THE SUBCARRIER MULTIPLEXING/WAVELENGTH DIVISION
MULTIPLEXING FOR RADIO OVER FIBER
ARIEF MARWANTO
UNIVERSITI TEKNOLOGI MALAYSIA
iv
To,
My Grandfather (Allahumagh firlahu warhamhu wa’afihi wa’fuanhu) who taught
me the spirit of science, My Beloved Father and Mother, My Father & Mother in
law, My lovely Soul of Heart Amalia Hayati, All of my lovely Brothers and Sisters
.
v
ACKNOWLEDGMENT
In the name of Allah, Most Gracious, and Most Merciful, I would like to
thank the many people who have made my master project possible. In particular I
wish to express my sincere appreciate to my supervisor, Dr. Sevia Mahdaliza Idrus,
for encouragement, guidance, critics and friendship. And also to Prof. Abu Bakar,
thank you very much for teach and introduce me about Optical Communication in
the class. Furthermore, I would like to sincere appreciation with deeply for all my
panels PM 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 my loving
support of my wife, my family and UNISSULA.
My sincere appreciation extends to all my best friends; Mahyuddin, Hilman
Haroun, Norizan, Hamim Nashoha, My Brother Muhammad Affan Jhoni, Ferdian
Yunazar, Evizal, Pak Qomar, Pak Imam MIS, “Pak Jendral” Prabowo Setyawan,
Pak Gatot R, Pak Ni’am, Bu Suryani Alifah, Mr. Akmad the PTC UTM staff, Mr.
Akhmed Bashir and others who have provide assistance. Their views and tips are
useful indeed. My deeply sincere appreciation also given to DR. Dr. Rofiq Anwar,
Sp.PA, Ir. H. Sumirin, MS., Ir. Muhammad Haddin, MT, for their support, advice
and spirit to me. Unfortunately, it’s not possible to list all of them in this limited
space. I am grateful having all of you beside me. Thanks you very much.
vi
ABSTRACT
In this project, we review the system configurations and performance of
high-speed digital optical transmission using sub-carrier multiplexing (SCM) and
Wavelength Division Multiplexing (WDM). The systems are setup to gain the
performance of channels spacing especially for bandwidth efficiency and compare it
by the modulation techniques.
A radio-over-fiber (RoF) distribution system
incorporating both SCM and WDM technologies is presented. In the system model,
the sixteenth input signals are modulated with different electrical carriers at
microwave frequencies and then they are merged by using a combiner. The
combined of the signal is then modulated by external modulation techniques using
Mach Zehnder Modulation that has own bandwidth is 20 GHz. In WDM, each of N
different wavelengths is transmitting at N times the individual CW laser speed,
providing a significant capacity enhancement. The WDM channels are separated in
wavelength to avoid cross-talk when they are demultiplexed by a non-ideal optical
fiber. In this scheme, multiple optical carriers are launched into the same optical
fiber through the WDM technique. Each optical carrier carries multiple SCM
channels using several microwave subcarriers. One can mix analog and digital
signals using different subcarriers or different optical carriers. At the receiver end,
the optical signal is converted back to an electrical domain by an APD photodetector
and filtered by Bandpass Rectangle filter. The particular signals then demultiplexed
and demodulated, using conventional detection methods. The outcomes of
bandwidth was increased to 60 GHz by applying of 16 channel of SCM combined to
WDM in optical fiber link. The combination of WDM and SCM provides the
potential of designing broadband passive optical networks capable of providing
integrated services (audio, video, data, etc.) to a large number of subscribers.
vii
ABSTRAK
Didalam projek ini, kita mengkaji performance dan konfigurasi dari
transmisi kecepatan tinggi optical digital menggunakan SCM yang dikombinasikan
dengan WDM. Efisiensi lebar pita diperoleh dengan membandingkan teknik
pemodulasi khususnya dari kanal - kanal di WDM. Sistem distribusi RoF dan
teknologi SCM/WDM telah dikembangkan pada projek ini. Di dalam sistem ini, 16
sinyal input dimodulasi menggunakan pembawa sinyal elektrik yang berbeda, yang
beroperasi pada frekuensi gelombang mikro, kemudian digabungkan menggunakan
pengkombinasi sinyal. Sinyal yang telah dikombinasikan, dimodulasi kembali
menggunakan sistem modulasi eksternal yaitu Mach Zehnder Modulator yang
mempunyai lebar pita sebesar 20 GHz. Di sistem WDM, setiap dari N laser
wavelength yang berbeda ditransmisikan dengan N, dikalikan kecepatan individual
pada CW laser sehingga menghasilkan peningkatan kapasitas yang signifikan. Kanal
– kanal dari WDM dipisahkan dalam wavelength dengan minimum jarak kanalnya
adalah 50 GHz, hal ini diaplikasikan untuk mencegah terjadinya cross-talk ketika didemultiplek oleh filter optik non ideal. Dalam skema ini, pembawa sinyal – sinyal
optik jamak ditransmisikan melalui kanal fiber optik menggunakan teknik WDM.
Setiap pembawa sinyal optik dibawa oleh beberapa kanal SCM menggunakan
subcarrier gelombang mikro. Pada sisi penerima, sinyal optik diubah menjadi sinyal
elektrik yang diterima oleh detektor optik APD dan ditapis oleh filter Band Pass
Rectangular. Sinyal yang telah diterima APD selanjutnya akan di demodulasi dan
demultiplek dengan menggunakan teknik deteksi konvensional. Hasil dari penerapan
sinyal input SCM sebanyak 16 kanal yang dikombinasikan dengan sistem WDM
menghasilkan lebar pita sebesar 60 GHz. Kombinasi SCM/WDM merupakan
teknologi yang paling potensial untuk dikembangkan untuk penyediaan komunikasi
broadband yang meng-integrasikan layanan audio, data, video, dan lain – lain
sehingga dapat meningkatkan kapasitas pelanggan dalam jumlah yang sangat besar.
viii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
DEDICATION
iv
ACKNOWLEDGEMENTS
v
ABSTRACT
vi
ABSTRAK
vii
TABLE OF CONTENTS
viii
LIST OF TABLES
xi
LIST OF FIGURES
xii
LIST OF ABBREVIATIONS
xv
LIST OF SYMBOLS
xvii
LIST OF APPENDIX
xviii
INTRODUCTION
1.1
Historical perspective
1
1.2
Problem Statement
3
1.3
Objective
4
1.4
Scope of the Work
1.5
1.6
Methodology
Thesis Outline
RADIO OVER FIBER COMMUNICATION
SYSTEM
2.1 Radio over Fiber
5
6
7
9
ix
2.1.1 Introduction to RoF Optical Links
10
2.1.2. Basic Radio Signal Generation and
11
Transportation Methods
2.1.3
2.2
3
RoF Link Configurations
Optical Modulator
15
2.2.1 Mach Zehnder modulator.
15
2.2.2 Principle of MZ modulator
16
2.2.3 Electrooptic Phase modulator
18
2.3
Light Source
19
2.4
Fiber link
20
2.4.1
Step-Index Fiber
20
2.4.2
Graded-Index Fiber
23
2.5
Optical amplifier
24
2.6
Literature Review
26
THE SCM/WDM FOR ROF COMMUNICATION
3.1. Introduction
29
3.2. Subcarrier Multiplexing (SCM)
30
3.3. Analog SCM
31
3.4. Digital SCM
32
3.5. Basic WDM Scheme
34
3.5.1 Multiplexer and Demultiplexer
3.6. The SCM/WDM System for Radio over Fiber
4
13
35
37
THE SCM/WDM SYSTEM MODEL
4.1. Introduction
39
4.2. The SCM/WDM System Model
40
4.3. The Transmitter Model
42
4.4. The Transmission Link Model
46
4.5. The Receiver Model
50
4.6. Conclusion
53
x
5
SIMULATION RESULT AND PERFORMANCE
ANALYSIS
5.1. Introduction
54
5.2. The Transmitter Simulation Results
56
5.3. The Transmission Link Simulation Results
60
5.4. The Receiver Simulation Results
63
5.5. The Eye Diagram
65
5.6. Performance Analysis of The SCM/WDM for RoF
System
68
5.7. Analysis of The Total Power to The EDFA Length
68
5.8. The Performance of WDM Mux/Demux
70
5.9. The Carrier-to-Noise Ratio (CNR) Performance
71
5.10. Analysis on The Number of Channels
72
5.11. The Performance Analysis of Signal-to-Noise-Ratio
(SNR)
73
5.12. Bit-Error Rate Performance
74
5.13. Analysis of Nonlinearity Due to Optical Power
75
Level
6
5.14. Analysis of RF Bandwidth Spectrum
78
5.15. Conclusion
79
CONCLUSIONS AND RECOMMENDATION
6.1
Discussions
80
6.2
Conclusions
81
6.3
Future Recommendations
82
REFERENCES
84
Appendix A
93
xi
LIST OF TABLES
TABLE NO.
TITLE
PAGE
2.1
Typical Step-Index Fiber characteristic
22
4.1
Global simulation setup
43
4.2
Subcarrier frequency allocation
44
4.3
Simulation setup for EDFA
47
4.4
Simulation setup for Single Mode Fiber
49
4.5
Simulation setup for APD Photo detector
51
5.1
WDM Mux Results
61
5.2
The WDM Demux Results
62
5.3
A basic simulation setup for SCM/WDM-RoF with
70
EDFA
5.4
Basic simulation setup for nonlinearity of power levels
76
xii
LIST OF FIGURES
TITLE
PAGE
FIGURE NO.
1.1
The methodology flow chart
6
2.1
Intensity-modulation direct-detection (IMDD) analog
12
optical link
2.2
Representative RoF link configurations.
14
(a) EOM, RF modulated signal.
(b) EOM, IF modulated signal,
(c) EOM, baseband modulated signal.
(d) Direct modulation.
2.3
Basic configuration of Optical modulator
15
2.4
Basic architecture of using Phase Modulator
18
2.5
Typical Layout of a Bidirectional Analog Optical Link
20
Using Direct Modulation of Laser Diodes
2.6
Step-Index Fiber
21
(a) Refractive index profile
(b) End view.
(c) Cross-sectional side view
2.7
Graded-Index Fiber
23
(a) Refractive Index profile
(b) End view
(c) Cross-sectional side view
2.8
Schematic diagram of a simple Doped Fiber Amplifier
24
xiii
2.9
(a) Transmitted data at STM-4 rate
27
(b) Received data at STM-1 rate for 70 km
27
(c) The eye pattern generated by SCM at 622 Mbps at
28
70 km.
3.1
Basic configuration of RF modulation
30
3.2
Schematic of a WDM fiber Link
34
3.3
Multiplexing and Demultiplexing in a Unidirectional
36
System
3.4
Multiplexing and Demultiplexing in a Bidirectional
36
System
3.5
The SCM/WDM Link Configuration
37
4.1
Model of eight channels the SCM/WDM-ROF system.
41
4.2
Transmitter for electrical (RF) domain
42
4.3
Transmitter for optical domain
45
4.4
Transmission link
47
4.5
The receiver for optical domain
51
4.6
The receiver for electrical domain
52
5.1
Signal wave modulated by PSK
56
5.2
(a) RF spectrum for data signal on first channel
57
(3.6GHz).
(b) RF spectrum for data signal on eleventh channel
(23.4 GHz).
(c) RF spectrum for data signal on fourth channel
(10.8 GHz).
(d) RF spectrum for data signal on tenth channel
(16.2 GHz).
5.3
RF spectrum for adding eight channels of SCM-1 and
58
SCM-2
5.4
Amplified composite RF signals
58
5.5 (a) & (b)
Spectrum of the signal after Mach Zehnder Modulator
59
xiv
5.6 (a) & (b)
WDM Mux Signal Spectrum output
60
5.7
WDM Demux Spectrum Signals
61
5.8
Optical Spectrum After EDFA
62
5.9
Optical spectrum and Optical Power after propagated
63
150 km in fiber
5.10
RF spectrum detected by photo detector
63
5.11
Amplified received RF spectrum
64
5.12
Demodulation sample of the RF spectrum for channel
64
5.13
Received electrical signal wave
65
5.14
Eye Diagram for the channel 1 of SCM-1 Channel
66
5.15
Eye Diagram for the channel 2 of SCM-1 Channel
66
5.16
Eye Diagram for the channel 3 of SCM-1 Channel
66
5.17
Eye Diagram for the channel 8 of SCM-1 Channel
67
5.18
Eye Diagram for the channel 2 of SCM-2 Channel
67
5.19
Eye Diagram for the channel 8 of SCM-2 Channel
67
5.20
(a): The performance of the total power to the Fiber
69
Length with and without EDFA in 100 km
(b) The performance of Total power to the fiber length
with and without EDFA in 150 km
5.21
The OSNR performance of the WDM Mux /Demux
71
5.22
CNR for SCM-1 Channels with power 1 mW
71
5.23
CNR performance for SCM-2 Channel with power
72
5mW
5.24
5.25
The performance of RF carrier signal for SCM-1 &
SCM-2
(a) SNR performance of SCM at 100 km
73
74
(b) SNR performance of SCM at 150 km
5.26 (a) & (b)
The performance of BER for SCM-1 and SCM 2
75
5.27
Nonlinearity Performance of the Total Power versus
76
Fiber Length
5.28 (a), (b), (c)
The performance of Total Power (0 dBm) to the Fiber
77
Length
5.29:
The performance of RF bandwidth Spectrum are
expanded to 60 GHz
78
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
λ
-
Wavelength
h
-
Blank’s Constant
C
-
Velocity of Light
Eg
-
Energy Gap
fc
-
Cut-off frequency
η
-
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
α
-
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
xviii
LIST OF APPENDIX
APPENDIX
A
TITLE
Accepted Paper: “The SCM/WDM
System Model for Radio over Fiber
Communication Link” RAFSS 2008 Ibnu
Sina Institute of UTM, 27 – 29 May 2008
PAGE
94
CHAPTER 1
INTRODUCTION
1.1 Historical Perspective
The prevalent utilization of Internet by business and consumer has been
generating a global demand for huge bandwidth. In recent years, as new bandwidth
hungry applications like internet video and audio and new access technologies such as
xDSL become more popular, optical communications networks are finally feeling the
bandwidth constraints already faced in many other communications networks such as
wireless and satellite communication systems. Service providers are searching for ways
to increase their fiber optic network capacity.
