MODELING AND PERFORMANCE ANALYSIS OF

THE OFDM SCHEME FOR RADIO OVER FIBER SYSTEM

FERDIAN YUNAZAR

UNIVERSITI TEKNOLOGI MALAYSIA

iii

MODELING AND PERFORMANCE ANALYSIS OF

THE OFDM SCHEME FOR RADIO OVER FIBER SYSTEM

FERDIAN YUNAZAR

A project report submitted in partial fulfilment of the

requirements for the award of the degree of

Master of Engineering (Electrical – Electronics and Telecommunications)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

MAY 2009

v

To,

My Beloved Father and Mother for their unwavering love, sacrifices and

inspirations. my lovely Brothers and my lovely soul of heart the one and only Mora

Shinta

vi

ACKNOWLEDGMENT

In the name of Allah, Most Gracious, and Most Merciful, I would like to

thank to the many people who have made my master project possible. First and

foremost I would like to express my sincere appreciate to my supervisor, Dr. Sevia

Mahdaliza Idrus, for being a dedicated mentor as well as for her valuable and

constructive suggestions that made this project to run smoothly. Furthermore, I

would like to sincere appreciation with deeply for all my panels Prof. Dr. Abu Bakar,

Assoc Prof. Dr. Abu Sahmah, Dr. Razali Ngah and Dr. Haniff Ibrahim for their

suggestion, critics and guidance of my thesis.

I would never have been able to make accomplishment without support from

my beloved parents, my brothers, my lovely Mora Shinta and her family.

My special thanks to my brother Arief Marwanto for his support and

encouragement. My sincere appreciation extends to all my best friends; Umar

Kalmar Nizar, Muhammad Affan Jhoni, Emad Fathi Aon, Elmuhtasib Abbas, Alawi

Ali Al Qushaibi, Mohammad Humaid, Muhammad Jhar Al-Faqi and others who

have provided any assistance for me. Their words of encouragement never failed to

keep me going even through the hardest of times and it is here that I express my

sincere gratitude to them. From the deepest of my heart, thank you very much.

vii

ABSTRACT

Radio over fiber (ROF) is an hybrid system that having both a fiber optic link

and free-space radio path. In such RoF systems using, broadband microwave data

signals are modulated onto an optical carrier at a central station, and then transported

to remote sites or base station using optical fiber. The base-stations then transmit the

RF signals over small areas using microwave antennas. Such system is important in a

number of applications, including mobile and satellite communications, wireless

local area networks (WLANs), wireless local loop and mobile broadband service, etc.

Orthogonal Frequency Division Multiplexing (OFDM) technique distributes the data

over a large number of carriers that are spaced apart at precise frequencies with

overlapping bands. The use of FFT for modulation provides orthogonality to the sub-

carriers, which prevents the demodulators from seeing frequencies other than their

own. Hence by incorporating OFDM along with the optical fiber, the RoF system can

be used for both short distance as well as long-haul transmission at very high data

rate. This improves the system flexibility and provides a very large coverage area

without increasing the cost and complexity of the system very much. In this project

author investigates the feasibility of OFDM as modulation technique for a RoF based

on WLAN system in consistency with IEEE 802.11g. Result from Optisystem model

shows the performance of OFDM signal through the RoF networks. The system was

utilized to carry data rates 20Mbps, using carrier frequency 2.4 GHz and the

modulation type for OFDM is 16QAM 4 bit per symbol. Total power of the signal

was decreasing while the fiber length of the RoF networks was increased from 10 –

50 km.

viii

ABSTRAK

Radio over Fiber (RoF) adalah suatu sistem hibrida yang mempunyai dua

jalur transmisi, serat optik dan jalur bebas radio. Dalam penggunaan sistem RoF,

gelombang mikro pita lebar sinyal data dimodulasikan pada pembawa optikal di

pemancar utama dan kemudian dikirimkan ke tempat yang berjauhan atau ke

pangkalan pemancar lainnya dengan menggunakan serat optik. Pangkalan pemancar

kemudian mentransmisikan sinyal RF untuk area yang lebih kecil dengan

menggunakan antena gelombang mikro. Sistem yang seperti ini sangatlah penting

dalam banyak aplikasi, termasuk komunikasi bergerak dan satelit, WLANs, dan

layanan pita lebar bergerak lainnya. Teknik modulasi OFDM mendistribusikan data

melalui banyak pembawa yang dipisahkan dengan frekuensi yang akurat dengan pita

yang saling berdekatan. Penggunaan FFT pada modulasi memberikan orthogonality

untuk setiap pembawa, yang dapat menghindarkan demodulator dalam membaca

frekuensi yang bukan miliknya. Kerana itu menggabungkan OFDM dengan serat

optik, sistem RoF dapat digunakan untuk transmisi jarak dekat maupun jauh dengan

kelajuan data yang tinggi. Ini meningkatkan kemudahan suai dan menyediakan

cakupan wilayah yang lebih besar tanpa menyebabkan kenaikan dalam biaya dan

kesukaran pada sistem. Dalam projek ini penulis meniliti kemungkinan modulasi

OFDM untuk RoF berdasarkan sistem WLAN IEEE 802.11g. Hasil yang didapat dari

model Optisystem, memperlihatkan performa sinyal OFDM melalui jaringan RoF.

Sistem ini dibuat untuk dapat membawa data 20 Mbps, menggunakan frekuensi

pembawa 2.4 GHz dan tipe modulasi untuk OFDM ialah 16QAM 4 bit untuk setiap

simbol. Total power yang dihasilkan oleh sinyal OFDM tersebut menurun seiring

dengan ditambahkannya panjang pada serat optik dalam jaringan RoF dari 10 hingga

50 Km.

ix

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION v

ACKNOWLEDGEMENTS vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF TABLES xii

LIST OF FIGURES xiii

LIST OF ABBREVIATIONS xv

LIST OF SYMBOLS xvii

1 INTRODUCTION 1

1.1 Project Background 1

1.2 Problem Statement 4

1.3 Objective 5

1.4 Scope of Work 5

1.5 Methodology 6

1.6 Thesis Outline 7

2 LITERATURE REVIEW 9

2.1 Introduction 9

2.2 Radio over Fiber (RoF) 10

2.2.1 Overview 10

x

2.2.2 Benefits of RoF Technology 12

2.2.3 Architecture of RoF Networks 13

2.3 Optical Transmission Link 15

2.3.1 Optical Fiber 15

2.3.1.1 Step Index Fiber 16

2.3.1.2 Graded Index Fiber 19

2.3.2 Optical Source (Laser) 20

2.3.3 Optical Modulation 21

2.3.4 Electro-Optic Modulation System 21

2.3.5 Electro-Optic Mach Zehnder Modulator 22

2.3.6 Optical Receiver (Photodetectors) 24

2.3.7 Optical Amplifier 24

2.4 Applications of Radio over Fiber Technology 26

2.4.1 Wireless LANs 26

2.4.2 Cellular Networks 27

2.4.3 Satellite Communications 27

2.4.2 Mobile Broadband Services 28

2.5 Conclusion 29

3 OFDM FOR RoF COMMUNICATIONS 30

3.1. Introduction 30

3.2. Orthogonal Frequency Division Multiplexing

(OFDM) 31

3.3. General Principles 33

3.3.1 Multicarrier Transmission 33

3.3.2 Fast Fourier Transform 37

3.3.3 Guard Interval and Implementation 38

3.4. Coded OFDM 39

3.4.1 Coded OFDM Systems 39

3.4.2 Trellis Coded Modulation 41

3.4.3 Bit Interleaved Coded OFDM 43

3.5. Advantages of OFDM 46

3.6. Disadvantages of OFDM 46

3.7. OFDM System for Radio over Fiber 47

xi

3.8 Conclusion 48

4 THE OFDM ROF SYSTEM MODEL 49

4.1. Introduction 49

4.2. The OFDM System Model 50

4.3. The Transmitter Model 51

4.4. The Transmission Link Model 53

4.5. The Receiver Model 55

4.6. Conclusion 56

5 SIMULATION RESULT AND PERFORMANCE

ANALYSIS 57

5.1. Introduction 57

5.2. The Transmitter Simulation Results 58

5.3. The Transmission Link Simulation Results 60

5.4. The Receiver Simulation Results 62

5.5. The Eye Diagram 64

5.6. Analysis of The Total Power to Length of Fiber 67

5.7. Conclusion 69

6 CONCLUSIONS AND RECOMMENDATION 70

6.1 Conclusions 70

6.2 Future Recommendations 71

REFERENCES 72

xii

LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Typical Step-Index Fiber characteristic 18

3.1 Data rates and modulation schemes for the 802.11 a

W-LAN system. 41

4.1 Global Parameter Setup 52

4.2 Sub carrier Allocation 53

4.3 Component’s Values of transmission link 54

xiii

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 General RoF systems 1

1.2 Project Flow Chart 6

2.1 A 900MHz Radio over Fiber System 11

2.2 Transport schemes for Fiber/Wireless systems 13

2.3 IF to RF up-conversion at the base stations 14

2.4 Optical Transmission�Link 15

2.5 Step-Index Fibers.

(a) Refractive index profile.

(b) End view.

(c) Cross-sectional side view. 17

2.6 Graded-Index Fiber.

(a) Refractive index profile.

(b) End view.

(c) Cross-sectional side view 19

2.7 Basic configuration of Optical modulator 23

2.8 Schematic diagram of a simple Doped Fiber Amplifier 24

3.1 (a) OFDM Symbol with Three Orthogonal Sub-

carriers in one.

(b) OFDM Symbol with Spectra of three OFDM sub-

carriers 32

3.2 A block diagram of an OFDM transmitter 34

3.3 Block diagram for multicarrier transmission (1) 36

3.4 Block diagram for multicarrier transmission (2) 36

xiv

3.5 Block Diagram IFFT/FFT 37

3.6 Guard Interval 38

3.7 (a) An example of 8-PSK modulation.

(b) An example of a trellis diagram for a coded

modulation scheme 42

3.8 A block diagram of Bit-interleaved coded OFDM 44

3.9 (a) Constellations of 16-QAM with Gray mapping.

