i

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

iii

To my beloved late mother, may her soul rest in Paradise.

To my beloved father and brothers and sisters.

iv

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.

vi

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.

v

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.

xv

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

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

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

vii

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

xiv

1 INTRODUCTION 1

1.1 Introduction

1.2 Objective of Project

1.3 Scope of Project

1.4 Problem Statement

1.5 Thesis outlines

1

2

2

3

3

viii

2 LITERATURE REVIEW

RADIO OVER FIBER(ROF)

5

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

5

5

7

8

8

9

10

10

11

11

12

12

12

13

13

13

14

14

15

15

16

3 PROJECT BACKGROUND RADIO ACCESS POINT

(RAP)

17

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

17

18

19

20

22

ix

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

23

24

24

25

25

27

28

28

28

29

30

30

30

31

32

35

35

38

39

39

40

4 METHODOLOGY 42

4.1 Introduction

4.2 Simulation using optisystem software

4.3 Simulation model

4.4 Simulation of the RAU

42

42

44

46

x

5

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

48

48

49

49

51

52

6

REFERENCES

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusion

6.2 Recommendation and future work

61

61

62

64

xi

LIST OF TABLES

TABLE

TITLES PAGE

3.1 Comparison of millimeter-wave generation & transport

techniques.

41

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

xii

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

36

3.9 Representative ROF link configurations .(c)EOM,IF baseband

Modulated signal (d) Direct modulation.

37

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

xiii

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

59

5.11 Bit error analyzer of the simulation diagram (c) threshold (d)

height

60

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

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

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

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

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

2

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

3

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.

4

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.

6

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

7

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.

8

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.

9

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

10

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.

11

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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