In order to solve this problem, people have been trying to make full use of the
huge bandwidth provided by optical fibers. Technologies like TDM, CDMA, SCM,
WDM and their combinations are used and improved.
2
One technology that can be used to increase the efficiency of bandwidth
utilization is the Sub-carrier multiplexing (SCM). It is an old technology that has been
studied and applied extensively in microwave and wireless communication systems.
The use of subcarrier multiplexing (SCM) transmission using an optical carrier
instead of the traditionally used super carrier over optical fibers is very attractive. This
technology has found wide spread application because of its simplicity and costeffectiveness. In optical domain, the most popular SCM application is the optical analog
video transmission and distribution.
Error correction coding techniques, such a block convolution, and trellis, have
advanced, further enhancing the noise immunity of multi state modulation scheme.
Thus, the type of modulation mentioned plus coding techniques can be very good
candidates for SCM application.
Later, as technology advanced, WDM came along. The WDM strategy is to
make better use of optical fiber bandwidth by stacking many TDM channels into the
same fiber. Wavelength division multiplexing (WDM) is recognised as a key
technology for increasing the capacity of distributive optical networks. To aid the
design of such WDM systems, simulation tools are needed to provide for accurate
synthesis and evaluation of proposed network architectures. Multichannel ligthwave
networks present challenges in terms of how to most effectively represent multiple
optical carrier signal formats and associated components and also with respect to
performance evaluation in the presence of more complicated effects such as crosstalk.
3
1.2 Problem Statement
Recent years have seen enormous growth in the deployment of mobile phones
and its service application. The revolution of mobile communication from the 1st
generation till latest generation are shown the needed of mobile communication.
Wireless access – fixed or mobile – is regarded as an excellent way to achieve
broadband services. Of course, it is the only possibility for mobile access (in particular
if global mobility is required), however wide application of fixed wireless broadband
access is also foreseen.
The mobile communication architecture shown that evolution from 1G to the
latest generation focused to efficiency and effectively of using frequency spectrum
allocation. Due to the channel allocation (dispersion limitation) provides are limited. On
the end-user side, the evolution are affected by enormous demand for services and
content application for mobile communication, which users day to day their needed of
easily communication, highly data rate and internet services, multimedia streaming
application and spreadsheets processing for they mobile phone.
The oldest method of wavelength generation for the mobile application between
the Mobile Switching Centre (MSC) and Base Station (BS) are using cooper cabling
and microwave radio for data transceiver. In this system the spectrum allocation and
bandwidth are required highest power, low data rate, highest attenuation and highly
losses is not sufficient to overall the traffic demand by end-users.
It is well known that both due to unavailability of lower microwave frequencies
and to the insufficient bandwidth of lower frequency ranges, next generation wireless
access systems – both mobile and fixed – will operate in the upper
4
microwave/millimeter wave frequency band. As in a cellular system both increased
traffic and propagation properties of millimeter-waves require small cells, further as
millimeter-wave circuits are rather expensive, the cost of base stations (BSs) will be of
determining role.
The main problems are dispersion limitation of the link. A radio-over-fiber
(RoF) distribution system incorporating both sub-carrier multiplexing and wavelength
division multiplexing (WDM) technologies is presented. This signal is directly
modulated onto three high-speed lasers. Bragg filters are employed at the receiver base
station in order to both demultiplex the required optical channel, and ensure that the
detected signal is single side band (in order to overcome dispersion limitations of the
link). System spectral efficiency is optimised by wavelength interleaving. The channel
spacing between the WDM channels is varied and the system performance for different
values of channel spacing and spectral efficiencies is investigated.
1.3 Objective
The main objective of this project is to increase radio channel allocation by
using Sub-carrier Modulation/Wavelength Division Multiplexing techniques with the
aimed of using fibre optic as part of an access link between the Mobile Switching
Centre and the Base Station (BS).
The specific objectives are modeling and simulate optical Sub Carrier
Multiplexing – Wavelength Division Multiplexing for Radio over Fiber (SCM/WDM -
5
ROF) System. To analyze the performance of the SCM/WDM-ROF System in term of
distortion, channel spacing, Eye Diagram (BER), distortion and optical spectrum.
1.4 Scope of Work
Provide a proper study concerning channel allocation using Sub-carrier
Multiplexing/Wavelength Division Multiplexing technique, the project begins with
literature study and full understanding of the optical transmitter system (Laser Diode,
SCM and WDM) and its characteristics. The literature concern on the SCM/WDM
techniques in order to increased channel allocation and bandwidth.
The system will be simulated in Opti System Software, with special focused to
the SCM/WDM characteristics. These characteristics will be modelled using by
wavelength interleaving, the channel spacing between the WDM channels is varied and
the system performance for different values of channel spacing and spectral efficiencies
and the relatively Optical Single Side Band (OSSB), Optical Double Side Band (ODSB)
which is the number of WDM channels are multiplexed are 4 x 4, 8 x 8 and 16 x 16
wavelength series (WDM).
6
1.5 Methodologies
The methodology of this project will follow the next flow chart:
START
Literature review on current development of the optical system especially Sub
Carrier Multiplexed/ Wavelength Division Multiplexing – Radio over Fiber
(SCM/WDM-ROF) System.
Identify and modeled system architecture.
Model the connection from transmitter to receiver.
Identify the simulation software.
Analysis of the system will be on to non linear distortion of the optical
modulator, channel spacing, Eye Diagram and optical spectrum.
Analysis also will be tested by two types of RF modulation scheme.
END
Figure 1.1: The methodology flow chart
7
First, full understand and literature review on current development of the optical
system especially SCM/WDM for radio over fiber (RoF) system. After that we have to
identified and modeled architecture system. Then the system will be modeled which
represented the connection from transmitter to receiver. The choosing of suitable
simulation software will be identified and applied to the system such as Opti System
software. Next stage is analyzed of the system will be on to non linear distortion of the
optical modulator, channel spacing, Eye Diagram (BER) and optical spectrum. And
finally we have to apply of testing and measurements by the RF modulation scheme.
1.6 Thesis Outline
This project provides a theoretical model for radio over fibre technique.
SCM/WDM are used to modelled the WDM channel spacing or channel allocation and
bandwidth from the transmitter over optical fibre links (SMF/MMF) with the EDFA as
an amplifier using comparative study of output-to-input approach. The main elements
for this thesis were divided into 6 chapters.
Chapter 1 gives an introduction to the project, along with this aims, objective,
and scope of work, problems statements, and methodology of this project.
Chapter 2 explains the general radio over fibre communication system and full
understanding of the parts of optical transmitter elements, multiplexing techniques,
wavelength, modulation techniques, and receiver components and their characteristics.
Also this chapter are focuses on literature review for previous work relative to this
project.
8
Chapter 3 review the characteristics of the wavelength transmission and the
frequency channel limited allocation (dispersion limitation), bandwidth distortion in
optical transmission medium and the techniques available to overcome it such as a subcarrier modulation/wavelength dense multiplexing (SCM/WDM) techniques.
Chapter 4 are discusses the methodology of the project were the SCM/WDM
technique and OPTIWAVE software are uses to model and implement the system is
presented. In this chapter are proposed SCM/WDM with BPSK modulation system for
radio over fibre which is using OSSB or ODSB and 4x4 WDM, 8x8 WDM, 16 x 16
WDM for channel multiplexer is presented.
Chapter 5 discussed the modelling of the radio over fibre using SCM/WDM and
presents the performance analysis of the plotted graph after implementing the model in
OPTIWAVE software.
Chapter 6 focuses the conclusion of the results and how far the overall
objectives were achieved. This chapter also provide the recommendation future
development and modifications of the findings.
CHAPTER 2
RADIO OVER FIBER COMMUNICATION SYSTEM
RoF is an analog optical link transmitting modulated RF signals. It serves to
transmit the RF signals down- and uplink, i.e. to and from central stations (CS) to
base stations (BS) called also radio ports. RF modulation is in most cases digital, in
any usual form such as PSK, QAM, TCM, etc.
2.1 Radio over Fiber
Radio over Fiber is a technique that modulates RF in microwave signals on
an optical carrier to take advantage of the relatively low loss of optical fibers [1, 2].
Many Radio over Fiber systems employ a Mach Zehnder modulator (MZM) to
amplitude modulate the light carrier [3].
10
MZMs typically have tremendous bandwidth that can easily exceed 40 GHz.
While this bandwidth is necessary for conventional fiber optic communications,
only a gigahertz or so of bandwidth is needed for radio over fiber applications. In
most data transmission and multi-point video/data distribution systems, information
is routed at baseband to the local transmission nodes, where it is up converted. The
signals are in analog form and often involve many individual digitally modulated
carriers spread over a GHz or more of bandwidth. Since only a fraction of the MZM
bandwidth is utilized in Radio over Fiber systems, linearization is a practical and
attractive method to achieve performance enhancement.
2.2.2
Introduction to RoF Optical Links
Unlike conventional optical networks where digital signal is mainly
transmitted, RoF is fundamentally an analog transmission system because it
distributes the radio waveform, directly at the radio carrier frequency, from a CS to a
BS. Actually, the analog signal that is transmitted over the optical fiber can either be
RF signal, IF signal or baseband (BB) signal. For IF and BB transmission case,
additional hardware for up converting it to RF band is required at the BS. At the
optical transmitter, the RF/IF/BB signal can be imposed on the optical carrier by
using direct or external modulation of the laser light.
In an ideal case, the output signal from the optical link will be a copy of the
input signal. However, there are some limitations because of non-linearity and
frequency response limits in the laser and modulation device as well as dispersion in
the fiber. The transmission of analog signals puts certain requirements on the
linearity and dynamic range of the optical link. These demands are different and
more exact than requirements on digital transmission systems [16].
11
2.2.2
Basic Radio Signal Generation and Transportation Methods
In this section, Kim Hong Bong [55] has gives a brief overview of how to
generate and transport radio signal over an optical fiber in RoF networks is given.
Virtually all of the optical links transmitting microwave/mm-wave signals apply
intensity modulation of light [13]. Essentially, three different methods exist for the
transmission of microwave/mm-wave signals over optical links with intensity
modulation: (1) direct intensity modulation, (2) external modulation, and (3) remote
heterodyning [17]. In direct intensity modulation an electrical parameter of the light
source is modulated by the information-bearing RF signal.
In practical links, this is the current of the laser diode, serving as the optical
transmitter. The second method applies an un-modulated light source and an
external light intensity modulator. This technique is called external modulation. In a
third method, RF signals are optically generated via remote heterodyning, that is, a
method in which more than one optical signal is generated by the light source, one
of which is modulated by the information-bearing signal and these are mixed or
heterodyned by the photodetector or by an external mixer to form the output RF
signal [55].
Direct intensity modulation is the simplest of the three solutions. So it is used
everywhere that it can be used. When it is combined with direct detection using PD,
it is frequently referred to as intensity-modulation direct-detection (IMDD) (Fig.
2.1). A direct-modulation link is so named because a semiconductor laser directly
converts a small-signal modulation (around a bias point set by a dc current) into a
corresponding small-signal modulation of the intensity of photons emitted (around
the average intensity at the bias point). Thus, a single device serves as the optical
source and the RF/optical modulator (Figure 2.1). One limiting phenomenon to its
use is the modulation bandwidth of the laser. Relatively simple lasers can be
modulated to frequencies of several gigahertzes, say, 5-10 GHz [55].
12
Although there are reports of direct intensity modulation lasers operating at
up to 40 GHz or even higher, these diodes are rather expensive or nonexistent in
commercial form. That is why at higher frequencies, say, above 10 GHz, external
modulation rather than direct modulation is applied. In entering into the millimeter
band a new adverse effect, such as the non-convenient transfer function of the
transmission medium, is observed. It turns out that the fiber dispersion and coherent
mixing of the sidebands of modulated light may cause transmission zeros, even in
the case of rather moderate lengths of fiber. For example, a standard fiber having a
one km length has a transmission zero at 60 GHz if a 1.55-μm wavelength light is
intensity modulated. Due to this phenomenon, optical generation rather than
transmission of the RF signal is preferable [55].
