satellite communications link optimization

[SATELLITE COMMUNICATIONS LINK OPTIMIZATION] November 26, 2012

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DEDICATION

I dedicate this work to my wife who has been a strong support to me

throughout our two years of marriage.

To my mother and all the members of my family who have made

enormous sacrifices for me.

To God through the intercession of Our Lady the Queen of Heaven most

especially, who has been the key to my protection and that of our

family.

November 26, 2012

SATELLITE COMMUNICATIONS LINK OPTIMIZATION

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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;

To the Director of IUT Douala who granted me the permission to

spent this academic year in his institution.

Mr. Emmanuel Chimi who has sacrificed so much time in review

this document; for always being available to answer my questions

whenever I knocked at his door.

Engineer Foumba Hyacinthe, who guided me in my choice of

project and furnished me with so many relevant documents

Engineer Tianang Germain for the deep inside of his advice and

the pertinent remarks he made to me.

Engineer Nyem Nestor who advised me to return to school and

who has always been there to assist me even in times of financial

difficulties.

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 during

this academic year.

Etoungou Olivier research teacher who greatly help me in the

presentation of my project.


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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

training among which are;

The initial training, which last for two years, at the end of which a diploma called “Diplôme Universitaire

de 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 trainings are;

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 Electrotecnique

FM/CM Mechanical Manufacturing/ Mechanical Construction

II Industrial Computing


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PROJECT SUMMARY

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 the availability and thus the reliability of the link

2) Satellite transponders have limited resources in terms of bandwidth and power, as such the transponder

leasing costs are determined by bandwidth and power used. The more bandwidth and power we use the more

we will have to pay for.

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

Increasing throughput without using additional transponder resources

Increasing link availability (margin) without using additional transponder resources

Or a combination to meet different objectives

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ABSTRACT

L'objectif de ce projet est de fournir des moyens d'optimiser un lien de communication par satellite. Le projet a

deux motivations;

i) La nécessité de réduire l'impact des pertubations atmosphériques, le bruit thermique, la non-linéarité

des chaînes satellitaires, des interférences sur les signaux, qui réduit la disponibilité et donc la fiabilité

de la liaison.

ii) Les transpondeurs satellitaire ont des ressources limitées en termes de bande passante et de la

puissance, ce titre, les frais de location du transpondeur sont déterminés par la bande passante et la

puissance utilisée. Plus la bande passante et la puissance que nous utilisons, plus nous aurons à payer.

Pour atteindre cet objectif, nous allons utiliser la modulation de pointe, gain de codage, l'adaptation fade

technologies d'annulation de porteuse, qui peut fournir des économies substantielles en bande passante,

améliorer la capacité, améliorer la fiabilité, ou les trois, tout en maintenant l'accord de services sous

contrat (SLA).

Le résultat de ce projet est qu'il y aura:

Réduire les dépenses d'exploitation (OPEX)

o Largeur de bande occupée et les ressources transpondeur réduira

Require les dépenses en capital(CAPEX)

o taille BUC / HPA et / ou la taille d'antenne

Augmenter le débit sans utiliser les ressources supplémentaires du transpondeur

Accroître la disponibilité lien (marge) sans utiliser les ressources supplémentaires transpondeur

Ou une combinaison pour répondre aux objectifs différents


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TABLE OF CONTENTS

Preface ........................................................................................................................................................................... 3

project summary ............................................................................................................................................................ 5

Abstract .......................................................................................................................................................................... 6

acronyms ...................................................................................................................................................................... 12

General introduction .................................................................................................................................................... 16

part I satellite communications system overview ........................................................................................................ 17

CHAPTER 1: INTRODUCTION TO SATELLITE COMMUNICATIONS ................................................................................. 18

1.1 Definition and Early History ........................................................................................................................ 18

1.2 Basic Satellite Communication System Definition ...................................................................................... 20

1.2.1 The Space Segment ................................................................................................................................ 20

.1.2.2 The Ground Segment ............................................................................................................................. 21

1.3. Satellite Link Parameters ........................................................................................................................ 21

1.4 Satellite Orbits ............................................................................................................................................ 22

1.5 Radio Regulations ....................................................................................................................................... 22

1.6 Space Radiocommunications Services ........................................................................................................ 23

