Tài liệu OPTICAL WIRELESS COMMUNICATION SYSTEM doc

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.

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

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To those who matters most to me

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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

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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

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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

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TABLE OF CONTENTS

DECLARATION DEDICATION ACKNOWLEDGEMENT

ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF ABBREVIATIONS

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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

2.3.1 Optical Wireless System

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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

3.6 Result Analysis 3.7 Presentation and Thesis Writing 3.8 Conclusions

4.4.1 IsOWC Transmitter Design 4.4.2 OWC Channel

4.4.3 IsOWC Receiver Design 4.5 Conclusions

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LIST OF TABLES

5.1 Maximum Q-factor recorded for respective signal

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LIST OF FIGURES

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

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Optical modulation process where input light is varied according to electrical signal to produce light pulses

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2.4 Optical antennae increase the signal divergence 12

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

2.9 Concept of data relay for inter orbit IsOWC 20

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

2.11 Constellations of satellite orbiting Earth 22

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4.1 IsOWC first design with basic subsystems 29

4.3 IsOWC full-duplex system between two satellites 30

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

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LIST OF ABBREVIATIONS

OWC - Optical Wireless Communication

IsOWC - Intersatellite Optical Wireless Communication

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

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

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

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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

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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

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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

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

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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

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

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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

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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

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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]

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

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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

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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

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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

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(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

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