Because the number of BSs is high in RoF networks, simple and costeffective components must be utilized. Therefore, in the uplink of a RoF network
system, it is convenient to use direct intensity modulation with cheap lasers; this
may require down conversion of the uplink RF signal received at the BS. In the
downlink either lasers or external modulators can be used [55].
Figure 2.1: Intensity-modulation direct-detection (IMDD) analog optical link [55]
13
2.2.2
RoF Link Configurations
In this section a typical RoF link configuration are discussed, which is
classified based on the kinds of frequency bands (baseband (BB), IF, RF bands)
transmitted over an optical fiber link [58].
Here, assumed that a BS has its own light source for explanation purpose;
however, BS can be configured without light source for uplink transmission. In each
configuration of the figure, BSs do not have any equipment for modulation and
demodulation, only the CS has such equipment. In the downlink from the CS to the
BSs, the information signal from a public switched telephone network (PSTN), the
Internet, or other CS is fed into the modem in the CS. The signal that is either RF, IF
or BB bands modulates optical signal from LD. As described earlier, if the RF band
is low, we can modulate the LD signal by the signal of the RF band directly. If the
RF band is high, such as the mm-wave band, we sometimes need to use external
optical modulators (EOMs), like electro-absorption ones [55].
The modulated optical signal is transmitted to the BSs via optical fiber. At
the BSs, the RF/IF/BB band signal is recovered to detect the modulated optical
signal by using a PD. The recovered signal, which needs to be upconverted to RF
band if IF or BB signal is transmitted, is transmitted to the MHs via the antennas of
the BSs [55].
14
Figure 2.2: Representative RoF link configurations [55].
(a) EOM, RF modulated signal.
(b) EOM, IF modulated signal,
(c) EOM, baseband modulated signal.
(d) Direct modulation.
This is especially important when RoF at mm-wave bands is combined with
dense wavelength division multiplexing (DWDM). However, this increases the
amount of equipment at the BSs because an upconverter for the downlink and a
down converter for the uplink are required. In the RF subcarrier transmission, the
BS configuration can be simplified only if an mm-wave optical external modulator
15
and a high-frequency PD are respectively applied to the electric-to-optic (E/O) and
the optic-to-electric (O/E) converters [55].
For the uplink from an MH to the CS, the reverse process is performed. In
the configuration shown in Figure 2.2 (a), the signals received at a BS are amplified
and directly transmitted to the CS by modulating an optical signal from a LD by
using an EOM. In the configuration (b) and (c), the signals received at a BS are
amplified and down converted to an IF or a baseband frequency and transmitted to
the CS by modulating an optical signal from a LD by using an EOM. In the
configuration (d), the signals received at a BS are amplified and down converted to
an IF or a baseband frequency and transmitted to the CS by directly modulating an
optical signal from a LD [55].
2.2
Optical Modulator
2.2.1
Mach Zehnder Modulator.
λ
Laser diode
Modulated
RF signal
Optical
Modulator
Modulated
Optical
Figure 2.3: Basic configuration of Optical modulator
16
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
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].
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].
2.2.2
Principle of MZ modulator
The principle of MZ modulator is very simple: an input light is coupled to
two waveguide branches of the same length and shape (the lengths could be different
for bias purpose), these two branches are fabricated by LiNbO3 and electrical field
can be applied on each of them separately. When the electrical field that is applied to
these arms changed, the effective refractive index of the waveguide will change, this
17
change can be seen as linearly related to the applied field and the corresponding
phase delay change also has a linear relationship with the applied electrical filed
intensity.
Thus the phase delay of light in each waveguide can be controlled by those
external electrical fields. The two light waves are then coherently added together at
the end of the waveguide by another coupler. If the input is Ei exp(− jω0t ) , the
output of such a modulator is
E0 =
(
)
1 − jφ1
e
− e − jφ 2 E i exp( − j ω 0 t )
2
(
)
1 − jφ 0 − jΔ φ / 2
e
e
− e − jΔ φ / 2 E i exp( − j ω 0 t )
2
⎛ Δφ ⎞
= sin ⎜
⎟ E i exp( − j φ 0 t ) exp( − j ω 0 t )
⎝ 2 ⎠
=
Here, φ1, φ2 are the phase delay of the two arms respectively, Δφ =
difference between the two arms. φ0 =
φ1 + φ2
2
(2.1)
φ1 - φ2 , is the
is the average phase delay of the two
⎛ ⎛ Δφ ⎞ ⎞
arms. ⎜⎜ sin ⎜
⎟ ⎟⎟ Represents the amplitude modulation, exp(− jφ0 ) is the phase
⎝ ⎝ 2 ⎠⎠
modulation term. By changing the value and relationship of φ1 , φ2 we can achieve
many different kind of modulation.
18
2.2.3
Electrooptic Phase modulator
Figure 2.4: Basic architecture of using Phase Modulator [61].
Previous studies state that another way to convert all subcarrier frequency
without O/E and E/O conversion is applying the Electrooptic Phase Modulator as the
optical modulator [61].
From Figure 2.4 shows the basic architecture of using the Phase Modulator
as optical modulator in the Sub carrier Multiplexed Radio over Fiber system.
Theoretically, laser diode generates a several low microwave frequency and electro
optic Phase Modulator will perform all-optical mixing. The modulated signal fed to
Electrooptic Phase Modulator applies ωLO as a local oscillator to up conversion the
frequency from 3 GHz into 11.5 GHz [61].
The phase modulator then will generate various optical sidebands after the
modulated signal has been up conversion by local oscillator. The frequency
deviations of the modulated signals were consisting of ωOP +/- ωSC, +/- ωLO, +/2ωSC, +/- 2ωLO. If phase modulator directly detected using a photo detector, no
mixing signals between the sub carriers with the local oscillator signal will be
detected. Only Intensity Modulated microwave signal generated at the laser diode
will be detected. However if the modulated signal transmitted through a dispersive
device, such as SMF, the phase relationship between all spectral lines will be change
to fully or partially in phase cause of the chromatic dispersion of the fiber.
19
2.3 Light Source
Radio-over-fiber systems (RoF) potentially have offered significant
flexibility, economic advantage, and large capacity in the access network. When
multiple subcarrier multiplexing (SCM) radio-frequency (RF) signals are transmitted
in RoF link, the undesired harmonics and intermodulation distortion products are
produced by nonlinearity of optical source such as distributed-feedback laser diode
(DFB LD) or external modulators. These products can lead to degradation of signal
quality of adjacent channels. In addition, it is more important to minimize thirdorder intermodulation distortion products (IMD3) because they cannot be easily
filtered in case of narrow band systems.
For optical fiber communication system requiring bandwidth greates than
200 MHz, the semiconductor injection laser diode is preferred over the LED, laser
diode typically have response times less than 1 ns, have optical bandwidth of 2 nm
or less, and, in general are capable of coupling several tens of milliwatts of useful
luminescent power into optical fibers with small cores and small mode-field
diameters. [1]
Figure 2.5 shows the layout of a simple bidirectional directly modulated RoF
link. In each direction the input RF signal is applied to a laser diode where it
modulates the intensity of the output light. In most case this light will have a
wavelength of either 1,300 or 1,550 nm for low transmission loss in silica fiber. The
fiber may be multimode or single mode, although the latter is preferred for link
spans of more than a few tens of meters for its low dispersion properties. The optical
receiver usually consists of a p-i-n diode, which provides an RF power output
proportional to the square of the input optical power [17].
20
Central Unit (CU)
Remote Antenna Unit (RAU)
LD
PD
Optical
Fiber
RF in/out
PD
RF in/out
LD
Figure 2.5: Typical Layout of a Bidirectional Analog Optical Link Using Direct
Modulation of Laser Diodes [17].
2.4
Fibre link
One of the components that are contributed to the performance of this
particular system is optical fiber. Recently, it was easy for optical communication
system designer to build a communication system using optical device because the
ability and facility provide by the design software make their job easier. However, it
was essential information for a new researcher to know a little background about
optical fiber. Basically there are two types of optical fiber, first so called as StepIndex Fiber and second Graded-Index Fiber [1].
2.4.1
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:
21
Sin θc = n2 / n1
(2.2)
and the functional refractive index change, Δ which is the important parameter for
the fiber given below:
Δ=
(n1 − n2 )
n1
(2.3)
Note that this parameter, Δ will give a positive value because value of n1 must be
larger than n2 in order for a critical angle to exist.
n2
n1
n2
n1
n
(a)
(b)
n2
CORE CLADDING
n1
θ
(c)
Figure 2.6: Step-Index Fiber. (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).
22
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 ratelength products. The NA of this fiber is small, making it difficult to couple light into
them efficiently.
Table 2.1: Typical Step-Index Fiber characteristic [1]
Construction
n1
n2
NA
α0
Δ
All glass
1.48
1.46
0.24
13.9o
0.0135
PCS
1.46
1.40
0.41
24.2o
0.041
All plastic
1.49
1.41
0.48
29o
0.054
Other than information jotted at Table 2.1, there another several parameters
needs to be considering before making a decision. Attenuation, dispersion, losses
and bandwidth are the most important parameters need to identify at first before laid
down the fiber.
All glass fibers have the lowest losses and the smallest intermodal pulse
spreading. Because of these properties, they are useful at moderate high information
rates or fairy long lengths. The low NA of the SI glass fiber results in large losses
during coupling from a light source. The low transmission loss partially
compensates for this problem. [1]
Because PCS fibers have a higher losses and large pulse spread than al-glass
fibers, they are mostly suitable for shorter links. Their higher numerical apertures
are able to increase the efficiency of the fiber in term of coupling the light into the
fiber. However, his advantages suddenly vanished by the highly absorption of the
23
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].
2.4.2
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.
n2
n1
2a
r
n2
z
nr
Figure 2.7: Graded-Index Fibre. (a) Refractive index profile. (b) End view.
(c) Cross-sectional side view [1].
24
Index variation for the Graded-Index fibre can be represented by the
following equation:
n(r) = n1 (1-2(r/a)α Δ) , r ≤ a
(2.4)
n(r) = n1 √ (1-2Δ)
(2.5)
,r≥a
Light rays travel through the fibre 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 2.4 and 2.5 cause them to be periodically
refocused. It can be easily illustrate this redirection by modelling the continuous
change in refractive index by a series of small step changes.
2.5
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
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
25
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 (sponteaneous
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 µm
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.
2.6 Literature Review
There are many research works carried out on the dispersion limitation on
radio over fibre of the optical communication systems. A number of authors have
reported an SCM/WDM improvement using wavelength interleaving or channel
spacing, this parts reviews some of these works.
The WDM technique works by modulating several Tributary data on
different wavelengths or carriers. Each WDM channel operates at a different
wavelength in order to avoid crosstalks when multiplexed and demultiplexed in nonideal optical fiber. Each channel will be recovered by detecting a selected
wavelength, this approach is capable of fully utilizing the fiber bandwidth, but it is
not an economical way for the slower speed channels to use the available THz
bandwidth capacity with the data that is being sent for monitoring purposes [22].
The deployment of multiple wavelengths closely spaced between each other gives
rise to the detrimental non-linear effects. Besides that, each node requires a second
laser to transmit the control packets, resulting in a substantial increase in the cost of
optoelectronics devices per node [23].
One way to avoid these difficulties is to use the Sub-carrier Multiplexing
(SCM) technique to implement the control channels by Mohd Fairuz Yusof, Malek
Al - Qdah, Siti Barirah Ahmad Anas, Mohamad Khazani Abdullah [24]. This
approach eliminates the need for a second laser at each node, and alleviates the
27
control channel contention problem by channeling the control packets. As such, the
control channel rate is kept low, typically in the 10 -100 M bit/s range, making it
feasible to process the control data with low cost silicon technology.
The recovered bit streams and the corresponding eye pattern are used as the
measurement criteria to determine the working condition of the systems. In this
paper, the results are based on one channel performance, to represent other channels.
They shows that the transmitted and recovered data of one of the sub-carrier
channels running at STM- 1 native rate for a total rate of 2 x 155 Mbps = 300 Mbps
over a single optical wavelength. The recovered pulses are similar to the original
ONES. Fairoz Yusof and friends [24] also depicts the eye-pattern of the received
signal showing a wide eye opening with corresponding Bit Error Rate (BER) of l.2 x
10-36 confirming the successfully transmission of the data. For the STM-1 study,
sub-carrier channels (500 MHz and 1 GHz) are used on a single optical wavelength.
(a)
(b)
28
(c)
Figure 2.9: (a) Transmitted data at STM-4 rate
(b) Received data at STM-1 rate for 70 km
(c) The eye pattern generated by SCM at 622 Mbps at 70km. [24]
Fig. 2.9(a) and 2.9(b) show the transmitted and recovered data running at
STM-4 native rate for a total rate of 8 x 622 Mbps = 5 Gbps over the eight channels
carried by two optical wavelengths. The corresponding eye-pattern is shown in Fig.