1.7 Frequency bands ......................................................................................................................................... 24

CHAPTER 2-SATELLITE ORBITS ...................................................................................................................................... 26

2.1 Kepler’s laws ............................................................................................................................................... 27

2.1.1 Kepler’s First Law .................................................................................................................................... 27

2.1.2 kepler’s second law ................................................................................................................................ 27

2.3 Kepler’s third law ........................................................................................................................................ 28

2.3 orbital parameters .......................................................................................................................................... 28

2.3 Orbits in common use ..................................................................................................................................... 29

2.3.1 Geostationary orbit .................................................................................................................................... 29

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2.3.2 Geosynchronous orbit ................................................................................................................................ 29

2.3.3 Low earth ORBIT (Leo) ................................................................................................................................ 30

2.3.4 Medium earth orbit .................................................................................................................................... 30

2.3.5 Highly elliptical orbit ................................................................................................................................... 30

2.3.6 Polar orbit ................................................................................................................................................... 30

2.3.7 Geometry of GSO Link ................................................................................................................................ 30

Chapter 3 – satellite subsystems .................................................................................................................................. 31

3.1 satellite bus ................................................................................................................................................. 33

3.1.1 Physical structure ........................................................................................................................................ 33

3.1.2 Power Subsystem ........................................................................................................................................ 34

3.1.3 Attitude control ........................................................................................................................................... 34

3.1.4 Orbital control ............................................................................................................................................. 35

3.1.5 Thermal Control .......................................................................................................................................... 35

3.1.6 Tracking, Telemetry, command and Monitoring ......................................................................................... 36

3.2 Satellite Payload ................................................................................................................................................. 37

3.2.1 Transponder ........................................................................................................................................... 37

3.2.1.1 frequency translation transponder .................................................................................................... 37

3.2.1.2 on-board processing transponder ..................................................................................................... 38

3.2.2 antennas ..................................................................................................................................................... 38

part II ............................................................................................................................................................................ 39

CHAPTER 4 noise .......................................................................................................................................................... 40

4.1 types of noise .............................................................................................................................................. 41

4.1.1 thermal noise ......................................................................................................................................... 42

4.2 interference ................................................................................................................................................ 43

4.3 intermodulation .......................................................................................................................................... 45

chapter 5- impairments ................................................................................................................................................ 45


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5.1 signal attenuation ....................................................................................................................................... 46

5.1.1 rain attenuation...................................................................................................................................... 46

5.1.2 GASEOUS attenuation ............................................................................................................................ 47

5.1.3 cloud attenuation ................................................................................................................................... 47

5.1.4 snow and ice attenuation ............................................................................................................................ 47

5.2 signal path effect related to refraction .............................................................................................................. 48

5.2.1 Tropospheric scintillation ............................................................................................................................ 48

5.2.2 signal polarization effects ........................................................................................................................... 48

part III ........................................................................................................................................................................... 50

chapter modulation and coding .................................................................................................................................. 52

6.1 types of modulation ........................................................................................................................................... 52

6.1.1 types of phase shift keying modulation and bandwidth efficiency ............................................................. 53

6.1.2 power efficiency of the various schemes .................................................................................................... 54

6.1.3 power requirement of various schemes-eb/no vs BER ................................................................................ 55

6.2 CHANNEL encoding ............................................................................................................................................ 56

6.2.1 Block encoding and convolutional encoding ................................................................................................... 56

6.2.1a block encoding .......................................................................................................................................... 56

6.2.1b convolution encoding ................................................................................................................................ 56

6.2.2 concatenated encoding ............................................................................................................................... 57

6.2.3 Turbo codes ................................................................................................................................................. 57

6.2.4 Low Density Parity check CODES (LDPC) ..................................................................................................... 57

6.3 channel decoding ............................................................................................................................................... 57

6.4 power-bandwidth tradeoff ................................................................................................................................. 59

6.4.1 coding with variable bandwidth .................................................................................................................. 59

6.4.2 coding with constant bandwidth ................................................................................................................. 59

chapter 7 SATELLITE LINK Budget ................................................................................................................................ 60

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7.1 configuration of a link ........................................................................................................................................ 60

7.2 antenna parameters ........................................................................................................................................... 61