(b) Constellations of 16-QAM with set partitioning 45

3.10 OFDM model for Radio Over Fiber Network� 47

4.1 Block Diagram of OFDM – RoF network system 50

4.2 OFDM Transmitter Design 51

4.3 RoF System Model 54

4.4 OFDM Receiver Design 55

4.5 OFDM – RoF MoDem 56

5.1 OFDM signal from transmitter 58

5.2 OFDM signal after amplified 59

5.3 OFDM optically modulated by MZM 60

5.4 OFDM signal after through optical fiber 61

5.5 OFDM signal from PIN photodetector 62

5.6 OFDM signal after amplified 63

5.7 Basic information contained in Eye diagram 64

5.8 (a) Eye diagram of received OFDM signal for in phase

signal

(b) Eye diagram of received OFDM signal for in

quadrature signal 65

5.9 Total power signal vs Fiber length after photodetector 67

5.10 Total power signal vs Fiber length after electrical

amplifier 68

xv

LIST OF ABBREVIATIONS

XPM - Cross Phase Modulation

SPM - Simple-Phase Modulation

LD - Laser Diode

PD - Photo Detector

LED - Light Emitting Diode

APD - Avalanche Photodiode

SCM - Sub-carrier Multiplexing

WDM - Wavelength Division Multiplexing

SNR - Signal to Noise Ratio

CNR - Carrier to Noise Ratio

DWDM - Dense Wavelength Division Multiplexing

BW - Bandwidth

OSSB - Optical Single Side Band

ODSB - Optical Double Side Band

OTDM - Optical time Division Multiplexing

OCDM - Optical Code Division Multiplexing

EAM - Electro Absorption Modulator

SMF - Single Mode Fiber

MMF - Multi mode Fiber

GRIN - Graded Index

RF - Radio Frequency

MZM - Mach-Zehnder Modulator

CSNRZ - Carrier Suppressed Non return to Zero

EDFA - Erbium Doped Fiber Amplifier

xvi

RZ - Return to Zero

NRZ - Non return to Zero

PMD - Polarization Mode Dispersion

PRBS - Pseudo Random Bit Sequence

RoF - Radio over Fiber

CW - Continuous Wave

IMD - Inter modulation distortion

OFDM - Orthogonal Frequency Division Multiplexing

ASK - Amplitude Shift Keying

FSK - Frequency Shift Keying

PSK - Pahse Shift Keying

QAM - Quadrature Amplitude Modulation

BPSK - Binary Phase Shift Keying

QPSK - Quadrature Phase Shift Keying

OQPSK - Offset Quadrature Phase Shift Keying

OOK - On Off Keying

BER - Bit Error rate

MPSK - Minimum Phase Shift Keying

CATV - Cable television

TDM - Time division multiplexing

OCDMA - Optical Code Division Multiple Access

FTTx - Fiber To The Home, curb, etc.

MH - Mobile Home

SONET - Synchronous Optical Network

DFB - Distributed Feedback Laser

SDH - Synchronous Digital Hierarchy

MAN - Metropolitan Area Network

LAN - Local Area Network

BS - Base Station

MS - Mobile Station

CS - Central Station

xvii

LIST OF SYMBOLS

� - Wavelength

h - Blank’s Constant

C - Velocity of Light

Eg - Energy Gap

fc - Cut-off frequency

� - Quantum Efficiency

ℜ - Responsivity

ip - Photocurrent

Po - Optical Power

q - Electron Charge

T - Temperature

K - Boltzmann Constant

B - Bandwidth

R - Nominally matched Resistance

Vth - The rms value for the thermal noise voltage

2PΔ - Mean square amplitude of the noise fluctuations

� - Mie Scattering Coefficient

P(Z) - The laser Power at Z

P(I) - Output optical power

I - The current injected to the active region

V - Volume of the active region

Q - Photon Density

�

Figure 1.1 General RoF systems

CHAPTER 1

INTRODUCTION

1.1 Project Background

�

Radio-over-fiber (RoF) is a technology used to distribute RF signals over

analog optical links. In such RoF systems, broadband microwave data signals are

modulated onto an optical carrier at a central station (CS), and then transported to

remote sites or base station (BS) using optical fiber. The base-stations then transmit

the RF signals over small areas using microwave antennas as shown below in Figure

1.1[1] [2].

2

Such technology is expected to play an important role in present and future

wireless networks since it provides an end user with a truly broadband access to the

network while guaranteeing the increasing requirement for mobility.

ROF is very attractive technique for wireless access network infrastructure,

because it can transmit microwaves and millimeter-waves through optical fibers for a

long distance. Moreover, 5 GHz ROF link using a direct modulation scheme has

been developed to support some important future wireless systems such as wireless

local area networks (WLAN) intelligent transport systems (ITS), and the 4th

generation cellular systems.

In particular, ROF is promising technique for WLAN infrastructures because

ROF technique can manage WLAN modems at a base station (BS) and can solve

serious interference problem between wireless signals caused by proliferated WLAN

access points (APs).

Orthogonal Frequency Division Multiplexing (OFDM) technique distributes

the data over a large number of carriers that are spaced apart at precise frequencies

with overlapping bands. The use of FFT for modulation provides orthogonality to the

sub-carriers, which prevents the demodulators from seeing frequencies other than

their own. Hence OFDM has the best spectral efficiency, resiliency to RF

interference, and lower multi-path distortion. LAN, which uses OFDM is the current

trend for indoor wideband communication with a drawback of limited coverage area

of few meters, but supports high data rate.

Hence by incorporating OFDM along with the optical fiber, the RoF system

can be used for both short distance as well as long-haul transmission at very high

data rate. This improves the system flexibility and provides a very large coverage

area without increasing the cost and complexity of the system very much. Recently,

3

it has been proved that OFDM is better compared to the conventional single carrier

modulation for long haul optical transmission.

There are so many previous research papers and works that have been done

by several people recently in terms of using OFDM modulation technique or

multicarrier transmission for sending and receiving data through Radio over Fiber

Networks. For example A. Marwanto and S. M. Idrus in their paper which titled

SCM/WDM Radio over Fiber for Broadband Communication, the author shown the

study about multiplexing carrier used for Radio Over Fiber Network Technology,

and their result that the outcomes of bandwidth was increased to 60 GHz by applying

of 16 Channel of SCM combined with WDM in optical fiber link. Another paper

from Guruprakash Singh and Arokiaswami Alphones which titled OFDM

Modulation Study for a Radio-over-Fiber System for Wireless LAN (IEEE 802.11,

they has made an analysis of theoretical performance for OFDM using different

technique of digital modulation such as PSK, BPSK, QPSK and QAM. Their result

shows that QAM provide better spectral efficiency and lower detection error

probability.

Meanwhile Dhivagar. B, Ganesh Madhan.M, and Xavier Fernando in

their paper works which titled Analysis of OFDM signal Through Optical Fiber for

Radio over Fiber Transmission has investigate the impact of fiber dispersion on the

transmission performance of OFDM based IEEE 802.11.g, WLAN signal for

different distances. The results show that using different fiber length it is clear that

significant coverage extension is possible with very minimum penalty. And another

work from I. A. Kostko, M. E. Mousa Pasandi, M. M. Sisto, S. Larochelle, L. A.

Rusch, and D. V. Plant which titled A radio-over-fiber link for OFDM transmission

without RF amplification. Their work is to increase OFDM signal transmission

quality over the optically amplified link by joint optimization of the PD impedance

matching and MZM bias. Their result show that the amplification can be moved from

electrical to optical, which allows having an optical amplifier at the central office and

simplifying the base station.

4

Based on those previous papers and studies, there are so many researches and

works in the field of using multicarrier transmission technique especially OFDM to

transmitted and received data through optical link in Radio over Fiber Networks.

Meanwhile in this project author working on modeling and analyze the

performance of the OFDM scheme for Radio over Fiber system to utilized

applications based on WLAN IEEE 802.11 b/g standard (2.4 GHz). The project

model has simulated by using commercial software, Optisystem 7.0.

1.2 Problem Statement

A key initiative in the deployment of new wireless services is to cost

effectively extends and enhance the network's radio coverage. In the case of further

wireless communication system significant effort is done to reduce the multipath

fading and small base station matched to demands made by the bigger number of

mobile cells and high frequency applications. To meet these requirements one of best

solution is the combination of Orthogonal Frequency Division Multiplexing (OFDM)

digital modulation and radio over fiber (ROF) technology.

The integration of both techniques emerged the possibility of cost-effective

and high data rate ubiquitous wireless networks. OFDM is seen as the modulation

technique for future broadband wireless communications because it provides

increased robustness against frequency selective fading and narrowband interference,

and is efficient in dealing with multipath delay spread [l]. While RoF is the next

generation communications system that can utilize the high capacity of optical

networks along with the mobility of wireless networks.

5

1.3 Objectives

The first objective of this project is to model and simulate the OFDM scheme

for RoF using commercial software, Optisystem 7.0 from Optiwave. The second

objective is to investigate and analyze the feasibility performance of OFDM for RoF

in term of Eye diagram and the effect of total length with the fiber dispersion.

1.4 Scope of Work

The scopes of this project are:

1. Understanding the basic principle of OFDM modulation technique and RoF

through literature study.

2. Modeling and simulation of OFDM signals through RoF network using

commercial software, Optisystem 7.0 from Optiwave.

3. The OFDM system is modeled for application based on WLAN IEEE 802.11

b/g standard (2.4 GHz).

6

Figure 1.2 Project Flow Chart

1.5 Methodology

The methodology of this project is described in the following flow chart

First the methodologies start with literature study and review on the RoF

system and OFDM modulation technique. Then understood the modeling design of

OFDM modulation technique for RoF system. After that followed by theoritical

analysis of OFDM and RoF system. Main thing to analyze here is the basic concept

of RoF system, OFDM modulation technique and incorporating OFDM along with

RoF. Sooner after fully understand about the theoretical part, then starting to design

and model the system and analyze characterization of the system modeled. The

OFDM-RoF system was modeled and simulated using commercial software, which is

Optisystem 7.0 from Optiwave.

7

Next is analyzing the result and system performance which is obtained from

the simulation model. While analyzing the result, the system is being optimize to get

a better performance and best simulation result. This would be done with referring to

the theoretical and numerical analysis part again to double check whether some part

is missing or some problem were occurring when understanding the theory.

Finally after all the simulation had been done and all the result derived,

compare the result with previous work and theoretical analysis. Then finished writing

also writes some publications.

1.6 Thesis Outline

These projects comprise of six chapters and organize as follows:

Chapter 1 is introductory part of this project which consists of the project

background, problem statement, and objective, scope of work, followed by

methodology and thesis outline.

Chapter 2 is literature review of this project which is explaining some basic

theory of Radio over Fiber, with the benefits and architecture of RoF. Also explain

the parts of optical transmission link and the applications of RoF technology.

Chapter 3 presents the Orthogonal Frequency Division Multiplexing (OFDM).

