Modeling Intersatellite Optical Wireless Communication System

14

14

15

19

19

21

22

23

25

25

25

26

26

27

27

28

28

29

30

31

33

33

35

36

37

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

6

CONCLUSIONS

6.1 Conclusions

6.2 Future Work Recommendations

REFERENCES

ix

40

42

44

45

46

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

1.2

2.1

2.2

Overview of IsOWC

Earth Satellite Communication Orbits

Optical intersatellite link between Artemis and

SPOT-4 first achieved in March 2003

IsOWC basic system block diagram for simplex

communication

Optical modulation process where input light is

2.3

varied according to electrical signal to produce

2.4

2.5

2.6

2.7

2.8

2.9

2.10

light pulses

Optical antennae increase the signal divergence

APD photodetector structure

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

GEO-GEO link

Optical satellite network creating a global

connection

High-level optical intersatellite communication

system

Concept of data relay for inter orbit IsOWC

Data relay methods (a) conventional (b) using

intersatellite data relay

2.11

Constellations of satellite orbiting Earth

2

4

9

10

12

12

13

17

18

19

20

21

22

xii

3.1

4.1

4.2

4.3

4.4

4.5

4.6

4.7

5.1

5.2

5.3

5.4

5.5

5.6

5.7

The project methodology

IsOWC first design with basic subsystems

IsOWC simplex design model

IsOWC full-duplex system between two satellites

NRZ encoding technique

Mach-Zehnder Modulator varies the light intensity

according to voltage

BER analyzer is connected to the transmitter when

without 3R regenerator

Connection of the BER analyzer to the 3R

regenerator

Maximum achievable Q-factor for variable distance

at 1550nm IsOWC link for bitrate up to 10Gbps

Eye-diagram for IsOWC system at distance

1000km and 10Mbps

Eye-diagram obtained at distance 3000km and

10Mbps bit rate

Eye diagram for IsOWC system with bit rate 1Gbps

at 1000km distance

Eye diagram for system at 850nm wavelength

System eye diagram obtained at wavelength 950nm

Eye diagram for 1550nm signal

Received power for respective optical antennae

24

29

29

30

31

32

34

35

37

38

39

39

40

41

41

5.8

diameter at distance up to 5000km and input power

44

of 10dBm

xiii

LIST OF ABBREVIATIONS

Optical Wireless Communication

Intersatellite Optical Wireless Communication

Radio Frequency

Signal Communication by Orbital Relay

Low Earth Orbit

Medium Earth Orbit

Geosynchronous Orbit

Bit Error Rate

National American Space Agency

Tracking and Data Relay Satellite System

European Space Agency

Satellite Pour L`Observation De La Terre 4

Telemetry, Tracking and Communication

Light Emitting Diode

Injecting Laser Diode

Avalanche Photodiode

Non-return Zero

Continuous wave

OWC

IsOWC

RF

SCORE

LEO

MEO

GEO

BER

NASA

TDRSS

ESA

SPOT-4

TT&C

LED

ILD

APD

NRZ

CW

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

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

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.

2

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.

1.2 Overview of Satellites

3

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.

1.4

Scope of Work

5

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].

Optical link

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

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.

10

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

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.

11

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

2.3.2 System Performance

14

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

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].

18

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

No

START

Receive project title from supervisor

Study on OWC system

Study on intersatellite link system

Design IsOWC model

Optimum design?

Yes

Simulation of Model

Result Analysis

Thesis Writing

Figure 3.1

The project methodology

3.2

Study on OWC System

25

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

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

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

26

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...