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 subcarriers, 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
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
LIST OF SYMBOLS
1
2
PAGE
xv
xvii
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
LITERATURE REVIEW
9
2.1 Introduction
9
2.2 Radio over Fiber (RoF)
2.2.1 Overview
10
10
x
2.2.2 Benefits of RoF Technology
12
2.2.3 Architecture of RoF Networks
13
2.3 Optical Transmission Link
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
3
15
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
OFDM FOR RoF COMMUNICATIONS
30
3.1. Introduction
30
3.2. Orthogonal Frequency Division Multiplexing
(OFDM)
3.3. General Principles
31
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
4
5
3.8 Conclusion
48
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
SIMULATION RESULT AND PERFORMANCE
ANALYSIS
6
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
CONCLUSIONS AND RECOMMENDATION
70
6.1 Conclusions
70
6.2 Future Recommendations
71
REFERENCES
72
xii
LIST OF TABLES
TABLE NO.
TITLE
2.1
Typical Step-Index Fiber characteristic
3.1
Data rates and modulation schemes for the 802.11 a
PAGE
18
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 TransmissionLink
15
2.5
Step-Index Fibers.
(a) Refractive index profile.
(b) End view.
(c) Cross-sectional side view.
2.6
17
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 Subcarriers in one.
(b) OFDM Symbol with Spectra of three OFDM subcarriers
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
5.9
5.10
quadrature signal
65
Total power signal vs Fiber length after photodetector
67
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
Ȝ
-
Wavelength
h
-
Blank’s Constant
C
-
Velocity of Light
Eg
-
Energy Gap
fc
-
Cut-off frequency
Ș
-
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
ΔP 2
-
Mean square amplitude of the noise fluctuations
Į
-
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
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].
Figure 1.1 General RoF systems
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
1.5
Methodology
The methodology of this project is described in the following flow chart
Figure 1.2 Project 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.
Figure 2.1 A 900MHz Radio over Fiber System
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
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).
Figure 2.2 Transport schemes for Fiber/Wireless systems
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
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].
Figure 2.3. IF to RF up-conversion at the base stations
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.
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.
Figure 2.4 Optical Transmission
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
16
theoretical upper bound of 50 THz of bandwidth. By using these large lowattenuation 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, ǻ which is the important parameter for
the fiber given below:
Δ=
(n1 − n2 )
n1
(2.2)
17
Note that this parameter, ǻ will give a positive value because value of n1 must
be larger than n2 in order for a critical angle to exist.
CORE CLADDING
θ
Figure 2.5 Step-Index Fibers. (a) Refractive index profile. (b) End view
(c) Cross-sectional side view [1].
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]
n1
n2
NA
Į0
1.48
1.46
0.24
13.9o
o
Construction
All glass
PCS
1.46
1.40
0.41
24.2
All plastic
1.49
1.41
0.48
29o
ǻ
0.0135
0.041
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
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.
Figure 2.6 Graded-Index Fiber. (a) Refractive index profile.
(b) End view. (c) Cross-sectional side view [1]
Index variation for the Graded-Index fiber can be represented by the
following equation:
n(r) = n1 (1-2(r/a)Į ǻ) , r ” a
(2.3)
n(r) = n1 ¥ (1-2ǻ)
(2.4)
,r•a
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 Machzehnder 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 returnto-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 ȝm
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 BISDN 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 (microcells). 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
carriers, it can recover each sub-carrier without ICI interference from other subcarriers.
Figure 3.1 (a) OFDM Symbol with
Figure 3.1 (b) OFDM Symbol with
Three Orthogonal Sub-carriers in
Spectra of three OFDM sub-carriers
one.
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 ǻf 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 intersubcarrier 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]
s (t ) = Re{¦ an h(t ) exp( j 2π f nt + φ )}
(3.1)
n
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 + nǻf.
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
cm =
1
N
N −1
¦a
n =0
n
§ 2π nm ·
exp ¨ − j
¸
N ¹
©
(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.
