
distribution. A case in point is the Multipoint Video Distribution Services (MVDS).
MVDS is a cellular terrestrial transmission system for video (TV) broadcast. It was
originally meant to be a transmit-only service but recently, a small return channel has
been incorporated in order to make the service interactive. MVDS can be used to
serve areas the size of a small town. Allocated frequencies for this service are in the
40 GHz band. At these frequencies, the maximum cell size is about 5km. To extend
coverage, relay stations are required.
In MVDS the coverage area is served by a transmitter, which is located either
on a mast or a tall building. The rooftop equipment can be simplified by employing
RoF techniques. For instance, instead of using Gunn oscillators with their own
antennas and heat pipes for frequency stabilisation, an optical fibre link may be used
to feed either a travelling wave tube or a solid state amplifier at the transmit
15
frequency [3]. This greatly reduces the weight and wind loading of the transmitter.
In addition, a single optical fibre could feed the transmitter unit from a distance of
several hundred metres.
2.5.4 Mobile Broadband Services
The Mobile Broadband System or Service (MBS) concept is intended to
extend the services available in fixed Broadband Integrated Services Digital Network
(B-ISDN) to mobile users of all kinds. Future services that might evolve on the B-
ISDN networks must also be supported on the MBS system. Since very high bit rates
of about 155 Mbps per user must be supported, carrier frequencies are pushed into
mm-waves. Therefore, frequency bands in the 60 GHz band have been allocated.
The 62-63 GHz band is allocated for the downlink while 65-66 GHz is allocated for
the uplink transmission. The size of cells is in diameters of hundreds of meters
(micro-cells). Therefore, a high density of radio cells is required in order to achieve
the desired coverage. The micro-cells could be connected to the fixed B-ISDN
networks by optical fibre links. If RoF technology is used to generate the mm-
waves, the base stations would be made simpler and therefore of low cost, thereby
making full scale deployment of MBS networks economically feasible [4].
2.5.5 Wireless LANs
As portable devices and computers become more and more powerful as well
as widespread, the demand for mobile broadband access to LANs will also be on the
increase. This will lead once again, to higher carrier frequencies in the bid to meet
the demand for capacity. For instance current wireless LANs operate at the 2.4 GHz
ISM bands and offer the maximum capacity of 11 Mbps per carrier (IEEE 802.11b).
Next generation broadband wireless LANs are primed to offer up to 54 Mbps per
carrier, and will require higher carrier frequencies in the 5 GHz band
(IEEE802.11a/D7.0) [10].
Higher carrier frequencies in turn lead to micro- and Pico-cells, and all the
difficulties associated with coverage discussed above arise. A cost effective way
16
around this problem is to deploy RoF technology. A wireless LAN at 60 GHz has
been realized [5] by first transmitting from the BS (Central Station), a stable
oscillator frequency at an IF together with the data over the fibre. The oscillator
frequency is then used to up-convert the data to mm-waves at the transponders
(Remote Stations). This greatly simplifies the remote transponders and also leads to
efficient base station design.
2.5.6 Vehicle Communication and Control
This is another potential application area of RoF technology. Frequencies
between 63-64 GHz and 76-77 GHz have already been allocated for this service
within Europe. The objective is to provide continuous mobile communication
coverage on major roads for the purpose of Intelligent Transport Systems (ITS) such
as Road-to-Vehicle Communication (RVC) and Inter-Vehicle Communication
(IVC). ITS systems aim to provide traffic information, improve transportation
efficiency, reduce burden on drivers, and contribute to the improvement of the
environment [6]. In order to achieve the required (extended) coverage of the road
network, numerous base stations are required. These can be made simple and of low
cost by feeding them through RoF systems, thereby making the complete system cost
effective and manageable.
CHAPTER 3
PROJECT BACKGROUND RADIO ACCESS POINT (RAP)
3.1 Introduction
Wireless networks based on ROF technologies have been proposed as a
promising cost effective solution to meet ever increasing user bandwidth and
wireless demands. Since it was first demonstrated for cordless or mobile telephone
service in 1990 [5], a lot of research has been carried out to investigate its limitation
and develop new and high performance ROF technologies. In this network a CS is
connected to numerous functionally simple BSs via an optic fiber. The main function
of BS is to convert optical signal to wireless one and vice versa. Almost all
processing including modulation, demodulation, coding, routing is performed at the
CS. That means, ROF networks use highly linear optic fiber links to distribute RF
signals between the CS and BSs. Fig. 3.1 shows a general ROF architecture. At a
minimum, an ROF link consists of all the hardware required to impose an RF signal
on an optical carrier, the fiber-optic link, and the hardware required to recover the RF
signal from the carrier. The optical carrier's wavelength is usually selected to
coincide with either the 1.3nm window, at which standard single-mode fiber has
minimum dispersion, or the 1.55 nm window, at which its attenuation is minimum.
This chapter, constituting two major parts, briefly covers basic optical fiber
transmission link and surveys state-of-the-art ROF technologies with an emphasis on
ROF system operating at mm-wave bands. The first part is dedicated to a description
of general optical transmission link, where digital signal transmission is assumed as
current optical networks. The second part mainly deals with ROF technologies and is
subdivided as follows:
18
(1) ROF link characteristics, requirements,
(2) RF signal generation and transportation techniques, and link configurations
(3) The state of the art on mm-wave generation and transport technologies. In
addition, ROF with wavelength division multiplexing (WDM) is described as it has
been one of the hot topics in this area.
Figure 3.1: General radio over fiber
3.2 Optical Transmission Links
In the first part of this section, a general optical transmission link , shown in
Fig. 3.2, is briefly described for which we assume that a digital pulse signal is
transmitted over optical fiber unless otherwise specified. The optical link consists of
an optical fiber, transmitter, receiver and amplifier, each of which is dealt with in the
subsequent subsections.