Consist of introduction, general principles and coded OFDM and also discusses the

advantages and disadvantages of OFDM.

8

Chapter 4 are discuss the methodology of the project were the OFDM - RoF

technique and Optisytem software are using to model and implement the system. In

this chapter proposed OFDM with 16 QAM modulation systems for radio over fiber

networks are presented.

Chapter 5 discusses the result and analysis of the OFDM-RoF system

simulation. The eye diagram and performance of the system are explained in this

chapter.

Chapter 6 gives the conclusion for the whole project and also provides the

recommendation future works for development and modifications of the system

presented in this project.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter is consist of three parts, first part covers about general

explanation of RoF systems, the benefits and RoF networks architectures. Then

second part briefly describes about the basic optical fiber transmission link, where

digital signal transmission is assumed as current optical networks. The third part

deals with applications of Radio over Fiber Technology, it tells about the today’s

applications of RoF for communications.

10

2.2 Radio over Fiber (RoF)

2.2.1 Overview

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.

ROF refers to a fiber optic link where the optical signal is modulated at radio

frequencies (RF) and transmitted via the optical fiber to the receiving end. At the

receiving end, the RF signal is demodulated and transmitted to the corresponding

wireless user. By implementing the above technique, ROF technology is able to shift

the system complexity away from the remote base station antenna and toward a

centralized radio signal processing installation. In a ROF link, laser light is

modulated by a radio signal and transported over an optical fiber medium. The laser

modulation is analog since the radio-frequency carrier signal is an analog signal. The

modulation may occur at the radio signal frequency or at some intermediate

frequency if frequency conversion is utilized. The basic configuration of an analog

fiber optic link consists of a hi-directional interface containing the analog laser

transmitter and photodiode receiver located at a base station or remote antenna unit

paired with an analog laser transmitter and photodiode receiver located at a radio

processing unit One or more optical fibers connect the remote antenna unit to the

central processing location.

11

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) [6] [7].

As shown in Figure 2.1, ROF systems were primarily used to transport microwave

signals and to achieve mobility functions in the CS.

For a multifunctional ROF system, the required electrical signal at the input

of the ROF system depends on the ROF technology and 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 fiber 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 fiber 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

Figure 2.1 A 900MHz Radio over Fiber System

12

location, thereby making it possible to use simpler remote sites. Furthermore, ROF

technology enables the centralizing of mobility functions such as macro- diversity for

seamless hangover. The benefits of having simple remote sites are many. They are

discussed in the following section.

2.2.2 Benefits of RoF technology

RoF transmission offers many advantages in wireless systems, some of that

are [9]:

• Low RF power remote antenna units (RAUs)

• Enabling of mobile broadband radio access close to the user in an

economically acceptable way.

• Capacity enhancement by means of improved trunking efficiency.

• Huge bandwidth that enables multiplexing several radio channels; each radio

channel may belong to a different system such as wireless LAN and cellular

radio.

• Inherent immunity to electromagnetic interference.

• Allowing for transparent operation because the RF to optical modulation is

typically independent of the baseband to RF modulation.

RoF also allows for easy integration and upgrades since the electrical to

optical conversion is independent of baseband to RF modulation format.

Conventional transmission mediums such as copper coaxial may not be completely

replaced by optical fiber, but in applications where factors such as RF power loss,

future system upgrades and transparency are considered, fiber is regarded as the most

practical and efficient medium. Even though the prospects of RoF are substantial,

there is still plenty of research to be carried out in this area before widespread

deployment can be considered [8].

13

2.2.3 Architectures of RoF Networks

Several RoF concepts have been developed. The main difference between

these concepts resides in techniques used for carrier generation and data distribution

over optical fiber. These concepts repose on three proposed architectures for RoF

links. They are implemented according to their frequency bands as shown in Figure

2.2, which are: (i) Radio Frequency (RF) band; (ii) Intermediate Frequency (IF) band,

and (iii) Base Band (BB).

In general, optical distribution techniques of electrical data signals can be

classified according to two approaches [10]:

1. The first approach consists in transporting the signal in either the BB or

the IF band. It leads to significant complexity reduction at the optical-link level. The

BB transport method consists of a direct modulation of a laser diode by the BB data

signal. Thereafter, the resulting optical signal is injected into an optical fiber and

transported from the CS to the BS (downlink). Symmetrically, in the up-link, the

transmitted BB electrical signals undergo the same process. For the BB transport

method, only low band-width optoelectronic components are necessary. However,

this optical-link simplification is accompanied by an increased complexity for the BS

Figure 2.2 Transport schemes for Fiber/Wireless systems

14

where BB to RF up-conversion and RF to BB down-conversion are required.

Moreover, this strengthens the complexity of the problem since the BS RF equipment

must be replicated by the number of hertzian network deposits. Furthermore, if new

channels need to be added the complexity of the BS makes the system

reconfiguration difficult. In the case of the IF transport option, the optical data

transmission is performed after optically modulating the electrical signal as shown in

Figure 2.3 below. The later should undergo a prior modulation by a common

generated IF carrier. At every BS, an IF-to-RF up-conversion is necessary before the

radio transmission [11].

2. The second approach consists of a maximum simplification of the BS

configuration, where the RF signal generation is centralized at the CS. Consequently,

the BS architecture is simplified. This centralized architecture of RoF systems allows

avoiding the problem of RF equipment duplications when several BSs are

interconnected with the CS. This reduces the network cost and increases its

flexibility. In this architecture, the BS as well as the CS should contain an external

optical modulator. This component is required for operating at the millimeter-wave

band frequencies (30-70 GHz). Photodetectors should also present high

performances in this frequency band. Thus, the BS role is reduced to three basic

functions only: RF amplification, multiplexing, demultiplexing, as well as downlink

Optical-to-RF and up-link RF-to-Optical conversions.

Figure 2.3. IF to RF up-conversion at the base stations

15

2.3 Optical Transmission Link

A general optical transmission links, shown in Figure 2.4 below 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.

2.3.1 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 reflections, light can propagate following the length of

a fiber with little loss.

Optical fiber has two low-attenuation regions [12]. Centered at approximately

1300 nm is a range of 200 nm in which attenuation is less than 0.50dB/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

Figure 2.4 Optical Transmission��� �

16

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[12]. 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 deplorable at

short notice. Also, fiber transmission is immune to electromagnetic interference and

does not cause interference. Basically there are two types of optical fiber, first so

called as Step-Index Fiber and second Graded-Index Fiber [1].

2.3.1.1 Step-Index Fiber

The main structure of step-index fiber is consist of central core and

surrounded by a cladding. The main characteristic of the this type of optical fiber are

the core of the fiber must have larger refractive index, n1 and lower refractive index,

n2 for the cladding. There for the critical angle, θc given the following equation:

Sin θc = n2 / n1 (2.1)

And the functional refractive index change, � which is the important parameter for

the fiber given below:

1

21 )(n

nn −=Δ (2.2)

17

Figure 2.5 Step-Index Fibers. (a) Refractive index profile. (b) End view

(c) Cross-sectional side view [1].

��� ��

���

θ �

��

���

���

���

���

CORE CLADDING

���

���

Note that this parameter, � will give a positive value because value of n1 must

be larger than n2 in order for a critical angle to exist.

Basically, step-index fibers have three typical forms that perform by first an

all glass fiber (a glass core and cladding), second a plastic-cladded silica fiber (PSC –

a silica glass core, cladded with plastic) and third an all plastic fiber (a plastic core

and cladding).

As with the slab waveguide, modal distortion and numerical aperture increase

with the refractive index different, n1 - n2. Because of this, the intermodal pulse spread

and NA are small for the all-glass fiber, larger for the PCS fiber, and highest for the

all-plastic fiber. Fibers with a little pulse spread have large rate-length products. The

NA of this fiber is small, making it difficult to couple light into them efficiently.

18

Table 2.1 Typical Step-Index Fiber characteristic [1]

Construction n1 n2 NA �0 �

All glass 1.48 1.46 0.24 13.9o 0.0135

PCS 1.46 1.40 0.41 24.2o 0.041

All plastic 1.49 1.41 0.48 29o 0.054

Other than information jotted at Table 2.1, there another several parameters

needs to be considering before making a decision. Attenuation, dispersion, losses and

bandwidth are the most important parameters need to identify at first before laid

down the fiber.

All glass fibers have the lowest losses and the smallest intermodal pulse

spreading. Because of these properties, they are useful at moderate high information

rates or fairy long lengths. The low NA of the SI glass fiber results in large losses

during coupling from a light source. The low transmission loss partially compensates

for this problem. [1]

Because PCS fibers have a higher losses and large pulse spread than al-glass

fibers, they are mostly suitable for shorter links. Their higher numerical apertures are

able to increase the efficiency of the fiber in term of coupling the light into the fiber.

However, his advantages suddenly vanished by the highly absorption of the fiber in a

long fiber. The more the large size of the core, the more efficient light coupled to the

fiber [1].

All plastic fibers are very limited by constrain of high propagation loss.

Therefore all plastic fibers are suitable for very short paths that usually around a few

tens of meters. The characteristic of the fibers which have a large core and large

numerical aperture let the coupling efficiency became higher make the fibers so

useful [1].

19

Figure 2.6 Graded-Index Fiber. (a) Refractive index profile.

(b) End view. (c) Cross-sectional side view [1]

��� � ����

��

���

���

��

��� ��

���

2.3.1.2 Graded-Index Fiber

The main structure of Graded-Index Fibers are consists of one core that has

refractive index decreases continuously with distance from the fiber axis.

Index variation for the Graded-Index fiber can be represented by the

following equation:

n(r) = n1 (1-2(r/a)� �) , r � a (2.3)

n(r) = n1 � (1-2�) , r � a (2.4)

20

Light rays travel through the fiber in the oscillatory fashion. The changing

refractive index continually causes the rays to be redirected toward the fiber axis, and

the particular variations in equation (2.3) and (2.4) cause them to be periodically

refocused. It can be easily illustrate this redirection by modeling the continuous

change in refractive index by a series of small step changes.

2.3.2 Optical Source (Laser)

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). There are three main types of laser that can be

considered for this type of application [13]:

1. VCSEL (Vertical Cavity Surface Emitting Laser); these lasers operate at

850 nm and are predominantly multi (transverse) mode. Cost is very low

because they are produced in high volume for data communications

applications.

2. FP (Fabry Perot laser); these lasers are edge emitters and predominantly

operate at longer wavelength (1310 or 1550 nm windows) with multiple

longitudinal modes. Cost is intermediate between VCSELS and DFBs.