QAM
modulator
Data
Serial to
Parallel
Converter
Parallel to
Serial
Converter
IDFT
an
D/A
cm
Figure 3.2 A block diagram of an OFDM transmitter
The OFDM system transmits the wideband data over many narrowband sub
channels. The symbol duration of each subcarrier becomes Nǻt, where ǻt 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 |Įi|2, then the received
signal-to-noise ratio (SNR) of the i-th channel becomes [11]
35
SNRi =
Pi | α i |2
BN 0
(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 |Įi|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 |Įi|2.
Piα =
1
| α i |2
(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
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 Block diagram for multicarrier transmission (1)
Figure 3.4 Block diagram for multicarrier transmission (2)
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
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
Figure 3.5 Block Diagram IFFT/FFT
38
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.
Figure 3.6 Guard Interval
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 applicationdependent. 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 ȝs. The
frequency spacing between subcarriers is 312.5 KHz, which is the inverse of 3.2 ȝs.
41
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]
Table 3.1 Data rates and modulation schemes for the 802.11 a W-LAN system.
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 (a) An example of 8-PSK modulation. (b) An example of a trellis
diagram for a coded modulation scheme
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
ǻ0 = 2sin(ʌ/8), ǻ1 = ¥2 and ǻ2 = 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 ǻ1 = ¥2. With this property, the minimum distance between two sequences
diverging from one state and merging into another is at least ǻ02+ ǻ12+ ǻ22. The
distance between two constellation points associated with the parallel transitions are
ǻ2 = 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]
d 2 free = min(Δ 2 2 , 2Δ12 + Δ 0 2 ) = Δ 23
(3.5)
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 A block diagram of Bit-interleaved coded OFDM
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
n = k / log 2 M
(3.6)
i = kMod log 2 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 ȡn. The received
signal yn at the receiver is given as
yn = ρnxn +nn
and nn is AWGN.
(3.8)
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]
C = max Pr{ y1,..., 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).
•
3.6
Facilitates Single Frequency Networks, i.e. transmitter macro diversity.
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
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.
Optical Link
Figure 3.10 OFDM model for Radio Over Fiber Network
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
48
processes at the receiver are the reverse process of the transmitter. The mathematical
equivalent of OFDM signal is expressed as
N −1
v(t) = ¦ X k e j 2π kt /T
k =0
0”t<T
(3.10)
Where, Xk are the data symbols and T is the OFDM symbol time. The subcarrier 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.
N −1
v(t ) = ¦ X k e j 2π kt /T
k =0
-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.
4.2
50
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.
Figure 4.1 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
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.
Figure 4.2 OFDM Transmitter Design
52
The global parameter setup that using in this simulation are shown in table
4.1 below:
Table 4.1 Global Parameter Setup
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 below shown the sub carrier frequency allocation for the OFDM
modulator:
Table 4.2 Sub carrier Allocation
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
The transmission link in this simulation model or RoF system model is
described by Figure 4.3 below:
Figure 4.3 RoF System Model
The value of each component which was used in transmission link system
parameter are shown in following Table 4.3 below:
Table 4.3 Component’s Values of transmission link
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
4.5
55
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:
Figure 4.4 OFDM Receiver Design
4.6
56
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:
Figure 4.5 OFDM – RoF MoDem
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
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:
Figure 5.1 OFDM signal from transmitter
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
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:
Figure 5.2 OFDM signal after amplified
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
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:
Figure 5.3 OFDM optically modulated by MZM
61
After optically modulated, the signal then transmitted through the fiber link
transmission as we could see from the Figure 5.4 below:
Figure 5.4 OFDM signal after through optical fiber
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
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:
Figure 5.5 OFDM signal from PIN photodetector
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 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.
Figure 5.6 OFDM signal after amplified
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
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 [ ].
Figure 5.7 Basic information contained in Eye diagram
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
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.
Figure 5.8 (a) Eye diagram of received OFDM signal for in phase signal
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
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.
Figure 5.9 Total power signal vs Fiber length after 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
And Figure 5.10 below here is OFDM received total signal power after being
amplified by electrical amplifier 10 dB.
Figure 5.10 Total power signal vs Fiber length after electrical amplifier
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 WLAN 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 fiberoptic 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