MODELING INTERSATELLITE OPTICAL WIRELESS

COMMUNICATION SYSTEM

AIDA HASFIZA BINTI HASHIM

UNIVERSITI TEKNOLOGI MALAYSIA

MODELING INTERSATELLITE OPTICAL WIRELESS

COMMUNICATION SYSTEM

AIDA HASFIZA BINTI HASHIM

A thesis submitted in fulfillment of the requirements

for the award of the degree

Bachelor of Electrical Engineering (Telecommunication)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

MAY 2009

ii

iii

To those who matters most to me.

iv

ACKNOWLEDGEMENT

My undivided gratitude to Allah S.W.T that has given me blessings and

strength to complete this project entitled “Modeling Intersatellite Optical Wireless

Communication System” successfully.

I wish to express my sincere appreciation to my Supervisor, Dr. Sevia M.

Idrus, for her constant guidance, counsels, and putting much effort upon the

completion of this project. I also would like to thank my lecturers in the Faculty of

Electrical Engineering who have taught me throughout the semesters. I am also

grateful to the Ministry of Higher Education for supporting this project under vote

number 78289.

Credits also given towards the researchers and staffs in Photonics

Technology Centre who are always willing to lend their hands and show me

guidance. Many thanks also to my mother, my sisters and the rest of my family

members for their encouragement, love and support.

Last but not least, for all my friends – Nadia, Norli, Farah, Huda, Julia and

the rest of my Electrical-Telecommunication Engineering classmates that had shared

their knowledge and experience with me throughout these four years of study. The

kindness, cooperation and support from all of them will always be remembered.

Thank you.

v

ABSTRACT

Optical communications systems have evolved from lengthy fibers to

powerful wireless system. This has hence resulted in the use of optical wireless

communication system in space communications. As the number of satellites

orbiting Earth increase year by year, a network between the satellites provides a

method for them to communicate with each other. This is important for satellites to

send information to one another and also to relay the information from one satellite

to another satellite and then to the ground stations. In this research, the intersatellite

communication link is studied and optical wireless communication was proposed for

the link. The intersatellite optical wireless communication (IsOWC) system was

designed and simulated for performance characterization. The intersatellite link was

modeled and simulated using a commercial optical system simulator named

OptiSystem by Optiwave. The findings of this project shows that by using laser

satellite communication system, the satellites can be connected with data rates up to

10Gbps. This thesis fully discusses the free space optic system technology for

intersatellite communication link for future development of large data transfer

between satellites with high Quality of Service (QoS). The system performance

including bit rates, receiver sensitivity and distance of LEO and GEO intersatellite

links were analyzed.

vi

ABSTRAK

Sistem komunikasi optik telah berkembang dari kabel-kabel fiber ke sistem

wayarles yang canggih. Ini telah mengembangkan penggunaan teknologi optik ke

sistem komunikasi angkasa lepas. Dengan pertambahan bilangan satelit di orbit dari

tahun ke tahun, jaringan perhubungan antara satelit-satelit ini dapat memberi satu

kaedah untuk ia berhubung. Ini adalah penting untuk satu satelit menerima dan

menghantar data dari satu satelit ke satelit yang lain dan juga ke bumi. Dalam kajian

ini, komunikasi antara satelit telah dipelajari dan komukasi optik wayarles telah

dicadangkan. Sistem komunikasi optik wayarles antara satelit (IsOWC) telah

direkabentuk dan disimulasi untuk kajian prestasi sistem. Talian antara satelit itu

telah dimodel dan disimulasi menggunakan perisian OptiSystem dari Optiwave.

Hasil daripada kajian ini telah menunjukkan bahawa dengan menggunakan sistem

komunikasi laser, satelit-satelit dapat dihubungkandengan kadar data mencecah

10Gbps. Tesis ini membincangkan secara keseluruhan sistem teknologi optik ruang

bebas untuk talian komunikasi antara satelit dengan perpindahan data yang besar dan

juga kualiti servis (QoS) yg tinggi untuk masa hadapan. Prestasi sistem seperti kadar

data, sensitiviti penerima, dan jarak antara satelit-satelit LEO dan GEO telah dikaji.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS

ii

iii

iv

v

vi

vii

x

xi

xiii

1 INTRODUCTION

1.1 Introduction

1.2 Overview of Satellites

1.3 Project Objectives

1.4 Scope of Work

1.5 Problem Statement

1.6 Thesis Outline

1

3

4

5

5

6

2 BASIC CONCEPT AND THEORIES

2.1 Introduction

2.2 Intersatellite Link Developments

2.3 Optical Wireless Communication Concepts

2.3.1 Optical Wireless System

8

8

9

10

viii

2.3.2 System Performance

2.3.3 Attenuation and Link Power Budget

2.4 Related Researches

2.5 Intersatellite Optical Link Applications

2.5.1 Data Relay for Inter Orbit Satellites

2.5.2 Connecting Constellations of Satellites

2.6 Conclusions

14

14

15

19

19

21

22

3 PROJECT METHODOLOGY

3.1 Introduction

3.2 Study the OWC System

3.3 Study the Intersatellite Link System

3.4 Design the IsOWC Basic System Model

3.5 IsOWC System Simulation of Model for

Characteristics Performance

3.6 Result Analysis

3.7 Presentation and Thesis Writing

3.8 Conclusions

23

25

25

25

26

26

27

27

4 SYSTEM MODELING

4.1 Introduction

4.2 OptiSystem Software

4.3 System Model in OptiSystem

4.4 System Components

4.4.1 IsOWC Transmitter Design

4.4.2 OWC Channel

4.4.3 IsOWC Receiver Design

4.5 Conclusions

28

28

29

30

31

33

33

35

5 RESULT ANALYSIS

5.1 Introduction

5.2 Relationship between Q-factor and Bit rates with

Distance of Intersatellite Link

36

37

ix

5.3 Relationship between Q-factor and Signal

Wavelength

5.4 Relationship between Diameter of Optical

Antennae with Received Power and Distance

5.5 Conclusions

40

42

44

6 CONCLUSIONS

6.1 Conclusions

6.2 Future Work Recommendations

45

46

REFERENCES 48

x

LIST OF TABLES

TABLE NO. TITLE PAGE

5.1 Maximum Q-factor recorded for respective signal

wavelengths 42

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Overview of IsOWC 2

1.2 Earth Satellite Communication Orbits 4

2.1 Optical intersatellite link between Artemis and

SPOT-4 first achieved in March 2003 9

2.2 IsOWC basic system block diagram for simplex

communication 10

2.3

Optical modulation process where input light is

varied according to electrical signal to produce

light pulses

12

2.4 Optical antennae increase the signal divergence 12

2.5 APD photodetector structure 13

2.6 Link attenuation for (a) LEO-LEO link and (b)

GEO-GEO link 17

2.7 Optical satellite network creating a global

connection 18

2.8 High-level optical intersatellite communication

system 19

2.9 Concept of data relay for inter orbit IsOWC 20

2.10 Data relay methods (a) conventional (b) using

intersatellite data relay 21

2.11 Constellations of satellite orbiting Earth 22

xii

3.1 The project methodology 24

4.1 IsOWC first design with basic subsystems 29

4.2 IsOWC simplex design model 29

4.3 IsOWC full-duplex system between two satellites 30

4.4 NRZ encoding technique 31

4.5 Mach-Zehnder Modulator varies the light intensity

according to voltage 32

4.6 BER analyzer is connected to the transmitter when

without 3R regenerator 34

4.7 Connection of the BER analyzer to the 3R

regenerator 35

5.1 Maximum achievable Q-factor for variable distance

at 1550nm IsOWC link for bitrate up to 10Gbps 37

5.2 Eye-diagram for IsOWC system at distance

1000km and 10Mbps 38

5.3 Eye-diagram obtained at distance 3000km and

10Mbps bit rate 39

5.4 Eye diagram for IsOWC system with bit rate 1Gbps

at 1000km distance 39

5.5 Eye diagram for system at 850nm wavelength 40

5.6 System eye diagram obtained at wavelength 950nm 41

5.7 Eye diagram for 1550nm signal 41

5.8

Received power for respective optical antennae

diameter at distance up to 5000km and input power

of 10dBm

44

xiii

LIST OF ABBREVIATIONS

OWC - Optical Wireless Communication

IsOWC - Intersatellite Optical Wireless Communication

RF - Radio Frequency

SCORE - Signal Communication by Orbital Relay

LEO - Low Earth Orbit

MEO - Medium Earth Orbit

GEO - Geosynchronous Orbit

BER - Bit Error Rate

NASA - National American Space Agency

TDRSS - Tracking and Data Relay Satellite System

ESA - European Space Agency

SPOT-4 - Satellite Pour L`Observation De La Terre 4

TT&C - Telemetry, Tracking and Communication

LED - Light Emitting Diode

ILD - Injecting Laser Diode

APD - Avalanche Photodiode

NRZ - Non-return Zero

CW - Continuous wave

CHAPTER 1

INTRODUCTION

1.1 Introduction

As of April 2009, there are 6124 satellites orbiting Earth and this number

increases year by year [1]. At the same time, the optical wireless communication

(OWC) technology has grown and advanced throughout the year. Laser

communication is now able to send information at data rates up to several Gbps and

at distance of thousands of kilometers apart. This has open up the idea to adapt

optical wireless communication technology into space technology; hence

intersatellite optical wireless communication (IsOWC) is developed.

IsOWC can be used to connect one satellite to another, whether the satellite is

in the same orbit or in different orbits. With light travelling at 3 x 108 m/s, data can

be sent without much delay and with minimum attenuation since the space is

considered to be vacuum. The advantages of using optical link over radio frequency

(RF) links is the ability to send high speed data to a distance of thousands of

kilometers using small size payload [2]. By reducing the size of the payload, the

mass and the cost of the satellite will also be decreased. Another reason of using

OWC is due to wavelength. RF wavelength is much longer compared to lasers hence

2

the beamwidth that can be achieved using lasers is narrower than that of the RF

system [3]. Due to this reason, OWC link results in lower loss compared to RF but

it requires a highly accurate tracking system to make sure that the connecting

satellites are aligned and have line of sight.

Figure 1.1 Overview of IsOWC

This project is done to study the intersatellite communication employing

optical communication link. The effects of distance between satellites, bit rates,

input power, optical antennae, and receiver sensitivity is studied and discussed in this

thesis while assuming that the satellites have line of sight.

3

1.2 Overview of Satellites

A satellite is an object that orbits or revolves around another object in space.

The Moon is a satellite to Earth and the Earth is a satellite to the Sun. Those are

natural satellite. In 1945, Arthur Clarke wrote on the possibilities of having man-

made satellites that could be able to relay telephone channels and broadcast

programs. Thirteen years later, the first communication satellite named SCORE

(Signal Communication by Orbital Relay) was launched and proved that Clarke’s

theory was indeed possible. Following the success of SCORE, many more satellites

were launched by the United States, Russia, United Kingdom and Canada. Since

then, satellites are launched up to space for many applications such as for

communication, remote sensing, scientific research and global positioning.

Satellites revolve around Earth at their own orbit and there are three

commonly used orbits for satellites. Low Earth Orbit (LEO) is the orbit closest to

Earth with altitude of 100km to 5,000km. LEO satellites take from 2 to 4 hours to

rotate around Earth. This orbit is commonly used for multi-satellite constellations

where several satellites are launched up to space to perform a single mission. The

Medium Earth Orbit (MEO) is from 10,000km to 20,000km altitude and the orbital

period is from 4 to 12 hours. MEO orbit is usually occupied by remote sensing

satellites. Communication satellites for broadcasting and telephone relay is placed in

the Geosynchronous Orbit (GEO) which has 36,000km altitude from Earth. A GEO

satellite takes 24 hours to rotate around Earth which makes it seem like stationary

from Earth’s point of view [4]. Figure 1.2 shows the satellite orbits around Earth.

4

Figure 1.2 Earth Satellite Communication Orbits

1.3 Project Objectives

This project was done to fulfill these objectives:

i) To study the optical wireless communication system for intersatellite links.

ii) To design the intersatellite optical wireless communication system for GEO

and LEO satellites.

iii) To model and simulate the intersatellite link for performance

characterization.