2.9(c), the BER achieved is 2.18475 x 10-27 expected, and the transmission quality at
STM-4 rate is less than that at STM-1 rate. The BER is higher and the eye opening
is smaller for the STM-4 rate. The results above clearly show the successful
application of the SCM technique. For example at 1.5 Gbps transmission rate, BER
of 10-20 requires the sub-carrier frequency to be at least 3 GHz while BER of 10-12
requires the sub-carrier frequency of 1.7 GHz. This study also shows that channels
can be easily added to the system.
Subsequently, to increase the capacity further, the same approach can be
repeated for different optical wavelengths as in WDM system. Only now, the
wavelength spacing can be made large enough to avoid the non-linear effects, yet
carrying the same amount of data as in the standard DWDM system. In the SCM
technique, one of the important issues is to identify the right sub-carrier frequency to
achieve the right transmission BER at different bit rates.
CHAPTER 3
THE SCM/WDM FOR RADIO OVER FIBER COMMUNICATION
3.1. Introduction
During the last decade, fiber optic transmission of microwave signals has
considerable attention in many applications. This happened because optical fiber
provides an excellent transmission medium for information distribution networks. The
use of sub carrier multiplexing (SCM) transmission using an optical carrier instead of
the traditionally used super carrier over optical fiber is very attractive. In SCM systems,
the available bandwidth of optical fiber is generally limited by the processing speed of
electronics. However in optical communication system, multiplexing techniques can be
combined with SCM to gain access to wider bands.
30
3.2. Sub Carrier Multiplexing
Basically the operation of the sub carrier multiplexing (SCM) was similar to Time
Division Multiplexing, such that TDM is commonly used in digital transmission system.
On other hand, SCM play an important role in analogue transmission system, however
multiplexing more conveniently carried out in frequency domain.
The main idea of SCM is combining two step of modulation which is operating
at different domain. First modulation was occupied at RF part such that several low
bandwidth RF channel carrying analogue or digital signal add up together by using
multiplexer. Thus the signal will be very close to each other in the frequency domain
depending to local oscillator frequency that applied in the modulation part. This
combined signal actually modulated onto higher frequency microwave carrier. The upconverted signals are in different frequency bands and can therefore be combined by a
microwave power combiner forming a microwave sub carrier multiplexed composite
signal.
Modulated RF
∑
f1
f2
f3
fn
Figure 3.1: Basic configuration of RF modulation
31
From figure 3.1, n number of digital signal were modulated by using a different
frequency at the local oscillator; f1, f2, f3.... fn. The modulation scheme applied was
depend on what kind of input signal (digital or analogue) was used and how good the
desired modulated signal. Second modulation was occupied at optical domain, the
modulated signal then convert to optical domain by using laser diode and optical
modulator.
3.3. Analog SCM
In analog SCM lightwave systems, each microwave subcarrier are modulated
using an analog format and the output of all subcarriers is summed using a microwave
power combiner. The composite signal is used to modulate the intensity of a
semiconductor laser directly by adding it to the bias current. The transmitted power can
be written as
⎡
⎤
N
P(t ) = P ⎢1 + ∑ m a cos(2πf + φ ⎥
b⎢
j j
j
j⎥
⎣ j =1
⎦
(3.1)
where Pb is the output power at the bias level and m j , aj, f j , and φ j are, respectively,
the modulation index, amplitude, frequency, and phase associated with the jth
microwave subcarrier; aj, fj, or φ j is modulated to impose the signal depending on
whether AM, FM, or phase modulation (PM) is used [55].
32
In the case of SCM systems, the carrier-to-noise ratio (CNR) is often used in
place of SNR. The CNR is defined as the ratio of RMS carrier power to RMS noise
power at the receiver and can be written as
CNR =
(mRP) 2 / 2
2
α S2 + α T2 + α I2 + α IMD
(3.2)
where m is the modulation index, R is the detector responsivity, P is the average
received optical power, and σs, σT , σI , and σIMD are the RMS values of the noise
currents associated with the shot noise, thermal noise, intensity noise, and IMD,
respectively [55].
3.4. Digital SCM
In SCM optical transmission systems, a large variety of modulation schemes
become feasible because all those modulation and demodulation can be done in the
microwave domain. Reng Xiang Huang has discussed that the major modulation formats
are OOK (on off keying) or ASK (amplitude shift keying), PSK (phase shift keying) and
QAM (Quadrature Amplitude Modulation). When BPSK is compared with ASK on a
peak envelope power (PEP) basis [31], for a given noise value of N0 (the only
detrimental factor is the additive white noise), 6 dB less (peak) signal power is required
for BPSK signaling to give the same BER as that for ASK [57].
33
The BER of BPSK is
⎛ A2 ⎞
c
⎟
BER = Q⎜
⎜ 8N 0 B ⎟
⎠
⎝
(3.3)
and the BER for ASK is
⎛ A2 ⎞
c
⎟,
BER = Q⎜
⎜ 2N 0 B ⎟
⎠
⎝
(3.4)
where A is the peak value of the signal, B is the noise bandwidth, N0 is the noise power
spectrum density and Q(x) represents a complementary error function.
Also refers to Reng Xiang Huang [57] are discussed, for BPSK, after the
transmitter filter; each base band signal is modulated to a sub carrier frequency by a
carrier suppressed microwave mixer. The function of the microwave mixer is simply
multiplication.
The
expression
for
the
output
of
such
a
mixer
is Y (t ) = k * X (t ) * cos(2πf RF ) , X (t) is the base band signal, and cos(2πf RF ) is the
microwave carrier. A separate mixer is need for each sub carrier channel at its own
frequency. Here the modulation frequency of each channel, fRF, and the modulation
index k of each channel can be changed or optimized.
34
3.5.
Basic WDM Scheme
The WDM technique corresponds to the scheme in which the capacity of a light
wave system is enhanced by employing multiple optical carriers at different
wavelengths. Each carrier is modulated independently using different electrical bit
streams (which may themselves use TDM and FDM techniques in the electrical domain)
that are transmitted over the same fiber. Figure 3.2 shows schematically the layout of
such a dispersion managed WDM link. The output of several transmitters is combined
using an optical device known as a multiplexer. The multiplexed signal is launched into
the fiber link for transmission to its destination, where a demultiplexer separates
individual channels and sends each channel to its own receiver.
Figure 3.2: Schematic of a WDM fiber Link [1]
The implementation of WDM networks requires a variety of passive and/ or
active devices to combine, distribute, isolate and amplify optical power at different
wavelengths. Passive device require no external control for their operation, so they are
somewhat limited in their application in WDM networks. These components are mainly
used to split and combine or tap off optical signals. The performance of active devices
can be controlled electronically, thereby providing a large degree of network flexibility.
Active WDM components include tunable optical filters, tunable sources and optical
amplifiers [1].
35
Under WDM, the optical transmission spectrum is carved up into a number of
non-overlapping wavelength (or frequency) bands, with each wavelength supporting a
single communication channel operating at whatever rate one desires, e.g., peak
electronic speed. Thus, by allowing multiple WDM channels to coexist on a single fiber,
one can tap into the huge fiber bandwidth, with the corresponding challenges being the
design and development of appropriate network architectures, protocols, and algorithms.
Also, WDM devices are easier to implement since, generally, all components in a WDM
device need to operate only at electronic speed; as a result, several WDM devices are
available in the marketplace today, and more are emerging [57].
3.5.1
Multiplexer and Demultiplexer
Because DWDM systems send signals from several sources over a single fiber,
they must include some means to combine the incoming signals. This is done with a
multiplexer, which takes optical wavelengths from multiple fibers and converges them
into one beam. At the receiving end the system must be able to separate out the
components of the light so that they can be discreetly detected. Demultiplexers perform
this function by separating the received beam into its wavelength components and
coupling them to individual fibers. Demultiplexing must be done before the light is
detected, because photodetectors are inherently broadband devices that cannot
selectively detect a single wavelength [57].
In a unidirectional system, there is a multiplexer at the sending end and a
demultiplexer at the receiving end. Two systems would be required at each end for
bidirectional communication, and two separate fibers would be needed.
36
Figure 3.3: Multiplexing and Demultiplexing in a Unidirectional System
In a bidirectional system, there is a multiplexer/demultiplexer at each end
and communication is over a single fiber pair.
Figure 3.4: Multiplexing and Demultiplexing in a Bidirectional System
Multiplexers and demultiplexers can be either passive or active in design.
Passive designs are based on prisms, diffraction gratings, or filters, while active designs
combine passive devices with tunable filters. The primary challenge in these devices is
to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how
well the channels are separated, while channel separation refers to the ability to
distinguish each wavelength.
37
3.6. The SCM/WDM System for Radio over Fiber
The basic configuration of SCM/WDM system is illustrated in Figure 3.5.
Generally, n numbers of signals were modulated individually with different frequency in
RF domain. Then the modulated RF signal will be added up by a RF multiplexer (or by
an adder) before transform the RF signal into Optical signal through optical source and
optical modulator on a single wavelength. All the operation above was perform by a
single transmitter [57].
Figure 3.5: The SCM/WDM Link Configuration [57]
Here,
assumed that the SCM/WDM has its own light source for explanation
purpose; however, the SCM/WDM can be configured without light source for uplink
transmission. In each configuration of the figure, the SCM/WDM does not have any
equipment for modulation and demodulation, only the CS has such equipment. In the
downlink from the CS to the BSs, the information signal from a public switched
telephone network (PSTN), the Internet, or other CS is fed into the modem in the CS.
38
The signal that is either RF, IF or BB bands modulates optical signal from LD. As
described earlier, if the RF band is low, we can modulate the LD signal by the signal of
the RF band directly. If the RF band is high, such as the mm-wave band, we sometimes
need to use external optical modulators (EOMs), like electro-absorption ones.
The modulated optical signal is transmitted to the SCM receiver’s via optical
fiber. At the SCM receiver’s, the RF/IF/BB band signal is recovered to detect the
modulated optical signal by using a PD. The recovered signal, which needs to be
upconverted to RF band if IF or BB signal is transmitted, is transmitted to the user’s via
the antennas of the SCM receiver’s.
In the configuration of Figure 3.5, the modulated signal is generated at the SCM
in an RF band and directly transmitted to the receivers by an EOM, which is called “RFover-Fiber”. At each receiver, the modulated signal is recovered by detecting the
modulated optical signal with a PD and directly transmitted to the users. Signal
distribution as RF-over-Fiber has the advantage of a simplified SCM receivers design
but is susceptible to fiber chromatic dispersion that severely limits the transmission
distance, the signals received at receivers are amplified and directly transmitted to the
SCM transmitters by modulating an optical signal from a LD by using an EOM [19].
This is especially important when RoF at mm-wave bands is combined with
Wavelength Division Multiplexing (WDM). However, this increases the amount of
equipment at the receivers (Base Station, WLAN, etc) because an upconverter for the
downlink and a down converter for the uplink are required. In the RF subcarrier
transmission, the receiver’s configuration can be simplified only if an mm-wave optical
external modulator and a high-frequency PD are respectively applied to the electric-tooptic (E/O) and the optic-to-electric (O/E) converters [57].
CHAPTER 4
THE SCM/WDM - ROF SYSTEM MODEL
4.1
Introduction
The main idea in this project is to combine the SCM model of RoF with
WDM. The integration of the two systems is responding to the demands for high
data rate applications and reasonable mobility. The employment of the SCM-RoF in
the WDM architecture allows reduction in cell size that increases the bandwidth,
thus improves the spectrum efficiency. In this project the SCM/WDM for some RoF
Model was designed by using OptiWave Software. The project is proposed with
considerable the some aspect of practical parameters of RoF communication link. As
in system design, this project aimed to provide some results for the RoF system
performance and analysis study.
40
4.2
The SCM/WDM System Model
The principle of subcarrier multiplexing is reasonably straightforward and it
is an easy technique to employ in practice. A RoF or microwave signal (subcarrier)
is used to modulate an optical carrier. This results in an optical spectrum consisting
of the original optical carrier. Multiple channels can be multiplexed onto the same
optical carrier by using multiple subcarriers. At the receiver the channels are
demultiplexed by using direct detection and then applying heterodyning and filtering
to the resultant RF signal.
The main essence of SCM system is to take all the modulating,
demodulating, multiplexing, demultiplexing, and amplifier which could be perform
optically, and instead perform them electrically [16]. In this model, the only optical
functions that remain are optical generation using a laser, optical transmission over
an optical fiber, optical detection using a photodiode and optical amplifier
introduced in the optical transmission.
The advantages of performing these functions electrically is that, under
current circumstances, electrical components are cheaper and more reliable than
optical components, and electrical filters can be of an efficient and near ideal
multipole design, whereas optical filters are only single pole.
At this moment, WDM offers an attractive solution to increasing LAN,
CATV, Cellular system bandwidth, without disturbing the existing embedded fiber,
which populates most buildings and campuses, and continue to be the cable of
choice for the near future. By multiplexing several relatively coarsely spaced
wavelengths over a single, installed multimode network, the aggregate bandwidth
can be increased by the multiplexing factor.
