Satellite Communications Link Optimization_revhya

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Rpublique du Cameroun Republic of Cameroon

Paix- Travail Patrie Peace- Work- Fatherland

Universit de Douala The University of Douala

Institut Universit de Technologie The University Institute of Technolo


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SATELLITE COMMUNICATIONS LINK OPTIMIZATIONDecember 15,

2012

ii

DEDICATION

I dedicate this work to my wife who has been pillar of strength to me

throughout this period.

To my mother and family as a whole for theirenormous sacrifices and

support.


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[SATELLITE COMMUNICATIONS LINK OPTIMIZATION] December 15, 2012

EKENKO FONJOCK COLUMBUS iii

ACKNOWLEDGEMENT

I sincerely thank those who have participated in one way or the other for the success of this project

I thank particularly;

The Director of IUT Douala who offered me the opportunity tospend this academic year in his institution.

Mr. Emmanuel Chimi who worked tirelessly to see that this work isrealised.

Engineer Foumba Hyacinthe, who guided me in my choice ofproject and provided me with relevant documents

Engineer Petra Nain who took so much time in correcting thedocument

Engineer Tianang Germain for the deep inside of his advice andthe pertinent remarks he made to me.

Engineer Nyem Nestor who advised me to return to go back toschool and who has been there to assist me in times of need.

To all my teachers at the University Institute of Technology(IUT),Douala, for all the lessons we received and the good time we had

during this academic year

To all my classmates and friends with whom we share ideas duringthis academic year.

Etoungou Olivier research teacher who helped me in thepresentation of my project.

Most especially to God who granted me the strength and wisdomto finish this work.


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PREFACE

Created by the Presidential degree N0008/CAB/PR of 19January 1993, the University Institute of Technology (IUT),

Douala is a professional training Institute, created with the aim of satisfying the requirements of Industrial and

Tertiary Companies, by putting at their disposal skilled workers.

IUT of Douala is situated at CAMPUS 2of the University of Douala, in NDOG-BONG, with modern infrastructure and

up to date equipment thanks to the French corporation and multitude of partners around the world. It offers many

trainingopportunities among which are;

The initial training, which last for two years, at the end of which a diploma called DiplmeUniversitairede Technologie(DUT), is issued; with the possibility of extension to the third year for a degree in

Technology

Permanent training based on specific programs Continuous training in which negotiations are carried out case-by-case with the Company that needs it.

The different fieldsare;

DUT

Platform Fields

PFTI( Industrial Technology) GIM(Maintenance Engineering)

GFE( Railway Engineering)

GTE( Mining Engineering)

GMP( Mechanical and Production Engineering)

PFTIN(Information and Digital Technology Platform) Electrical and Industrial Computer Engineering

GI(Computer Engineering)

GRT(Networking and Telecommunications Engineering)

GBM(Biomedical Engineering)

PFTT(Platform of Tertiary Technologies) GAPMO: Applied Management of Small and Medium

Size Company

GLT: Logistics and Transport Engineering

OGA: Organization and Administrative Management


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EKENKO FONJOCK COLUMBUS v

For BTS

ACO Commerce

CGE Enterprise Management Accounting

ET Electrotechnique

FM/CM Mechanical Manufacturing/ Mechanical Construction

II Industrial Computing


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ABSTRACT

The goal of this project is to provide means of optimizing a satellite

communications link. The project has two motivations;

1) The need to reduce the effect of atmospheric impairments, thermal noise, non-linearity of satellite channels and interferences on signals, which reduces

availability and thus the reliability of alink

2) Satellite transponders resources such as bandwidth and power are limited, assuch the transponder leasing costs are determined by bandwidth and power

used. The more bandwidth and power we use the more costly the services

provided.

To achieve this goal, we will use advanced modulation, coding gain, fade

adaptation, and carrier cancelling technologies which can provide substantial

savings in bandwidth, improve capacity, improve reliability or all three while

maintaining contracted service agreement (SLA).

The outcome of this project is that there will be:

Reduce Operational Expenditure(OPEX)o Occupied bandwidth and transponder resources will reduce

Reduce Capital Expenditure(CAPEX)o BUC/HPA size and/or antenna size

Increase in throughput without the use ofadditional transponder resources Increase in link availability (margin) without the use of additional

transponder resources

Or a combination to meet different objectives


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EKENKO FONJOCK COLUMBUS vii

RESUME

L'objectif de ce projet est de fournir des moyens d'optimisation dune liaison de communication

par satellite. Ce projet a deux grandes motivations:

i) La ncessit de rduire l'impact des perturbations atmosphriques, le bruitthermique, la non-linarit des chanes satellitaires, des interfrences sur les signaux,

qui ont un impact ngatif sur la fiabilit de la liaison.

ii) La capacit de la charge utile : les transpondeurs satellitaires ont des ressourceslimites en termes de bande passante et de la puissance, ce titre, les frais de

location du transpondeur sont dtermins par la bande passante et la puissance

utilise. Plus la bande passante et la puissance sont utilises, plus nous aurons

payer.

Pour atteindre cet objectif, nous aurons utiliser des techniques de modulation avance,

gain de codage, l'adaptation dvanouissement, technologies d'annulation de porteuse,

qui peuvent fournir des conomies substantielles en bande passante, amliorer la

capacit, amliorer la fiabilit, ou les trois, tout en maintenant l'accord de services sous

contrat (ASC).

Les rsultats attendus de ce projet sont:

Rduire les dpenses d'exploitation (OPEX)o Largeur de bande occupe et les ressources transpondeur seront rduits

Rduire les dpenses en capital(CAPEX)o Taille BUC / HPA et / ou la taille d'antenne

Augmenter le dbit sans utiliser les ressources supplmentaires du transpondeur Accrotre la disponibilit lien (marge) sans utiliser les ressources supplmentaires

du transpondeur

Ou encore une combinaison pour rpondre aux objectifs diffrents


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TABLE OF CONTENTS

Preface .......................................................................................................................................................................... iv

ABSTRACT ...................................................................................................................................................................... vi

Resume .......................................................................................................................................................................... vii

Acronyms ..................................................................................................................................................................... xiv

General introduction ...................................................................................................................................................... 1

CHAPTER 1: INTRODUCTION TO SATELLITE COMMUNICATIONS ................................................................................... 2

1.1 Definition and Early History .......................................................................................................................... 2

1.2 Basic Satellite Communication System Definition ........................................................................................ 4

1.2.1 The Space Segment .................................................................................................................................. 4

.1.2.2 The Ground Segment ............................................................................................................................... 5

1.3. Satellite Link Parameters .......................................................................................................................... 5

1.4 Satellite Orbits .............................................................................................................................................. 6

1.5 Radio Regulations ......................................................................................................................................... 6

1.6 Space Radiocommunications Services .......................................................................................................... 7

1.7 Frequency bands ........................................................................................................................................... 8

CHAPTER 2-SATELLITE ORBITS ...................................................................................................................................... 10

2.1 Keplers laws ............................................................................................................................................... 11

2.1.1 Keplers First Law.................................................................................................................................... 11

2.1.2 keplers second law ................................................................................................................................ 11

2.3 Keplers third law ........................................................................................................................................ 11

2.3 orbital parameters .......................................................................................................................................... 12

2.3 Orbits in common use ..................................................................................................................................... 13

2.3.1 Geostationary orbit .................................................................................................................................... 13

