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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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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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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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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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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
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