41
Figure 4.1 shows the proposed eight channels of the SCM/WDM-ROF
system. The digital data was modulated by BPSK using a ~gigahertz subcarriers
electrical wave, by then the composite electrical wave modulates the ~terahertz
optical carrier wave. BPSK was setup for signal modulated in electrical domain for
carrier. In this project, the 1.8 Gbps data are electrically mixed with the electrical
subcarrier, producing sum and difference frequencies (fc ± fi) as result in standard
heterodyning. An electrical bandpass filter is used to allow only one product,
typically the sum frequency to pass and be transmitted.
Figure 4.1: Model of eight channels the SCM/WDM-ROF system.
4.3
The Transmitter Model.
42
In this SCM/WDM-RoF system, transmitter system was split into two
domains; electrical domain and the optical domain.
Figure 4.2: Transmitter for electrical (RF) domain.
Figure 4.2 shows the transmitter system considered in electrical domain. The
transmitter was consist eight channels that carried digital data generates by PRBS.
Each of the data will be modulated by BPSK modulator with varies number of
subcarrier which was in gigahertz. One subcarrier may carry digital data, while
another may be modulated with an analogue signal such as video or telephone
traffic. In addition, the data used to modulate the subcarriers need not be of the same
kind. An interesting aspect of the BPSK format is that the optical intensity remains
constant during all bits and the signal appears to have a CW form. Coherent
detection is a necessity for PSK as all information would be lost if the optical signal
were detected directly without mixing it with the output of a local oscillator. The
43
implementation of BPSK requires an external modulator capable of changing the
optical phase in response to an applied voltage. The following table shows the basic
simulation setup in this design.
Table 4.1: Global simulation setup
Parameter
Bit rate
Value
1.8 Gbps
Time window
7.11e-0.08
Sample Rate
115.2 GHz
Sequence length
Sample per bit
Number of samples
128 bits
64
8192
Each of data has a bit rate 1.8 Gbps for each subcarrier channel. The data bits
are generated by a pseudo random number generator; it generates a bit sequence of 0
and 1 with equal probability. In this simulation, the length of the bit sequence is
usually set to 128. This number is equal to l to 27 that was one of PRBS (pseudorandom bit sequence) used in most BER tester. This PRBS has 7 consecutive 1s and
6 consecutive 0s. We employed this PRBS in our simulation.
Then an ideal rectangle baseband signal is generated according to the data
bits. The number of samples per bit will determine the simulation bandwidth. The
simulation bandwidth which is the highest frequency that the simulated signal could
be is 0.5/ dT, in which the dT is the time interval between samples. For nonlinear
system, the bandwidth of the output signal is usually larger than that of the input.
Usually we use 64 samples in each bit for one wavelength simulation and use 256
samples per bit for four wavelengths simulation.
44
In order to get rid of the side peaks in the spectrum of an ideal rectangle
waveform, we use a transmitter filter to shape the output bit. These transmitter filters
are usually are set as 6-order Butterworth filters with 3dB bandwidth of about 1.0 *
bit rate. It will effectively remove the second side peak and effectively reduce the
inter-channel interference when several subcarriers are multiplexed together.
Table 4.2: Subcarrier frequency allocation
Operation frequency
SCM Frequency 1
SCM Frequency 2
f1
3.6 GHz
5.4 GHz
f2
7.2 GHz
9 GHz
f3
10.8 GHz
12.6 GHz
f4
18.0 GHz
14.4 GHz
f5
21.6 GHz
16.2 GHz
f6
23.4 GHz
19.8 GHz
f7
25.2 GHz
28.8 GHz
f8
27 GHz
30.6 GHz
The generated data then will be modulated in the RF frequency. The first
data that been up converted 3.6 GHz PSK considered as channel 1. The same
processed has been applied to the 16th data that up converted to 5.4 GHz, 7.2 GHz,
9 GHz, 10.8 GHz, 12.6 GHz, 14.4 GHz, 16.2, 18.0 GHz, 19.8 GHz, 23.4 GHz, 25.2
GHz, 28.8 GHz, 27 GHz and 30.6 GHz respectively as shown in the Table 4.2.
All generated signals were multiplexed by an electrical multiplexer. This was
a part where the electrical transmitter plays a role in the SCM/WDM-ROF system.
Whereby Figure 4.3 shows how the optical transmitter merges with electrical
transmitter in this particular system.
45
SCM-1 Channel
SCM-2 Channel
Figure 4.3: Transmitter for optical domain.
Figure 4.3 illustrated that the composite electrical signal that has been
generated by the electrical transmitter that was amplified to10 dB by an electrical
amplifier and transform to optical domain through external optical modulator, MZM
and CW laser was applied as the optical source.
There are two ways of modulating the light source. The laser diode can itself
be modulated directly by using the appropriate RF signal to drive the laser bias
current. The second option is to operate the laser in continuous wave (CW) mode
and then use an external modulator such as the Mach-Zehnder Modulator (MZM), to
46
modulate the intensity of the light. In both cases, the modulating signal is the actual
RF signal to be distributed. The RF signal must be appropriately premodulated with
data.
In this project, the external modulation was made by MZM for the
SCM/WDM – RoF system model. CW laser established the light source of the 1500
nm wavelength and the power was setup at 0 dBm.
4.4
The Transmission Link Model
In any transmission system, loss is the main factor that needs to be
considered as main priority in designing and modelling any communication link. In
many years, much research and invention has been done to introduce a components
or designs for losses compensations. There are absorption, scattering and others are
referred as attenuation through the fiber. The attenuation can be expressed by the
formula,
Attenuation
⎛P ⎞
10
⎛ dB ⎞
⎟ = − log10 ⎜⎜ in ⎟⎟ ≈ 4.343α
L
⎝ km ⎠
⎝ Pout ⎠
α⎜
(4.1)
Where, if L is expressed in kilometres, the loss is defined in units of decibels per
kilometre (dB/km) and refers to it as the fiber-loss parameter
The other factor that considerable as system design in optical link is fiber
bandwidth, this parameter refers to the frequency
f = f 3dB , where f3dB is the
optical bandwidth of the fiber as the optical power drop by 3 dB at this frequency
compared with the zero-frequency response.
47
From Figure 4.4, Erbium Doped Fiber Amplifier (EDFA) was introduced
before the optical signal propagated through the optical fiber. The main reason the
EDFA occupied in the system was to encounter the attenuation and dispersion
occurred through the optical fiber.
Figure 4.4: Transmission Link Diagram
The basic operation of the fiber amplifier is similar to that of the
semiconductor amplifier. The fiber amplifier contains a gain medium that must be
inverted by a pump a sourse. A signal initiates stimulated emission resulting in gain,
and spontaneous emission occurs naturally, which results in noise. The fiber
amplifier is circular, not rectangular, thus eliminationg significant attenuation when
it is coupled to a standard optical fiber as well as removing any polarization
dependence in the gain. Table 4.3 shows the simulation setup for the EDFA.
Table 4.3: Simulation setup for EDFA
Parameters
Core radius
Er doping radius
Er metastable lifetime
Numerical aperture
Er ion density
Loss at 1550 nm
Forward pump power
Backward pump power
Length
Value
2.2 um
2.2 um
10 ms
0.24
1e+25 m-3
0.1 dB/km
100 mW
0 mW
0m–5m
48
The carrier lifetime of erbium ions is milliseconds, whereas that
semiconductor carrier is nanoseconds. That difference reduces significantly the two
nonlinear problems in multichannel systems of intermodulation distortion (fourwave mixing) and bit-rate-dependent cross-talk due to gain saturation.
Wavelength Division Multiplexing was installed to multiplexing optical
signal carrier to the link, the basic operation of the WDM is several base bandmodulated channels are transmitted along a single fiber but with each channel
located at a different wavelength. Each of N different wavelength lasers is operating
at the slower Gbps speeds, but the aggregate system is transmitting at N times the
individual laser speed, providing a significant capacity enhancement. The WDM
channels are separated in wavelength to avoid cross-talk when they are
(de)multiplexed by a non-ideal optical fiber. Each laser is modulated at a given
speed, and the total aggregate capacity being transmitted along the high-bandwidth
fiber is the sum total of the bit rates of the individual lasers. The wavelengths can be
individually routed through a network or individually recovered by wavelengthselective components. In a simple WDM system, each laser must emit light at a
different wavelength, with all the lasers light multiplexed together onto a single
optical fiber [62].
On the other hand, the driving force motivating the use of multichannel
optical systems is the enormous bandwidth available in optical fiber. The highbandwidth characteristic of the optical fiber implies that a single optical carrier can
be base band modulated at ~25,000 Gbps, occupying 25,000 GHz surrounding 1550
nm, before transmission losses of the optical fiber would limit transmission.
Obviously, this bit rate is impossible for present-day optical devices to achieve,
given that heroic lasers, external modulators, switches or detectors have bandwidths
< 100 GHz. As such, a single high-speed channel takes advantage of an extremely
small portion of the available fiber bandwidth [62].
49
Table 4.4: Simulation setup for Single Mode Fiber
Parameters
References wavelength
Length
Attenuation
Dispersion
Dispersion slope
Value
1500 nm
1 km - 150 km
0.2 dB/km
16.75 ps/nm/km
0.075 ps/nm2/k
Choosing the incorrect and unsuitable fiber into the system can be so much
attenuation and dispersion existence. Therefore, a single mode fiber was the perfect
match for this system according to the characteristic of the fiber. A single mode fiber
conducts only one mode and also capable to eliminate higher order modes.
Attenuation in a single mode fiber is smaller than in a multimode fiber because in
the single mode fiber less light will encounter absorption and scattering effects.
However, attenuation (macro bending effect) in single mode fiber increases as
operating wavelength increases and bend radius decreases.
Linear dispersion in a single mode fiber is mainly cause by chromatic
dispersion. The bit rate (BR) that can be transmitted over the fiber is defined as;
BR(Gbps) <
1
{4Δt (ns)}
Where Δt is dispersion-caused pulse spreading.
(4.2)
50
4.5
The Receiver Model
After transmission through the fiber and direct detection on a APD
photodiode the photocurrent will be a replica of the modulating RF signal applied
either directly to the laser or to the external modulator at the transmitter.
However, electrical amplifier that introduced in the terminated components
able to gain up the received signal power. A better filter in electrical part was proven
that improve the performance of the system. The filter type is Band Pass Rectangle;
filter was the most vital components to take care of after the photo detector.
Figure 4.5 shows the connection between receiver for optical and electrical
domain. APD photo detector was introduced in this system to obtain the desired
signals. One of the major parameter that is in the top priority in decides photo
detector is sensitivity. Sensitivity of the photo detector is present the minimum light
power a photo detector can detect. This parameter determines the length of a fiberoptic link imposed by a power limitation. The more sensitive the photodiode, the
longer the link can afford. In this project an ideal WDM Demux was installed as
function as optical signal demultiplexer. It is works as optical filtering that
compress, split, and filtering desire optical signals. After being transmitted through a
high-bandwidth optical fiber, the combined optical signals must be demultiplexed at
the receiving end by distributing the total optical power to each output port and then
requiring that each receiver selectively recover only one wavelength by using a
tunable optical filter. The two wavelengths have been detected for this project in the
range of 1500 nm to 1550 nm.
51
Figure 4.5: The receiver for optical domain
The setup parameters can be found in Table 4.5 below. APD photo detector
was used as the photo detector and amplifier work. However, for this particular
where the optical fiber length was expanded to 150 km without repeater it is
expected that the gain provided was not enough to magnify the signal. Therefore an
existence of electrical amplifier was reasonable and acceptable.
Table 4.5: Simulation setup for APD Photo detector
Parameters
Gain
Responsivity
Ionization ratio
Dark current
Value
3
1 A/W
0.9
10 nA
Figure 4.6 shows the connection of the components to receive the RF signal.
In order to obtain high quality signals in electrical domain, an electrical amplifier
was employed. The function of this amplifier is to amplify the signals after the long
52
distance of optical link without repeater. With the gain is 10 dB, all signals that
distorted, attenuated when transmitted along the link can be recoverable at the
receiver side
Figure 4.6: The receiver for electrical domain
On the other side, to gain the subcarriers frequency that transmitted the
splitter breakdown for the eight signals into two parts, each of parts consists of four
subcarriers channel frequency. Every subcarrier will be filtered by Band Pass
Rectangle Filter respectively to the setup subcarriers transmitter frequency.
53
4.6
Conclusion
The systems are designed incorporate three part domain of the system. Mmwave domains are generated by applying of RF channels as SCM, Optical signal
carrier is generating by Mach Zehnder Modulator and Multiplexing are construct by
Wavelength Division Multiplexing. The mm-wave consist of 16 RF channels and
separate into 2 SCM group. MZM generates of 20 GHz optical signal. A WDM ideal
has 50 GHz channel spacing. Those systems are considered in bandwidth present.
By focusing how to increase the bandwidth, we proposed the combine of SCM
techniques applied into WDM technology. In this simulation the parameters was
setup to measure the capacity in the systems such as SNR, BER, Number of Carrier,
Total Power, etc. The intention of the SCM/WDM system afforded the bandwidth
capacity of the fiber.
CHAPTER 5
SIMULATION RESULT AND PERFORMANCE ANALYSIS
5.1 Introduction
In this chapter, we present the simulation results from the system design.