19
Figure 3.2: optical transmission link
3.2.1 Optical Fiber
Optical fiber is a dielectric medium for carrying information from one point
to another in the form of light. Unlike the copper form of transmission, the optical
fiber is not electrical in nature. To be more specific, fiber is essentially a thin
filament of glass that acts as a waveguide. A waveguide is a physical medium or path
that allows the propagation of electromagnetic waves, such as light. Due to the
physical phenomenon of total internal reflection, light can propagate following the
length of a fiber with little loss (Fig. 3.4).
Optical fiber has two low-attenuation regions . Centered at approximately
1300 nm is a range of 200 nm in which attenuation is less than 0.5 dB=km. The total
bandwidth in this region is about 25 THz. Centered at 1550 nm is a region of similar
size with attenuation as low as 0.2 dB/km. Combined, these two regions provide a
theoretical upper bound of 50 THz of bandwidth. By using these large low-
attenuation areas for data transmission, the signal loss for a set of one or more
wavelengths can be made very small, thus reducing the number of amplifiers and
repeaters actually needed. In single channel long-distance experiments, optical
signals have been sent over hundreds of kilometers without amplification. Besides its
enormous bandwidth and low attenuation, fiber also offers low error rates.
Communication systems using an optical fiber typically operate at BER's of
less than 10-11. The small size and thickness of fiber allows more fiber to occupy the
same physical space as copper, a property that is desirable when installing local
networks in buildings. Fiber is flexible, reliable in corrosive environments, and
20
deployable at short notice. Also, fiber transmission is immune to electromagnetic
interference and does not cause interference.
3.2.1.1 Optical Transmission in Fiber
Light can travel through any transparent material, but the speed of light will
be slower in the material than in a vacuum. The ratio of the speed of light in a
vacuum to that in a material is known as the material's refractive index (n) and is
given by n = c=v, where c is the speed in a vacuum and v is the speed in the material.
When light travels from one material of a given refractive index to another material
of a different refractive index (i.e., when refraction occurs), the angle at which the
light is transmitted in the second material depends on the refractive indices of the
two materials as well as the angle at which light strikes the interface between the two
materials. According to Snell's law, we have na sinθa = nb sin θb, where na and nb
are the refractive indices of the first substance and the second substance,
respectively; and θa and θb are the angles from the normal of the incident and
refracted lights, respectively. From Fig. 3.3, we see that the fiber consists of a core
completely surrounded by a cladding (both of which consist of glass of different
refractive indices). Let us first consider a step-index fiber, in which the change of
refractive index at the core-cladding boundary is a step function. If the refractive
index of the cladding is less than that of the core, then the total internal reflection can
occur in the core and light can propagate through the fiber as shown in Fig. 3.4. The
angle above which total internal reflection will take place is known as the critical
angle and is given by θc.
………………………………… (3.1)
where nclad and ncore are the refractive indices of cladding and core, respectively.
Thus, for a light to travel down a fiber, the light must be incident on the core-
cladding surface at an angle greater than θc. For the light to enter a fiber, the
incoming light should be at an angle such that the refraction at the air-core boundary
results in the transmitted light's being at an angle for which total internal reflection
21
can take place at the core-cladding boundary. The maximum value of θair can be
derived from
……………………….. (3.2)
We can rewrite it as:
……………………………… (3.3)
The quantity nair sin θair is referred to as the numerical aperture (NA) of the fiber
and θair is
The maximum angle with respect to the normal at the air-core boundary, so
that the incident light that enters the core will experience total internal reflection
inside the fiber.
Figure 3.3: multimode (a) and single mode (b) optical fiber (unit:nm).
22
Figure 3.4:light traveling via total internal reflection within an optical fiber
3.2.1.2 Multimode Versus Single-Mode Fiber
A mode in an optical fiber corresponds to one of the possible multiple ways
in which a wave may propagate through the fiber. It can also be viewed as a standing
wave in the transverse plane of the fiber. More formally, a mode corresponds to a
solution of the wave equation that is derived from Maxwell's equations and subject to
boundary conditions imposed by the optical fiber waveguide. Although total internal
reflection may occur for any angle θ that is greater than θc, light will not necessarily
propagate for all of these angles. For some of these angles, light will not propagate
due to destructive interference between the incident light and the reflected light at the
core-cladding interface within the fiber. For other angles of incidence, the incident
wave and the reflected wave at the core cladding interface constructively interfere in
order to maintain the propagation of the wave. The angles for which waves do
propagate correspond to modes in a fiber. If more than one mode propagates through
a fiber, then the fiber is called multimode. In general, a larger core diameter or high
operating frequency allows a greater number of modes to propagate. The advantage
of multimode fiber is that, its core diameter is relatively large; as a result, injection of
light into the fiber with low coupling loss can be accomplished by using inexpensive,
large-area light sources, such as light-emitting diodes (LED's). The disadvantage of
multimode fiber is that it introduces the phenomenon of intermodal dispersion. In
multimode fiber, each mode propagates at a different velocity due to different angles
of incidence at the core-cladding boundary. This effect causes different rays of light
from the same source to arrive at the other end of the fiber at different times,
resulting in a pulse that is spread out in the time domain. Intermodal dispersion
23
increases with the distance of propagation, so that it limits the bit rate of the
transmitted signal and the distance that the signal can travel. Thus, in ROF networks
multimode fiber is not utilized as much as possible, instead, single-mode fiber is
widely used.
Single-mode fiber allows only one mode and usually has a core size of about
10 nm, while multimode fiber typically has a core size of 50.100 nm. It eliminates
intermodal dispersion and hence can support transmission over much longer
distances. However, it introduces the problem of concentrating enough power into a
very small core. LED's cannot couple enough light into a single-mode fiber to
facilitate long-distance communications. Such a high concentration of light energy
may be provided by a semiconductor laser, which can generate a narrow beam of
light.
3.2.1.3 Attenuation In Fiber
Attenuation in an optical fiber leads to a reduction of the signal power as the
signal propagates over some distance. When determining the maximum distance that
a signal can propagate for a given transmitter power and receiver sensitivity, one
must consider attenuation. Let P (L) be the power of the optical pulse at distance L
km from the transmitter and A be the attenuation constant of the fiber (in dB/km).