5

1.4 Scope of Work

A few guidelines are proposed so that this project is narrowed to a certain

boundaries. This is to ensure that this project achieves its objectives.

Firstly, the environment surrounding the satellites is assumed to be vacuum.

It is also assume that the satellites for intersatellite link are aligned and have line of

sight. Hence, the presence of any large particle that may obstruct the line of sight is

not studied in this project.

This project models basic optical communication system where no advanced

modulation, multiplexing or coding technique is used. The software that is used to

model the IsOWC system is Optiwave’s OptiSystem. Therefore, the result of system

performance relies on the software and the channel characteristic follows the

software’s OWC channel characteristic.

To analyze the system, parameters that can affect the system performance

such as distance between the transmitter and receiver, data bit rate, input power and

wavelength are varied. The performance of the system is measured in terms of the

bit error rate (BER), Q-factor and received power retrieved from the software.

1.5 Problem Statement

Conventional communication between satellites and also to Earth is by using

RF system. The problem with RF system is that there are many limitations in the

6

systems, limitation that optical links can recover. Frequency is one of the many

limitations of RF links as there are regulations and license to the frequency that can

be used for satellite communications. The regulations are not applicable in optical

link. Since optical link are able to transmit very high frequency which is up to

194THz for wavelength 1550nm, therefore it can support high data rate transmission

[5]. For intersatellite communications, signal need to travel thousands of kilometers

from one satellite to another. If RF system is to be employed, the size of the

transmitting and receiving antenna that is needed would be very big (about meters

wide) and also heavy, compared to using optical link that would only need an optical

antenna of several centimeters big. Reducing size and weight of the satellite’s

payload can reduce the cost of the satellite, which is what every satellite designer

aims for.

1.6 Thesis Outline

This thesis consists of six chapters. Chapter 1 introduces the project and

discusses the basics of satellites. Chapter 1 also presents the objective of the project

and the scope of work to achieve the objectives. That is followed by the problem

statement which explains why this project is done.

Chapter 2 discusses the theory and literature review of the project. The

chapter begins with brief explanations on intersatellite developments. Then, the

chapter discusses on OWC concepts where the fundamentals of OWC system,

attenuation and power budget calculation is explained. The chapter then presents the

OWC system performance analysis method and compares fiber optic system to OWC

system. The chapter also presents some researches that are relevant to the project.

Finally, some applications of optical intersatellite links are presented at the end of the

chapter.

7

In Chapter 3, the methodology of the project is presented and explained in

details of each steps in the methodology. The chapter explains the process of this

project from research study of OWC and satellites to modeling, simulating, gathering

results, analyzing and writing this thesis report.

Chapter 4 presents the system model that had been designed. The

OptiSystem software that is used to model the IsOWC system is briefly discussed.

Each subsystem in the system is also explained in the chapter.

The findings of this project are presented in Chapter 5. The system

performance is presented in graphs and figures and is then discussed. System

performance is measured in Q-factor and the signal received power. The relationship

of these parameters with varying input parameter such as bit rates and distance is

conferred in this chapter.

The final chapter is Chapter 6 where the overall project is concluded. The

chapter answers on whether the project objectives are successfully accomplished and

then concludes the findings of this project. Lastly, some recommendations were

stated on future work that can be continued and improved from this project.

CHAPTER 2

BASIC CONCEPT AND THEORIES

2.1 Introduction

In this chapter, the concept and theories of intersatellite links and OWC

system will be discussed. The development of intersatellite communication and the

important theories related to OWC is covered in this chapter.

2.2 Intersatellite Link Developments

Intersatellite links have been employed on several satellite systems such as

Iridium and National American Space Agency (NASA)’s Tracking and Data Relay

Satellite System (TDRSS) where RF is used to link the satellites. However, optical

links has been proven to provide higher bit rates and better efficiency than RF link.

Hence, several satellites have been implanted with OWC intersatellite links such as

European Space Agency (ESA)’s Artemis and Japan’s Kirari satellites. The first

9

intersatellite communication employing optical link was successfully achieved on

March 2003 between Artemis and French satellite named Satellite Pour

L`Observation De La Terre 4 or SPOT-4 [6]. The simplex communication from

Artemis to SPOT-4 was done by using data transmitted at 50Mbps with signal

wavelength of 850nm and optical signal with the power of 120mW. Artemis was

placed in the GEO satellite while SPOT-4 was in LEO at altitude of 832km. In

December 2005, a full-duplex communication between Artemis and Kirari was

achieved. These two experiments have shown that IsOWC is possible. Figure 2.1

shows the overview of optical communication link between Artemis and SPOT-4 [7].

Figure 2.1 Optical intersatellite link between Artemis and SPOT-4 first achieved

in March 2003

2.3 Optical Wireless Communication Concepts

Different from RF links, OWC uses light at near-infrared frequency to

communicate. OWC system still consists of three main communication parts which

Optical link

10

are transmitter, propagation channel and receiver. The OWC system is not much

different from free space optics and fiber optic communication where the difference

relies in the propagation medium. OWC channel is considered to be outer space

where it is assumed to be vacuum and free from atmospheric attenuation factors.

2.3.1 Optical Wireless System

As mentioned, the optical wireless system consists of transmitter, propagation

medium and receiver. Figure 2.2 shows the basic block diagram of an IsOWC

system where the transmitter is in the first satellite and the receiver is in the second

satellite. The free space between the satellites is the propagation medium is the

OWC channel that is use to transmit the light signal.

Figure 2.2 IsOWC basic system block diagram for simplex communication

11

The IsOWC transmitter receives data from the satellite’s Telemetry, Tracking

and Communication (TT&C) system. The data that usually transmitted by a satellite

are such as the satellite position and attitude tracking, captured image for remote

sensing satellite, or even voice data for telephone network relaying satellite.