2.3.2 Geosynchronous orbit ................................................................................................................................ 13


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2.3.3 Low earth ORBIT (Leo) ................................................................................................................................ 14

2.3.4 Medium earth orbit .................................................................................................................................... 14

2.3.5 Highly elliptical orbit ................................................................................................................................... 14

2.3.6 Polar orbit ................................................................................................................................................... 15

2.3.7 Geometry of GSO Link ................................................................................................................................ 15

Chapter 3 satellite subsystems .................................................................................................................................. 16

3.1 satellite bus ................................................................................................................................................. 17

3.1.1 Physical structure ........................................................................................................................................ 17

3.1.2 Power Subsystem ........................................................................................................................................ 18

3.1.3 Attitude control ........................................................................................................................................... 18

3.1.4 Orbital control ............................................................................................................................................. 19

3.1.5 Thermal Control .......................................................................................................................................... 19

3.1.6 Tracking, Telemetry, command and Monitoring ......................................................................................... 20

3.2 Satellite Payload ................................................................................................................................................. 21

3.2.1 Transponder ........................................................................................................................................... 21

3.2.1.1 frequency translation transponder .................................................................................................... 21

3.2.1.2 on-board processing transponder ..................................................................................................... 22

3.2.2 antennas ..................................................................................................................................................... 23

CHAPTER 4 noise .......................................................................................................................................................... 23

4.1 types of noise .............................................................................................................................................. 24

4.1.1 thermal noise ......................................................................................................................................... 25

4.2 interference ................................................................................................................................................ 27

4.3 intermodulation .......................................................................................................................................... 29

chapter 5- impairments ................................................................................................................................................ 29

5.1 signal attenuation ....................................................................................................................................... 30

5.1.1 rain attenuation...................................................................................................................................... 30


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5.1.2 GASEOUS attenuation ............................................................................................................................ 31

5.1.3 cloud attenuation ................................................................................................................................... 31

5.1.4 snow and ice attenuation ............................................................................................................................ 32

5.2 signal path effect related to refraction .............................................................................................................. 32

5.2.1 Tropospheric scintillation ............................................................................................................................ 32

5.2.2 signal polarization effects ........................................................................................................................... 33

chapter 6: modulation and coding .............................................................................................................................. 35

6.1 types of modulation ........................................................................................................................................... 35

6.1.1 types of phase shift keying modulation and bandwidth efficiency ............................................................. 36

6.1.2 power efficiency of the various schemes .................................................................................................... 37

6.1.3 power requirement of various schemes-eb/no vs BER ................................................................................ 38

6.2 CHANNEL encoding ............................................................................................................................................ 39

6.2.1 Block encoding and convolutional encoding ................................................................................................... 39

6.2.1.1 block encoding ......................................................................................................................................... 39

6.2.1.2 convolution encoding ............................................................................................................................... 40

6.2.2 concatenated encoding ............................................................................................................................... 40

6.2.3 Turbo codes ................................................................................................................................................. 40

6.2.4 Low Density Parity check CODES (LDPC) ..................................................................................................... 40

6.3 channel decoding ............................................................................................................................................... 41

6.4 power-bandwidth tradeoff ................................................................................................................................. 42

6.4.1 coding with variable bandwidth .................................................................................................................. 42

6.4.2 coding with constant bandwidth ................................................................................................................. 42

chapter 7 SATELLITE LINK Budget ................................................................................................................................ 43

7.1 configuration of a link ........................................................................................................................................ 43

7.2 antenna parameters ........................................................................................................................................... 44

7.2.1 antenna gains .............................................................................................................................................. 44


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7.2.2 radiation pattern and angular beamwidth .................................................................................................. 45

7.2.3 Polarization.................................................................................................................................................. 46

7.3 radiated power ................................................................................................................................................... 48

7.3.1 effective isotropic radiated power (EIRP) ................................................................................................... 48

7.3.2 power flux density ....................................................................................................................................... 48

7.4 Received signal power ........................................................................................................................................ 49

7.4.1 Power captured by the receiving antenna and free space path loss .......................................................... 49

7.5 additional losses ................................................................................................................................................. 50

7.5.1 attenuation in the atmosphere ................................................................................................................... 51

7.5.2 LOSSES IN THE TRANSMITTING AND RECEIVING EQUIPMENT .................................................................... 51

7.5.3 DEPOINTING LOSSES ................................................................................................................................... 52

7.5.4 losses due to polarization mismatch ........................................................................................................... 52

7.5.5 conclusion ................................................................................................................................................... 53

7.6 noise power spectral density at the receiver input ............................................................................................ 53

7.6.1 origin of noise .............................................................................................................................................. 53

7.6.2 Noise CHARACTERIZATION .......................................................................................................................... 53

7.6.3 noise temperature of a noise source .......................................................................................................... 54

7.6.4 noise figure .................................................................................................................................................. 54

7.6.5 EFFECTIVE INPUT NOISE TEMPERATURE OF AN ATTENUATOR ................................................................... 54

7.6.6 effective input noise temperature of cascaded elements .......................................................................... 54

7.6.7 EFFECTIVE INPUT NOISE TEMPERATURE OF A RECEIVER ............................................................................ 55

7.6.8 antenna noise temperature ........................................................................................................................ 55

7.6.8 noise temperature of a satellite antenna .................................................................................................... 55

7.6.9 noise temperature of an earth station ANTENNA (downlink) ..................................................................... 56

7.7 SYSTEM NOISE TEMPERATURE ........................................................................................................................... 56

7.7.1 conclusion ................................................................................................................................................... 57


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7.8 individual link performance ................................................................................................................................ 57

7.8.1 carrier to noise power spectral density ratio at the receiver input ............................................................ 58

7.8.2 clear sky condition ....................................................................................................................................... 59

7.9 link performance under rain conditions ............................................................................................................. 63

7.9.1 uplink performance ..................................................................................................................................... 63

7.9.2 downlink performance ................................................................................................................................ 64

7.9.3 conclusion ................................................................................................................................................... 64

7.10 overall link performance with a transparent satellite ...................................................................................... 65

7.10.1 characteristics of the satellite channel...................................................................................................... 65

7.10.2 satellite power flux density at saturation ................................................................................................. 66

7.10.3 satellite eirp at saturation ......................................................................................................................... 67

7.10.4 satellite repeater gain ............................................................................................................................... 67

7.10.5 input AND OUTPUT BACK-OFF .................................................................................................................. 68

7.10.6 carrier power at the satellite receiver input ............................................................................................. 68

7.10.7 expression for without interference from other systems or intermodulation............................... 697.10.8 expression for taking account of INTERFERENCE and intermodulation ......................................... 70

chapter 8 optimization ................................................................................................................................................. 70

8.1 link Margin.......................................................................................................................................................... 70

8.2 Power restoral techniques ................................................................................................................................. 71

8.2.1 beam diversity ................................................................................................................................................. 71

8.3 power control ..................................................................................................................................................... 72

8.3.1 uplink power control ................................................................................................................................... 72

8.4 site diversity ....................................................................................................................................................... 73

8.5 signal modification techniques .......................................................................................................................... 74

8.5.1 Optimization By Doubletalk carrier-in-carrier............................................................................................. 74

8.5.6 Double Talk Carrier-in-carrier cancellation process ........................................................................................ 76


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8.6 adaptive coding and MODULATION (ACM) ........................................................................................................ 77