7.2.1 antenna gains .............................................................................................................................................. 61

7.2.2 radiation pattern and angular beamwidth .................................................................................................. 61

7.2.3 Polarization.................................................................................................................................................. 63

7.3 radiated power ................................................................................................................................................... 64

7.3.1 effective isotropic radiated power (EIRP) ................................................................................................... 64

7.3.2 power flux density ....................................................................................................................................... 64

7.4 Received signal power ........................................................................................................................................ 65

7.4.1 Power captured by the receiving antenna and free space path loss .......................................................... 65

7.5 additional losses ................................................................................................................................................. 66

7.5.1 attenuation in the atmosphere ................................................................................................................... 67

7.5.2 LOSSES IN THE TRANSMITTING AND RECEIVING EQUIPMENT .................................................................... 67

7.5.3 DEPOINTING LOSSES ................................................................................................................................... 68

7.5.4 losses due to polarization mismatch ........................................................................................................... 69

7.5.5 conclusion ................................................................................................................................................... 69

7.6 noise power spectral density at the receiver input ............................................................................................ 70

7.6.1 origin of noise .............................................................................................................................................. 70

7.6.2 Noise CHARACTERIZATION .......................................................................................................................... 70

7.6.3 noise temperature of a noise source .......................................................................................................... 70

7.6.4 noise figure .................................................................................................................................................. 70

7.6.5 EFFECTIVE INPUT NOISE TEMPERATURE OF AN ATTENUATOR ................................................................... 71

7.6.6 effective input noise temperature of cascaded elements .......................................................................... 71

7.6.7 EFFECTIVE INPUT NOISE TEMPERATURE OF A RECEIVER ............................................................................ 71

7.6.8 antenna noise temperature ........................................................................................................................ 72

7.6.8 noise temperature of a satellite antenna .................................................................................................... 72


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7.6.9 noise temperature of an earth station ANTENNA (downlink) ..................................................................... 72

7.7 SYSTEM NOISE TEMPERATURE ........................................................................................................................... 73

7.7.1 conclusion ................................................................................................................................................... 74

7.8 individual link performance ................................................................................................................................ 75

7.8.1 carrier to noise power spectral density ratio at the receiver input ............................................................ 75

7.8.2 clear sky condition ....................................................................................................................................... 76

7.9 link performance under rain conditions ............................................................................................................. 80

7.9.1 uplink performance ..................................................................................................................................... 80

7.9.2 downlink performance ................................................................................................................................ 81

7.9.3 conclusion ................................................................................................................................................... 81

7.10 overall link performance with a transparent satellite ...................................................................................... 82

7.10.1 characteristics of the satellite channel ...................................................................................................... 82

7.10.2 satellite power flux density at saturation ................................................................................................. 83

7.10.3 satellite eirp at saturation ......................................................................................................................... 84

7.10.4 satellite repeater gain ............................................................................................................................... 84

7.10.5 input AND OUTPUT BACK-OFF .................................................................................................................. 84

7.10.6 carrier power at the satellite receiver input ............................................................................................. 85

7.10.7 expression for without interference from other systems or intermodulation............................... 85

7.10.8 expression for taking account of interference and intermodulation ............................................. 86

chapter 8 optimization ................................................................................................................................................. 87

8.1 link Margin.......................................................................................................................................................... 87

8.2 Power restoral techniques ................................................................................................................................. 88

8.2.1 beam diversity ................................................................................................................................................. 88

8.3 power control ..................................................................................................................................................... 89

8.3.1 uplink power control ................................................................................................................................... 89

8.4 site diversity ....................................................................................................................................................... 90

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8.5 signal modification techniques .......................................................................................................................... 91

8.5.1 Optimization By Doubletalk carrier-in-carrier ............................................................................................. 91

8.5.6 Double Talk Carrier-in-carrier cancellation process ........................................................................................ 93

8.6 adaptive coding and MODULATION (ACM) ........................................................................................................ 94

8.6.1 acm background .......................................................................................................................................... 95

8.6.2 requirements for ACM ................................................................................................................................ 96

9.0 general conclusion ................................................................................................................................................. 97

Bibliographic references .............................................................................................................................................. 97

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


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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

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

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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

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


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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 today’s “wired world.” Satellite communications are at the core of a global, automatically switched telephony network. Today’s 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.