Light source is the most important component in optical signal since

communication is done by transmitting light. Light-emitting diode (LED) and

injected laser diode (ILD) are two types of optical light source commonly used in

optical communication. These devices are commonly made from semiconductor

materials whereby the interaction between positively charge semiconductor and

negatively charge semiconductor produces photons or light energy [8]. The output

light emitted by the ILD is monochromatic, coherent and has high radiance which

makes it suitable for long distance free space transmission. The light generated by

the laser can travel much further than the light emitted by LED. Hence, ILD is used

for IsOWC system.

The electrical signal from TT&C system and optical signal from the laser will

be modulated by an optical modulator before it is transmitted out to space. An

optical modulator varies the intensity or amplitude of the input light signal from ILD

according to the electrical signal. This is done by changing optical parameters such

as refractive index, reflection factor and transmission factor of the optical modulator

that is made from fiber waveguides. Figure 2.3 illustrates the modulation process of

an optical modulator [9].

12

Figure 2.3 Optical modulation process where input light is varied according to

electrical signal to produce light pulses

The output light pulses from the optical modulator are transmitted in the

transmission medium to the receiving satellite. In the case of IsOWC system, the

transmission is the optical wireless channel. Different from free space optics that is

subjected to many losses due to weather and atmospheric attenuation, the OWC

channel is vacuum and free from atmospheric losses. At an ideal case, the only cause

of signal attenuation is the distance of the transmission. Optical antennae or optical

lenses can be used at the transmitter and the receiver. The optical antennae allow

wider light beam divergence and detection. An optical antenna is actually a lens or a

telescope that is place before and after the transmission medium as shown in Figure

2.4.

Figure 2.4 Optical antennae increase the signal divergence

13

The receiving end of the IsOWC signal consists of a photodiode and a low

pass filter. A photodiode is a device that detects the received light signal and

converts it into electrical signal. Like an optical light source, photodiodes is made

from positive and negatively charged semiconductor junction that is connected in

reverse bias. When photons strike the junction, electrical signal will be created.

Avalanche photodiode (APD) is used in long distance free space optical data

transmission due to its characteristics of producing high amplification for low or

weak light signals. Amplification in APD photodetector or avalanche phenomenon

occurs when charged electrons are introduced in such high electric field area and

collide with neutral semiconductor atoms, thus generating other carriers and this

collision. This process is then repeated to effectively amplify the limited number of

carriers. Figure 2.5 shows the structure of APD photodetector that consists of two p-

n junctions which produces the internal gain [10].

Figure 2.5 APD photodetector structure

14

2.3.2 System Performance

The system performance can be evaluated in many ways such as by analyzing

the BER and Q-factor. BER can be said to be the ratio of the number of bit errors

detected in the receiver and the number of bits transmitted. Bit errors happen as the

result of incorrect decisions being made in a receiver due to the presence of noise on

a digital signal [11]. Meanwhile, Q-factor is a measurement of the signal quality. It

is proportional to the system’s signal to noise ratio. In optical system, the BER is

typically too small to measure hence Q-factor is more suitable to be used. The

relationship between BER and Q-factor can be given as

(2.1)

From the equation 2.1, it can be seen that the BER is inversely proportional to

Q-factor. Therefore, if the system’s error increases, the Q factor will thus decrease.

2.3.3 Attenuation and Link Power Budget

Link power budget is done by calculating the power received by the system.

The power received is the resultant signal of the input signal degradation due to

losses and attenuation and also amplification of the transmitter and receiver gain.

Equation 2.2 can be used to calculate the received power in an OWC system [5].

(2.2)

where PR = Received power

PT = Transmit power

15

ηT = Optics efficiency of the transmitter

ηR = Optics efficiency of the receiver

λ = Signal wavelength

Z = Distance between transmitter and receiver

GT = Transmitter optical antenna gain

GR = Receiver optical antenna gain

LT = Transmitter pointing loss

LR = Receiver pointing loss

The gain of the transmitter and receiver optical antennae can be given by

G=(πD/λ)2 where D is the diameter of the optical antenna. Most optical system

transmitter uses laser diode with narrow-beam-divergence angle and the receiver has

narrow field view; therefore, pointing loss can be a major contributor to signal

degradation. Pointing loss factor can be approximate by L=exp(-Gθ2) where θ is the

divergence angle.

2.4 Related Researches

Several researchers had written on the optical intersatellite link.

Pfennigbauer and Leeb (2003) in their paper entitled Free-space Optical Quantum

Key Distribution Using Intersatellite Links had presented the employment of

quantum cryptography in intersatellite links [12]. The paper also discussed the link

properties where attenuation, A, for intersatellite link and satellite to Earth link was

calculated using the equation 2.3.

16

(2.3)

where L is the distance between the transmitter and receiver, λ is the wavelength, θatm

is the atmospheric turbulence that causes divergence, DR is the receiver’s optical

antenna diameter, LP is the pointing loss, Aatm is the attenuation of the atmosphere,

TT and TR are the transmission factors for the transmitter and the receiver

respectively. θT is the divergence angle at the transmitter where it can be given by

θT=λ/DT and DT is the diameter of the transmitter’s optical antenna. Since the

intersatellite link communication is not affected by the atmospheric turbulence and

attenuation of the atmosphere, equation 2.3 can be reduced into

(2.4)

The results from calculations of attenuation for LEO-LEO and GEO-GEO

intersatellite link were presented in the paper as shown in Figure 2.6 (a) and (b)

respectively where DT and DR used is equal [12]. The paper concludes that for larger

optical antennae are needed for longer transmission distance.

17

(a)

(b)

Figure 2.6 Link attenuation for (a) LEO-LEO link and (b) GEO-GEO link

Another relevant research is found in Chan (2003) in his paper Optical

Satellite Networks [2]. In the paper, the author discussed the feasibility in

constructing high speed optical satellite network as part of a larger integrated space-

terrestrial network. According to Chan, optical wireless intersatellite links can be

used as a backbone for global networking creating local area networks and wide area

networks all over Earth. The intersatellite links can also be connected to remote

18

sensing satellites, airplanes, ships, submarines and spacecrafts far away from Earth.

The usage of intersatellite link proposed in the paper is shown in Figure 2.7 [2].