8.6.1 acm background .......................................................................................................................................... 78

8.6.2 requirements for ACM ................................................................................................................................ 79

9.0 general conclusion ................................................................................................................................................. 80

Bibliographic references .............................................................................................................................................. 81


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ACRONYMS

ACI ADJACENT CHANNEL

INTERFERENCE

ES EARTH STATION

ADC ANALOG TO DIGITAL CONVERSION FDM FREQUENCY DIVISION MULTIPLEX

ADM ADAPTIVE DELTA MODULATION FEC FORWARD ERROR CORRECTION

ADPCM ADAPTIVE PULSE CODE

MODULATION

FES FIXED EARTH STATION

ALC AUTOMATIC LEVEL CONTROL FGM FIXED GAIN MODE

AM AMPLITUDE MODULATION FM FREQUENCY MODULATION

AMSS AERONAUTIC AL MOBILE SATELLITE

SERVICE

FSS FIXED SATELLITE SERVICES

APSK AMPLITUDE PHASE SHIFT KEYING GC GLOBAL COVERAGE

AR AXIAL RATIO GCS GROUND CONTROL STATION

BEP BIT ERROR PROBABILITY GEO GEOSTATIONARY EARTH ORBIT

BER BIT ERROR RATE GSO GEOSTATIONARY SATELLITE ORBIT

BPF BAND PASS FILTER HEO HIGHLY ELLIPTICAL ORBIT

BPSK BINARY PHASE SHIFT KEYING HIO HIGHLY INCLINED ORBIT

BS BASE STATION HPA HIGH POWER AMPLIFIER

BSS BROADCAST SATELLITE SERVICE HPB HALF POWER BANDWIDTH

BW BANDWIDTH IBO INPUT BACK-OFF


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CAMP CHANNEL AMPLIFIER IF INTERMEDIATE FREQUENCY

CCI CO CHANNEL INTERFERENCE IMUX INPUT MULTIPLEX

CDMA CODE DIVISION MULTIPLE ACCESS INMARSAT INTERNATIONAL MARITIME SATELLITE

ORGANIZATION

D/C DOWN CONVERTER INTELSAT INTERNATIONAL TELECOMMUNICATIONS

SATELLITE CONSORTIUM

DA DEMAND ASSIGNMENT IOT IN ORBIT TEST

dB DECIBEL ISL INTER SATELLITE LINK

DE Differentially ENCODED ITU INTERNATIONAL TELECOMMUNICATIONS

UNION

DEMOD Demodulator

EIRP EFFECTIVE ISOTROPIC RADIATED

POWER

LEO LOW EARTH ORBIT PLMN PUBLIC LAND MOBILE NETWORK

LHCP LEFT HAND CIRCULAR

POLARIZATION

PM PHASE MODULATION

LNA LOW NOISE AMPLIFIER POL POLARIZATION

LNB LOW NOISE BLOCK PSK PHASE SHIFT KEYING

LO LOCAL OSCILLATOR PSTN PUBLIC SWITCHED TELEPHONE NETWORK

LPF LOW PASS FILTER PTN PUBLIC TELECOMMUNICATIONS NETWORK

MCPC MULTIPLE CHANNEL PER CARRIER PTO PUBLIC TELECOMMUNICATIONS OPERATOR

MEO MEDIUM EARTH ORBIT QoS QUALITY OF SERVICE

MES MOBILE EARTH STATION QPSK QUADRATURE PHASE SHIFT KEYING

MF MULTIFREQUENCY RF RADIO FREQUENCY


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MOD MODULATOR RHCP RIGHT HAND CIRCULAR POLARIZATION

MODEM MODULATOR/DEMODULATOR RS REED SOLOMON(coding)

MSK MINIMUM SHIFT KEYING RX RECEIVER

MSS MOBILE SATELLITE SERVICE SC SUPPRESSED CARRIER

MUX MULTIPLEXER SCPC SINGLE CHANNEL PER CARRIER

MX MIXER SEP SYMBOL ERROR PROBABILITY

NASA NATIONAL AERONAUTIC AND SPACE

ADMINISTRATION

SL SATELLITE

N-GSO NON-GEOSTATIONARY SATELLITE

ORBIT

SNR SIGNAL-TO-NOISE RATIO

OBO OUTPUT BACK-OFF TWTA TRAVELING WAVE TUBE AMPLIFIER

OBP ON BOARD PROCESSING TX TRANSMITTER

PCM PULSE CODE MODULATION VSAT VERY SMALL APERTURE TERMINAL

PCS PERSONAL COMMUNICATION

SYSTEM

XPD CROSS POLARIZATION DISCRIMINATION

PDF PROBABILITY DENSITY FUNCTION XPI CROSS POLARIZATION ISOLATION

PLL PHASE LOCKED LOOP Xponder TRANSPONDER


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GENERAL INTRODUCTION

Since their introduction in the mid-1960s, satellite communications have grown from a futuristic

experiment into an integral part of todays wired world. Satellite communications are at the core

of a global, automatically switched telephony network.

Todays communications satellite have extensive capabilities in applications involving data, voice

and video with services provided to fixed, broadcast, mobile, personal communications and private

users.

But Satellite communication is highly affected by propagation impairments at the atmosphere, non-

linearity of the satellite channel, Thermal noise, Interferences and also regulatory constraints.

Therefore a good knowledge and modeling of the propagation channel is necessary for the

performance assessment. This is thus a major preoccupation of most satellite operators.

This project is organized as follows:

The first three chapters give a general overview of the satellite communication system.

Chapters 4 and 5 presents a brief description of the impairments encountered in this domain.

Chapter 6 briefly describes modulation and coding. Chapter 7 presents the parameters necessary to

calculate the performance of a link and concludes with the calculation of link performance, for an

uplink, a downlink and overall link from transmitter through satellite to receiver.

Chapter 8 presents the different means of optimizing a satellite link. The first part, using power

restoral techniques and the second part using signal modification techniques.


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CHAPTER 1: INTRODUCTION TO SATELLITE COMMUNICATIONS

1.1 DEFINITION AND EARLY HISTORYA communications satellite is an orbiting artificial earth satellite that receives a communications signal

from a transmitting ground station, amplifies and possibly processes it, then transmits it back to the

earth for reception by one or more receiving ground stations. Communications information neither

originates nor terminates at the satellite itself. The satellite is an active transmission relay, similar in

function to relay towers used in terrestrial microwave communications.

The Commercial communication Satellite exists since the mid-1960s.Within a space of about 50years, it

has grown from an alternative technology to a mainstream transmission technology. Todays

communication satellites offer extensive capabilities in applications involving data, voice and video, with

services provided to fixed, broadcast, mobile and personal communication and private network users

Communications Satellites offer advantages that are not readily available in other alternative modes of

transmissions such as terrestrial microwave, cable or fiber optic networks, such as:

Distance Independent cost: The cost is the same, regardless of the distance between thetransmitting and the receiving earth stations.

Fixed Broadcast Cost: Broadcast from an earth station to a number of other earth station isindependent of the number of earth stations receiving the transmission.

High capacity: Capacity ranges from 10s of megabits to 100s of Mbps Low error rate: Bit errors on a digital satellite link turns to be random, allowing statistical

detection and error correction techniques to be used. Error rates of one error in 10

6

bits andhigher can be seen commonly.