Attenuation is characterized by
…………………………………… (3.4)
Where P (0) is the optical power at the transmitter.
24
3.2.1.4 Dispersion In Fiber
Dispersion is the widening of a pulse duration as it travels through a fiber. As
a pulse widens, it can broaden enough to interfere with neighboring pulses (bits) on
the fiber, leading to intersymbol interference. Dispersion thus limits the bit spacing
and the maximum transmission rate on a fiber-optic channel. As described earlier,
one form of the dispersion is an intermodal dispersion. This is caused when multiple
modes of the same signal propagate at different velocities along the fiber. Intermodal
dispersion does not occur in a single-mode fiber.
Another form of dispersion is material or chromatic dispersion. In a
dispersive medium, the index of refraction is a function of the wavelength. Thus, if
the transmitted signal consists of more than one wavelength, certain wavelengths will
propagate faster than other wavelengths. Since no laser can create a signal consisting
of an exact single wavelength, chromatic dispersion will occur in most systems.
A third type of dispersion is waveguide dispersion. Waveguide dispersion is
caused as the propagation of different wavelengths depends on waveguide
characteristics such as the indices and shape of the fiber core and cladding. At 1300
nm, chromatic dispersion in a conventional single-mode fiber is nearly zero. Luckily,
this is also a low-attenuation window (although loss is higher than 1550 nm).
Through advanced techniques such as dispersion shifting, fibers with zero dispersion
at a wavelength between 1300.1700 nm can be manufactured.
3.2.1.5 Nonlinearities In Fiber
Nonlinear effects in fiber may potentially have a significant impact on the
performance of WDM optical communications systems. Nonlinearities in fiber may
lead to attenuation, distortion, and cross-channel interference. In a WDM system,
these effects place constraints on the spacing between adjacent wavelength channels,
limit the maximum power on any channel, and may also limit the maximum bit rate.
The details of the optical nonlinearities are very complex and beyond the scope of
25
the dissertation. It should be emphasized that they are the major limiting factors in
the available number of channels in a WDM system.
3.2.2 Optical Transmitters
3.2.2.1 How a laser works
The word “laser” is an acronym for light amplification by stimulated
emission of radiation. The key word is stimulated emission, which is what allows a
laser to produce intense high-powered beams of coherent light (light that contains
one or more distinct frequencies).
To understand stimulated emission, we must first acquaint ourselves with the energy
levels of atoms.
Atoms that are stable (in the ground state) have electrons in the lowest
possible energy levels. In each atom, there are a number of discrete levels of energy
that an electron can have, which are referred to as .states. To change the level of an
atom in the ground state, the atom must absorb energy. When an atom absorbs
energy, it becomes excited and moves to a higher energy level. At this point, the
atom is unstable and usually moves quickly back to the ground state by releasing a
.photon. a particle of light.
There are certain substances, however, whose states are quasi-stable, which
means that the substances are likely to stay in the excited state for longer periods of
time without constant excitation. By applying enough energy (in the form of either
an optical pump or an electrical current) to a substance with quasi-stable states for a
long enough period of time, population inversion occurs, which means that there are
more electrons in the excited state than in the ground state. This inversion allows the
substance to emit more light than it absorbs.
Fig. 3.5 shows a general representation of the structure of a laser. The laser
consists of two Mirrors that form a cavity (the space between the mirrors), a lasing
26
medium, which occupies the cavity, and an excitation device. The excitation device
applies current to the lasing medium, which is made of a quasi-stable substance. The
applied current excites electrons in the lasing medium, and when an electron in the
lasing medium drops back to the ground state, it emits a photon of light. The photon
will reflect off the mirrors at each end of the cavity and will pass through the
medium again.
Stimulated emission occurs when a photon passes very close to an excited
electron. The photon may cause the electron to release its energy and return to the
ground state. In the process of doing so, the electron releases another photon, which
will have the same direction and coherency (frequency) as the stimulating photon.
Photons for which the frequency is an integral fraction of the cavity length.
Figure 3.5: The general structure of a laser
Will coherently combine to build up light at the given frequency within the
cavity. Between normal and stimulated emission, the light at the selected frequency
builds in intensity until energy is being removed from the medium as fast as it is
being inserted. The mirrors feed the photons back and forth, so further stimulated
emission can occur and higher intensities of light can be produced. One of the
27
mirrors is partially transmitting, so that some photons will escape the cavity in the
form of a narrowly focused beam of light. By changing the length of the cavity, the
frequency of the emitted light can be adjusted.
The frequency of the photon emitted depends on its change in energy levels.
The frequency is determined by the equation
………………………………………… (3.5)
where f is the frequency of the photon, Ei is the initial (quasi-stable) state of the electron, Ef is the final (ground) state of the electron, and h is Planck's constant (= 6:626 * 10-34 J .s). 3.2.2.2 Semiconductor Diode Lasers
The most useful type of a laser for optical networks is the semiconductor
diode laser. The simplest implementation of a semiconductor laser is the bulk laser
diode, which is a p.n junction with mirrored edges perpendicular to the junction (see
Fig. 3.6).
Figure 3.6: structure of semiconductor laser diode.
28
3.2.2.3 Optical Modulation
To transmit data across an optical fiber, the information must first be
encoded, or modulated, onto the laser signal. Analog techniques include amplitude
modulation (AM), frequency modulation (FM), and phase modulation (PM). Digital
techniques include amplitude shift keying (ASK), frequency shift keying (FSK), and
phase shift keying (PSK). Of all these techniques, binary ASK currently is the
preferred method of digital modulation because of its simplicity. In binary ASK, also
known as on-off keying (OOK), the signal is switched between two power levels.
The lower power level represents a 0 bit, while the higher power level represents a 1
bit.