Figure 2.7 Optical satellite network creating a global connection

The paper also presents an advanced block diagram of an intersatellite optical

transceiver as in Figure 2.8 [2]. The transceiver system includes tracking system and

attitude control system to point the transceiver to the other satellite.

19

Figure 2.8 High-level optical intersatellite communication system

2.5 Intersatellite Optical Link Applications

There are many applications of IsOWC, applications of where satellites need

to communicate with each other. One of the applications is data relay between inter

orbit satellites and another is to connect satellites in constellations.

2.5.1 Data Relay for Inter Orbit Satellites

Unlike GEO satellites, LEO and MEO satellites orbit are not stationary from

Earth. This means that the satellite is not constantly in its Earth station’s view. By

using intersatellite link, data can be sent a LEO and MEO satellite at any time by

using a GEO satellite as relay. Data can also be relayed from one LEO satellite to

another if they have line of sight. This concept is shown in Figure 2.9.

20

Figure 2.9 Concept of data relay for inter orbit IsOWC

Relaying data using intersatellite links also reduce the time it takes to send a

data from one part of the world to anther. The conventional way of relaying data is

as shown in Figure 2.10(a) while Figure 2.10(b) shows relaying data by using

intersatellite links [13]. Transmitting data from Earth to satellite has high time delay;

therefore by using IsOWC, the time delay can be reduced.

21

(a) (b)

Figure 2.10 Data relay methods a) conventional b) using intersatellite data relay

2.5.2 Connecting Constellations of Satellites

Some missions and applications require more than one satellite such as the

global tracking system (GPS) satellites and Iridium satellites. To connect these

satellites, the fastest and most efficient way is by using optical intersatellite links.

For example, the Iridium system has 66 satellites up in space at an altitude of 700km.

The satellites are placed in 6 orbits and each orbit has 11 satellites. These satellites

are to become the base stations for cellular mobile communication. Iridium satellites

employ RF intersatellite links to connect with each other at Ka-band frequency. Due

to the short distance between the satellites and low data rate, the intersatellite link is

applicable. However, by using IsOWC, the data rate can be improved and more

mobile user can be supported. Figure 2.11 shows constellation of satellites orbiting

Earth.

22

Figure 2.11 Constellations of satellite orbiting Earth

2.6 Conclusions

In this chapter, the concepts and theories of intersatellite communication and

OWC system was discussed. Previous researches of [2] and [12] were also

presented. Several IsOWC applications which are data relaying and connecting

constellations of satellites were also conferred.

CHAPTER 3

PROJECT METHODOLOGY

3.1 Introduction

Methodologies are steps taken to achieve the project objectives. This chapter

explains steps by steps taken to fulfill this project where there are several steps to be

completed. The flow diagram in Figure 3.1 shows the steps taken to accomplish this

project.

24

Figure 3.1 The project methodology

START

Receive project title from supervisor

Study on OWC system

Study on intersatellite link system

Design IsOWC model

Optimum design?

Simulation of Model

Result Analysis

Thesis Writing

Yes

No

25

3.2 Study on OWC System

This first step is important in order to understand what the project is all about.

This is also important in order to achieve the first objective of the project which is to

study OWC for intersatellite links. The research sources are mainly from books,

library and online databases of previous researches.

3.3 Study on Intersatellite Link System

The second step is to study about the satellites and intersatellite links. It is

important to know how satellite works and what IsOWC will achieve in connecting

the satellites. The basic theories about satellite were learned in class, but books and

previous researches provide extra information on the topic. There are not many

existing intersatellite links, therefore to find a relevant paper on this topic is hard.

Therefore, the most relevant experiment relevant of this project is from the Artemis

and Kirari optical intersatellite link.

3.4 Design the IsOWC Basic System Model

The second objective of this project is to design the IsOWC system model.

This is done by using Optiwave’s OptiSystem software. It took several days to study

the software and to get used to it. Several designs were built to get an optimum

design. The first design consists of basic OWC communication subsystems which

26

are optical transmitter, OWC channel and optical receiver. The design was then

improved by builing the optical transmitter and receiver block by block.

3.5 IsOWC System Simulation of Model for Characteristics Performance

Using the model designed in OptiSystem, parameters such as bit rate,

distance between satellites and power were varied to study the system’s performance.

Further study on the software was done at this step to understand the visualizers to be

used to analyze the output of the system. The visualizer that is most important in this

design is the BER Analyzer where the system BER, Q-factor and eye diagram were

obtained from it. Apart from that, the relationship between optical antennae

diameter, attenuation, and receives power were plotted using Microsoft Excel. The

result of the plot shows the advantage of using optical antennae where the received

power improved as the antennae diameter increased.

3.6 Result Analysis

From the simulation results, analyses were made to see the system

performance. Graphs were plotted to have a clearer view on the system’s

performance. Observations were then made from the graphs and conclusions were

drawn from it. The results ware compared with previous researches and the theory to

prove its validity.

27

3.7 Presentation and Thesis Writing

After the conclusions were drawn, the project is considered complete. The

final step to be taken is to present the project and to write the thesis. This step is

done as the requirements in completing the course.

3.8 Conclusions

In this chapter, the steps and methodology of this project were presented and

discussed. The steps were done in order to achieve what the project aims for. At the

end, everything there is to know about this project is written in the thesis.

CHAPTER 4

SYSTEM MODELING

4.1 Introduction

This chapter discusses on the IsOWC system design. The OptiSystem

software used for designing is also conferred here. The system model is then

presented and finally this chapter explains each and every subsystems of the model.

4.2 OptiSystem Software

The OptiSystem software is developed by Optiwave to perform complex

optical communication simulation. It provides an easy user interface which is

common to many other electrical engineering tools. The OptiSystem software is

suitable to be used to model and simulate fiber optic system, free space optic system

and also OWC system. The OWC channel provided in the software is specifically

designed to suit the intersatellite environment.

29

4.3 System Model in OptiSystem

Several designs were built to obtain the optimum design for the IsOWC

system. Based on the basic IsOWC block diagram presented in Chapter 2, the

system is modeled with basic communication subsystems as stated in the project’s

scope of work. The first design consists of basic OWC communication system and is

shown in Figure 4.1. The design was then improved by expanding the optical

transmitter and receiver with specific subsystems. This is shown in Figure 4.2.