Diverse User Network. Due to its large coverage area, it can be used to interconnect land, seaand air users who can be mobile or fixed


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The idea of an artificial orbiting satellite capable of relaying communication to and from the earth is

attributed to Arthur C. Clarke. Below is a table with information concerning the early satellites, their

launched dates, and basic information concerning the satellites.

Satellite name Launched date Basic Function/use

SPUNIK1 1957 USSR

SCORE 1958 By USA Relayed a recorded voice message with delay

ECHO1 &2 1960 BY NASA

COURIER October 1960 First to employ solar cells for power

WESTFORD 1963 by US

Army

Voice and frequency shift keying transmission.

TELSTAR 1&2 1962 and 1963 Multichannel telephone, telegraph, facsimile and television transmission

RELAY1 & 2 1962 and 1964 Extensive telephony and network television transmission between USA,

Europe and Japan

SYNCOM2 & 3 1963 and 1964 First communication from a synchronous satellite

EARLY BIRD 1965 First commercial communication from a synchronous satellite.

Later called INTELSAT

ATS-1 1966 First multiple access communication from synchronous orbit

ATS-3 1967 Multiple access communication with Orbit Control

ATS-5 1969 Design to provide propagation data on the effect of the atmosphere on Earth-

Space communication.

INTELSAT 1964 Created , becoming the recognized international legal entity satellite

communication

Table1.1 satellite history

These early accomplishments and events led to the rapid growth of the satellite communications

industry, beginning in the mid-1960s. INTELSAT was the prime mover in that time focusing on the

benefits of satellite communication to many nations


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1.2 BASIC SATELLITE COMMUNICATION SYSTEM DEFINITIONSatellite communications system is broken down into two main segments: the space segment and the

ground (or earth) segment.

1.2.1 THE SPACE SEGMENTThe elements of the space segment in a satellite communications system are shown in figure 1.1.The

space segment includes the satellite (or satellites) in orbit and the ground station that provide the

operational control of the satellite(s) in orbit. This ground station is sometimes referred to as Tracking,

Telemetry and Command (TT&C) or Tracking, Telemetry, Command and Monitoring (TTC&M)station.

The TTC&M station provides essential space craft management and control functions to keep the

satellite operating in Orbit.

The TTC&M Links between the spacecraft or

satellite are usually from the user

communications link. Most of the time,

TTC&M is accomplished through separate

earth terminal facilities, design for this

purpose.

Figure 1.1 TTC&M


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.1.2.2 THE GROUND SEGMENT

It consists of the earth terminal(s) that make use of the communication capabilities of the spacesegment. It should be noted that the TTC&M do not make part of the ground segment.

The ground segment terminals could be one of the following:

Fixed Terminals Transportable Terminals Mobile Terminals1.3.SATELLITE LINK PARAMETERS

Satellite communications link is defined by several parameters as shown in figure 1.2. These parameters

are used in the evaluation of a satellite communication link. The portion of the link from the earth station

to the satellite is called uplink, while the portion from the satellite to the ground station is called

downlink. Either station in the figure has an uplink and a downlink. The electronics in the satellites that

receives the uplink signal, amplifies and possibly processes the signal and then reformat and retransmit

the signal back to the downlink is called a transponder. It is indicated by the triangular symbol in the

figure. The Antennas of the satellite that receives the signal and transmit it on the downlink are not

included as part of the transponder electronics. A channel is defined as a one way link from A-to-S-to-B

or from B-to-S-to-A. Aduplex link from A-to-S-to-B and from B-to-S-to-A is called a circuit. A Half-Circuit

is the link from an earth station to the satellite and back. That is A-to-S and S-to-A is a half-circuit.

Figure 1.2 Basic Link Parameters of a satellite Communications Link


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1. 4SATELLITE ORBITSA detail description of satellite orbits will be given in chapter 2. We introduce here the four most

commonly used orbits, their altitudes and one way delay time. This information is given in table 1.2

below.

Satellite Orbit Orbital Altitude One-way delay

Geostationary Earth

Orbit(GSO)

36000km 260ms

Low Earth Orbit(LEO) 160-640km 10ms

Medium Earth

Orbit(MEO)

1600-4200km 100ms

High Earth Orbit(HEO) 40000km 10 to 260ms

Table1.2: common satellite orbit

1. 5RADIO REGULATIONSRadio Regulations are necessary to ensure an efficient use of the radio frequency spectrum by all

communication systems including terrestrial and satellite. All satellite operators must operate within the

constraints of regulations related to fundamental parameters and characteristics of the satellite

communications system. The satellite communication parameters that are regulated include the

following;

Radiating frequency Maximum allowable radiated power Orbit Location(slot) for GSO

The purpose of the regulation is to minimize radio frequency interference and to some extent, physical

interference between systems. Potential radio interferences are not only from other satellite systems but

also from other terrestrial systems operating in the same frequency band. Two levels of regulations and

allocation are involved in the process: International and domestic. The primary organization responsible

for international satellite communication system regulation and allocation is the International

Telecommunication Union (ITU), with headquarters in Geneva, Switzerland.

ITU has three primary functions:

Allocation and Use of the radio- frequency spectrum; Telecommunications standardization; Development and expansion of worldwide telecommunication


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These functions are accomplished through the three sectors of ITU organization: The

Radiocommunications Sector (ITU-R), responsible for the frequency allocations and use of the radio-

frequency spectrum. The Telecommunications Standard Sector (ITU-T), responsible for

telecommunications standards and the Telecommunications Development Sector (ITU-D), responsible for

the development and expansion of worldwide telecommunications.

The International regulations developed by ITU are processed by each country, where domestic level

regulations are developed. Each Country is left to manage and enforce the regulations within its

boundaries.

In Cameroun this is managed by the Telecommunication Regulations Agency (ART).

1. 6SPACE RADIOCOMMUNICATIONS SERVICESTwo attributes determine the specific frequency band and other regulatory factors for a particular

satellite system.

Service(s) to be provided by the particular satellite system/Network; and The Location(s) of the ground terminals

Services applicable to satellite systems as designated by ITU are:

Aeronautical Mobile Satellite Services(AMSS) Aeronautical Radionavigation Satellite Service(ARSS) Amateur Satellite Service(ASS)

Broadcasting Satellite Service(BSS) Earth-exploration Satellite Service(ESS) Fixed Satellite Service(FSS) Inter-satellite Service(ISS) Land Mobile satellite Service(LMSS) Maritime Mobile Satellite(MMSS) Maritime Radionavigation Satellite(MRSS) Meteorological Satellite(MSS) Mobile Satellite(MSS) Radionavigation Satellite(RSS)

Space Operations(SOSS)

Space Research(SRSS) Standard Frequency Satellite(SFSS)

Some of the services are divided into sub areas. For example the mobile satellite service (MSS) is further

divided into Aeronautical Mobile Satellite Service (AMSS), Land Mobile Satellite Service (LMSS), and

Maritime Mobile Satellite Service (MMSS), with respect to the location of the ground terminals.


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The Location of the satellite system ground terminal, which is the second attribute, depends on the

service region. ITU divides the globe into three Telecommunications Service regions. Region1 consist of

Europe and Africa, Region2 the Americas, Region3 the Pacific Rim countries. Each of these regions is

treated independently in terms of frequency allocation. It is assumed that systems operating in any of

these regions are protected from those in another because of the geographical separation between

them.