In systems employing OOK, modulation of the signal can be achieved by
simply turning the laser on and off (direct modulation). In general, however, this can
lead to chirp, or variations in the laser's amplitude and frequency, when the laser is
turned on. A preferred approach for high bit rates (10 Gb/s) is to have an external
modulator that modulates the light coming out of the laser. To this end, the
MachZehnder interferometer or electroabsorption modulation are widely utilized .
3.2.3 Optical Receivers
3.2.3.1 Photodetectors
In receivers employing direct detection, a photodetector converts the
incoming photonic stream into a stream of electrons. The electron stream is then
amplified and passed through a threshold device. Whether a bit is a logical zero or
one depends on whether the stream is above or below a certain threshold for a bit
duration. In other words, the decision is made based on whether or not light is
present during the bit duration. The basic detection devices for direct.detection
optical networks are the PN photodiode (a p-n junction) and the PIN photodiode (an
intrinsic material is placed between p- and n- type material).
29
In its simplest form, the photodiode is basically a reverse-biased p-n junction.
Through the photoelectric effect, light incident on the junction will create electron-
hole pairs in both the .n. and the .p. regions of the photodiode. The electrons released
in the .p. region will cross over to the .n. region, and the holes created in the .n.
region will cross over to the .p. region, thereby resulting in a current flow.
3.2.4 Optical Amplifers
Although an optical signal can propagate a long distance before it needs
amplification, both long-haul and local lightwave networks can benefit from optical
amplifers. All-optical amplification may differ from optoelectronic amplification in
that it may act only to boost the power of a signal, not to restore the shape or timing
of the signal. This type of amplification is known as 1R (regeneration), and provides
total data transparency (the amplification process is independent of the signal's
modulation format). 1R amplification is emerging as the choice for the transparent
all-optical networks of tomorrow. Today's digital networks [e.g., Synchronous
Optical Network (SONET) and Synchronous Digital Hierarchy (SDH)], however,
use the optical fiber only as a transmission medium, the optical signals are amplified
by first converting the information stream into an electronic data signal and then
retransmitting the signal optically. Such amplification is referred to as 3R
(regeneration, reshaping, and relocking). The reshaping of the signal reproduces the
original pulse shape of each bit, eliminating much of the noise. Reshaping applies
primarily to digitally modulated signals but in some cases it may also be applied to
analog signals. The relocking of the signal synchronizes the signal to its original bit
timing pattern and bit rate. Relocking applies only to digitally modulated signals.
Another approach to amplification is 2R (regeneration and reshaping), in which the
optical signal is converted to an electronic signal, which is then used to modulate a
laser directly. The 3R and 2R techniques provide less transparency than the 1R
technique, and in future optical networks, the aggregate bit rate of even just a few
channels might make 3R and 2R techniques less practical. Optical amplification uses
the principle of stimulated emission, similar to the approach used in a laser. The two
basic types of optical amplifers are semiconductor laser amplifers and rare-earth-
doped- fiber amplifers.
30
3.2.4.1 Doped-Fiber Amplifier
Optical doped-fiber amplifiers are lengths of fiber doped with an element
(rare earth) that can Amplify light. The most common doping element is erbium,
which provides gain for wavelengths of 1525.1560 nm. At the end of the length of
fiber, a laser transmits a strong signal at a lower wavelength (referred to as the pump
wavelength) back up the fiber. This pump signal excites the do pant atoms into a
higher energy level. This allows the data signal to stimulate the excited atoms to
release photons. Most erbium doped fiber amplifiers (EDFA's) are pumped by lasers
with a wavelength of either 980 or 1480 nm. A limitation to optical amplification is
the unequal gain spectrum of optical amplifiers. While an optical amplifier may
provide gain across a range of wavelengths, it will not necessarily amplify all
wavelengths equally. This characteristic . accompanied by the fact that optical
amplifiers amplify noise as well as signal and the fact that the active region of the
amplifier can spontaneously emit photons, which also cause noise . limits the
performance of optical amplifiers. Thus, a multiwavelength optical signal passing
through a series of amplifiers will eventually result in the power of the wavelengths'
being uneven.
3.3 Radio over Fiber Optical Links
3.3.1 Introduction to ROF Analog Optical Links
Unlike conventional optical networks where digital signal is mainly
transmitted, ROF is Fundamentally an analog transmission system because it
distributes the radio waveform, directly at the radio carrier frequency, from a CS to a
BS. Actually, the analog signal that is transmitted over the optical fiber can either be
RF signal, IF signal or baseband (BB) signal. For IF and BB transmission case,
additional hardware for upconverting it to RF band is required at the BS. At the
optical transmitter, the RF/IF/BB signal can be imposed on the optical carrier by
using direct or external modulation of the laser light. In an ideal case, the output
signal from the optical link will be a copy of the input signal. However, there are
31
some limitations because of non-linearity and frequency response limits in the laser
and modulation device as well as dispersion in the fiber. The transmission of analog
signals puts certain requirements on the linearity and dynamic range of the optical
link. These demands are different and more exact than requirements on digital
transmission systems [10].
3.3.2 Basic Radio Signal Generation and Transportation Methods
In this section, a brief overview of how to generate and transport radio signal
over an optical fiber in ROF networks is given. Virtually all of the optical links
transmitting microwave/mm-wave signals apply intensity modulation of light [13].
Essentially, three different methods exist for the transmission of microwave/mm-
wave signals over optical links with intensity modulation: (1) direct intensity
modulation, (2) External modulation, and (3) remote heterodyning. In direct intensity
modulation an Electrical parameter of the light source is modulated by the
information-bearing RF signal 1. In practical links, this is the current of the laser
diode, serving as the optical transmitter. The second method applies an unmodulated
light source and an external light intensity modulator. This technique is called
.external modulation.. In a third method, RF signals are optically generated via
remote heterodyning, that is, a method in which more than one optical signal is
generated by the light source, one of which is modulated by the information-bearing
signal and these are mixed or heterodyned by the photodetector or by an external
mixer to form the output RF signal. The external modulation and heterodyne
methods are discussed in more detail in subsection 3.3.4. In this subsection, we
consider only direct intensity modulation.