The organization of the project (Dissertation) is as follows:

Part 1 describes a general overview of the satellite communication system in three chapters.

Part2 present a brief description of the impairments encountered in this domain in three chapters.

Part3 briefly talks on modulation and coding in one chapter. It also presents the parameters

necessary to calculate the performance of a link. It is concluded with the calculation of link

performance, for an uplink, a downlink and overall link from an uplink through a satellite to a

downlink.

Part4 presents the different means of optimizing a satellite link, in two parts. The first part, using

power restoral techniques and the second part using signal modification techniques.


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PART I SATELLITE COMMUNICATIONS SYSTEM OVERVIEW

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CHAPTER 1: INTRODUCTION TO SATELLITE COMMUNICATIONS

1.1 DEFINITION AND EARLY HISTORY

A 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. Today’s 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 over 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 the transmitting and

the receiving earth stations.

Fixed Broadcast Cost: Broadcast from an earth station to a number of other earth station is independent

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 106 bits and higher can be seen

commonly.

Diverse User Network. Due to its large coverage area, it can be used to interconnect land, sea and 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

These early accomplishments and events led to the rapid growth of the satellite communication’s 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 DEFINITION

Satellite communications system is broken down into two main segments: the space segment and the ground (or

earth) segment.

1.2.1 THE SPACE SEGMENT

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

segment include 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)

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 it is accomplished through separate earth terminal facilities, design for this purpose.

Figure 1.1


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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 space segment. 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 Terminals

1.3. 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 the

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. A duplex 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.

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1.4 SATELLITE ORBITS

A detail description of the 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

1.5 RADIO REGULATIONS

Radio Regulations are necessary to ensure an efficient use of the radio frequency spectrum by all communication

systems including terrestrial and satellite. This does not prevent each state from regulating its telecommunications

sector. 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 at Geneva, Switzerland.

ITU has three primary functions:

Allocation and Use of the radio- frequency spectrum;

Telecommunications standardization;

Development and expansion of the worldwide telecommunication


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These functions are accomplished through the three sectors of the 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 the

worldwide telecommunications.

The International regulations developed by ITU are process 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).

1.6 SPACE RADIOCOMMUNICATIONS SERVICES

Two 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 satellite system ground terminals

Services applicable to satellite systems as designated by ITU are:

Aeronautical Mobile Satellite(AMSS)

Aeronautical Radionavigation Satellite(ARSS)

Amateur Satellite(ASS)

Broadcasting Satellite(BSS)

Earth-exploration Satellite(ESS)

Fixed Satellite(FSS)

Inter-satellite(ISS)

Land Mobile Satellite(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 service areas are divided into sub areas. For example the mobile satellite service (MSS) area 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 one of these regions are protected from those in

another because of the geographical separation between them.

1.7 FREQUENCY BANDS

The frequency of operation is one of the major factors in the design and performance of a satellite communication

system. As it’s 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


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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 licence

Ku-Band -Smaller antennas -Smaller RF power amplifiers

Greater effect from rain Smaller footprint (beam) coverage

Ka-Band Smaller antenna Smaller RF power amplifier

– Greater effect from rain

– Smaller footprint (beam) coverage

– High equipment cost

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CHAPTER 2-SATELLITE ORBITS

The same laws of motion that governs 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 , can be

represented as

Fin= m (

) ….2.1

and Fout=m (

)….2.2

where 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 =Kepler’s constant

(Geocentric

gravitational constant)

=3.9864002x Km3/s2.

If the gravitational 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.


27

2.1 KEPLER’S LAWS

The laws of Kepler apply to any two bodies in space that interact through gravitation.

2.1.1 KEPLER’S FIRST LAW

Kepler’s 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 satellite will 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 KEPLER’S SECOND LAW

For equal time interval, the satellite sweeps out equal area in the orbital plane. This is shown in figure 2.2. 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

This result shows that the satellite orbital velocity is not constant; the satellite moves much faster at locations near

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

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2.3 KEPLER’S THIRD LAW

The square of the periodic time of orbit is proportional to the cube of the mean distance between the two bodies.