3. DFB (distributed feedback laser); these lasers are edge-emitters and

predominantly operate at longer wavelength (1310 or 1550nm windows)

with a single longitudinal mode. Cost is higher than for VCSEL or FP.

21

2.3.3 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 (110 Gb/s) is to have an external

modulator that modulates the light coming out of the laser. To this end, the Mach-

zehnder interferometer or electroabsorption modulation is widely utilized [9].

2.3.4 Electro-optic Modulation System

There are two primary methods for modulating light in telecommunication

systems: direct and external modulation. Direct modulation refers to the modulation

of the source, i.e: turning a laser on and off to create pulses, while external

modulation uses a separate device to modulate the light. External modulation has

become the dominant method for high-speed long haul telecommunication systems.

22

External modulators can be implemented using a variety of materials and

architectures although typically electro-optic materials the permitivity of the material

is affected by the presence of electric fields. Many electro-optic materials are also

birefringent. Since the indexes of the refraction are refraction are related to the

permitivities of the materials, the phase velocities of the light inside the materials can

changed by the application of electric fields. These changes are exploited in various

ways to achieve optical modulation.

A material used in many commercial electro-optic modulators is lithium

niobate, LiNbO3. Lithium niobate is a crystalline material that is optically

transparent as well as birefringent. Currently most 10 GHz per channel long-haul

telecommunication systems are based on lithium niobate modulators. Several

companies are now offering 40 GHz modulators in anticipate the development of

higher bandwidth systems. As products are developed to meet future needs there is

an emphasis on higher operating speeds and greater levels of device integration.

Many electro-optic materials are currently being investigated for use in developing

optoelectronic devices. Several groups are working with nonlinear optical polymers

such as NLOPs to create electro-optic devices like optical modulators. NLOPs have

various characteristics that may facilitate that development of high speed, low

voltage electro-optic devices.

2.3.5 Electro-Optic Mach Zehnder Modulator

The electro-optic Mach-Zehnder modulator has become a ubiquitous device

for high speed optical communication systems. It is customarily used as an intensity

modulator for typical systems making use of the non return-to-zero (NRZ) or return-

to-zero (RZ) modulation formats, and has recently demonstrated its potential for

phase modulation in future systems making use of the differential phase-shift keying

(DPSK) format. Such modulators are made from an electro-optic crystal (typically

23

lithium-niobate, LiNbO3), who’s refractive index depends on the electric field, hence

voltage, which is applied to it. The electrical data can thus modulate the refractive

index of the crystal, hence the phase of the incoming light wave. Incorporating the

crystal into an interferometric structure (Mach-Zehnder interferometer) in turn

converts the phase modulation into intensity modulation [55].

Figure 2.7 Basic configuration of Optical modulator

Although the principle of such a modulator is fairly simple, its operation can

present many degrees of freedom and resulting trade-offs. The purpose of this project

is therefore to explore the operation modes of electro-optic Mach-Zehnder

modulators and their consequences on the quality of the modulated optical signal.

One particular task will be to establish relations between the extinction ratio (defined

as the ratio of the power transmitted into a binary `1´ and `0´) of the modulated

optical signal and its frequency chirping, depending on the chirp generation

mechanism (optical or electrical imbalance of the Mach-Zehnder modulator) [60].

������������������

������������

��������������������

���������� ���������������

24

2.3.6 Optical Receivers (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 a logical zero or one is depends on

whether the stream is above or below a certain threshold for 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 Pm photodiode (an intrinsic material is placed

between p- and n- type material). 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" in a region, thereby

resulting in a current flow [14].

2.3.7 Optical Amplifier

Figure 2.8 Schematic diagram of a simple Doped Fiber Amplifier [54]

Doped fiber amplifiers (DFAs) are optical amplifiers which use a doped

optical fiber as a gain medium to amplify an optical signal. They are related to fiber

lasers. The signal to be amplified and a pump laser are multiplexed into the doped

25

fiber, and the signal is amplified through interaction with the doping ions. The most

common example is the Erbium Doped Fiber Amplifier (EDFA), where core of a

silica fiber is doped with trivalent Erbium ions (Er+3), can be efficiently pumped

with a laser at 980 nm or at 1,480 nm, and exhibits gain the 1,550 nm region [54].

Amplification is achieved by stimulated emission of photons from dopant

ions in the doped fiber. The pump laser excites ions into a higher energy from where

they can decay via stimulated emission of a photon at the signal wavelength back to a

lower energy level. The excited ions can also decay spontaneously (spontaneous

emission) or even trough non radiative processes involving interactions with phonons

of the glass matrix. These last two decay mechanisms compete with stimulated

emission reducing the efficiency of light amplification [54].

Although the electronic transitions of an isolated ion are very well defined,

broadening of the energy levels occurs when the ions are incorporated into the glass

of the optical fiber and thus the amplification window is also broadened. This

broadening is both homogeneous (all ions exhibit the same broadened spectrum) and

inhomogeneous (different ions in different glass locations exhibit different spectra).

Homogeneous broadening arises from the interactions with phonons of the glass,

while inhomogeneous broadening is caused by differences in the glass sites where

different ions are hosted. Different sites expose ions to different local electric fields,

which shifts the energy levels via the Stark effect. In addition, the Stark effect also

removes the degeneracy of energy states having the same total angular momentum

(specified by the quantum number J). Thus, for example, the trivalent Erbium

ion(Er+3) has a ground state with J = 15/2, and in the presence of an electric field

splits into J +1/2 = 8 sublevels with slightly different energies. The first excited state

has J = 13/2 and therefore a Stark manifold with 7 sublevels. Transitions from the J =

13/2 excited state to the J= 15/2 ground state are responsible for the gain at 1.5 �m

wavelength. The gain spectrum of the EDFA has several peaks that are smeared by

the above broadening mechanisms. The net result is a very broad spectrum (30 nm in

silica, typically) [54].

26

The broad gain-bandwidth of fiber amplifiers make them particularly useful

in wavelength-division multiplexed communications systems as a single amplifier

can be utilized to amplify all signals being carried on a fiber and whose wavelengths

fall within the gain window.

2.4 Applications of Radio-over-Fiber Technology

There are many applications of ROF technology. This section gives brief

explanation to some of them

2.4.1 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].

27

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

around this problem is to deploy ROF technology. A wireless LAN at 60 GHz has

been realized [10] by first transmitting from the BS (Central Station), a stable

oscillator frequency at an IF together with the data over the fiber. The oscillator

frequencies were 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.

2.4.2 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

SMF technology. Other ROF functionalities such as dynamic capacity allocation

offer significant operational benefits to cellular networks.

2.4.3 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 fiber links of less than 1

km and operating at frequencies between 1 GHz and 15 GHz are used. By so doing,

high frequency equipment can be centralized.

28

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 kilometers 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 relating to the cost of premises,

without requiring to be in the vicinity of the earth station antennas.

2.4.4 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 fiber 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.

29

2.5 Conclusion

In this chapter a brief description and explanation of the basic theory about

Radio over Fiber are presented. The basic theories are about the overview of RoF

technologies nowadays, the benefits and the architecture of RoF. Also the

explanation of optical transmission link has been described. The applications of RoF

in nowadays communication technologies also have been described, but only tells

some of them.

CHAPTER 3

OFDM SYSTEM

FOR RADIO OVER FIBER COMMUNICATIONS

3.1 Introduction

In this chapter author want to explain some brief theory of the basic concept

and general principles of OFDM such as multicarrier transmission, Fast Fourier

Transform (FFT) and Guard interval and its implementation.

Also coded OFDM is very important concept to understand. And here were

also discussed about the advantages and disadvantages of OFDM and continued by

implementation of OFDM for RoF Communication in the last part of this chapter.

31

3.2 Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal Frequency Division Multiplexing (OFDM) is seen as the

modulation technique for future broadband wireless communications because it

provides increased robustness against frequency selective fading and narrowband

interference, and is efficient in dealing with multi-path delay spread. To achieve this,

OFDM splits high-rate data streams into lower rate streams, which are then

transmitted simultaneously over several sub-carriers.

By so doing, the symbol duration is increased. The advantage of this is that

the relative amount of dispersion in time caused by multi-path delay spread is

decreased significantly. Furthermore, introducing a guard time in every OFDM

symbol eliminates Inter-Symbol Interference (ISI) almost completely. Within the

guard time, the OFDM symbol is cyclically extended to avoid Inter-Carrier

Interference (ICI). OFDM can in fact be considered as both a multiplexing method as

well as a modulation method.

As stated above, OFDM uses multiple subcarriers to transmit low rate data

streams in parallel. The subcarriers are themselves modulated by using Phase Shift

Keying (PSK) or Quadrature Amplitude Modulation (QAM) and are then carried on

a high frequency microwave carrier (e.g. 5 GHz). This is similar to conventional

Frequency Division Multiplexing (FDM) or Sub-Carrier Multiplexing, except for the

stringent requirement of orthogonality between the sub-carriers Sub-carrier

orthogonality can be viewed in two ways, namely the time, and the frequency

domains. In the time domain, each sub-carrier must have an integer number of cycles

during each OFDM symbol interval (duration). In other words, the number of cycles

between adjacent sub-carriers differs by exactly one as shown in Figure 3.1.a In the

frequency domain, the amplitude spectra of individual sub-carriers (which are PSK

or QAM modulated) overlap as shown in Figure3.1.b However, at the maximum of

each subcarrier spectrum, all other sub-carrier spectra are zero. Since the OFDM

receiver calculates the spectrum values at the maximum points of individual sub-

32

Figure 3.1 (a) OFDM Symbol with

Three Orthogonal Sub-carriers in

one.

Figure 3.1 (b) OFDM Symbol with

Spectra of three OFDM sub-carriers

carriers, it can recover each sub-carrier without ICI interference from other sub-

carriers.

OFDM is processor intensive because the basic OFDM signal is formed using

the Inverse Fast Fourier Transform (IFFT), adding a cyclic extension and performing

windowing to get steeper spectral roll off. In the receiver, the sub-carriers are

demodulated by using Fast Fourier Transformation (FFT). The requirement for the

intensive computations accounts for the complexity of OFDM transmitters and

receivers. In comparison to single-carrier modulation systems, OFDM is more

sensitive to Frequency offset and phase noise. Furthermore, OFDM has a relatively

large peak-to-average power ratio, which reduces the power efficiency of the RF

amplifier. OFDM is already used in many access network technologies including

High-bit-rate, Digital Subscriber Lines (HDSL), Asymmetric Digital Subscriber

Lines (ADSL), Very high-speed Digital Subscriber Lines (VDSL), Digital Audio

Broadcasting (DAB), and High Definition Television (HDTV) broadcasting. It is

discussed in detail in [10].