Direct intensity modulation is the simplest of the three solutions. So it is used
everywhere that it can be used. When it is combined with direct detection using PD,
it is frequently referred to as intensity-modulation direct-detection (IMDD) (Fig.
3.7). A direct-modulation link is so named because a semiconductor laser directly
converts a small-signal modulation (around a bias point set by a dc current) into a
corresponding small-signal modulation of the intensity of photons emitted (around
the average intensity at the bias point). Thus, a single device serves as both the
32
optical source and the RF/optical modulator (Fig. 3.7). One limiting phenomenon to
its use is the modulation bandwidth of the laser. Relatively simple lasers can be
modulated to frequencies of several gigahertz, say, 5.10 GHz. Although there are
reports of direct intensity modulation lasers operating at up to 40 GHz or even
higher, these diodes are rather expensive or nonexistent in commercial form. That is
why at higher frequencies, say, above 10 GHz, external modulation rather than direct
modulation is applied. In entering into the millimeter band a new adverse effect, such
as the nonconvenient transfer function of the transmission medium, is observed. It
turns out that the fiber dispersion and coherent mixing of the sidebands of modulated
light may cause transmission zeros, even in the case of rather moderate lengths of
fiber. For example, a standard fiber having a one km length has a transmission zero
at 60 GHz if 1.55 nm wavelength light is intensity modulated. Due to this
phenomenon, optical generation rather than transmission of the RF signal is
preferable. Because the number of BSs is high in ROF networks, simple and cost-
effective components must be utilized. Therefore, in the uplink of an ROF network
system, it is convenient to use direct intensity modulation with cheap lasers; this may
require down conversion of the uplink RF signal received at the BS. In the downlink
either lasers or external modulators can be used.
3.3.3 ROF Link Configurations
In this section we discuss a typical ROF link configuration, which is
classified based on the kinds of frequency bands (baseband (BB), IF, RF bands)
transmitted over an optical fiber link. Representative ROF link configurations are
schematically shown in Fig. 3.8 . Here, we assume that a BS has its own light source
for explanation purpose, however, as will be seen in section 3.3.4 BS can be
configured:
33
Figure 3.7: Intensity-modulation direct-detection (IMDD) analog optical link.
Without light source for uplink transmission. In each configuration of the
figure, BSs do not have any equipment for modulation and demodulation, only the
CS has such equipment.
In the downlink from the CS to the BSs, the information signal from a public
switched telephone network (PSTN), the Internet, or other CS is fed into the modem
in the CS. The signal that is either RF, IF or BB bands modulates optical signal from
LD. As described earlier, if the RF band is low, we can modulate the LD signal by
the signal of the RF band directly. If the RF band is high, such as the mm-wave band,
we sometimes need to use external optical modulators (EOMs), like
electroabsorption ones. The modulated optical signal is transmitted to the BSs via
optical fiber. At the BSs, the RF/IF/BB band signal is recovered to detect the
modulated optical signal by using a PD. The recovered signal, which needs to be
upconverted to RF band if IF or BB signal is transmitted, is transmitted to the MHs
via the antennas of the BSs. In the configuration shown in Fig. 3.8 (a), the modulated
signal is generated at the CS in an RF band and directly transmitted to the BSs by an
EOM, which is called .RF-over-Fiber. At each BS, the modulated signal is recovered
by detecting the modulated optical signal with a PD and directly transmitted to the
MHs. Signal distribution as RF-over-Fiber has the advantage of a simplified BS
design but is susceptible to fiber chromatic dispersion that severely limits the
transmission distance. In the configuration shown in Fig. 3.8 (b), the modulated
signal is generated at the CS in an IF band and transmitted to the BSs by an EOM,
34
which is called .IF-over-Fiber.. At each BS, the modulated signal is recovered by
detecting the modulated optical signal with a PD, upconverted to an RF band, and
transmitted to the MHs. In this scheme, the effect of fiber chromatic dispersion on
the distribution of IF signals is much reduced, although antenna BSs implemented for
ROF system incorporating IF-over- Fiber transport require additional electronic
hardware such as a mm-wave frequency LO for frequency up- and downconversion.
In the configuration (c) of the figure, the modulated signal is generated at the CS in
baseband and transmitted to the BSs by an EOM, which is referred to as .`BB-over-
Fiber. At each BS, the modulated signal is recovered by detecting the modulated
optical signal with a PD, upconverted to an RF band through an IF band or directly,
and transmitted to the MHs. In the baseband transmission, influence of the fiber
dispersion effect is negligible, but the BS configuration is the most complex. Since,
without a subcarrier frequency, it has no choice but to adopt time-division or code
division multiplexing. In the configuration shown in Fig. 3.8 (d), the modulated
signal is generated at the CS in a baseband or an IF band and transmitted to the BSs
by modulating a LD directly. At each BS, the modulated signal is recovered by
detecting the modulated optical signal with a PD, upconverted to an RF band, and
transmitted to the MHs. This is feasible for relatively low frequencies, say, less than
10 GHz. By reducing the frequency band used to generate the modulated signal at the
CS such as IF over- Fiber or BB-over-Fiber, the bandwidth required for optical
modulation can greatly be reduced. This is especially important when ROF at mm-
wave bands is combined with dense wavelength division multiplexing (DWDM) as
will be discussed in section 3.3.5. However, this increases the amount of equipment
at the BSs because an up converter for the downlink and a down converter for the
uplink are required. In the RF subcarrier transmission, the BS configuration can be
simplified only if a mm-wave optical external modulator and a high-frequency PD
are respectively applied to the electric-to-optic (E/O) and the optic-to-electric (O/E)
converters. For the uplink from an MH to the CS, the reverse process is performed.