That is T2= [

]a

3, where T=orbital period in seconds s, a= distance between the bodies in km and µ=Kepler’s

constant=3.986004x105km

3/s

2

2.3 ORBITAL PARAMETERS

Important 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 to north

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


29

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 earth’s 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 earth’s

rotation is called a Prograde orbit, inclination angle 0 < < 90. A satellite in a retrograde orbit moves in the

opposite direction to earth rotation, inclination angle 90 < < 180

Most satellites are launched in Prograde orbit because the earth’s rotational velocity enhances the satellite orbital

velocity, reducing the amount of energy required to launch and place the satellite in orbit.

2.3 ORBITS IN COMMON USE

2.3.1 GEOSTATIONARY ORBIT

Kepler’s third law shows that there is a fixed relationship between orbit radius and the period of revolution of the

satellite. If we carefully choose an orbit radius we can determine the orbit period.

If an orbit radius is chosen so that the period of revolution of the satellite is exactly set to the period of rotation of

the earth. Also if the orbit is circular (e = 0) and the orbit is in the equatorial plane ( =0), the satellite will appear to

hover motionless above the earth. This orbit is called Geostationary Earth Orbit (GEO). This orbit radius is

42104Km. The GEO is an ideal orbit that cannot be achieved for real artificial satellites because there are many

other forces acting on the satellite apart of the earth gravity. In addition to this, extensive station keeping and a

vast amount of fuel is necessary to maintain the satellite in this orbit.

2.3.2 GEOSYNCHRONOUS ORBIT

It is one whose inclination angle is slightly greater than zero and possibly with an eccentricity above zero. It’s at an

altitude of 36000Km. Most current communications satellites operate in geosynchronous orbit.

Advantages -It’s the most common orbit -Fixed slant path -little or no ground station tracking required -2 to 3 satellites for global coverage (accept at the poles) -period of revolution is 23hours, 56minutes Disadvantages -Large path loss and significant latency (approximately 260ms for a duplex communication) -cannot provide coverage to high latitude locations Coverage can be increase by using high elevation angle but this produces problems such as increase ground station antenna tracking, which increases cost and system complexity.

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2.3.3 LOW EARTH ORBIT (LEO)

Operate typically at an altitude from 160 – 2400Km and is near circular. 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 orbit Disadvantages -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 ORBIT

It 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 ORBIT

Popular 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.

2.3.6 POLAR ORBIT

Circular orbit with an inclination near

Useful for sensing and data gathering services

2.3.7 GEOMETRY OF GSO LINK

GSO 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


31

Azimuth and elevation angle are called the look angle of

the earth station to the satellite. This is shown in figure

2.5

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

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 control 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

We will focus on 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.

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32

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

A satellite may have more than one payload


33

3.1 SATELLITE BUS

The 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 area Solar 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 equipment on a communication satellite is obtained

primarily from solar cells, which convert incident sunlight into electrical energy. Solar

cells operate at an efficiency of at 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.

t Two types of batteries:

t Specific energy density Nickel - cadmium: 25 - 30 W.hr/Kg

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

GEO LEO

t 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, earth’s gravitational field force.


35

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 the absence 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 called East-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 maintain

with little or no North-South station keeping, to extend on-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 of space. 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 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.

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36

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 different frequency 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.

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.


37

3.2 SATELLITE PAYLOAD

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

3.2.1 TRANSPONDER

The Transponder in a communications satellite is the series of components that provide the communications

channel or link between the uplink signal received at the uplink antenna and the downlink signal transmitted at the

downlink antenna. A typical 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.

3.2.1.1 FREQUENCY TRANSLATION TRANSPONDER

It is the most frequently use of the two types. The Frequency Translation Transponder also referred to as a Non-

Regenerative or Bent Pipe receives the uplink signal and after amplification, retransmits it with only a translation in

carrier frequency. Figure 3.5 shows a dual frequency translation transponder, where the uplink radio frequency, ,

converted into an intermediate lower frequency, , amplified and then converted back up to the downlink ,

for transmission to earth. Frequency translation

transponders are used for FSS, BSS, and MSS

applications. The uplink and downlink are

codependent meaning any degradation

introduces in the uplink will be transferred to

the downlink, affecting the total

communications link performance.

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3.2.1.2 ON-BOARD PROCESSING TRANSPONDER

The 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 available for

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 and after 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.