Figure 4.1 IsOWC first design with basic subsystems

Figure 4.2 IsOWC simplex design model

Optical

Transmitter

Optical

Receiver

30

In the design model shown, note that the model is for simplex system. Which

means the model is for one way data transmission from one satellite to another. The

full-duplex system model consists of two simplex systems. Hence it can be used for

two way data transmission from one satellite to another and back. Figure 4.3 shows

the final system model for full-duplex communication between two satellites.

Figure 4.3 IsOWC full-duplex system between two satellites

4.4 System Components

This part will explains each subsystems used in modeling the IsOWC system.

The part will be divided into three, the transmitter, OWC channel and the receiver

part.

31

4.4.1 IsOWC Transmitter Design

The transmitter consists of four subsystems. The first subsystem is the

pseudo-random bit sequence generator. This subsystem is to represent the

information or data that wants to be transmitted. The data usually come from the

satellite’s TT&C system. In this project the bit rate is varied to observe the system

performance and the relationship between bit rate and distance.

The second subsystem is the NRZ pulse generator. This subsystem encodes

the data from the pseudo-random bit sequence generator using the non-return zero

encoding technique. Figure 4.4 shows the NRZ encoding technique.

Figure 4.4 NRZ encoding technique

The third subsystem in the satellite IsOWC transmitter is the CW laser. CW

stands for continuous wave where the output signal of the laser is nonstop and un-

modulated. Lasers are used instead of LED for this system because of its ability to

transmit at further distance. The frequency of the light is chosen to be 1550nm or

193.1THz with input power of 10dBm. These parameters can be change and the

effect of varying the frequency is discussed in the next chapter.

32

The last subsystem in the transmitter is the Mach-Zehnder Modulator. It is an

optical modulator that functions is to vary intensity of the light source from the laser

according to the output of the NRZ pulse generator. The Mach-Zehnder modulator

consists of two couplers and two waveguides of equal-length as shown in Figure 4.5.

The input optical signal from the laser will split in to two and go through phase

shifting process in the waveguides. Phase-shifting happens due to the electro-optic

effect where the output electrical pulse from the NRZ pulse generator will vary the

voltage hence varying the refractive indices of the waveguides. The output of the

Mach-Zehnder modulator will be transmitted to the other satellite through the space

of OWC channel.

Figure 4.5 Mach-Zehnder Modulator varies the light intensity according to

voltage

33

4.4.2 OWC Channel

The free space between two connecting satellites is considered as OWC

channel which is the propagating medium for the transmitted light. In the

OptiSystem software, the OWC channel is between an optical transmitter and optical

receiver with 15cm optical antenna at each end. The transmitter and receiver gains

are 0dB. The transmitter and receiver antennae are also assumed to be ideal where

the optical efficiency is equal to 1 and there are no pointing errors. Additional losses

from scintillation and mispointing are also assumed to be zero. Due to the altitude of

the satellites that is above the Earth’s atmospheric layers, there is no attenuation due

to atmospheric effects.

4.4.3 IsOWC Receiver Design

The receiver of the data consists of an APD photodiode, low pass filter and

3R regenerator. The photodiode acts as a front-end receiver that receives the optical

signal and converts it into electrical signal. The APD photodiode has an internal gain

which allows for the reduction of noisy external amplifiers in optical detection

systems. Therefore, in this system model, no optical amplifier is needed. Apart from

that, the APD photodiode is useful in low, weak or reduced light applications

because of the avalanche phenomenon utilized by the device provides high

amplification. Hence it is ideal to be used in this system where the long distance

transmission reduces the intensity of the light. APD photodiode used in the

OptiSystem model has a multiplication factor of 3 and default dark current used is

10nA. The frequency of the photodiode is set to 193.1THz.

34

The low pass filter (LPF) after the photodiode is used to filter out the

unwanted higher frequency signals. Bessel LPF is used with cut-off frequency of

0.75 x bit rate of the signal. The order of the Bessel funtion is 4 and the maximum

attenuation of the filter is 100dB. The 3R regenerator is the subsystem use to

regenerate electrical signal of the original bit sequence, and the modulated electrical

signal as in the transmitter to be used for BER analysis. It is as if the BER analyzer

is connected to the output of the pseudo-random bit sequence generator and NRZ

pulse generator in the transmitter to be compared with the received signal at the LPF

output. Figure 4.6 shows an example of connection of the BER analyzer without 3R

regenerator and Figure 4.7 shows the connection with 3R regenerator. Note the

result of the connection is equivalent. The output of the 3R regenerator is connected

to the satellite’s TT&C system for further signal processing.

Figure 4.6 BER analyzer is connected to the transmitter when without 3R

regenerator

35

Figure 4.7 Connection of the BER analyzer to the 3R regenerator

4.5 Conclusions

In this chapter, the IsOWC system that was modeled using OptiSystem is

presented. Brief information on the OptiSystem software were conferred and details

of each and every subsystems used in the model were explained.

CHAPTER 5

RESULT ANALYSIS

5.1 Introduction

IsOWC system designed was modeled and simulated for performance

characterization. Several parameters of the system were varied to obtain optimum

system performance. The main parameter that was considered is the light

propagation distance of the specific OWC channel, either inter LEO satellites, inter

GEO satellites or between LEO and GEO satellites. Furthermore, other OWC link

performance characteristics were also inconsidered which includes the system bit

rate, the frequency of light carrier signal of CW laser and the optical receiver

sensitivity. From the simulation, two observation was done which is the relationship

of the Q-factor and the bit rate at varying distance and also the relationship between

Q-factor and the signal wavelength.

Apart from that, the system attenuation and link budget was done by using

equation 2.4. The relationship between optical antennae diameters and distance with

the received power were drawn and discussed.

37

5.2 Relationship between Q-factor and Bit rates with Distance of

Intersatellite Link

By varying the bit rate and the distance between the satellites in the IsOWC

system, the system performance in terms of Q-factor was obtained and plotted in

Figure 5.1. The distance was set from 0km up to 5000km and the input power is set

at a constant value of 10dBm and signal wavelength of 1550nm is used. The bit rate

was set at 5 levels which are 1Mbps, 10Mbps, 100Mbps, 1Gbps and 10Gbps.