33

3.3 General Principles

3.3.1 Multicarrier transmission

A multicarrier communication system with orthogonal subcarriers is called an

Orthogonal Frequency Division Multiplex (OFDM) system. In an OFDM system, the

carrier spacing �f is 1/NT, where N is the number of the carriers and 1/T is the

overall symbol rate. With this carrier spacing, the sub channels can maintain

orthogonality, although the sub channels overlap. Therefore, there is no inter-

subcarrier interference with ideal OFDM systems. The number of subcarriers N is

chosen so that the sub channel bandwidth is less than the channel coherence

bandwidth. Under this condition, each sub channel does not experience significant

Inter-symbol Interference (ISI).

The transmitted signal of an OFDM system for one OFDM symbol period is

of following form[11]

( ) Re{ ( )exp( 2 )}n nn

s t a h t j f tπ φ= +� (3.1)

Where an is the transmitted data symbol for the n-th subcarrier, h(t) is the

pulse shaping filter response, fn is the n-th subcarrier frequency fn = fc + n�f.

As the number of OFDM subcarriers increases, the complexity of the

modulator and demodulator is increased accordingly. However, the OFDM

modulator and demodulator can be implemented easily by the inverse discrete

Fourier transform (IDFT) and discrete Fourier transforms (DFT) respectively. Figure

3.2 shows a block diagram of the OFDM modulator. The time domain coefficients

cm can be computed by IDFT as [11]

34

Figure 3.2 A block diagram of an OFDM transmitter

QAM modulator

Serial to Parallel

Converter

IDFT

D/A

Parallel to

Serial Converter

Data an cm

1

0

1 2exp

N

m nn

nmc a j

NN

π−

=

� �= −� �

� �� (3.2)

Where N is the input of the IDFT block which is the data symbol for n-th

subcarrier, cm is the m-th output of the IDFT block. After the IDFT operation, the

parallel output of IDFT block cm(m = 1, · · · ,N − 1) is converted to a serial data

stream. From the IDFT operation of Equation (3.2), we can convert the frequency

domain data symbols into a series of time domain samples. The digital samples are

digital to analog converted, filtered and converted to a carrier frequency fc.

At the receiver, the received signal is down converted to base band and

sampled at the symbol rate 1/T. Then, N serial samples are converted to parallel data

and passed to a DFT which converts the time domain signal into parallel signals in

the frequency domain.

The OFDM system transmits the wideband data over many narrowband sub

channels. The symbol duration of each subcarrier becomes N�t, where �t is the

symbol duration of the input data symbols. We assume that each sub channel

experiences flat-fading. If the gain of the i-th channel is |�i|2, then the received

signal-to-noise ratio (SNR) of the i-th channel becomes [11]

35

2

0

| |i

Pi iSNR

BN

α= (3.3)

Where Pi is the signal power at the transmitter, B is the bandwidth of each

sub channel and N0 is the noise spectral density.

If the i-th sub channel has low channel gain |�i|2, the sub channel will have

low received SNRi for constant transmission power. This can cause a higher symbol

error rate in the sub channel with low channel gain. There are several schemes to

compensate for the degradation due to low gain sub channels. One of the techniques

is presetting transmission power with the channel inversion [11][3]. With the channel

inversion presetting, the transmitter allocates more power to the sub channels with

lower channel gain. The transmission power of i-th sub channel Pi is proportional to

the inverse of the channel gain |�i|2.

2

1

| |ii

Pαα

= (3.4)

With channel inversion, the received SNR of each subcarrier is the same and

we can maintain the symbol error rate of each subcarrier the same. However, channel

inversion is not a power efficient technique in fading channels since it can waste

much power at the sub channels with low gain. Furthermore, a large amount of

power would be required for a Rayleigh fading channel. For example, for the

frequency response, infinite power is required for channel inversion. Therefore,

channel inversion is not widely used in practical multi-carrier communication

systems. Instead, there are two other transmission techniques for compensating the

frequency selectivity. One technique is coding across the tones and the other

technique is transmission with adaptive bit and power loading. These transmission

techniques will be discussed in detail in the following sections.

36

Figure 3.3 Block diagram for multicarrier transmission (1)

Figure 3.4 Block diagram for multicarrier transmission (2)

Multicarrier modulation methods transmit data on several different subcarrier

frequencies. The multicarrier method most frequently used in wireless

communications is orthogonal frequency-division multiplexing (OFDM). OFDM is a

part of several standards for WLANs (IEEE 802.11a,g), fixed broadband wireless

access (IEEE 802.16), digital audio broadcasting (DAB), digital TV (DVB-T), and

wireless personal area networks (IEEE 802.15). It is also a promising candidate for

future "beyond 3G" communication systems.

Figure 3.3 above describe that the parallel data stream excites replicas of the

same pulse-shaping filter g(t), and the filtered signals are modulated on the different

carriers and summed up before transmission. Figure 3.4 above shows the parallel

data stream excites a filter bank of K (or K + 1) different band pass filters. The filter

outputs are then summed up before transmission.

37

Figure 3.5 Block Diagram IFFT/FFT

3.3.2 Fast Fourier Transform

At the transmitter of an OFDM system, data are apportioned in the frequency

domain and an IFFT is used to modulate the data into the time domain. The FFT

output data are guaranteed to be real-valued if conjugate symmetry is imposed on the

input data.

In the receiver, an FFT is used to recover the original data. The FFT allows

an efficient implementation of modulation of data onto multiple carriers [11]. Due to

the similarity between the forward and inverse transform, the same circuitry, with

trivial modifications, can be used for both modulation and demodulation in a

transceiver. Figure 3.5 below shows the block diagram of IFFT/FFT operation of

OFDM transmitter and receiver

38

Figure 3.6 Guard Interval

3.3.3 Guard interval and its implementation

The orthogonality of sub channels in OFDM can be maintained and

individual sub channels can be completely separated by the FFT at the receiver when

there are no intersymbol interference (ISI) and intercarrier interference (ICI)

introduced by transmission channel distortion. In practice these conditions cannot be

obtained. Since the spectra of an OFDM signal is not strictly band limited (sinc(f)

function), linear distortion such as multipath cause each sub channel to spread energy

into the adjacent channels and consequently cause ISI. A simple solution is to

increase symbol duration or the number of carriers so that distortion becomes

insignificant. However, this method may be difficult to implement in terms of carrier

stability, Doppler shift, FFT size and latency.

One way to prevent ISI is to create a cyclically extended guard interval,

where each OFDM symbol is preceded by a periodic extension of the signal itself. In

Figure 3.6 above shows the guard interval with symbol. The total symbol duration is

Ttotal=Tg+T, where Tg is the guard interval and T is the useful symbol duration.

When the guard interval is longer than the channel impulse response, or the

multipath delay, the ISI can be eliminated. However, the ICI, or in-band fading, still

exists. The ratio of the guard interval to useful symbol duration is application-

dependent. Since the insertion of guard interval will reduce data throughput, Tg is

usually less than T/4. The reason to use a cyclic prefix for the guard interval is to

maintain the receiver carrier synchronization; some signals instead of a long silence

must always be transmitted; and cyclic convolution can still be applied between the

OFDM signal and the channel response to model the transmission system.

39

3.4 Coded OFDM

Coded OFDM (COFDM) is one of the widely used transmission techniques

for overcoming the frequency selectivity of the channel. The basic idea of coded

OFDM is to encode input data and interleave the coded symbols. The interleaved

symbols are split into several sub channels to achieve frequency diversity. Even

though the uncoded symbol error rate is high for the subcarriers with low channel

gains, with the channel coding and interleaving it is possible to correct the errors in

the low gain channels. With the channel coding and interleaving, coded OFDM

provides a robust communication link for many wireless channel environments.

This technique is very effective for channels with narrow coherence

bandwidth. However, if the coherence bandwidth is large, then the channel gains of

neighboring sub channels are highly correlated and this may limit the diversity gain

of coded OFDM systems.

3.4.1 Coded OFDM Systems

Coded OFDM is a transmission technique which puts equal amounts of data

bits and transmission power on each sub channel. Originally, it was designed for a

broadcasting channel where a feedback link does not exist from the receivers to the

transmitter [12].

In many wireless environments, optimization of bit and power allocation is

not possible due to rapid change of the channel responses. Coded OFDM is also used

in time-varying wireless channels to recover errors or erasures in the sub channels

located close to the nulls of the frequency response.

40

The information data is coded by a channel encoder. One of the most popular

forms for the channel code is a trellis code. The encoded symbols are interleaved by

an interleaver. The interleaved symbols are modulated onto a carrier at an OFDM

modulator.

An example of a coded OFDM system is the 802.11a Wireless Local Area

Network (W-LAN) system in the 5GHz band [14]. The IEEE 802.11a standard uses a

coded OFDM scheme which demultiplexes coded symbols into 52 separate

subcarriers. Data rates of the 802.11a system are 6, 9, 12, 18, 24, 36 and 48 Mbps.

Data rates below 24 Mbps are mandatory. Table 3.1 presents the data rates and

modulation schemes for the 802.11a coded OFDM system. Coded data symbols are

transmitted over 48 subcarriers. The remaining 4 subcarriers are used as pilot

subcarriers. A pseudo-random sequence is transmitted through the pilot subcarriers to

avoid spectral lines. The receiver knows the signal sent on the pilot subcarriers and

uses the pilot subcarriers for channel estimation.

For channel coding, a convolutional code with constraint length k = 7 and

rate 1/2 is used. A rate 3/4 code is made by puncturing a coded symbol among three

symbols.

In the time domain, a guard period is inserted between two OFDM symbols

to prevent overlapping of two consecutive symbols by multipath delay spread. The

guard period contains a copy of the end of the IDFT output and is called a cyclic

prefix. The symbol rate of the system is 250 KHz and the guard period is 0.8 �s. The

frequency spacing between subcarriers is 312.5 KHz, which is the inverse of 3.2 �s.

41

Table 3.1 Data rates and modulation schemes for the 802.11 a W-LAN system.

3.4.2 Trellis Coded Modulation

Coded Modulation is a technique to combine the channel coding and

modulation for multi-level modulation. Ungerboeck first designed trellis coded

modulation for AWGN channels with the concept like the following Table 3.1 below

here.[12]

Data Rate (Mbps) Modulation Coding Rate

6 BPSK 1/2

9 BPSK 2/4

12 QPSK 1/2

18 QPSK 3/4

24 16-QAM 1/2

36 16-QAM 3/4

48 64-QAM 1/2

54 64-QAM 3/4

The main advantage of the trellis-coded modulation is significant coding gain

without a loss of bandwidth efficiency.