In the configuration shown in Fig. 3.8 (a), the signals received at a BS are amplified
and directly transmitted to the CS by modulating an optical signal from a LD by
using an EOM. In the configuration (b) and (c), the signals received at a BS are
amplified and down converted to an IF or a baseband frequency and transmitted to
the CS by modulating an optical signal from a LD by using an EOM. In the
configuration (d), the signals received at a BS are amplified and down converted to
35
an IF or a baseband frequency and transmitted to the CS by directly modulating an
optical signal from a LD.
3.3.4 State-of-the-Art Millimeter-wave Generation and Transport
Technologies
Recently, a lot of research has been carried out to develop mm-wave
generation and transport techniques, which include the optical generation of low
phase noise wireless signals and their transport overcoming the chromatic dispersion
in fiber. Several state-of-the-art techniques that have been investigated so far are
described in this section, which are classified into the following four categories :
i. Optical heterodyning.
ii. External modulation.
iii. up- and down-conversion.
iv. Optical transceiver.
3.3.4.1 Optical Heterodyning
In optical heterodyning technique, two or more optical signals are
simultaneously transmitted and are heterodyned in the receiver. One or more of the
heterodyning products is the required RF signal. For example, two optical signals
with a wavelength separation of 0.5 nm at 1550 nm will generate a beat frequency of
around 60 GHz. Heterodyning can be realized by the PD itself or the optical signals
can be detected separately and then converted in an electrical (RF) mixer. In a
complete (duplex) system, the PD can be replaced by an electroabsorption
transceiver.
36
Figure 3.8: Representative ROF link configurations: (a) EOM, RF modulated signal.
(b) EOM, IF modulated signal,
37
Figure 3.9: Representative ROF link configurations. (c) EOM, baseband modulated signal. (d) Direct modulation.
Because phase noise is a key problem in digital microwave/mm-wave
transmission, care must be taken to produce a small phase noise only by the
heterodyned signals. This can be achieved if the two (or more) optical signals are
phase coherent; in turn, this can be realized if the different frequency optical signals
are somehow deduced from a common source or they are phase-locked to one master
source. Benefits of this approach are that (1) it overcomes chromatic dispersion
effect and (2) it offers a flexibility in frequency since frequencies from some
megahertz up to the terahertz-region is possible. However, it uses either a precisely
biased electrooptic modulator or sophisticated lasers . Fig 3.8 Fig. 3.9 shows a
typical design of optical heterodyning [20]. The master laser's intensity is modulated
by the unmodulated RF reference signal; several harmonics of the reference signal
and consequently several sidebands are generated. The reference laser is injection
locked by one of these and the signal laser by another one in such a way that the
38
difference of their frequencies corresponds to the mm-wave local oscillator
frequency. And, as seen, the optical field generated by the signal laser is
alsomodulated by the information-bearing IF signal.
Figure 3.10: Optical heterodyning.
3.3.4.2 External Modulation
Although direct intensity modulation is by far the simplest, due to the limited
modulation bandwidth of the laser this is not suitable for mm-wave bands. This is the
reason why at higher frequencies, say, above 10 GHz, external modulation rather
than direct modulation is applied. External modulation is done by a high speed
external modulator such as electro-absorption modulator (EAM). Its configuration is
simple, but it has some disadvantages such as fiber dispersion effect and high
insertion loss. Representative configurations are shown in Fig. 3.8 (a).(c), where
intensity modulation is employed. In conventional intensity modulation, the optical
carrier is modulated to generate an optical field with the carrier and double sidebands
(DSB). When the signal is sent over fiber, chromatic dispersion causes each spectral
component to experience different phase shifts depending on the fiber link distance,
modulation frequency, and the fiber dispersion parameter. If the relative phase
39
between these two components is 180, the components destructively interfere and the
mm-wave electrical signal disappears.
To reduce such dispersion effects, optical single-sideband (SSB) is widely
used . Specially designed EAM was developed and experimented at 60 GHz band
ROF system in , while a Mach- Zehnder modulator (MZM) and a fiber Bragg grating
filter were used in [24] and [25], respectively, to produce single-sideband optical
modulation.
3.3.4.3 Up- and Down-conversion
In this technique IF band signal is transported over optical fiber instead of RF
band signal. The transport of the IF-band optical signal is almost free from the fiber
dispersion effect, however, the electrical frequency conversion between the IF-band
and mm-wave requires frequency mixers and a mm-wave LO, resulting in the
additional cost to the BS. Another advantage of this technique is the fact that it
occupies small amount of bandwidth, which is especially beneficial when the system
is combined with DWDM. as is described in section 3.3.5. A representative
configuration is shown in Fig. 3.8 (b).
3.3.4.4 Optical Transceiver
The simplest BS structure can be implemented with an optical transceiver
such as electro-absorption transceiver (EAT). It serves both as an O/E converter for
the downlink and an E/O converter for the uplink at the same time. Two wavelengths
are transmitted over an optical fiber from the CS to BS. One of them for downlink
transmission is modulated by user data while the other for uplink transmission is
unmodulated (Fig. 3.10). The unmodulated wavelength is modulated by uplink data
at the BS and returns to the CS. That is, an EAT is used as the photodiode for the
data path and also as a modulator to provide a return path for the data, thereby
removing the need for a laser at the remote site. This device has been shown to be
40
capable of full duplex operation in several experiments at mm-wave bands [31] [32]
[33] [34]. A drawback is that it suffers from chromatic dispersion problem. Fig. 3.10
shows a ROF system based on EAT developed in [34]. Note that two wavelengths
are always needed for up- and downlink communication, and full-duplex operation is
possible.
3.3.4.5 Comparison of mm-wave Generation and Transport Techniques
Table 3.1 summarizes the advantages and the disadvantages of the four
techniques described above [11]. In addition, Table 3.2 shows some experimental
results reported in the literature. It suggests that at mm-wave bands very high bit rate
up to 155 Mbps is easily feasible. This implies that together with small cell size (Pico
cell) ROF technology can provide much higher capacity than conventional wireless
networks at microwave bands such as 2.4 or 5 GHz.
Figure 3.11: Electroabsorption transceiver (EAT).