Generally, the system has a three main function with an individual task in each part.
The first part is transmitter, which are dividing into two main functions such as RF
domain and External Modulator.
The second scope is transmission link, which consists of three main
functions, there are Optical Signal Multiplexing - Demultiplexing (WDM), Optical
Amplifier (EDFA) for encountered the effects of attenuation, distortion and
Rayleigh scattering. And the last parts of the second scope is Optical Link (SMF),
the types of the SMF determine how signal travels over the link with the level of
quality. In this project we setup the attenuation factor for the link is 0.2 dB/km, the
distance is varies from 100 km to 200 km.
55
The third part of the system is the receiver. This part shows selective of the
RF domain signals. The processing of signal converting has done in this part.
Optical signal converted into RF signals by using APD Photo detector which are
consider level of the sensitivity. The reason of using APD PD is the level of
sensitivity is better than PIN Photo detector. On the other hand, electric amplifier
has a significant act for signal processing in the level of RF domain. The amplifier
has increases of the received power level RF signals. In this case the power level of
receiver is must be equal or less than the transmitter power level. The last device in
this part is filter. The filter that using has refers to the region of the desire signals,
with the result that power level, amplitude and phase of the signals appropriate with
the transmitter signals.
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 the gain slope of the
EDFA, the frequency or wavelength dependence of the dispersion compensation
modules etc. Second, the number of parameters that are considered in the simulation
is still large and the combination of different values is huge.
In the previous chapter, this project was carried out by modelling and
studying the performance analysis for single transmitter that carrying four channels
through the optical fibre link and detected back at the receiver part. First, it is
important to make the selections of basic simulation parameters, such as the
modulation techniques, RF frequency allocation plan, the bandwidth of the filters
etc. This optimization is not necessarily the best; however it will give a possible
range within which the system may perform better than out of this range. Or it will
give guidance when select the parameters.
56
5.2 The Transmitter Simulation Results.
At this moment, the simulation will only consider some important
combinations of parameters that dominant in optical data transmission. Therefore,
on this project, two parameters that will vary in order to analyze the performance are
PMD coefficient at the fibre link and type of laser source. Usually newer SMF will
have better PMD coefficient which is typically less than 0.1 ps /sqrt(km) while old
SMF may have PMD coefficient as high as 0.5 ps /sqrt(km). However, for this initial
simulation, PMD coefficient was setup at 0.5 ps / sqrt(km). Nevertheless, the PMD
coefficient on this state was not giving effect at the transmitter part because the
PMD coefficient was contribute at fiber link.
Figure 5.1: Signal wave modulated by PSK
Figure 5.1 Shows some signal that modulate by PSK modulation at
sixteenths difference frequency carrier which are at 5.4 GHz, 7.2 GHz, 9 GHz, 10.8
GHz, 12.6 GHz, 14.4 GHz, 16.2, 18.0 GHz, 19.8 GHz, 23.4 GHz, 25.2 GHz, 28.8
GHz, 27 GHz and 30.6 GHz. After the signal has been modulated individually in
electrical domain, every single signal will through the band pass rectangle filter
which was initially consider as ideal filter. Then, the multiplexed signal will be
modulate in optical domain by the external modulation (Mach-Zehnder Modulator).
The optical modulated signal then carried out by CW Laser (as light source) through
WDM and optical fiber. Figure 5.2 (a), (b), (c) and (d) were shows the frequency
spectrum of the signal after surpass the band pass filter. Each signals that shown
below basically carried by their own frequency carrier.
57
(a)
(b)
(c)
(d)
Figure 5.2: (a) RF spectrum for data signal on first channel (3.6 GHz).
(b) RF spectrum for data signal on eleventh channel (23.4 GHz).
(c) RF spectrum for data signal on fourth channel (10.8 GHz).
(d) RF spectrum for data signal on tenth channel (16.2 GHz).
In modulation process, the signal will be added up together by a multiplexer.
Figure 5.3 shows all the eight signals or channels are allocated near by to each other
and there no interference or aliasing occurred. If there is aliasing, it is important to
reallocate the frequency carrier because the effect of aliasing basically on the data
recovery at the receiver part. In this particular project, there was no data recovery at
the receiver because it is not the main concern however frequency allocation was
vital process to get low BER and good Quality Factor.
58
(a) SCM-1
(b) SCM-2
Figure 5.3: RF spectrum for adding eight channels of SCM-1 and SCM-2.
The composite electrical signals that have been generated by electrical
transmitter were amplified by the electrical amplifier. The amplified signals were
represented by Figure 5.4. The signals were gain up by 10 dB, however the noise
that existence in the system also amplified.
Figure 5.4: Amplified composite RF signals
59
The most important part for this project is the optical domain which was all
the parameters will be analyzed in this particular only domain. Figure 5.5 below
show the optical signal which is carried the channel used single wavelength. In
addition, the existing of the signal can be easily track by looking at the wavelength
width. There were eight signals existed in the positive and negative side. The
existence of duplicated signal at the negative side will double BW requirement. This
phenomenon will reduce number of signal that can be coupled.
(a) At the SCM-1, 8 PSK RF signals modulated in single Wavelength of MZM
(b) At the SCM-2, 8 PSK RF signals modulated in single Wavelength of MZM
Figure 5.5 (a) & (b): Spectrum of the signal after Mach Zehnder Modulator.
60
5.3 The Transmission Link Simulation Results
WDM has a significant factor that increases the bandwidth capacity in
optical communication system. The result of WDM shows that RF domain carried
by single optical wavelength and multiplexed. WDM has function as filter with
tuneable factor that setting the wavelength. In this simulation we use two
wavelengths for a sample to represented SCM channels. The minimum channel
spacing of WDM Mux is 50 GHz.
(a)
(b)
Figure 5.6 (a) and (b): WDM Mux Signal Spectrum output
Table 5.1 shows the results of WDM Mux, parameter of the signal power,
noise power in dBm and Watt, OSNR in dB. Frequency that we use is 1500 nm –
1600 nm.
61
Table 5.1: WDM Mux Results
Signal Power (dBm)
Signal Power (W)
Noise Power (dBm)
Noise Power (W)
OSNR (dB)
Min value
-3.569.488
0.00043959343
-48.882.016
1,29E-01
45.312.528
Max Value
-3.569.488
0.00043959343
-48.882.016
1,29E-01
45.312.528
Total
-3.569.488
0.00043959343
-48.882.016
1,29E-01
Frequency at min
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
Frequency at max
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
Wavelength at min
1500 nm
1500 nm
1500 nm
1500 nm
1500 nm
Wavelength at max
1550 nm
1550 nm
1550 nm
1550 nm
1550 nm
At the WDM Demux, wavelength demultiplexing into the original optical
signals. This process works as tunable filter that capture the desire optical signals.
The results shown in Table 5.2 and Figure 5.7 has result effect of the demultiplexing
particullary in optical signal power, noise power and OSNR of WDM Demux.
Figure 5.7: WDM Demux Spectrum Signals
Table 5.2 shows the results of the WDM Demux, the main parameter are
signal power, noise power in dBm and Watt, OSNR in dB. Frequency that we use is
1500 nm – 1600 nm.
62
Table 5.2: The WDM Demux Results
Signal Power (dBm)
Signal Power (W)
Noise Power (dBm)
Noise Power (W)
OSNR (dB)
Min value
-25.672.648
2,71E+01
-44.902.387
3,23E-01
19.229.739
Max Value
-25.672.648
2,71E+01
-44.902.387
3,23E-01
19.229.739
Total
-25.672.648
2,71E+01
-44.902.387
3,23E-01
Frequency at min
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
Frequency at max
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
19.986.164 THz
Wavelength at min
1500 nm
1500 nm
1500 nm
1500 nm
1500 nm
Wavelength at max
1500 nm
1500 nm
1500 nm
1500 nm
1500 nm
From the Table 5.2, operating frequency of the SCM/WDM for RoF was
establish in 1500 nm, the total WDM Demux signal power was decreases to -25.67
dBm due to optical loss propagation, distortion or attenuation when travels over the
optical fiber. One factor that affects the WDM Demux spectrum is Optical Signal to
Noise Ratio (OSNR). Compare with WDM Mux, OSNR in WDM Demux has
decreased to 19.23 dB.
Figure 5.8 illustrated the optical signal has been magnified by EDFA. The
power signal was increased from -60 dBm up to -20 dBm as well as amplitude of the
optical carrier.
Figure 5.8: Optical spectrum after EDFA.
63
Figure 5.9: Optical spectrum and Optical Power after propagated 150km in fiber.
5.4 The Receiver Simulation Results
At the receiver part, the sensitivity of the photo detector was one of the main
factors in improving system performances instead of EDFA. In the system design,
RF signal separated by Bandpass Rectangle filter assignment to gain the desire
signals. Figure 5.10 shows the ablities of photo detector to absorb the optical signals
and transform back into electrical.
Figure 5.10 : RF spectrum detected by photo detector
64
The power of received signal however were to small and will contributed to
the uneccepted Carrier-to-Noise Ratio (CNR). Therefore, an electrical amplifier
encounter this problem that can be reserved the amplified output signal as shown in
Figure 5.11 below.
Figure 5.11: Amplified received RF spectrum
The maturity of electrical components than optical could be an advantages to
this system where the filtering process was done in electrical domain. Figure 5.12
shows the electrical spectrum after Rectangular Band pass filter. This particular
filter will track back the signal from first channel. The electrical signal wave can be
found in Figure 5.12. The performance of received signal wave was represented in
eye diagram.
Figure 5.12: Demodulation sample of the RF spectrum for channel 2
65
Figure 5.13: Received electrical signal wave
5.5 The Eye Diagram
Eye diagram is one of the powerful diagrams in that illustrated the system
performances. This diagram is used to show the existence of harmonic or phase
error, noise, and carrier frequency effected.
In this simulation, two channel of SCM has been proposed, but in this report
only the random sample of SCM-1 and SCM-2 channel are displayed. There were
seven eye diagram Figure 5.14, 5.15, 5.16, 5.16, 5.17, 5.18 and 5.19 and each of the
eye diagram shows the random performance of the signal transmission for each
channel; channel 1, 2, 3, 8 from SCM-1 channel by the channel 2, and 8 respectively
from SCM-2 channel. Basically, the noise occurs vary way in the transmitter,
transmission link and receiver part. The eye diagram obviously describes the
existence multiple number of harmonic signals pass trough the optical signal.
Providing a proper filter in the system was a main factor in order to reduce the
harmonic signals.
66
Max. Q Factor
Min. BER
Eye Height
Threshold
Decision Inst.
2.91199
0.0017943
-0.000163081
-2.78907e-005
0.609375
Figure 5.14: Eye Diagram for the channel 1 of SCM-1 Channel.
Max. Q Factor
Min. BER
Eye Height
Threshold
Decision Inst.
0
1
0
0
0
Figure 5.15: Eye Diagram for the channel 2 of SCM-1 Channel.
Max. Q Factor
Min. BER
Eye Height
Threshold
Decision Inst.
Figure 5.16: Eye Diagram for the channel 3 of SCM-1 Channel.
0
1
0
0
0
67
Max. Q Factor
Min. BER
Eye Height
Threshold
Decision Inst.
0
1
0
0
0
Figure 5.17: Eye Diagram for the channel 8 of SCM-1 Channel.
Max. Q Factor
Min. BER
Eye Height
Threshold
Decision Inst.
2.12249
0.0168583
-0.00560727
0.000314705
0.484375
Figure 5.18: Eye Diagram for the channel 2 of SCM-2 Channel.
Max. Q Factor
Min. BER
Eye Height
Threshold
Decision Inst.
2.03727
0.0206909
-0.00156004
-3.33138e-005
0.4375
Figure 5.19: Eye Diagram for the channel 8 of SCM-2 Channel.
68
The eye diagrams illustrated that the carrier frequency influence the open eye
area. It clearly seen that, the higher frequency goes to the less open eye area. In
addition, the number of existing eyes was also proportional to the modulation
frequency. From the eye diagrams, for 3.6 GHz, there were 4 eyes appear in the
range 0 – 1 second (time period) otherwise there were no eyes appear in the range of
28.8 GHz to 30.6 GHz.
5.6 Performance Analysis of the SCM/WDM for RoF System
In this section, there were six parameters that have been deeply concentrated
for the performances analysis of the SCM/WDM-RoF system, fiber length, EDFA
length, a nonlinear power and the number of channels. The performances analysis
depend on the Carrier-to-Noise Ratio (CNR), Signal-to-Noise Ratio (SNR), Bit
Error Rate (BER), Total Power, Optical Signal-to-Noise Ratio (OSNR) and Eye
Diagram
5.7 Analysis of the Total Power to the EDFA Length
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 EDFA or without EDFA. The distance link was setup for 100 km and
150 km to evaluate how total power and EDFA have an effect to the link.
69
(a)
(b)
Figure 5.20 (a): The performance of the total power to the Fiber Length with
and without EDFA in 100 km
(b): The performance of Total power to the fiber length with and
without EDFA in 150 km
Figure 5.20, illustrates the performance of total power to the length with and
without EDFA. Figure 5.20 (a) shows that EDFA can influence the total power to
the link distance 100 km, where significantly increases 0.025 Watt to 0.037 Watt. In
other hand, the total power also increases to the level of -20 dBm for the distance
link of 150 km as shown in Figure 5.20 (b). It is mean that the power will be reduce
or attenuated over the link without EDFA.