1. 7FREQUENCY BANDSThe frequency of operation is one of the major factors in the design and performance of a satellite

communication system as its wavelength will determine the interaction effect of the atmosphere, and

the resulting link degradation. Two types of designations are used; The Letter Designation and the

designation which divides the spectrum from 3Hz to 300GHz. These are shown in the tables below

Designation Frequency

C 6GHz up/ 4GHz down

X 8GHz up/ 7GHz down

Ku 14GHz up/ 11GHz down

Ka 30GHz up/20GHz down

Table: 1.3 Frequency bands used in satellite communications


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Below is a table that briefly summarizes the advantages and disadvantages of the most commonly used

frequency bands in satellite communications

Frequency Band Advantages Disadvantages

C-Band-Wide footprint coverage-Minor effects from rain

-Lower cost for earth

station antenna

-Requires large antennas-Requires Larger RF power amplifiers

-Affected by terrestrial interference

-Difficult to obtain transmit license

Ku-Band-Smaller antennas

-Smaller RF power

amplifiers

Greater effect from rain

Smaller footprint (beam) coverage

Ka-BandSmaller antenna

Smaller RF power

amplifier

Greater effect fromrain

Smaller footprint(beam) coverage

High equipment cost

Table: 1.4 summary of advantages and disadvantages of main satellite frequency bands


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CHAPTER 2-SATELLITE ORBITS

The same laws of motion that govern the movement of the planets

around sun also control the movement of artificial satellites around

the earth. Satellite Orbital determination is based on the laws of

motion developed by Kepler and later refined by newton.

Competing forces act on the satellite; gravity turns to pull the

satellite in towards the earth, while its orbital velocity turns to pull

the satellite away from the earth. These forces are shown in figure

2.1

The gravitational force, Fin and the angular velocity, Fout are represented

as

Fin= m ( ) .2.1and Fout=m

( ).2.2where

m=the satellite

mass, v= the

satellite velocity

in the plane of

its orbit,

r=orbital radius(distance from

the center of the earth); and =Keplers constant (Geocentric

gravitational constant) =3.9864002xKm3/s2. If thegravitational force from the sun, moon and other bodies are

neglected, then Fin=Fout and the velocity necessary to keep the

satellite in orbit will be

V= (

) ..2.3

The orbital locations of

the spacecraft in a

communications

satellite system play a

major role in

determining the

coverage and

operational

characteristics of the

services provided by

that system. This

chapter describes the

general characteristics

of satellite orbits and

summarizes the

characteristics of the

most popular orbits for

communications

applications.


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2. 1KEPLERS LAWS Keplers laws apply to any two bodies in space that interact through gravitation.

2.1.1 KEPLERS FIRST LAW Keplers first law as applied to artificial satellite orbits goes thus: the path followed by a satellite around

the earth will be an ellipse, with the center of mass

of the earth as one of the two foci of the ellipse.

If no other forces are acting on the satellite, either

intentionally by orbit control or unintentionally as

in gravity forces from other bodies, the satellitewill eventually settle in an elliptical orbit, with the

earth as one of the foci of the ellipse. The size of

the ellipse will depend on the satellite mass and its

angular velocity.

2.1.2 KEPLERS SECOND LAW For equal time interval, the satellite sweeps out

equal area in the orbital plane. This is shown in

figure 2.3.The shaded area A1

shows the area

swept out in the orbital plane by the orbiting satellite in one hour time period at a location near the

earth. According to the second law, the area A2, swept out around the point furthest from the earth is

also equal to A1. That is A1=A2

Thisresult shows that the satellite orbital velocity is not constant; the satellite moves faster at locations

near the earth, and slows down at locations around the apogee.

2. 3KEPLERS THIRD LAWThe square of the periodic time of orbit is proportional to the cube of the mean distance between the

two bodies.

That is T2= [

]a3, where T=orbital period in seconds s, a= distance between the bodies in km and=Keplers constant=3.986004x10

5km

3/s

2


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2.3 ORBITAL PARAMETERSImportant orbital parameters used for defining earth-orbiting satellite characteristics are:

Apogee-The point furthest from the earth. Perigee-The point of closest approach to earth Line of Apsides-the line joining the perigee and apogee through the center of the earth Ascending Node-The point where the orbit crosses the equatorial plane going from south to

north

Descending Node-The point where the orbit crosses the equatorial plane going from south tonorth

Lines of Nodes- The line joining the ascending and the descending nodes through the center ofthe earth.

Argument of Perigee,- The angle from ascending node to perigee, measured in the orbital. The eccentricity-is a measure of the circularity of the orbit. It is determined from Where e=eccentricity of the orbit;

ra=distance from the center of the

earth to the apogee point, rp=distance from the center of the earth to the perigee point.


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A circular orbit is a special case of an ellipse with equal major and minor axes (e=o)

That is for Elliptical orbit 0 < e < 1 and for Circular Orbit e = 0.

Inclination Angle

is the angle between the orbital plane and the earths equatorial

plane.

A satellite that is in an orbit with some inclination angle is said to be in an Inclined Orbit. A satellite that

is in orbit in the equatorial plane (inclination angle = 0) is in an Equatorial Orbit. A satellite in an orbit

with inclination angle of is said to be in a polar orbit.All these orbits may be circular or elliptical depending on the orbital velocity and the direction of motion

imparted to the satellite on insertion into orbit. An orbit in which the satellite moves in the same

direction as the earths rotation is called a Prograde orbit, inclination angle 0


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-2 to 3 satellites for global coverage (except at the poles)

-period of revolution is 23hours, 56minutes

Disadvantages

-Large path loss and significant latency (approximately 260ms for a duplex communication)

-cannot provide reliablecoverage to high latitude locations. Coverage can be improved by using highelevation angle but this produces problems such as increase ground station antenna tracking, which

increases cost and system complexity.

2.3.3 LOW EARTH ORBIT (LEO)Operate typically at an altitude from 160 2400Km and is near circular and requires earth tracking

terminals for continuous service.

Advantages

-Shorter earth satellite link, leading to lower path loss as such smaller power and smaller antenna

systems

-can cover high latitude locations

-the satellite is much smaller in size, as such requires less energy to put it in orbitDisadvantages

-A constellation of multiple LEO (12, 24, 66 etc.) to provide global coverage

-approximately 8 to 10 minutes per pass of an earth terminal

-Requires earth antenna tracking

-Oblateness or non-spherical nature of the earth causes major perturbations to LEO obit.

2.3.4 MEDIUM EARTH ORBITIt is situated at an altitude from 10,000 to 20,000Km similar to LEO, but higher circular orbit.

One to two hours per pass for an earth terminal

Requires a constellation of satellite to provide global coverage, for example GPS requires up to 24

satellites.

It is mostly used for meteorological, remote sensing and position location application

2.3.5 HIGHLY ELLIPTICAL ORBITPopular for high latitude or polar coverage

Often referred to as MOLNIYA orbit

Eight to ten hour per pass for an earth terminal

Typical MOLNIYA orbit has a perigee altitude of 1000Km and an apogee altitude of nearly 40,000Km.


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2.3.6 POLAR ORBITCircular orbit with an inclination near Useful for sensing and data gathering services

2.3.7 GEOMETRY OF GSO LINKGSO is the dominant orbit in use for communication satellites. Three key parameters of the GSO orbit are

used for evaluation of satellite link performance.