41
Table 3.1: Comparison of Millimeter-wave Generation and Transport Techniques
Table 3.2: Millimeterwave-band RoF Experiments
CHAPTER 4
METHODOLOGY 4.1 Introduction
This chapter highlights the techniques and methods employed to study the
Design for radio access point (RAP) for RoF as well as to analyze the modeling
results obtained. Details of the methods will be given in the proceeding sections
4.2 Simulation Using Optisystem Software
OptiSystem software is a numerical simulation enables users to plan, test and
simulate almost every type of optical link in the physical layer across the broad
spectrum of optical networks. Algorithms are included for dispersion map design, bit
error rate calculation, system penalty estimations, and link budget calculations. Each
layout can have certain component parameters assigned to be in sweep mode. The
number of sweep iterations to be performed on the selected parameters could be
defined. The value of the parameter changes through each sweep iterations; which
produces a series of different calculation results, based on the parameter values.
These processing parameters effect on the results are channel pacing, input power,
effective area and dispersion of the fiber. Figure 4.1 describe the flow charge which
used it to achieved the project objective in details.
43
Figure 4.1: The flow chart of the methodology of the project
ROF RAP
STUDY THE TYPES RADIO OVER FIBER
LITERATURE REVIEW OF RADIO OVER FIBER
USING (EAM) IN THE RADIO ACCESS
POINT (RAP)
DESIGN THE RADIO OVER FIBER USING OPTISYSTEM
SIMULATION THE RADIO ACCESS POINT
EVALUATE THE PERFORMANCE
GET THE DESIRED RESULT OUTPUT
END
44
4.3 The Simulation Model
There are two technologies for modulation, direct or without external
modulation as shown in Figure 4.2 which the RF signal directly varies the bias of a
semiconductor laser diode
Figure 4.2: Direct modulation
The other technology is the external modulators are typically either integrated
Mach-Zehnder interferometers or electroabsorption modulators as shown in Figure
4.3 which the constant wave (CW) laser (always on bright), and the light is
modulated by an external lithium-niobate electro-optic modulator. External
modulation is currently preferred over any other form of modulation because it has
best performance, in spite of high cost.
45
Figure 4.3: externally modulated
Using Optisystem software, two types of simulation models have been
developed to design radio access point. The two models are with external modulated
signal and without external modulated signal as shown in the Figure 4.4 and 4.5,
respectively. The frequency of the phase modulator drive signal was kept at 2.4 GHz.
The phase modulator has been used to sweep the optical frequency, it was necessary
to first integrate the drive signal
Figure 4.4: Simulation model with external modulated signal.
46
Figure 4.5: Simulation model without external modulated signal
4.4 Simulation of the (RAU):
Each component in both simulation models, shown in Figures 4.4 and 4.5,
has its own role, to play in the process. The Pseudo Random Bit Sequence Generator
is a device or algorithm, which outputs a sequence of statistically independent and
unbiased binary digits. NRZ Pulse Generator (non-return-to-zero) refers to a form of
digital data transmission in which the binary low and high states, represented by
numerals 0 and 1, are transmitted by specific and constant DC (direct-current)
voltages.
In positive-logic NRZ, the low state is represented by the more negative or
less positive voltage, and the high state is represented by the less negative or more
positive voltage. In negative-logic NRZ, the low state is represented by the more
positive or less negative voltage, and the high state is represented by the less positive
or more negative voltage.
The continues wave (CW) Generator is a generator of continuous-wave
millimeter-wave optical signals. The spectral line width of the generated millimeter
wave signals is 2 kHz. The power of the measured cw millimeter-wave signals is
almost in proportion to the power multiplication of the two input optical signals. The
Mach-Zehnder Modulator is a modulator, which has two inputs, one for the laser
47
diode and the other for the data from the channels. The WDM Multiplexer is a
method of transmitting data from different sources over the same fiber optic link at
the same time whereby each data channel is carried on its own unique wavelength.
The Optical Fiber is a component, used in the simulation is a single mode fiber
(SMF-28), where the dispersive and nonlinear effects are taken into account by a
direct numerical integration of the modified nonlinear Schrödinger (NLS) equation.
Besides the above components there are three types of components, which used for
visualizing purposes
i. RF specturm Visualizer
ii. Oscilloscope Spectrum Analysis
CHAPTER5
SIMULATION RESULT AND DISCUSSION
5.1 Introduction
This Chapter presents the simulation results .The RoF system designed in this
was simulated in optisystem software. The basic model used to simulate the RoF
System is given in Figure 5.1. This is the same system given in Figure 3.10, with the
fibre link, the RF amplifier, and the transmitting antennas. The fibre link was not
included because there was no in-built multi-mode fibre model in optisystem, and no
other suitable alternative model capable of processing electrical fields of an optical
signal could be found.
Figure 5.1: The Basic model used to simulate the ROF system.
49
5.2 Optical Transmitters
The role of the optical transmitter is to: convert the electrical signal into
optical form, and launch the resulting optical signal into the optical fiber. The optical
transmitter consists of the following components ,optical source ,electrical pulse
generator Optical modulator (see Figure 5.3).
The launched power is an important design parameter, as indicates how much
fiber loss can be tolerated. It is often expressed in units of dBm with 1 mW as the
reference level as show in Figure 5.3.
Figure 5.2 Transmitter components
5.3 Parameter Values of Components
The parameter values of the model used in simulation studies were
represented in Table 5.1, 5.2 as information that was modulated by using PSK after
that amplified the signal table 5.3 followed by filter in Table 5.4:
50
Table 5.1: Pseudo Random Bit sequence generator.
Table 5.2 Electrical PSK modulator
51
Table 5.3: transimpedance amplifier
5.4 External mach–zehnder modulator (MZM) with carrier waves (CW)
In Table 5.4 illustrate carrier wave (CW) modulated with PSK signal by using
(MZM) which has parameter in Table 5.5 the optical intensity-modulated signal
from a laser diode is subsequently intensity modulated by an external Mach–Zehnder
modulator (MZM) which is biased at its inflexion point of the modulation
characteristic and driven by a sinusoidal signal at half the microwave frequency.