3.2.2 ANTENNAS

The 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.


39

PART II

NOISE AND IMPAIRMENTS ARE THE MAJOR SOURCES OF DEGRADATION ON A SATELLITE COMMUNICATIONS

LINK. THIS PART PRESENTS ALL THE TYPES OF NOISE AND IMPAIRMENTS THAT CAN BE ENCOUNTERED ON A

COMMUNICATION LINK. A GOOD KNOWLEDGE OF THIS NOISE AND IMPAIRMENTS WILL HELP AN OPERATOR

BETTER OPTIMIZE PERFORMANCE.

THIS PART IS DIVIDED INTO TWO MAIN CHAPTERS. CHAPTER FOUR PRESENTS ALL THE NOISE ON A SYSTEM

WHILE CHAPTER FIVE PRESENTS THE IMPAIRMENTS.

PART ∥ NOISE AND IMPAIRMENTS

ON SATELLITE COMMUNICATIONS

LINK

November 26, 2012


40

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.

At can also be seen that from the point the noise is injected into the signal, it follows the same path as the signal

and therefore goes through the same attenuation and gain stages

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.


41

4.1 TYPES OF NOISE

The 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

– Intermodulation Noise

In the High Power Amplifier(HPA) of the transmit terminal

In the satellite High Power Amplifier(HPA)

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42

4.1.1 THERMAL NOISE

Every 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.

We can group thermal noise into Uplink Thermal Noise (satellite receive system) and Downlink Thermal Noise

(Terminal Receive System)

4.1.1A UPLINK 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


43

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

4.2 INTERFERENCE

Interference 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

November 26, 2012


44

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

There are two kinds

Uplink ASI

Downlink ASI


45

4.3 INTERMODULATION

Non-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

November 26, 2012


46

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

Signal attenuation due to

o Atmospheric gases-primarily oxygen and water vapor

o Rain and snow

o Clouds

Signal polarization effects

o Depolarization due to rain

o Faradays rotation

Signal path effects related to refraction

o Tropospheric scintillation- variation in refractive index

5.1 SIGNAL 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 ATTENUATION

Rain is a major weather effect of concern 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 elevation angle 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 general

characteristics 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

99% link availability which corresponds to 1%

outage.


47

5.1.2 GASEOUS ATTENUATION

Gaseous 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 around 60GHz 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 ATTENUATION

Cloud 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 . The cloud attenuation is

seen to increase with frequency and decrease elevation angle.

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 as compared to rain.

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48

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 around 30MHz 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 a 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

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.


49

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 earth’s surface

Vertical Polarization: The electric field lies in a plane perpendicular to the earth’s 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.

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.

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- Sun-spot activity can increase Faraday rotation.

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

PART III PART ΙΙΙ

OPTIMIZATION TECHNIQUES AS APPLIED TO

SATELLITE COMMUNICATIONS LINK


51

The figure 6.1 below shows the basic communications elements in the transmitting and receiving earth stations. It

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

November 26, 2012


52

This section is divided into three chapters.

Chapter six deals with modulation and coding; how they can be used in tradeoff analysis between bandwidth and

power, to optimize satellite communications link.

Chapter seven covers satellite Link Budget; which is used to analyze the link performance of a satellite

communications system. In this chapter we first consider the individual link performance, providing tools to

evaluate the carrier-power budget and the noise-contribution budget. We then introduce the concept of link

performance for the overall link from origin to the destination station, for a transparent satellite.

CHAPTER 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 and compared to frequency shift keying(FSK), it provide 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 generator

The 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


53

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 of bits 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 hybrid amplitude 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 earth station transmitter and

at the satellite.

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6.1.2 POWER EFFICIENCY OF THE VARIOUS SCHEMES

The error performance of various modulation schemes can be compared as follows:

The square of the distance from the origin is the power corresponding to each symbol. Using this, the

average power per bit (P) for the modulation scheme can be computed.

The square of half the distance between two closest symbols is the minimum noise power (E) required to

cause an error. It is a measure of the error tolerance of the modulation scheme.

If two schemes have the same E, the one requiring the lower P is more power efficient.