(distance) from the earth(Earth Station) to the satellite, in km from the earth station to the satellite in degrees from the earth station to the satellite in degrees

Azimuth and elevation angles are called the look angle of the earth station to the satellite. This is shownin figure 2.4

Input parameters that can be used with software tools for determining the look angle are:

- - -Le=Earth Station Latitude

-Ls=Satellite latitude

There are also software tools which require just the Country, name of the town and antenna size to find

the look angle


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CHAPTER 3 SATELLITE SUBSYSTEMS

A basic satellite system consists of a satellite (satellites) in space, relaying information between two or more users

through ground terminals and the satellite. The information relayed may be voice, data, video or a combination of

the three. The satellite is controlled from the ground through a satellite control facility, often called the Master

Control Center (MCC), which provide tracking, telemetry, command and monitoring for the system.

The Space Segment of the satellite system consist of the orbiting satellite (or satellites) and the ground satellite

control facilities necessary to keep the satellite(s) operational.

The Ground Segment or Earth Segment of the satellite system, consist of the transmit and receive earth stations

and the associated equipment to interface with the user network, as shown in figure 3.1

Focus will beon the space segment of a general communication satellite

The Space segment equipment on-board the satellite can be divided into: BUS and

PAYLOAD.

-BUS: It refers to the basic satellite structure and the subsystem that supports the

satellite.

The BUS subsystems are: Physical Structure, Power Subsystem, Attitude and Orbital

Control subsystems, command and telemetry subsystem.

-PAYLOAD: It is the equipment that provide the service or services intended for the

satellite

A communication payload can be further divided into Transponder and antenna

subsystems as shown in figure 3.2


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3. 1SATELLITE BUSThe basic characteristics of a BUS subsystem are described below.

3.1.1 PHYSICAL STRUCTURE

It contains the other components of the satellite.

The basic shape of the structure depends on the method of stabilization employed to keep the satellite stable and

pointing to the desired direction; usually to keep the antenna properly oriented towards the earth.

Two methods of stabilization are employed: Spin Stabilization and three-axis or body stabilized. These are shown

below

Spin stabilized 1 fig 3.3a

Three-axis stabilized 1 fig 3.3b

3-Axis stabilized

Larger solar cells areaSolar arrays can be

Slewed to provide more or

Less power as required

Spin stabilized

Solar Cells are spinning

Solar cell efficiency due to limited visibility

to the sun

Antenna is de-spun to keep

it pointing towards the earth


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3.1.2 POWER SUBSYSTEM

The electrical power for operating equipmen

t on a communication satellite is obtained primarily from solar cells, which convert

incident sunlight into electrical energy. Solar cells operate at an efficiency ofat the Beginning of Life (BOL) and can degrade to at the End of Life (EOL),usually considered to be 15years. In addition large number of cells connected in serial-

parallel arrays, are required to support the communication satellite electronic system.

Two types of batteries: Specific energy density Nickel - cadmium: 25 - 30 W.hr/Kg

Nickel - Hydrogen: 25 - 60 W.hr/Kg

GEO LEO

Depth of discharge (DOD) Nickel - cadmium 50% 10-20%Nickel hydrogen 70% 40-50%

3.1.3 ATTITUDE CONTROL

The attitude of a satellite refers to the orientation in space with respect to the earth. It helps the narrow

directional beam antenna to be pointed correctly to earth. Several forces can interact to affect the

attitude of a spacecraft. These forces are gravitational forces from the sun, moon and planet, solar

pressure acting on the spacecraft body, antenna and solar panels, earths gravitational field force.


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The orientation is monitored on the spacecraft by Infrared Horizon Detectors. Four detectors are used to

establish a reference point; usually the center of the earth and any shift in orientation is detected by one

or more of the sensors. A control signal is generated that is used to activate attitude control devices to

restore proper orientation.

Gas jets, ion thrusters and momentum wheels are used to provide active attitude control on

communications satellites. Since the earth is not a perfect sphere, the satellite will be accelerated

towards one of the stable points in the equatorial plane. This locations are and. In theabsence of orbital control, the satellite will drift and settle in one of these stable locations.

3.1.4 ORBITAL CONTROL

Orbital Control often referred to as Station Keeping, is the process required to maintain the satellite in

its proper orbit location. It is similar to though not the same as attitude control. GSO satellites will

undergo forces that will cause the satellite to drift in the East-West (longitude) direction and the North-South (Latitude) direction. Orbital Control is usually maintained using Gas jets, Ion thrusters and

momentum wheels.

The non-spherical properties of the earth primarily exhibited as an equatorial bulge, cause the satellite to

drift slowly in longitude along the equatorial plane. Control jets are pulsed to impart an opposite velocity

component to the satellite, causing the satellite to drift back to its nominal position. This is calledEast-

West Station Keeping Maneuvers, which are accomplished every two to three weeks.

North-South Station Keeping requires more fuel than East-West Station Keeping and often satellites are

maintained with few or no North-South station keeping to extend the satellites life orbit life.

The quantity of fuel that must be carried on-board the satellite to provide orbital and attitude control is

usually a determinant factor in the on-orbit life of a communication satellite.

3.1.5 THERMAL CONTROL

Thermal radiation from the sun heats on one side of the spacecraft, while the side facing the outer space

is exposed to extremely low temperature. Most of the equipment in the satellite itself generates heat,

which must be controlled.

Satellite thermal control is design to control the large thermal gradient generated in the satellite by

removing or relocating the heat to provide as stable as possible temperature environment for the

satellite.

-Thermal Blankets and Thermal Shield are placed at critical locations to provide insulation. Radiation

Mirrors are placed around electronic subsystems to protect critical equipment. Heat Pumps are used to

relocate heat from power devices such as Traveling Wave Tube Amplifiers (TWTA) to outer walls or heat


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sinks. Thermal heaters can also be used to maintain adequate temperature conditions for some

components, such as propulsion lines or thrusters, where low temperature would cause severe

problems.

Satellite antennas are highly affected by the heat from the sun. Large aperture antenna can be twisted.

3.1.6 TRACKING, TELEMETRY, COMMAND AND MONITORING

Tracking, Telemetry, Command and Monitoring (TTC&M) provide essential spacecraft management and

control functions to keep the satellite operating safely in orbit.

The TTC&M links between the spacecraft and the

ground are usually separated from the

communications system links. TTC&M links may

operate in the same frequency bands or differentfrequency bands as the communications links.

Separate earth terminal facilities specifically design

for the complex operation required to maintain the

spacecraft in orbit are used. A single TTC&M facility

may maintain several spacecraft simultaneously in

orbit through TTC&M links to each vehicle. Figure

3.4 shows typical TTC&M facility elements.

TTC&M is divided into the satellite TTC&M

subsystem and the earth TTC&M subsystem.

The satellite TTC&M subsystem comprises the

antenna, command receiver, tracking and telemetry

transmitter, and possibly tracking sensors.

Telemetry data are received from the other

subsystems of the spacecraft, such as the payload,

power, attitude and thermal control.

Command data are relayed from the command receiver to the other subsystems to control such

parameters as antenna pointing, transponder modes of operation, battery and solar cell charges etc.

The ground TTC&M subsystem comprise the antenna, telemetry receiver, command transmitter, tracking

subsystem and associated processing and analysis functions

Satellite control and monitoring is accomplished through monitors and keyboard interface. Major

operations of TTC&M are automated, with minimal human interface required.


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Tracking refers to the determination of the current orbital position and the movement of the spacecraft.