Thus, at the MZM’s output port, a two-tone optical signal emerges, with a tone
spacing equal to the microwave frequency. After heterodyning in a photodiode, the
desired amplitude-modulated microwave signal is generated. The transmitter may
also use multiple laser diodes, and thus a multiwavelength radio-over-fiber.
52
Table 5.4: CW laser properties.
5.5 Optical Modulation Converter And Method For Converting The
Modulation Format Of An Optical Signal
This invention relates to an optical modulation converter and method for
converting the modulation format of an optical signal. The invention also relates to a
receiver employing said modulation converter and method for receiving and
detecting a modulated optical signal.
In present optical transmission systems, communications traffic is conveyed
by optical carriers whose intensity is modulated by the communications traffic that is
the optical carrier is Amplitude Modulated (AM). Generally the communications
traffic used to modulate the optical carrier will have a Non Return to Zero (NRZ)
format though sometimes it can have a Return to Zero (RZ) format.
53
Intensity-modulation (IM) is preferred mainly due to the simplicity of the
corresponding optical receiver/detector that is based on a photodetector, for example
a photodiode, which operates as a simple amplitude threshold detector. For particular
applications, in general for the soon coming 40Gbit/s optical communication
systems, it has been proposed to use other modulation formats which have greater
immunity against non-linear propagation effects and also for greater polarization
mode dispersion (PMD) and chromatic dispersion (CD) tolerance. These
characteristics can open the road to a new design of optical transmission systems for
example with higher transmission powers and longer sections free of repeaters.
Although these alternative modulation formats are typically taken from
specific works in the theory of communications there are often difficulties in
applying them directly into real optical communications
Figure 5.3: laser intensity carrier wave
54
Table 5.5: Mach-Zehnder Modulator
55
Figure 5.4 Simulation diagram for radio over fiber using (EAM)
From above diagram simulation Figure 5.4 the output of the pseudo-random
bit sequence which presented the information or data and after modulated signal with
PSK the output is shown in Figure 5.5.
56
(a)
(b)
Figure 5.5: Output of the RF spectrum analyzer for (a) Pseudo-Random Bit
Sequence (b) Electrical PSK Modulator
57
Figure 5.6: Output of the optical fiber and MZM modulator
Figure 5.7 : Output of the signal (pseudo random bit sequence)
58
Figure 5.7 : Output of signal using PSK modulation
( (a)
59
(a) optical time domain visualize (b) RF spectrum analyzer
(c ) RF spectrum analyzer (d) spectrum analyzer
60
Figure 5.8: Output of EAM for different specturms
61
62
(a)
(b) Figure 5.9 : Bit error analyzer of simulation diagram (a) Q factor (b) Min BER
63
(c)
(d)
Figure 5.9: Bit error analyzer of the simulation diagram(c) threshold (d) height
64
65
CHAPTER 6
CONCLUSIONS AND RECOMMENDATIONS
6.1 Conclusion
As discussed earlier, the remote antenna units (RAU) in these conventional
radios over fiber systems contain a laser, photodiode, circulator, amplifiers, control
circuit, and power supplies .Although these can be housed in quite small enclosures ,
they are reasonably complex. A recently proposed new approach replace the laser ,
photodiode, and circulator with single photo electronic device,an electroabsorption
modulator that act as transceiver. This device consist of a semiconductor optical
waveguide inside pseudo noise (PN) junction,where wave guide core is
electroabsorbsorptive, that is absorption of light in the waveguide can be controlled
by a dc bias voltage .
A Radio-over-Fibre system capable of generating modulated wireless LAN
microwave carriers has been simulated and analyzed. The design objective in general
is to exploit the low installation and maintenance costs associated with using multi-
mode fibre in general, and polymer optical fibre in particular, in in-home and access
network environments. The limitation in multi-mode fibre bandwidth caused by
modal dispersion is overcome by employing a novel optical frequency multiplication
technique.
62
The radio access unit (RAU) can be made even simpler if the radio coverage
is restricted to small open space. In this case no amplifier is required and the
electroabsorption modulator can be operated without a bias voltage so that the RAU
consist only of the electroabsorption modulator and antenna, amplifier, control
circuit, and power supplies can be removed from the radio access point (RAP), which
take s simplification the limited. Because we are relying solely on the RF signal
power generated by electorabsorption modulator from the downstream light, the
range of the radio link is confined to around 10-100 depending on available optical
power, propagation environment, antenna type, and radio system. The designed
radio-over-fibre is able to transmit and up-convert radio signals having both linear
and constant envelope data modulations such as ASK, BPSK, as well as QAM.
This chapter has looked at the use of ROF technologies for cellular radio
communications system .it has described the generic advantages and disadvantages
of ROF and has discussed characteristic and requirement of cellular radio .we have
seen that ROF technology can result in substantial cost savings for in building
coverage compared to a conventional distributed radio architecture or to distributed
antenna systems based on coaxial cable. The performance of ROF systems for next
generation cellular networks has been analyzed and the limits evaluated. The main
conclusion from this project are summarized
• Simplification of the RAU leads to reduced installation and maintenance cost
compared to conventional distributed radio.
• Centralization of the system complexity lead to significant efficiency savings
through resource sharing. For example savings 50% have been calculated for
GSM based GSM based on a four –cell installation and 1% call blocking
probability.
• Fiber cables are easier and cheaper to install than coaxial cable.
6.2 Recommendations for Future Work
It is recommended that the radio-over-fibre system be validated in
experiments EAM using in optical heterodyned , in the optical heterodyned link ,the
current problem is the high noise due to the laser’s diode phase and RIN noise and
63
the photo detector’s shot noise . The RIN noise could be cancelled out by balanced
detection ,but the phase noise and the photodetector’s shot noise would remain the
development of low RIN and low phase noise lasers is very important issue for
optical heterodyned link.
64
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