42

Figure 3.7 above shows an example of an 8-PSK trellis-coded modulation.

We can see from Figure 3.7 (a) an 8-PSK modulation scheme. Each constellation

point of the 8-PSK is denoted with a number and the distance values in the figure are

�0 = 2sin(/8), �1 = �2 and �2 = 2. Figure 3.7 (b) describes a trellis diagram of a rate

2/3 encoder. There are four states in the encoder which are the contents of the shift

register. With a new information input, the state of the encoder changes. There are

four possible state transitions with 2 bit inputs. There are two pairs of parallel state

transitions from each state. The number on the transition indicates the constellation

point of the modulation associated with the state transition.

Consider the state transitions merging to the zero state or diverging from the

zero state. The distance between two constellation points for these transitions are at

least �1 = �2. With this property, the minimum distance between two sequences

diverging from one state and merging into another is at least �02+ �1

2+ �22. The

distance between two constellation points associated with the parallel transitions are

�2 = 2, which is the maximum distance in the constellation. With the trellis diagram

and modulation mapping in Figure 3.7, we can obtain d2 free as [12]

2 2 2 2 3

2 1 0 2min( ,2 )d free = Δ Δ + Δ = Δ (3.5)

Figure 3.7 (a) An example of 8-PSK modulation. (b) An example of a trellis

diagram for a coded modulation scheme

43

Compared with the d2 free = 2 of 4-PSK modulation, we observe about 3 dB

coding gain with the modulation and trellis diagram of Figure 3.7. The performance

of a communication link can be improved by 3 dB in an AWGN channel with a

simple four state TCM scheme without reducing the data rate or increasing the

bandwidth. If more states are allowed at the encoder, it is possible to achieve coding

gains up to 6 dB [16]. In 1984, a TCM scheme was adopted for voice band modems

by the International Telephone and Telephone Consultative Committee (CCITT) [12].

3.4.3 Bit-interleaved Coded OFDM

Trellis coded modulation is a transmission technique to combine channel

coding and modulation. However, the combination of channel coding and

modulation may not be the best solution for fading channels, especially for fast

Rayleigh fading channels.

In fading channel environments, we can achieve better performance by

separating the channel coding and modulation [20][21]. Bit-interleaved Coded

OFDM is a technique which separates the coding and modulation with a bit-level

interleaver. For the decoding of the received signal, it is necessary to obtain a soft-bit

metric for the input of the Viterbi decoder.

44

Figure 3.8 above is showing a block diagram of a transmitter and receiver

using bit-interleaved coded modulation. Input data are encoded with a channel

encoder which is characterized by the coding rate and the minimum distance between

two nearest data sequences. The coded bits are interleaved at the interleaver. The

interleaved bits are mapped to a point in a modulation constellation. The modulation

scheme can be M-ary PSK or QAM, where M is assumed to be a power of 2. A

mapper is a memory less mapper between the interleaved bits bk and a constellation

symbol xn that will be sent over the channel. log2M interleaved bits are mapped to

one modulation symbol. The relation between the position k of interleaved bit

position bk and the position of the modulation symbol is

2/ logn k M= (3.6)

2logi kMod M= (3.7)

Where i is the location of the interleaved bit in a modulation symbol. The

channel response for the n-th modulation symbol is assumed to be �n. The received

signal yn at the receiver is given as

n n n ny x nρ= + (3.8)

and nn is AWGN.

Figure 3.8 A block diagram of Bit-interleaved coded OFDM

45

At the receiver, a demapper computes the maximum-likelihood (ML) soft

metric of each interleaved bit based on the received signal. The ML soft metric is

deinterleaved and sent to a Viterbi decoder. The Viterbi decoder chooses a sequence

C which satisfies [12]

�

max Pr{ 1,..., / }C y yN C= � � � � � � � (3.9)

Where C is the set of all possible data sequence and N is the number of

modulation symbols.

In following Figure 3.9 below shows mappers for a 16-QAM modulator for

bit-interleaved coded modulation and for trellis coded modulation. The mapping for

each axis is a Gray mapping for bit-interleaved coded modulation. The mapping for

trellis coded modulation is designed based on the set partition.

The bit-interleaved code modulation is a sub-optimal coding scheme for

channel capacity in static channel environments. However, it provides frequency

diversity which is essential in wireless system designs that encounter multipath

fading.

Figure 3.9 (a) Constellations of 16-QAM with Gray mapping.

(b) Constellations of 16-QAM with set partitioning

46

3.5 Advantages of OFDM

OFDM has several advantages such as high data rate in mobile wireless

channel and it is conveniently implemented using IFFT and FFT operations.

However OFDM is a spectrally efficient modulation technique and it handles

frequency selective channels well when combined with error as well as Correction

coding. OFDM also has good tolerance to inter symbol interference (ISI). Here some

advantages of OFDM:

• Can easily adapt to severe channel conditions without complex equalization.

• Robust against narrow-band co-channel interference.

• Robust against Intersymbol interference (ISI) and fading caused by multipath

propagation.

• High spectral efficiency.

• Efficient implementation using FFT.

• Low sensitivity to time synchronization errors.

• Tuned sub-channel receiver filters are not required (unlike conventional

FDM).

• Facilitates Single Frequency Networks, i.e. transmitter macro diversity.

3.6 Disadvantages of OFDM

As OFDM has a lot of advantages it also has some disadvantages. The most

effective disadvantage of OFDM is the complexity, where OFDM is a multicarrier

modulation which is more complex than single-carrier modulation as well as OFDM

requires a more linear power amplifier. Here the summary of OFDM disadvantages:

• Sensitive to Doppler shift.

• Sensitive to frequency synchronization problems.

• High peak-to-average-power ratio (PAPR), requiring linear transmitter

circuitry, which suffers from poor power efficiency.

47

Figure 3.10 OFDM model for Radio Over Fiber Network

3.7 OFDM System for Radio over Fiber

The basic building blocks for a OFDM transceiver system for Radio over

Fiber[8,9] are shown in Figure 3.10. First the user data is converted into symbols by

m-ary signaling. These symbols are the converted into frames of N parallel rows.

Also pilot symbol is added at the starting of each frame, which is used at the receiver

for synchronization and also for channel equalization. The N sub symbols are then

sent to an Inverse Fast Fourier Transform (IFFT) block that performs an N-point

Inverse Fast Fourier Transform. Hence, the original input data is treated by OFDM as

though it is in the frequency-domain. The output of the Inverse Fast Fourier

Transform block is N time-domain samples. This complex signal is transmitted on

orthogonal carriers at RF frequency. The RF signal then carried by optical

modulation in RoF transmitter to be sent over single mode fiber. At the RoF receiver

the optical signal received would converted back onto electrical domain signal by

photodetector.

In the receiver the pilot symbol is used for synchronization at the starting

stage and the same symbol is used for equalization at the later stage. All the other

Optical Link

48

processes at the receiver are the reverse process of the transmitter. The mathematical

equivalent of OFDM signal is expressed as

12 /

0

( )N

j kt Tk

k

v t X e π

−

=

=� 0 � t < T (3.10)

Where, Xk are the data symbols and T is the OFDM symbol time. The sub-

carrier spacing of 1/T makes them orthogonal over each symbol period. To avoid

intersymbol interference in multipath fading channels, a guard interval of length Tg

is inserted prior to the OFDM block. During this interval, a cyclic prefix is

transmitted such that the signal in the interval –Tg � t < 0 equals the signal in the

interval (T–Tg) � t < T.

12 /

0

( )N

j kt Tk

k

v t X e π

−

=

=� -Tg � t < T (3.11)

The OFDM signal with cyclic prefix is thus can be derived from equation

3.11 above.

3.8 Conclusion

Chapter 3 gives a brief explanation of OFDM for RoF communications, this

chapter also include the general principles of OFDM, the advantages and

disadvantages of OFDM. In the OFDM – RoF networks system, the signal

modulated from OFDM transmitter will be optically modulated by RoF transmitter

then sending through the fiber network, while in RoF receiver part the signal then

converted back into electrical signal before being received in OFDM receiver.

��

CHAPTER 4

THE OFDM - ROF SYSTEM MODEL

4.1 Introduction

The main idea of this project is to incorporate the OFDM modulation

technique into RoF system networks. Because the advantages of both, while OFDM

could distribute the data over a large number of carriers that are spaced apart at

precise frequencies with overlapping bands and RoF is a next generation

communication systems that can utilize the high capacity of optical networks along

with the mobility of wireless networks. Hence by incorporating OFDM along with

RoF, the system can be used for both short distance as well as long-haul transmission

at very high data rate. This improves the system flexibility and provides a very large

coverage area without increasing the cost and complexity of the system very much.

In this chapter shows the OFDM-RoF system model of this project which is

done by using commercial system from Optiwave, Optisystem 7.0. The chapter also

includes of whole the OFDM-RoF system model, and then would explain the system

from each part of the transmitter part, the transmission link and the receiver model.

And the last part of this chapter is the conclusion of the model. The parameters that

have been used in this project were also clearly stated.

� 50

Figure 4.1 Block Diagram of OFDM – RoF network system

4.2 The OFDM-RoF System Model

The principle of OFDM is split high-rate data streams into lower rate streams,

which are then transmitted simultaneously over several subcarriers. By doing so, the

symbol duration is increased. The advantage of this is that the relative amount of

dispersion in time caused by multi-path delay spread is decreased significantly. The

critical advantage of using OFDM in optical fiber communications includes better

spectral efficiency, elimination of sub channel and symbol interference using the fast

Fourier transform (FFT) for modulation and demodulation, which does not require

any equalization and dispersion tolerance [4].

In this project the OFDM-RoF system were modeled for application based on

WLAN IEEE 802.11 b/g standard which is using frequency carrier 2.4 GHz. The

data rate of the system is 20 Mbps with modulation type 16 QAM/ 4 bit per symbol.

Length of the fiber for transmission link are from 10 – 50 Km. Figure 4.1 below

described the block diagram of OFDM – RoF network system.

From Figure 4.1, the digital data input first would be split into parallel

streams data by OFDM transmitter, then this data would carry onto the optical fiber

link. In the optical link, CW laser would emit a continuous beam or a train of short

pulses of a laser. In the MZM the electrical wave signal from OFDM transmitter are

combined with the continuous wave light from CW laser, this two wave are then

modulated by MZM to form optical signal which is would sent through the optical

� 51

Figure 4.2 OFDM Transmitter Design

fiber. After sending through the optical fiber, then the data would be received first by

photodetector. This photodetector converts the incoming photonic stream back into a

stream of electrons, so the optical signals are converted back into electrical signals.