Telemetry involves the collection of data from sensors on-board the spacecraft and relay of this

information to the ground. Command is the complementary function of telemetry. The command

systems relay specific control and operations information from ground to the spacecraft, most often in

response to telemetry.

3.2 SATELLITE PAYLOAD

A communications satellite payload is made up of two subsystems: Transponder and Antenna

subsystems

3.2.1 TRANSPONDERAtransponder in a communications satellite is aseries of interconnectedcomponents that provides a

communications channel from the output of the receive antenna to the input of the transmit antenna. Atypical communications satellite will contain more than one transponder and some of the equipment

may be common to more than one transponder.

Each transponder generally operate in a different frequency band, with the allocated frequency band

divided into slots (sub bands), with a specified center frequency and operating bandwidth. For example a

500MHz frequency band allocated for FSS can be divided among 12 transponders each of 36MHz

bandwidth, width 4MHz guard band between each. Typical commercial communications satellites can

have 24 to 48 transponders.

The number of transponders can be doubled by the use of polarization frequency reuse. We can also

spatial separation of the signal in the form of narrow spot beam, which allow the reuse of the same

carrier in spatially separated locations on earth.

Communications satellite transponders can be implemented in two general types; Frequency Translation

and On-Board Processing Transponder.

3.2.1.1FREQUENCY TRANSLATION TRANSPONDERIt is the most frequently use of the two types. The Frequency Translation Transponder also referred to as

a Non-Regenerative or Bent Pipe, receives an uplink signal and after amplification, retransmits it with

only a translation in carrier frequency. Figure 3.5 shows a dual frequency translation transponder, wherethe uplink radio frequency,, is converted into an intermediate lower frequency,, amplified andthen converted back up to the downlink, for transmission to earth. Frequency translationtransponders are used for FSS, BSS, and MSS applications. The uplink and downlink are codependent

meaning any degradation introduced in the uplink will be transferred to the downlink, affecting the total

communications link performance.


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3.2.1.2ON-BOARD PROCESSING TRANSPONDERThe On-Board processing transponder also called a Regenerative Repeater or Demo/Remod

transponder or Smart Satellite is shown in figure 3.6

The uplink signals at is demodulated to baseband,. The baseband signal is then availablefor processing on-board, including reformatting and error correction. The baseband information is then

remodulated to the downlink carrier at, possibly in a different modulation format to the uplink andafter final amplification is transmitted to the downlink. The Demodulation/Remodulation process

removes the uplink noise and interference from the downlink, while allowing additional on board

processing to be accomplished. Thus the uplink and downlink are independent with respect to the

evaluation of the overall link performance

This type of satellite turns to be more expensive than frequency translation satellites, but do offer

significant performance advantages.

Travelling wave tube amplifiers (TWTA) or Solid State Power Amplifiers (SSPA) are used to provide final

output amplification for each transponder channel.


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3.2.2 ANTENNASThe antenna system is a critical part of the satellite communications system, because it is an essential

element in increasing the strength of the transmitted or received signal to allow amplification, processing

and eventual retransmission. The most important parameters that define the performance of an antenna

are; antenna gain, antenna beamwidth, and antenna side lobes.

The gain defines the increased in strength achieved in concentrating the radio wave energy. The

beamwidth usually express as 3-dB beamwidth or half power beamwidth is a measure of the angle over

which the maximum gain occurs. The sidelobe is defined as the amount of gain in the off-axis direction.

The common types of antennas used in satellite communications are: Linear dipole, horn antenna,parabolic reflector and array antenna.

CHAPTER 4 NOISE

The figure 4.1 below shows the path taken by a signal from the transmitter to the receiver and the level

of noise present in the signal.

From the graph it can be seen that signal power and noise power are almost equal at the input of the

receive terminal. That is it is possible to confuse noise and carrier power.

It can also be seen that from the point the noise is injected into the signal, it follows the same path as thesignal and therefore goes through the same attenuation and gain stages


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Noise can be introduced into a communication link at various points

At the transmit terminal At the receive system of the satellite In the satellite non-linear amplifier At the transmit system of the satellite At the receive terminal of the earth station.

4. 1TYPES OF NOISEThe following (figure4.2) are the major types of noise experienced in a satellite communication link

Thermal Noise

In the satellite receive system In the receive system of the earth terminal

Interference From the carriers in the same transponder From carriers in other transponders in the same satellite From other carriers in other satellites


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Intermodulation Noise In the High Power Amplifier(HPA) of the transmit terminal In the satellite High Power Amplifier(HPA)

4.1.1 THERMAL NOISEEvery object in the universe generates thermal noise. Thermal noise is very weak, so it is important only

when the signal itself is very weak, that is at the input of the receive system of the satellite or the receive

system of the receive earth station.

Thermal noise is measured in terms of noise temperature T. The gain (G) to noise temperature (T) ratio

of a receive system, G/T is a key performance parameter of the receive system.

Thermal noise can be grouped into Uplink Thermal Noise (satellite receive system) and

Downlink Thermal Noise (Terminal Receive System)


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4.1.1AUPLINK THERMAL NOISE (SATELLITE RECEIVE SYSTEM)

It comes from the following sources:

From the electronic components of the satellite. Space and other celestial bodies. Earth

This is shown in figure 4.3


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4.1.1B DOWNLINK THERMAL NOISE (TERMINAL RECEIVE SYSTEM)It comes from the sun, cloud and rain, sky, moon and other celestial bodies, ground and terrestrial noise

sources. This is shown in figure 4.4 below

4. 2INTERFERENCEInterference is the unwanted power contribution of other carriers in the frequency band occupied by the wanted

carrier. The three major types of interferences are

Adjacent Satellite Interference(ASI); Interference from a signal on an adjacent satellite Co-channel Interference(CCI); Interference from a carrier in a co-channel transponder on the same satellite Adjacent carrier Interference(ACI);Interference from an adjacent carrier in the same transponder

These are all shown in figure 4.5 below


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Adjacent Satellite Interference (ASI) is the most complex form of interference on a satellite link

There are two kinds

Uplink ASI Downlink ASI


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4. 3INTERMODULATIONNon-linear devices such as Traveling Wave Tube Amplifiers (TWTA) Or Solid State Power Amplifiers

(SSPA) at the satellite transponders or any High Power Amplifier (HPA) at the transmit terminal will

generate intermodulation noise when multiple carriers pass through them. The nature of the

intermodulation noise depends on the carriers and the non-linear device.

A precise computation of intermodulation noise is vital in predicting the performance close to saturation,

for maximum output performance.

CHAPTER 5- IMPAIRMENTS

The atmosphere offers an RF window for satellite communications.

At low frequencies the ionosphere cannot be penetrated by radio waves and acts as a reflector At high frequencies the atmospheric gases absorb and severely attenuate the radio waves


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Propagation impairment at frequencies above 1GHz can be grouped into the following classes

Signal attenuation due too Atmospheric gases-primarily oxygen and water vaporo Rain and snowo Clouds

Signal polarization effectso Depolarization due to raino Faradays rotation

Signal path effects related to refractiono Tropospheric scintillation- variation in refractive index

5. 1SIGNAL ATTENUATION

Attenuation is the absorption and scattering of radio wave energy as it travels along the propagation medium.

Signal attenuation can be caused by Atmospheric gases, rain, snow and cloud.