70
Table 5.3: A basic simulation setup for SCM/WDM-RoF with EDFA
Parameter
Value
Duplexing
SDD
RF Modulation
PSK
Optical Modulation
MZM
Channel BW
1.8 GHz
Bit Rate/Sub Carrier Channel
1.8 Gbps
Sample/bit
64
Sequence Length
128 bits
EDFA length
0m-5m
Fiber length
1 km – 150 km
5.8 The Performance of WDM Mux/Demux
Traffic monitoring, analysis, and aggregation are responsible collecting data
traffic statistic from the networks elements. In order to monitoring the optical
channels, Optical Signal to Noise Ratio (OSNR) is used to resolve the outcomes. As
shown in the section 5.3, Figure 5.21 shows that the WDM Mux/Demux results for
both channels climbs from 19 dB to 61 dB throughout the fiber length. Various
types of traffic monitoring are possible in a WDM system, including schemes based
on the power spectral density of the WDM signals.
71
Figure 5.21: The OSNR performance of the WDM Mux /Demux
5.9 The Carrier-to-Noise Ratio (CNR) Performance
Figure 5.22 shows the performance of the system in term of CNR value
across 8 channels. The figure illustrates that overall CNR reading were climbed from
37 dB toward 45 dB at the beginning except for channel 6, 7 and 8 where the CNR
reading drop along the fiber length for SCM-1.
Figure 5.22: CNR for SCM-1 Channels with power 1 mW
72
Figure 5.23: CNR performance for SCM-2 Channels with power 5mW
The CNR performance of SCM-2 illustrated that some of the channel were
augmented from 27 dB towards to 37 dB, except for channel 8, where the signal
drop along the fiber length, in this case the distance is 50 km and the power was
setup 5 mW.
5.10
Analysis On The Number of Channels
One of the interests of this analysis is looking for how good the performance
as the number of electrical carrier increases. As mentioned before the SCM/WDMRoF permits more than one electrical carrier is modulated by a single optical carrier.
Many researchers have been studied to increase the number of channel in the optical
fiber by modified the ordinary SCM system with other such as SCM/WDM and
hybrid SCM.
Nevertheless, hybrid SCM system can transport up to 80 analog and 30
digital channels using a single optical transmitter. If using QAM format, the number
of digital channels is limited to about 80.
73
.
Figure 5.24: The performance of RF carrier signal for SCM-1 and SCM-2
As shown in Figure 5.24, the power of overall RF signal carrier exist on -15
dBm to -10 dBm except for channel 5, 6 and 14 are fall to -25 dBm. Note that the
existence EDFA in the system able to boost the signal amplitude thus increases CNR
of the system.
5.11
The performance Analysis of Signal-to-Noise Ratio (SNR)
Signal to noise ratio is discussed in this section; SNR in optical link
communication was used to evaluate the minimum energy per pulse that is required
to achieve a prescribed maximum bit-error rate. SNR depend on the total noise in
the systems. Total noise is accumulated from shot noise, thermal noise, shunt noise
and series noise. These parameters have an effect on BER and Q factor. So the Q
factor is related to the signal-to-noise ratio required to the desire bit error rate.
Figure 5.25 (a) and (b) illustrates the SNR performance to the some various
distance links (100 km and 150 km). In 150 km SNR, some channels of SCM fall
down from 30 dB closed to 5 dB over the distance link. It is meaning that an error
probability for SNR is increases according to the fiber length.
74
(a)
(b)
Figure 5.25: (a) SNR performance of SCM at 100 km
(b) SNR performance of SCM at 150 km
5.12
Bit-Error Rate Performance
Figure 5.26 shows the BER performances of all electrical carrier frequency
across 1 km until 150 km with length of EDFA 5 m. The SCM-1 is illustrated in
Figure 5.26 (a), for channel 1, 2 and 6 the optimum fiber length is 30 km. For
channel 4 and 8 the optimum fiber length is 75 km. Note that the existence of EDFA
somehow affected the performance of the system in term of BER. In the SCM-2, the
channels are arbitrary (fluctuate) in unsystematic line due to highest bit-error rate, as
shown in Figure 5.26 (b). The preamplifier in optical domain has a primary
75
drawback that call as Amplified Spontaneous Emission (ASE) noise. This kind of
noise not only affect the BER but able to degrade CNR of the system.
(a)
(b)
Figure 5.26 (a) & (b): The performance of BER for SCM-1 and SCM 2
5.13
Analysis of Nonlinearity Due To Optical Power Level.
The system performances depend on nonlinearity due to optical power level.
The moving to deploy high bit rate (>10 Gbps per optical channel) in the system
cannot be done without considering nonlinear effects and reducing their impact on
these system. This is why nonlinear effects are today the most significant factor
76
determining the performance of high bit rate long haul fiber optic communications
systems and why design engineers must take them carefully into account.
Figure 5.27: Nonlinearity Performance of the Total Power versus Fiber Length
There were several researches and studies of nonlinearity effects on Radio
over Fiber system, however in this performance analysis, the study has been
specified to SCM/WDM-RoF system which was differs to other typical Radio over
Fiber system. The simulation setup was following the Table 5.4.
Table 5.4: Basic simulation setup for nonlinearity of power levels
Parameters
Value
Duplexing
SDD
RF Modulation
PSK
Optical Modulation
MZM
Channel BW
1.8 GHz
Bit Rate/Sub Carrier Channel
1.8 Gbps
Sample/bit
64
Sequence Length
128 bits
Fiber length
150 km
EDFA length
5m
Power level
0 dBm – 10 dBm
77
(a)
(b)
(c)
Figure 5.28 (a), (b) and (c): The performance of Total Power (0 dBm) to the Fiber
Length
According to the figure 5.28 (a) and (b), the total power of SCM-1 and SCM2, both of channels in the graph are gained up at the spectrum of 1525 nm due to
high attenuation and dispersion of the laser properties. The total powers are climbed
from -30 dBm to -10 dBm respectively. In figure 5.28 (c), illustrated that the best
link distance for 0 dBm is ≤ 50 km, more than 50 km should be required more
power.
78
In optics, the terms of linear and nonlinear mean “power independent” and
“power dependent” phenomena, respectively. From this perspective, all the effects
that have been considered do far are linear because their mechanisms are not the
function of power. Until recently, nonlinear effects in optical fibers were an area of
research.
5.14
Analysis of RF Bandwidth Spectrum
In this section, we present the attaining of the RF bandwidth outcomes from
the SCM/WDM-RoF system. By employing an external modulation of using MachZehnder Modulation, totally gain bandwidth of MZM is 20 GHz we derive 60 GHZ
of RF bandwidth spectrum as shown in Figure 5.29. The total capacity of RF
bandwidth was increased by utilize the number of channels. In this simulation we
setup for 16 channels
Figure 5.29: The performance of RF bandwidth Spectrum are expanded to 60 GHz
79
5.15
Conclusion
The performance of the SCM/WDM for RoF is resolved by many factors
such as BER, SNR, Power, distortion and attenuation. In this simulation 16 channels
of SCM are employed to the WDM over fiber. Mm-wave was generated to obtain
RF bandwidth. An external modulation of MZ Modulator was utilized to obtain
optical bandwidth. And the WDM employed multiplexing/demultiplexing of RF
signal that carried by optical signal carrier to resolve the huge bandwidth. The
outcomes of bandwidth was increased to 60 GHz by applying of 16 Channel of SCM
combined with WDM in optical fiber link.
CHAPTER 6
CONCLUSION & RECOMMENDATION
6.1
Discussions
The use of subcarrier multiplexing (SCM) transmission using an optical
carrier instead of the traditionally used super carrier over optical fibers is very
attractive. This technology found in wide spread application because of its simplicity
and cost-effectiveness. In optical domain, the most popular SCM application is the
optical analog video transmission and distribution. The SCM signal encompasses the
multiplexing of both multichannel of analog and/or digital signals. These signals can
carry either voice, data, video, digital audio, high-definition video or any other
analog or digital information.
In the SCM/WDM for RoF system, the sixteenth input signals are modulated
with different electrical carriers at microwave frequencies and then they are merged
by using a combiner. The combined signal is then modulated by external modulation
techniques using Mach Zehnder Modulation that has own bandwidth is 20 GHz.
After modulated and converted into optical carrier, the wavelengths then
81
multiplexed by WDM. In WDM, each of N different wavelength lasers is operating
at the slower Gbps speeds, but the aggregate system is transmitting at N times the
individual laser speed, providing a significant capacity enhancement. The WDM
channels are separated in wavelength (minimum channel spacing is 50 GHz) to
avoid cross-talk when they are demultiplexed by a non-ideal optical fiber. At the
receiver end, the optical signal is converted back to an electrical domain by an APD
photodetector and filtered by Bandpass Rectangle filter. The particular signals then
demultiplexed and demodulated, using conventional detection methods.
6.2
Conclusions
In digital communication systems, one of the focuses topics by the researcher
is bandwidth provided. The SCM and WDM technique and application offers very
attractive technique by applying in any model optical communication. The optical
fibers as medium guide present the capability of delivery many applications in any
modulation/multiplexing format. The mm-wave generation of SCM in different
frequency carriers was combined and carried in optical signals improved the
bandwidth capacity. Optical SCM also offers very attractive bandwidth performance
by conjunction with WDM technique. The SCM/WDM has been explored to present
bandwidth capacity for delivery any application in communication system such as
CATV, GSM, 3G, etc.
The project intended to explore and developed the SCM/WDM which
applied in RoF applications. The purpose is to perform significantly of the
bandwidth capacity by employed the SCM/WDM for RoF technology. The
characteristic of the systems then evaluated by BER, SNR, Power level and getting
standard parameter outcomes that can be used in real application.
82
In this project the SCM/WDM system introduced in Chapter 4 was
implemented in OptiSystem, with special attention shown to bandwidth
characteristics. These characteristics were modeled using BER analysis, the number
of carrier analysis, and important point to note is, that as long as the estimated of
bandwidth in this case is raise up in comparison to the conventional SCM
technology without WDM, the level of bandwidth expected is much the same.
This work shows that combination of 16 channels of SCM, MZM external
modulation and the WDM system products over 60 GHz bandwidth capacity, which
demonstrate a considerable enhanced is possible with this technique, and this has
shown better results compared to available published paper.
The results from this work are quote significant for papers publication. The
full paper was accepted for publication of the RAFSS 2008 Conference at Ibnu Sina
Institute that can be found in appendix A.
6.3
Future Recommendations
The SCM system has long been used to carry the signals in RF domain. In
this domain, subcarrier frequencies are limited in maximum subcarrier frequencies
and data rates by the available bandwidth of the electrical and optical components. It
can be described as the number of carrier with the total power of RF carrier. The
OFDM and OCDMA offer unlimited subcarrier frequencies conjunction with WDM
techniques that can improved the number of maximum subcarrier frequencies and
bandwidth capacity than the SCM technology. Therefore we suggest in the research
shall be done to fully characterized the SCM/WDM system. The system could be
enhance use other techniques, such as OFDM/WDM and OCDMA/WDM.
83
Furthermore, the model could be simulating by use commercial simulators
such as MATLAB and C++. These software tools have more significant to the
numerical measurement than OptiSystem to simulate the optical devices and derives
an improved diagrams outcomes. Practical measurements should be conducted
hence a good comparison can be made with the simulation and the theoretical
results.
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APPENDIX A
Accepted Paper
1
A. Marwanto, 2Sevia M. Idrus and 3Norizan M. Nawawi
“The SCM/WDM System Model for Radio over Fiber Communication Link”
Ibnu Sina Institute, UTM, Johor Bahru Malaysia
27 – 29 May 2008
The SCM/WDM System Model for Radio over Fibre Communication Link
1
A. Marwanto, 2S. M. Idrus and 3N. M. Nawawi
Photonic Technology Centre,
Faculty of Electrical Engineering,
University Technology of Malaysia,
81310, Skudai, Johor Darul Takzim.
Tel : +607-5535302
1
2
3
Email: a_marwanto@yahoo.com, sevia@fke.utm.my, norizan1123@yahoo.com
Abstract
Subcarrier Multiplexing (SCM) is multiple radio frequency (RF) carrying signal to transmit through
optical fiber using single wavelength. The most significant advantage of SCM in optical
communications is its ability to place different optical carriers together closely. On the other hand,
Wavelength Division Multiplexing (WDM) is a multiplexer at the transmitter to join the signals together,
and a demultiplexer at the receiver to split them apart. In WDM each laser is modulated at a given
speed, and the total aggregate capacity being transmitted along the high-bandwidth fiber is the sum
total of the bit rates of the individual lasers. In this work, we investigate various issues in this scenario
in order to provide a cost-effective, high performance solution for high speed data rates by the
available bandwidth of the electrical and optical components. Therefore, SCM must be used in
conjunction with WDM to utilize any significant fraction of the fiber bandwidth. This paper was focus
on a link between two station; one transmitter and receiver. The link was applying SCM on WDM and
RoF systems, the schemes that have been applied to perform the communication system were PSK
as RF modulation techniques in different frequencies. By setup 1.8 Gbps for high bit rate and applying
in 16 channels of SCM through WDM 60 GHz and modulated in single wavelength with L band of CW
laser channel, the bandwidth was significantly increase the capacity. The results is present higher
bandwidth for long distance communication system (SMF, 150 km) by using SCM/WDM for Radio
over Fiber. Therefore, the efficiency of bandwidth utilization of SCM is expected to be much better
than conventional optical WDM.