The signal then would recombine again in the OFDM receiver to get the original data

back.

4.3 The Transmitter Model.

In this OFDM – RoF system, transmitter system were consist of OFDM

modulation block system. The transmitter is split into 4 sub carriers with 20 Mbps of

data rate for each sub carriers. The data then modulated using 16 QAM/ 4 bit per

symbol, then carried by different frequency of each sub carriers to have the

orthoganality between the sub carriers. Figure 4.2 below shows the OFDM

transmitter design for one sub carrier.

� 52

Table 4.1 Global Parameter Setup

The global parameter setup that using in this simulation are shown in table

4.1 below:

Parameter Value

Bit rate 128 Mbps

Time window 1.6e-005 s

Sample Rate 8.192 GHz

Sequence length 2048 bits

Sample per bit 64

Number of samples 131072

The data bits in OFDM transmitter are generated by a pseudo random number

generator; it generates a bit sequence of 0 and 1 with equal probability. In this

simulation the sequence of length is 2048 bits. Optisystem requires a power of two

sequence length, while in this simulation is using 16 QAM which is using 4 bits per

symbol. This means that it will have not only a binary sequence that will use the

parameter sequence length, but also a M-ary sequence with sequence length divided

by 4 (after the QAM sequence generator). Sample rate 8.192 GHz were used to

accommodate the frequency carrier 2.4 GHz.

To achieve the orthogonality of each sub carriers, the frequencies are set to

have precise different frequencies so the data would not interfere with each other. In

this simulation model the number of sub-carriers are 4, while each of data rate = 20

Mbps. Then we might place our QAM carrier modulators as shown below:

• With Rs = 20, and B = 20 x 1.25 = 25 MHz

• Each carrier may be placed (25 + 2.5) MHz = 27.5 MHz apart allowing for

a 10 % guard band.

� 53

Table 4.2 Sub carrier Allocation

Table 4.2 below shown the sub carrier frequency allocation for the OFDM

modulator:

Operation Frequency Value (in GHz)

f1 2.345

f2 2.3725

f3 2.4275

f4 2.455

The output of electrical signals from each sub carrier then multiplexed before

sending through the optical fiber in transmission link.

4.4 The Transmission Link Model

The transmission link part were consist of RoF system network, the signal

modulation from OFDM transmitter would be sent through the RoF system network

before being received by the OFDM receiver. These RoF system networks in this

simulation model are electrical amplifier, CW laser, MZM, optical fiber and

photodetector PIN.

The electrical amplifier was used to gain the electrical signal from the OFDM

transmitter before being sent through the optical fiber. The CW laser and MZM were

used to convert the electrical signal to the optical wave so the signal could be

modulated in optical fiber. Then photodetector were used to convert back the optical

signal into electrical signal so the OFDM receiver could derive the data back to the

original.

� 54

Figure 4.3 RoF System Model

Table 4.3 Component’s Values of transmission link

The transmission link in this simulation model or RoF system model is

described by Figure 4.3 below:

The value of each component which was used in transmission link system

parameter are shown in following Table 4.3 below:

Component Value

CW Laser Frequency = 1550 nm

Electrical Amplifier (2 pcs) Gain = 10 dB

MZM Extinction Ratio = 30 dB

Optical Fiber Length = 10 up to 50 Km

Photodetector PIN Responsitivity = 1 A/W

� 55

Figure 4.4 OFDM Receiver Design

4.5 The Receiver Model

After the data being transmitted as optical signals through the optical fiber in

RoF system transmission link. Then the data were received back in OFDM

demodulation or OFDM receiver part. The data in optical signals already converted

back by the photodetector into electrical signals and the signal power also being

gained by electrical amplifier. This signals then demultiplexed back to their own

carrier by QAM demodulator. From the demodulator the next step is to detect the I

and Q electrical signals using a M-ary Threshold Detector. By using this, we can

recover the original QAM sequence and then decode the sequence into the original

binary signal. Last component in this receiver part is the eye diagram. Optisystem

can plot and estimate the BER (Bit Error Rate) of an optical system for two level

(binary) signals but when using M-ary signals, we cannot estimate the values of BER

directly, however we can still plot the eye diagram and analyze the system by the

pattern of eye diagram. The whole system of the receiver model for one sub carrier

that already explained above could be shown by the Figure 4.4 below:

� 56

Figure 4.5 OFDM – RoF MoDem

4.6 Conclusion

The OFDM – RoF system are designed to incorporate three part domain of

the system, which are Transmitter part, Transmission link part and Receiver part. The

transmitter and receiver part are consist of 4 sub carriers which is in electrical

domain. And the transmission link is consisting of the optical domain. In this

simulation, the parameters are setup to measure the effect of fiber length to the total

power signal and to analyze the characteristic of the system by the pattern of eye

diagram.

The whole system of the OFDM – RoF system modulator demodulator

(MoDem) were shown in figure 4.5 below here:

CHAPTER 5

SIMULATION RESULT AND PERFORMANCE ANALYSIS

5.1 Introduction

In this chapter, we present the simulation results from the system design. As

described and explained before in the previous chapter, the system has three main

part, which are transmitter part, transmission link part and receiver part. The result

that would be shown in this chapter is based on that three main part. It has to be

mentioned that there are many factors that are not included or considered in the

simulation while they exist in reality, such as number of sub carriers of OFDM

transmitter and receiver and etc. At this moment, the simulation only considers some

important combinations of parameters that dominant in optical data transmission for

project simulation purpose. Therefore, the results for this project were carried out

based on some assumption.

For the analysis result, the consideration is focused only in the eye diagram

and the effect of fiber length to signal power from received OFDM signals.

� 58

Figure 5.1 OFDM signal from transmitter

5.2 The Transmitter Simulation Results.

Up to this moment, the simulation only considers some important

combinations of parameters that dominant in optical data transmission. All

parameter that been used in this transmitter part are already explained in the

previous chapter. The result for the transmitter part which is in electrical domain

was shown in Figure 5.1 below:

The result shown in Figure 5.1 above is derived from OFDM signal with 4

sub carrier that has been multiplexed before optically modulated. We could see from

that figure the power of the signal is 38.067 dBm and frequency carrier of the signal

is 2.4 GHz.

� 59

Figure 5.2 OFDM signal after amplified

Before optically modulated the signal of OFDM is to be amplified by the

electrical amplifier, with gain is 10 dB. These OFDM signals after amplified were

shown in Figure 5.2 below:

The OFDM signal after amplified with electrical amplifier gain 10 dB, as we

could see from the Figure 5.2 above is now have signal power 48.067 dBm. The

amplified is necessary to be done before the signal will optically modulated, to

reduce the effect of loss possibility from the signal when it’s modulated through the

fiber optic.

� 60

Figure 5.3 OFDM optically modulated by MZM

5.3 The Transmission Link Simulation Results

In this section we present the result from the transmission link, which is in

optical domain. The transmission link function is to convert the electrical domain

signal from the transmitter part into optical domain signal so the OFDM signal could

be send through the fiber transmission. To do this we use CW laser and external

modulator like MZM to modulate the signal of OFDM. The wavelength for CW

laser is set to 1550 nm, while the rest are to be set into default value from the

Optisystem. The electrical attenuation is 10 dB and the fiber length for the

transmitting the signals is varied from 10 up to 50 Km.

The result from transmission link after OFDM signal optically modulated by

MZM were shown in Figure 5.3 below here:

� 61

Figure 5.4 OFDM signal after through optical fiber

After optically modulated, the signal then transmitted through the fiber link

transmission as we could see from the Figure 5.4 below:

From both Figure 5.3 and 5.4 above, we could see that the wavelength is

1550 nm, but the power from both signal are different. The power of signal after

transmitted through optical fiber was decreased from -20 dBm (10-5 W) to -30 dBm

(10-6 W). According to the theory that typical maximum received signal power of

wireless network is -10 to -30 dBm, and this project is modeled for application

based on WLAN IEEE 802.11 b/g, so the result is agree with the theory.

� 62

Figure 5.5 OFDM signal from PIN photodetector

5.4 The Receiver Simulation Results

After optically modulated and being transmitted over fiber link transmission,

the optical domain signal then converted back into electrical domain signal by PIN

photodetector. The PIN photodetector were used in this project simulation have

responsitivity 1 A/W. The OFDM signal that has been converted and retrieved back

in receiver part is shown in Figure 5.5 below here:

The received OFDM signal in Figure 5.5 have total power is -55.372 dBm

(2.9e-9 W) and frequency carrier is 2.4 GHz. This signal then amplified with gain 10

dB by electrical amplifier, before demodulated back to get the original data that has

been sent from the transmitter part.

� 63

Figure 5.6 OFDM signal after amplified

Figure 5.6 below here shown the OFDM signal after amplified by electrical

amplifier with gain = 10 dB. This electrical amplifier value is the default from the

Optiwave just to consider for simulation of this project only. The actual value for

electrical amplifier based on what we want to apply the system in the real situation

could be different with simulation of this project.

As shown in Figure 5.6 above, the power of OFDM signal after being

amplified now is -45.372 dBm (2.9e-8 W). The typical range of wireless (802.11x)

received signal power over a network is (-60 to -80 dBm) [ ]. But because here we

are using fiber as the network transmission, so there is also loss to concern from the

fiber characterization in the system.

� 64

Figure 5.7 Basic information contained in Eye diagram

5.5 The Eye Diagram

The eye diagram is a useful tool for the qualitative analysis of signal used in

digital transmission. It provides at-a-glance evaluation of system performance and

can offer insight into the nature of channel imperfections. Careful analysis of this

visual display can give the user a first-order approximation of signal-to-noise, clock

timing jitter and skew.

Figure 5.7 below here described the basic information contained in the eye

diagram. The most important are size of the eye opening (signal-to-noise during

sampling), plus the magnitude of the amplitude and timing errors [ ].

Based on the basic diagram form Figure 5.7 above, we could derive some

information from the system such as amount of distortion (Signal to Noise Ratio),

time variation of zero crossing and etc. The best time sample (decision point) is

most open part of eye meaning best signal to noise ratio.

� 65

Figure 5.8 (a) Eye diagram of received OFDM signal for in phase signal

In this project simulation we are using 4 sub carriers for transmitter and

receiver part. Since this system using QAM as the modulator and demodulator so we

would have In phase and in Quadrature signals for each sub carriers. So there would

be total of 8 eye diagrams for the whole system. But since the most of eye diagram

has same pattern, that’s why the author decided to represent just two eye diagrams

from one sub carrier only.