Figure 5.1 Maximum achievable Q-factor for variable distance at 1550nm

IsOWC link for bitrate up to 10Gbps

From the graph of Figure 5.1, it can be observed that at longer distance the

maximum Q-factor of the system decrease. This shows that the error in the received

0

100

200

300

400

500

600

700

800

900

0 1000 2000 3000 4000 5000

Max

Q-f

acto

r

Distance (km)

Bitrate 1Mbps

Bitrate 10Mbps

Bitrate 100Mbps

Bitrate 1Gbps

Bitrate 10Gbps

38

signal increases as the distance increase. The graph also shows that with higher bit

rate, maximum Q-factor is reduced. At the distance of 5000km, only the 1Mbps can

be used as the Q-factor received does not equals to zero. It was also observed that

data transmission at high bit rate of 10Gbps can only be used for shorter distance of

less than 500km. This however, can be increased by using higher optical input

power or by improving the modulation technique.

Figure 5.2 shows the eye diagram of an ideal transmission where the distance

is 1000km and the bit rate is 10Mbps. The Q-factor recorded is 48.04. When the

distance is incresed from 1000km to 3000km with the same bit rate, the eye diagram

consists of more jitter and the opening of the the eye decreases. The same happens

when the data rate is incresed to 1Gbps and the distance is constant at 1000km.

Figure 5.3 and 5.4 shows the eye diagrams for these situtions. However, the system

shown in the two figures are still acceptable as the bit rate recorded was still below

the common communication bit error rate threshold value of 10-5

.

Figure 5.2 Eye-diagram for IsOWC system at distance 1000km and 10Mbps

39

Figure 5.3 Eye-diagram obtained at distance 3000km and 10Mbps bit rate

Figure 5.4 Eye diagram for IsOWC system with bit rate 1Gbps at 1000km

distance

40

5.3 Relationship between Q-factor and Signal Wavelength

At long-haul transmission, the common wavelength used is 1550nm but

shorter wavelengths can also be used. The following figures 5.5, 5.6 and 5.7 shows

the effect on the system performance when the variable wavelengths are used. For

this simulation, the distance between the satellites is set constant at 200km and bit

rate 1Gbps are used. Table 5.1 shows the Q-factor received at varying wavelength.

Figure 5.5 Eye diagram for system at 850nm wavelength

41

Figure 5.6 System eye diagram obtained at wavelength 950nm

Figure 5.7 Eye diagram for 1550nm signal

42

Table 5.1 Maximum Q-factor recorded for respective signal wavelengths

Signal Wavelength (nm) Maximum Q-factor

850 82.12

950 70.25

1550 32.72

From the previous figures and table, it can be observed that at higher

wavelength, more error is produced due to lower value of Q-factor. However, by

using longer wavelength, the effect of scattering can be reduced. Attenuation due to

Rayleigh and Mie scattering is inversely proportional to the wavelength. Though in

this project it is assume that the are no particles obstructing the light signal, but small

and large particles by the means of space dusts and meteorites can happen to be

within the light signal’s way. Therefore, the longest possible wavelength is to be

used which is 1550nm. Another reason of using 1550nm is because the

compatibility with current technology and devices.

5.4 Relationship between Diameter of Optical Antennae with Received

Power and Distance

In Chapter 2, the link budget and attenuation calculation for IsOWC system

have been presented with equation 2.4 for attenuation. From the equation, the

diameters of the receiver and transmitter optical antennae, DR and DT respectively,

are assumed to be equal, pointing loss, LP, is set to 0.2 and the transmission factors,

TR and TT, equal to 0.8 according to Pfennigbauer [12]. The operated optical

window was set at a wavelength of 1550nm, thus the propagation attenuation, A,

becomes as in equation 4.1,

43

(4.1)

where D = DT = D

R.

The attenuation of the IsOWC system is calculated for varying antennae

diameters of 20cm up to 40cm and distance between satellites of range 0km until

5000km. The values of antenna diameters were chosen from current optical systems

manufactured by LightPointe® and also taking the reference from Artemis and Kirari

optical antennae of 25cm. The received power for the system is the difference

between the input power and the attenuation in decibels. The graph of received

power against distance between satellites for respective optical antennae diameter is

plotted in Figure 5.8.

From equation 4.1, it is noted that attenuation is inversely proportional to D4.

Meanwhile, from the graph of Figure 5.8, the power received is observed to be

increasing with the increment of optical antennae diameter. However, increasing the

distance between the satellites reduce the level of received power. Optical received

power can be associated with the receiver’s sensitivity where increasing the received

power would result in increasing the sensitivity. Therefore it can be concluded that

by increasing the diameter optical antennae, the receiver’s sensitivity is also

increased. It can also be conferred that lower sensitivity is obtained for longer

intersatellite distance. Higher sensitivity will allow more throughputs and reducing

the error received by the system.

44

Figure 5.8 Received power for respective optical antennae diameter at distance

up to 5000km and input power of 10dBm

5.5 Conclusions

In this chapter, the results of the simulation for IsOWC system model are

presented and discussed. The system performance was analyzed when several

parameters of the system characteristics are varied. Several conclusions were made

and will be discussed further in the following chapter.

-80.0

-70.0

-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

0 1000 2000 3000 4000 5000 6000

Rec

eived

Pow

er (

dB

m)

Distance (km)

40cm

35cm

30cm

25cm

20cm

Diameter

CHAPTER 6

CONCLUSIONS

6.1 Project Conclusions

More and more satellites are deployed to space to perform many applications

for the benefit of mankind. The future of space technology aims for satellites that

can send its research data and images from any parts of the world, and also satellites

that can give high speed internet connection and provide mobile cellular services to

people at any places and anytime. This project is done to analyze a method to

connect and network these satellites by using optical link. It has been discussed that

IsOWC can provide intersatellite communication at higher speed and much further

distance compared to RF links.