Keywords: SCM, Radio over Fiber, WDM, Optical Fiber Communication, Laser Channels,
Microcellular System
Introduction
Radio over Fiber
RoF is an analog optical link transmitting modulated RF signals. It serves to transmit the RF
signals down- and uplink, i.e. to and from central stations (CS) to base stations (BS) called
also radio ports. RF modulation is in most cases digital, in any usual form such as PSK,
QAM, TCM, etc. Optical modulation might have in principle also various forms, however,
intensity modulation (IM) is only dealt with here as different schemes were virtually never
proposed.
Radio Over Fiber is a technique that modulates RF in microwave signals on an optical
carrier to take advantage of the relatively low loss of optical fibers [1,2]. Many Radio over
Fiber systems employ a Mach Zehnder Modulator (MZM) to amplitude modulate the light
carrier [3]. MZMs typically have tremendous bandwidth that can easily exceed 40 GHz.
While this bandwidth is necessary for conventional fiber optic communications, only a
gigahertz or so of bandwidth is needed for radio over fiber applications. In most data
transmission and multi-point video/data distribution systems, information is routed at
baseband to the local transmission nodes, where it is up converted. The signals are in
analogue form and often involve many individual digitally modulated carriers spread over a
GHz or more of bandwidth. Since only a fraction of the MZM bandwidth is utilized in Radio
over Fiber systems, linearization is a practical and attractive method to achieve
performance enhancement.[3]
Sub Carrier Multiplexing
Basically the operation of the sub carrier multiplexing (SCM) was similar to Time Division
Multiplexing, such that TDM is commonly used in digital transmission system. On other
hand, SCM play an important role in analogue transmission system, however multiplexing
more conveniently carried out in frequency domain.
The main idea of SCM is combining two step of modulation which is operating at different
domain. First modulation was occupied at RF part such that several low bandwidth RF
channel carrying analogue or digital signal add up together by using multiplexer. Thus the
signal will be very close to each other in the frequency domain depending to local oscillator
frequency that applied in the modulation part. This combined signal actually modulated onto
higher frequency microwave carrier. The up-converted signals are in different frequency
bands and can therefore be combined by a microwave power combiner forming a
microwave subcarrier multiplexed composite signal. Second modulation was occupied at
optical domain, the modulated signal then convert to optical domain by using laser diode
and optical modulator as shown in figure 1.
Figure 1: Basic configuration of RF Subcarrier modulation
From figure 1, n number of digital signal were modulated by using a different frequency at
the local oscillator; f1, f2, f3.... fn. The modulation scheme applied was depend on what kind
of input signal (digital or analogue) was used and how good the desired modulated signal.
[8, 9]
Wavelength Division Multiplexing (WDM)
Wavelength-division multiplexing (WDM) is an approach that can exploit the huge optoelectronic bandwidth mismatch by requiring that each end user's equipment operate only at
electronic rate, but multiple WDM channels from different end-users may be multiplexed on
the same fiber. Under WDM, the optical transmission spectrum is carved up into a number
of non-overlapping wavelength (or frequency) bands, with each wavelength supporting a
single communication channel operating at whatever rate one desires, e.g., peak electronic
speed. Thus, by allowing multiple WDM channels to coexist on a single fiber, one can tap
into the huge fiber bandwidth, with the corresponding challenges being the design and
development of appropriate network architectures, protocols, and algorithms. Also, WDM
devices are easier to implement since, generally, all components in a WDM device need to
operate only at electronic speed; as a result, several WDM devices are available in the
marketplace today, and more are emerging.
The channel frequencies (or wavelengths) of WDM systems have been standardized by the
International Telecommunication Union (ITU) on a 100-GHz grid in the frequency range
186–196 THz (covering the C and L bands in the wavelength range 1530–1612 nm). For
this reason, channel spacing for most commercial WDM systems is 100 GHz (0.8 nm at
1552 nm). This value leads to only 10% spectral efficiency at the bit rate of 10 Gb/s. More
recently, ITU has specified WDM channels with a frequency spacing of 50 GHz. The use of
this channel spacing in combination with the bit rate of 40 Gb/s has the potential of
increasing the spectral efficiency to 80%. [15].
Methodology
In this paper, we focus into integration of the SCM techniques which modulated in RF area
and second modulation by optical modulator to improve the bandwidth capacity. The
system modeled by the optisym simulation tools with refers to the real parameters was
setup. The goals are to improve the optimization of the SCM and WDM for bandwidth
utilizing in BS and CS. The model was setup in two parts, the first part is transmitter and
optical link, the second part is receiver.
Transmitter Parts:
In the SCM/WDM transmitter parts consists of 2x8 channels RF modulated which
modulated in single wavelength. CW Laser and MZM Modulator carried the RF modulated
data in single wavelength. The power of CW Laser is 1 mw or 0 dBm. The frequency is
beginning from 3.6 GHz as a licenses frequency and the space of frequency is 1.8 GHz.
The bit rate was setup for 1.8 Gbps to gain the bandwidth for 60 GHz assigned in 16
channels of vary frequencies. For this experiment, we take two sample of SCM channel
consisting of 2x8 channels that carried digital data generates by PRBS. Each of the data
will be modulated by BPSK modulator with varies number of subcarrier which was in
gigahertz. One subcarrier may carry digital data, while another may be modulated with an
analogue signal such as video or telephone traffic. The composite electrical signal that has
been generated by the electrical transmitter that was amplified to10 dB by an electrical
amplifier and transform to optical domain through external optical modulator, MZM and CW
laser was applied as the optical source.
There are two ways of modulating the light source. The laser diode can itself be modulated
directly by using the appropriate RF signal to drive the laser bias current. The second option
is to operate the laser in continuous wave (CW) mode and then use an external modulator
such as the Mach-Zehnder Modulator (MZM), to modulate the intensity of the light. In both
cases, the modulating signal is the actual RF signal to be distributed. The RF signal must
be appropriately premodulated with data. In this project, the external modulation was made
by MZM for the SCM/WDM – RoF system model. CW laser established the light source of
the 1500 nm wavelength and the power was setup at 0 dBm.
The WDM was setup for multiplexing a single wavelength in order to transmit through SMF
optical link. Two port channel setup for two link SCM channels for multiplexing in single
wavelength. The Wavelength Division Multiplexing was installed to multiplexing optical
signal carrier to the link; the basic operation of the WDM is several base band-modulated
channels are transmitted along a single fiber but with each channel located at a different
wavelength. Each of N different wavelength lasers is operating at the slower Gbps speeds,
but the aggregate system is transmitting at N times the individual laser speed, providing a
significant capacity enhancement. The WDM channels are separated in wavelength to
avoid cross-talk when they are (de)multiplexed by a non-ideal optical fiber. Each laser is
modulated at a given speed, and the total aggregate capacity being transmitted along the
high-bandwidth fiber is the sum total of the bit rates of the individual lasers.
In the optical link distance varies between 20 km up to 150 for long distance communication
it’s refers to the low cost distance and resources efficient. The scenarios for optical amplifier
will be setup in pre-amplifier and post-amplifier, pre-amplifier applied before WDM Mux and
post amplifier assigned after WDM Mux in link of optical fiber. EDFA utilize as an optical
amplifier which the wavelength value between 2 m up to 5 m. The optical fiber are setting
with a single mode fiber were conducts only one mode and also capable to eliminate higher
order modes. Attenuation in a single mode fiber is smaller than in a multimode fiber
because in the single mode fiber less light will encounter absorption and scattering effects.
However, attenuation (macro bending effect) in single mode fiber increases as operating
wavelength increases and bend radius decreases.
Receiver parts:
At the receiver optical signal demultiplexing by WDM Demux and it’s converted into
electrical signal by Photodetector and filtering by Band Pass Rectangle Filter which split into
each of SCM frequency. A low noise amplifier (LNA) then amplifies the detected signal level
in order to overcome the losses and noise figures of the subsequent mixer chain and other
electronics. The APD photo detector was introduced in this system to obtain the desired
signals. One of the major parameter that is in the top priority in decides photo detector is
sensitivity. Sensitivity of the photo detector is present the minimum light power a photo
detector can detect. This parameter determines the length of a fiber-optic link imposed by a
power limitation. The more sensitive the photodiode, the longer the link can afford. In this
project an ideal WDM Demux was installed as function as optical signal demultiplexer. It is
works as optical filtering that compress, split, and filtering desire optical signals. After being
transmitted through a high-bandwidth optical fiber, the combined optical signals must be
demultiplexed at the receiving end by distributing the total optical power to each output port
and then requiring that each receiver selectively recover only one wavelength by using a
tunable optical filter.
Figure 2: Propose of basic configuration of the SCM/WDM system
The basic configuration of SCM/WDM system is illustrated in Figure 2. Generally, n
numbers of signals were modulated individually with different frequency in RF domain. Then
the modulated RF signal will be added up by a RF multiplexer (or by an adder) before
transform the RF signal into Optical signal through optical source and optical modulator on
a single wavelength. All the operation above was perform by a single transmitter.
Results and Discussion
In this work, two parameters that will be considered to analyze the performance are PMD
coefficient at the fibre link and type of laser source. Usually newer SMF will have better
PMD coefficient which is typically less than 0.1 ps /sqrt(km) while old SMF may have PMD
coefficient as high as 0.5 ps/sqrt (km). However, for this initial simulation, PMD coefficient
was setup at 0.5 ps/sqrt (km). Nevertheless, the PMD coefficient on this state was not
giving effect at the transmitter part because the PMD coefficient was contribute at fibre link.
Figure 3: RF Spectrum of 60 GHz Bandwidth
Figure 4: The width of Optical Spectrum
Bandwidth is 60 GHz
The available bandwidth of optical fiber generally limited by the processing speed of
electronics, however, the 16-PSK techniques in SCM combine of a continuous-wave tone
and transmits the phase of carrier signal. It has been proven very useful for band-limited
applications. In Figure 3 shows that range of bandwidth is 60 GHz, but the power
significantly degrades at 30 GHz beginning from -87 dBm due to losses and attenuation and
nonlinearities in Laser Diode. Figure 4 show that the gained bandwidth capacity for optical
spectrum is 60 GHz.
Figure 5: Total powers are degrades at 70 km less than 1 mw (0 dBm)
Figure 5 illustrated that the total power are degrades to length due to non-linearity of laser
diode, attenuation and dispersion in optical link. Where the optical power generated by a
laser diode is linearity proportional to the input electric driving current. In this graph, the
signal are dropped at 70 km to the below level of 0 dBm. It means that the total attenuation,
dispersion and non-linearity are influences in the total power of this system. To avoid the
shortcoming in this system we applied an optical amplifier that can be amplified the total
power of optical signals. As shown in Figure 6 (a) and (b), optical amplifier significantly is
increases the total power in case of for 100 km and 150 km. The EDFA optical amplifier
was setup in the range of 1m – 5 m to boost up the optical signal power in SCM/WDM
system model.
Figure 6(a): Total power at 100 km, with and without EDFA amplification
Figure 6(b): Total power at 150 km, with and without EDFA amplification
Figure 6 (a) has illustrated that EDFA can influence the total power to the link distance 100
km, where significantly increases 0.025 Watt to 0.037 Watt. Figure 6 (b) shows that EDFA
was able to boost the total power value for all channels about -20 dB from -50 dB (Optical
length = 150 km). It is reasonable to use EDFA in the system with length above 5m for 100
km optical fiber. It is mean that the power will be reduce or attenuated over the link without
EDFA.
Conclusion
The SCM and WDM model has been proposed as solution for bandwidth demand. The
combination of two different types of modulated has been perform to provide high bit rate
data and bandwidth in cellular communication, in particular between CS and BS. The L
band of CW Laser, MZM modulation and EDFA amplified applied in SCM-WDM model, the
60 GHz bandwidth was achieved with bit rate at 1.8 Gbps for 16 channels of the SCM. In
WDM each laser is modulated at a given speed, and the total aggregate capacity being
transmitted along the high-bandwidth fiber is the sum total of the bit rates of the individual
lasers. For long haul communication, the combined of SCM/WDM must be conjunction with
optical amplifier such as EDFA in order to avoid attenuation and dispersion of optical signal.
The combination of WDM and SCM provides the potential of designing broadband passive
optical networks capable of providing integrated services (audio, video, data, etc.) to a large
number of subscribers.
Acknowledgment
The authors acknowledge the Ministry of Science, Technology & innovation Malaysia for the
financial support through Escience. Our gratitude also goes to the administration of
Universiti Teknologi Malaysia (UTM) especially for Research Management Centre (RMC)
for the financial support through VOT number 79236
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