Figure 5.8 (a) and (b) below here shows the eye diagram from the received

OFDM signal for in phase and in quadrature signals.

� 66

Figure 5.8 (b) Eye diagram of received OFDM signal for in quadrature signal

The eye diagrams that illustrated in Figure 5.8 above as we could see most of

the eye opening is not so wide and high. Based on the theory as explained before in

the first paragraph of this section, most open part eye means best signal to noise

ratio. So this system might have some noise distortion and because of that it might

have some modification in the future works. And we could not estimate the BER

performance directly because the system model in this project is using M-ary

signaling, this reason already explained before in the previous chapter.

� 67

Figure 5.9 Total power signal vs Fiber length after photodetector

5.6 Analysis of the Total Power to the Length of Fiber

In the optical communication link, power is one of the components that used

to transmit optical signal. Through the link, naturally power drops due to

attenuation, distortion and losses. In this simulation, the results will illustrate the

effect of using SMF which varies from 10 up to 50 Km without using any repeater.

The results shown in Figure 5.9 below here is the OFDM received total

signal power graphic from the receiver part in photodetector.

As we could see from Figure 5.9 above, the total power signals are

decreasing while the length of fiber were being increased in simulation from 10 to

50 Km. The total power received in 50 Km length fiber is -55.372 dBm (2.9e-9 W)

� 68

Figure 5.10 Total power signal vs Fiber length after electrical amplifier

And Figure 5.10 below here is OFDM received total signal power after being

amplified by electrical amplifier 10 dB.

From Figure 5.10, after being amplified by 10 dB, the total power of OFDM

received signal is -45.372 dBm (2.9e-8 W). To increase the total power of received

signal we could put repeater in every each 50 Km or using lower attenuation 0.1

dB/Km. Else we could also increase the Photodetector sensitivity, in this simulation

model we are using -60 dBm for PD sensitivity.

� 69

5.7 Conclusion

The OFDM –RoF system model is already simulated. The results were

shown in the previous section of this chapter. In this simulation 4 sub carriers of

OFDM modulator and demodulator are employed to transmit and received the

signals over a fiber network transmission which varies from 10 to 50 Km length.

The performance of this model measured by the effect of length of fiber to total

power signal that being received. The total power is decreasing while the length of

optical fiber was increasing. The system model has been analyzed by the pattern of

the eye diagram of received signals, which most of the part eyes were not open wide

and high, which means the system has to be improved due some noise distortion

inside the system model.

CHAPTER 6

CONCLUSION AND RECOMMENDATION

6.1 Conclusions

The study of incorporating OFDM modulation technique with RoF

technology is essential in nowadays implementation of optical commnunication.

With the combination of the advantages from OFDM and RoF, the system can be

used for both short distance as well as long haul transmission at very high data rate.

This improves the system flexibility and provides a very large coverage area of

telecommunication networks without increasing the cost and complexity of the

system very much. Also recently, it has been proved by many researchers that

OFDM is better compared to the conventional single carrier modulation for long

haul optical transmission.

In this project the simulation of modeling the OFDM scheme for Radio over

Fiber system has been completely done by using commercial software Optisystem.

Some consideration has been added to system design in order to meet the applicable

design for using in the reality. The system model has been design to accomodate W-

LAN IEEE 802.11 b/g which is using data rate 20 Mbps over frequency carrier 2.4

GHz.

71

This project shows that total power of the OFDM signal that being carried

over the fiber network is decreasing while the length of fiber is increasing from 10 to

50 Km. It is observed that as distance increases, the performance is degraded due to

fiber dispersion.

Overall, the project offered vast learning opportunity in the OFDM and Radio

over Fiber technology and how to simulate using Optisystem software. The studying of

OFDM modulation and radio over fiber technology became very important because both

of them have been developed to support some important future wireless systems.

6.3 Future Recommendations

The future work is to complement and extend the results further, since the

system model of this project using only QAM for the digital modulation technique so

there are possibilities to try the system with other modulation technique such as BPSK

or QPSK etc.

The system could have an improvement by occupying the WDM

(Wavelength Division Multiplexing) technique, so it would be OFDM-WDM

modulation for RoF. This kind of system could provide more bandwidth efficient

and manage traffic communication better.

The model of OFDM – RoF system could also be done in other software

such as Matlab Simulink or Linux NS2. This provide an alternative way to analyze

the performance and characteristic of the OFDM modulation for RoF transmission.

REFERENCES

[1] A. Marwanto and S.M.Idrus, ‘SCM/WDM Radio over Fiber for Broadband

Communication’, INCOMTIS2008, Semarang Indonesia, 14-15 May 2008

[2] M. Arsat, S.M.Idrus and N.M.Nawawi, ‘Performance Analysis of Sub Carrier

Multiplexed System for Radio over Fiber Technology’, Proceeding of National

Conference on Telecommunication Technologies and Malaysia Conference on

Photonics 2008 (NCTT-MCP 2008), 26-28 August 2008

[3] A. Marwanto and S.M. Idrus, ‘System Performance Analysis for 150km

SCM/WDM Radio over Fiber’, International Graduate Conference on

Engineering and Science 2008 (IGSES2008), Johor Baharu, 23-24

December2008

[4] Guruprakash Singh and Arokiaswami Alphones.,” OFDM Modulation Study

for a Radio-over-Fiber System for Wireless LAN (IEEE 802.11a)”,

Proceedings of Fourth International Conference on Information,

Communications & Signal Processing, ICICS- PCM 2003, Singapore, pp 1460

– 1464

[5] Dhivagar. B, Ganesh Madhan.M, and Xavier Fernando., “Analysis of OFDM

signal Through Optical Fiber for Radio over Fiber Transmission”, Madras

Institute of Technology Campus, Anna University Chromepet, Chennai 600

044, INDIA

73

[6] Zhang Bo, Lu Yinghua, Zhang Jinling, Yang Biao., “Nonlinear Effect of

OFDM in Radio-over-Fiber transmission”, Beijing University of Posts and

Telecommunications. BeiJing, CHINA, 100876

[7] Hamed Al Raweshidy and Shozo Komaki (eds),” Radio over fiber technologies

for mobile communications networks”, Artech House, London.(2002)

[8] Van Nee, Richard, and Prasad, Ramjee. OFDM for Wireless Multimedia

Communications. Boston: Artech House, 2000.

[9] J. Lowery, L. Du, and J. Armstrong, “Orthogonal frequency division

multiplexing for adaptive dispersion compensation in long haul WDM

systems,” in Optical Fiber Communication Conference and Exposition and The

National Fiber Optic Engineers Conference, Technical Digest, (Anaheim, CA,

USA, 2006), Paper PDP39.

[10] Shi, Q., ”Error performance of OFDM-QAM in subscriber multiplexed fiber-

optic transmission”, IEEE Photonics Technol. Lett., 1997, 9, (6), pp. 845–847

[11] I. B. Djordjevic and B. Vasic, “Orthogonal frequency division multiplexing for

high-speed optical transmission,” Opt. Express 4, 3767-3775 ( 2006).[11] H.

Chettat, L. M. Simohamed, Y. Bouslimani and H. Hamam, “RoF Networks: A

Comprehensive Study” , IEEE journals, ISWPC 2008

[12] Hiroyuki S., Tsutomu N.,Kuniaki U. and Susumu M.” Radio-over-Fiber

Transmission performance of OFDM Signal for Dual-Band WirelessLAN

systems” International Topical Meeting on Microwave Photonics 10-12, Japan,

September,2003.

[13] P'eter Horv'ath, Istv'an Frigyes. Effects of the Nonlinearity of a Mach- Zehnder

Modulator on OFDM Radio-over-Fiber Transmission. IEEE Communications

Letters. Volume 9, Issue 10, Oct. 2005 Page(s):921 -923.

74

[14] Sigit Puspito W.J., “Mengenal Teknologi Orthogonal Frequency Division

Multiplexing (OFDM) pada Komunikasi Wireless”, Elektro Indonesia, Nomor

24, Tahun V, Januari 1999

[15] Charan Langton, ”Orthogonal Frequency Division Multiplexing Tutorial”,

www.complextoreal.com, 2002

[16] J.E. Mitchell, “Performance of OFDM at 5.8 GHz using radio over fibre

link,”Electronics Letters, Vol. 40, No. 21, Oct. 2004

[17] A. Bahai and B.R. Saltzberg, Multicarrier Digital Communication: Theory and

Applications of OFDM. Norwell, MA: Kluwer, 1999.

[18] I. A. Kostko, M. E. Mousa Pasandi, M. M. Sisto, S. Larochelle, L. A. Rusch,

and D. V. Plant. “A radio-over-fiber link for OFDM transmission without RF

amplification”. Department of Electrical and Computer Engineering, McGill

University, Montreal and Centre d'optique, photonique et lasers (COPL), Laval

University, Quebec, QC, Canada

[19] H. Harada, K. Sato and M. Fujise. A Radio-on-Fiber Based Millimeter-Wave

Road-Vehicle Communication�System by a Code Division Multiplexing Radio

Transmission Scheme. IEEE Trans.� Intelligent Transport. System, vol. 2, no. 4,

pp. 165.179, Dec. 2001.

[20] U. Gliese, S. Norskow, and T. N. Nielsen. Chromatic Dispersion in Fiber-Optic

Microwave and Millimeter-Wave Links. IEEE Trans. Microwave Theory

Tech., vol. 44, no. 10, pp. 1716.1724, Oct.1996.

[21] Ramaswami R., Sivarajan K.N. Optical Networks A Practical Perspective. 2nd

Edition. Morgan Kaufmann, San Francisco,CA, 2002.

[22] Govind P. Agrawal. Fiber-optic Communication Systems. John Wiley & Sons,

New York, 2nd edition, 1997.

75

[23] H. Ogawa, D. Polifko, and S. Banba. Millimeter-wave fiber optics systems for

personal radio communication. IEEE Transactions on Microwave Theory and

Techniques 40(12), 2285–2293 (1992).

[24] Hong Bong Kim and Adam Wolisz. A Radio over Fiber based Wireless Access

Network Architecture for Rural Areas. In Procceeding of 14th IST Mobile &

Wireless Communication Summit, Dresden, Germany, June 2005

[25] X. N. Fernando and S. Z. Pinter. “Radio over Fiber for Broadband Wireless

Access”, Department of Electrical and Computer Engineering, Ryerson

University, Toronto, Canada, 2005