5.1.1 RAIN ATTENUATIONRain is a major weather effect which isof greatconcern particularly for earth-space communication in frequency

bands above 3GHz. It is particular significant for frequencies of operation above 10GHz.

Rain attenuation occurs because when the signal passes through rain drops, some of the signal energy get absorbed

and converted to heat, thus resulting in degradation of the reliability and performance of the link.

The amount of rain attenuation depends on:

The frequency (wavelength relative to the size of raindrops) The rain intensity or rain rate(amount of

water in the path per unit distance)

The elevation angle(lower elevationangle means signal has to travel a longer

path through the rain)

Figure 5.2 shows the rain attenuation measured

for the worst 1% of the year. Several generalcharacteristics can be derived from the figure;

rain attenuation increases with increasing

frequency and decreasing elevation angle. Rain

attenuation levels can be very high particularly

for frequencies above 30GHz.The plots are for


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99% link availability which corresponds to 1% outage.

5.1.2 GASEOUS ATTENUATIONGaseous attenuation is primarily due to signal absorption by oxygen and water vapor. Signal degradation

can be minor or severe depending on the frequency, temperature, pressure and water vapor

concentration

The absorption is high for frequencies that represent the resonant frequency of the elements that make

up the gases. Only oxygen and water vapor have absorbable resonant frequencies in the band of

interest. The figure 5.3 shows the total gaseous attenuation observed on a satellite path located in

Washington DC, for elevation angles from to . The stark effect of oxygen absorption lines around60GHz is seen. Water vapor absorption lines around 22.3GHz is observed. As the elevation angle

decreases, the path length through the troposphere increases, and the resulting total attenuation

increases.

5.1.3 CLOUD ATTENUATIONCloud attenuation behaves similarly to rain attenuation but it is generally a small effect. The figure 5.4

shows the total cloud attenuation as a function of frequency, for elevation angles from . Thecloud attenuation is seen to increase with frequency and decrease elevation angle.


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5.1.4 SNOW AND ICE ATTENUATION

The effects of snow and ice are generally included in rain impairments. Snow and ice generally attenuate

the signal to a small extent compared to rain.

5.2 SIGNAL PATH EFFECT RELATED TO REFRACTION

The main signal path effect related to refraction is scintillation. The scintillation effects occur at the

ionosphere and at the troposphere. The ionospheric scintillation mostly affects frequencies

around30MHz to 300MHz. Therefore are main concern will be tropospheric scintillation

5.2.1 TROPOSPHERIC SCINTILLATION

Tropospheric scintillation describes a rapid fluctuation in the received signal level as a result of variation

in the refractive index of the atmosphere. It is generally negligible at frequencies below 10GHz and at

high elevation angles but it becomes a significant problem for frequencies below 10GHz and low

elevation angles.

There are generally two kinds: Amplitude and Phase Scintillations


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5.2.2 SIGNAL POLARIZATION EFFECTS

5.2.2.1 POLARIZATION

The wave radiated by an antenna consists of electric field component and a magnetic field component.

These two components are orthogonal and perpendicular to the direction of propagation of the wave.

Polarization is the directional aspects of the electrical field of a radio signal. Two common types in

satellite communications are Linear Polarization and Circular Polarization.

Linear Polarization: The electric field is wholly in one plane containing the direction of propagation.

There are two types; Horizontal and Vertical Polarization.

Horizontal Polarization: The electric field lies in a plane parallel to the earths surface

Vertical Polarization: The electric field lies in a plane perpendicular to the earths surface.

Circular Polarization: The electric field radiates energy in both the horizontal and vertical planes and all

planes in between.

Right Hand Circular Polarization (RHCP) The electric field is rotating in the clockwise direction as seen by

an observer towards whom the wave is moving

Left Hand Circular Polarization (LHCP) The electric field is rotating in the counterclockwise direction as

seen by an observer towards whom the wave is moving.

5.2.2.2 RAIN DEPOLARIZATION

It refers to the change in the polarization characteristics of a radio wave. A depolarized radio wave will

have its polarization state altered such that power is transferred from the desired polarization state to an

undesired polarization channel.

Rain depolarization can be a problem in the frequency bands above about 12GHz, particularly for

frequency reuse systems communications links the same frequency bands to increase channel capacity.


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5.2.2.3 FARADAYS ROTATION

Faraday rotation is an ionospheric effect.

-The ionosphere is a charged layer of the atmosphere.

- When the electromagnetic RF signal passes through the ionosphere, the electric field rotates the polarization

plane of the signal.

- Therefore, the plane of polarization of linearly polarized signals (H / V) twists.

- Faraday rotation has no effect on circular polarization.

- Faraday rotation is dependent on the charged state of the atmosphere, which is dependent on solar activity.

- Sun-spot activity can increase Faraday rotation.

- This polarization rotation causes signal depolarization and increased cross-pol interference.


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The figure 6.1 above shows the basic communications elements in the transmitting and receiving earth

stations. It also indicates measures of performance at various points of the link.

CHAPTER 6: MODULATION AND CODING

6.1 TYPES OF MODULATION

In digital communications, we have three types of modulations: Amplitude, Frequency and Phase

Modulations.

Amplitude Shift keying(ASK): The bit information is carried in the amplitude of the signal Frequency Shift Keying(FSK): The bit information is carried in the frequency of the signal Phase Shift Keying(PSK):The bit information is carried in the phase of the signal

In satellite communications Phase Shift Keying is most frequently used because it has the advantage of a

constant envelope as compared to frequency shift keying(FSK), it provides better spectral

efficiency(number of bits transmitted per radio frequency bandwidth)

The figure 6.2 below shows the principle of a modulator. It consists of;

A symbol generator An encoder or mapper A signal generatorThe symbol generator generates symbols with M states, where M=2m, from m consecutive bits of

the input bit stream.

The encoder establishes a correspondence between M states of these symbols and M possible

states of the transmitted carrier


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6.1.1 TYPES OF PHASE SHIFT KEYING MODULATION AND BANDWIDTH EFFICIENCY

Depending on the number m, of bits per symbol, different M-ary Phase Shift Keying modulation can be

considered.

Binary Phase Shift Keying (BPSK): If a single bit is used to defined a symbol, a basic two state modulation

(M=2) is defined called BPSK

Quadrature Phase Shift Keying (QPSK): if two consecutive bits are grouped to define a symbol, a four

state modulation (M=4) is defined called QPSK

8-Phase Shift Keying (8PSK): If three consecutive bits are grouped to define a symbol, an eight state

modulation (M=8) is defined called 8-PSK, as shown in figure 6.3 below.

Higher Order Modulation (M=16, 32): This can be obtain for m=4, 5 etc. bits per symbol. As the order of

the modulation increases, the spectral (bandwidth) efficiency increases with increase in the number ofbits per symbol. That is: BPSK uses one bit per symbol

QPSK two bits per symbol- use half the bandwidth

8-PSK three bits per symbol- use one third of the bandwidth

With a modulation of higher order M , better performance is achieved by considering hybridamplitude and phase shift keying (APSK), also called Quadrature Amplitude Modulation (QAM). The

state of the carrier corresponds to given values of carrier phase and carrier amplitude (2 for 16APSK, 3

for 32APSK)

16-QAM for example takes four bit per symbol and uses one fourth of the bandwidth.

As we move from 8-PSK to 16-APSK, 32APSK, the

drawback is that the signal is also affected by the non-

linear components like the amplifiers at the ea

Satellite Communications Link Optimization_revhya

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