The project first objective which is to study OWC system for intersatellite

links is accomplished successfully and discussed in Chapter 2. The design of

IsOWC system was presented in Chapter 4 where the system model using

OptiSystem software was explained in extensively. The project also aimed to

simulate the system model for performance characterization and this has been

conferred in Chapter 5. From the model simulation and the result findings, several

conclusions can be drawn out. The conclusions are as the followings:

46

i. The received error increases as the distance between satellites increase.

ii. Optical signal with lower bit rate can be used for further distance between

satellites since the system performance is better at lower bit rates.

iii. Longer signal wavelength produces more errors but transmission at 1550nm

is used to reduce the effect of scattering and for its compatibility with existing

devices.

iv. Receiver sensitivity increases with the use of optical antennae where bigger

antennae improve the receiver’s sensitivity.

The findings of this project in significant to benefit and contribute to Malaysia’s

development in the space technology. This project is part of Fundamental Research

Grand Scheme (FRGS) by Ministry of Higher Education of vote number 78289.

Other than that, this project was presented and winner of the Grand Prize in

Telecommunication Exhibition 2009.

6.2 Future Work Recommendations

There more topics and area’s that can be improved for this project. Due to

that, the following topics are recommended for the IsOWC and intersatellite

communication system improvements:

i. Path prediction model for intersatellite optical communication link

In this project, the OWC channel is assumed to be ideal where losses

scattering and pointing errors are neglected. Therefore, a path prediction

model for IsOWC system can be developed to have a better approximation of

the real OWC channel.

47

ii. Development of intersatellite optical data network protocol

The network protocol can be beneficial for constellation of satellites

that provides internet access or cellular mobile network. The encoding and

channel multiple access methods can also be developed to improve the

system performance.

iii. System Quality of Service (QoS) performance analysis by comparing to RF

links

Several papers and books have mentioned the advantages of using

optical link over RF link for intersatellite communication but none has really

study in depth on the topic. QoS of the system is important to really prove

that the optical link is much more favorable than RF link.

REFERENCES

1. SATCAT Boxscore website. Retrieved April 29, 2009, from

http://celestrak.com/satcat/boxscore.asp

2. Sun, Z. Satellite Networking - Principles and Protocols. UK: John Wiley

& Sons. 2005.

3. Chan, V.W.S. Optical Satellite Networks. Journal of Lightwave

Technology, 2003. 21(11): 2811–2817.

4. Jamalipour, A. Low Earth Orbital Satellites for Personal Communication

Networks. UK: Artech House Publisher. 1999.

5. Ramirez-Iniguez, R., Idrus, S.M. and Sun, Z. Optical Wireless

Communication: IR for Wireless Connectivity. USA: CRC Press. 2008.

6. Sodnik, Z., Furch, B. and Lutz, H. Free-Space Laser Communication

Activities in Europe: SILEX and beyond. 19th Annual Meeting of the

IEEE, 2006.

7. Fletcher, G.D., Hicks, T.R. and Laurent, B. The SILEX optical interorbit

link experiment. Electronics & Communication Engineering Journal,

1991. 273-279.

8. Palais, J.C. Fiber Optic Communications. 5th ed. Upper Saddle River,

N.J.: Prentice Hall. 2005.

9. IBM Optical Modulator. Retrieved April 29, 2009, from http://www-

03.ibm.com/press/us/en/photo/22808.wss

10. Molecular Expressions Microscopy Primer: Digital Imaging in Optical

Microscopy - Concepts in Digital Imaging - Avalanche Photodiodes.

Retrieved April 29, 2009, from http://www.microscopy.fsu.edu/

primer/digitalimaging/concepts/avalanche.html

11. Marcuse, D., Calculation of Bit-Error Probability for a Lightwave System

with Optical Amplifiers and Post-Detection Gaussian Noise. Journal of

Lightwave Technology, 1991. 9 (4): 505-513.

49

12. Pfennigbauer, M. and Leeb, W.R. Free-Space Optical Quantum Key

Distribution Using Intersatellite Links. CNES - Intersatellite Link

Workshop. November 2003.

13. Freeman, B. S. An Initial Analysis of Cost for Laser Communications and

Optical Intersatellite Links, SCEA Conference, 2002.

14. Sibley, M.J.N. Optical Communications. London: Macmillan Education

Ltd. 1990.

15. Tafazoli, R. Technologies for the Wireless Future, Vol. 2, England: John

Wiley & Sons Ltd. 2006.

16. Jono, T., Takayama, Y., Shiratama, K., Mase, I., Demelenne, B., Sodnik,

Z., Bird, A., Toyoshima, M., Kunimori, H., Giggenbach, D., Perlot, N.,

Knapek, M. and Arai, K. Overview of the inter-orbit and orbit-to-ground

laser communication demonstration by OICETS, Proceedings of SPIE, the

International Society for Optical Engineering, 2007.

17. Toyoshima, M., Takizawa, K., Kuri, T., Klaus, W., Toyoda, M., Kunimori,

H., Jono, T., Takayama, Y., Mokuno, M. and Arai, K. Results of Ground-

to-Space Optical Communications Experiments using a Low Earth Orbit

Satellite. 19th Annual Meeting of the IEEE , 80-81, 2006.

18. Sbardellati, A., Alenia Spazio S.p.A., Sassorossi, T., Marinelli, M., and

Giubilei, R. AIAA-1994-905, 15th AIAA International Communications

Satellite Systems Conference. Feb 28-Mar 3, 1994, Washington: American

Institute of Aeronautics and Astronautics, 1994. 22-32.

19. Jamroz, W.R., Kruzelecky, R.V. and Haddad, E.I. Applied

Microphotonics, USA: CRC Press. 2006.

20. Krainak, M.A. Intersatellite communications optoelectronics research at

the Goddard Space Flight Center. Telesystems Conference, May 19-20,

1992. 14/9 -1412.

21. Fujise, M., Nohara, M., Uehara, K. and Chujo, W., Broadband mobile

satellite communication system by LEO-SATs and optical ISLs. Global

Telecommunications Conference, Dec 6-9, 1992. Communication for

Global Users:IEEE. 437-442.