55

6.1.3 POWER REQUIREMENT OF VARIOUS SCHEMES-EB/NO VS BER

The power required to achieve a certain bit error rate (BER) is often express as a relationship between the Eb/No

and BER. Bit error rate is a measure of the performance of a digital communications system at the output of a

demodulator. Figure 6.4 shows the power requirement of various modulation schemes.

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56

6.2 CHANNEL ENCODING

The figure6.5 below illustrates the

principle of channel encoding.

It has the objective of adding to the

information bit, redundant bits, which are

used at the receiver to detect and correct

errors.

This technique is called Forward Error

Correction (FEC). The code rate is defined

as , where r is the number

of redundant bits added to n information

bits. The bit rate at the encoder input

is , at the output, it is greater and equal

to . Hence,

(bit/s)

6.2.1 BLOCK ENCODING AND CONVOLUTIONAL ENCODING

6.2.1A BLOCK ENCODING

The encoder associates bits of redundancy with each block of information bits; each block is coded

independently of the others. The code bits are generated by a linear combination of the corresponding block

Some of the most commonly used block codes are:

Hamming codes; which can correct a single error

Reed-Solomon codes; which can correct multiple errors.

An reed-Solomon code can correct

errors. Here [x] represents the largest integer less than

or equal to x. For example a (219,201) RS encodes blocks of 201 bits onto code words of length 219 bits.

This can correct 9 simultaneous bits errors in the 219 bits code word.

Bose, Chaudhari and Hocquenghem (BCH) codes

6.2.1B CONVOLUTION ENCODING

A convolutional code process a stream of data. For every K bits it take in, it generates n bits at the output The

choice between block codes and convolutional encoding is dictated by the types of errors that are expected at the

output of the demodulator. The distribution of errors depends on the nature of the noise and the propagation

impairments encountered on the satellite link.


57

Under stable propagation conditions and Gaussian noise, errors occur randomly and convolutional encoding is

mostly used. Under fading conditions, errors occur mostly in bursts, compared with convolutional encoding; block

encoding is less sensitive to bursts of errors, so block encoding is preferred.

6.2.2 CONCATENATED ENCODING

The concatenated coding wraps a convolutional code inside a Reed-Solomon code, with an interleaver. The

convolutional code corrects must of the channel errors. When a convolutional code causes errors, the errors are in

bursts. The interleaver spreads the bursts of errors over multiple Reed-Solomon code words. The Reed-Solomon

code then corrects the remaining errors.

Concatenated coding provides very significant improvement in performance over either types of coding alone.

INPUT

OUTPUT

Overall code rate =

*

6.2.3 TURBO CODES

There are a complete replacement for convolutional and Reed-Solomon codes

6.2.4 LOW DENSITY PARITY CHECK CODES (LDPC)

LDPC codes have been found to offer better performance than Turbo codes

LDPC block codes (just like RS block codes) are often used as part of a concatenated coding schemes e.g. the

DVB-S2 standard uses LDPC inner codes and BCH outer codes. This concatenated coding yields better performance.

6.3 CHANNEL DECODING

With FEC, the decoder uses the redundancy introduced at the encoder to detect and correct errors. Various

possibilities are available for decoding block codes and convolutional encoding. Convolutional codes are mostly

RS Encoder Interleaver Convolutional

Encoder

Channel

Convolutional

decoder

De-interleaver RS Decoder

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58

decoded with the use of VITERBI decoding algorithm, for best performance. The figure 6.6 shows the performance

of a modulation and coding scheme

The Bit error probability (BEP) is express as a function of Eb/N0, where Eb is the energy per information bit. And

Eb=C/Rb, where C is the carrier energy present after demodulation and Rb is the bit rate.

Therefore

The Decoding gain is defined as the difference

in decibel (dB) at a considered value of BEP or

BER between the required value of Eb/N0 with

and without coding, assuming equal

information rate Rb.

Table 6.1 below shows typical values of coding gain.

Bit error rate (BER): It is used to measure the

performance of a digital communications system at the

output of the demodulator. It is a very important

performance parameter.


59

6.4 POWER-BANDWIDTH TRADEOFF

Coding allows bandwidth to be exchanged for power (that is it permits us to use more bandwidth put less power).

As a result of this link performance can be optimize in terms of cost (cost of earth station)

6.4.1 CODING WITH VARIABLE BANDWIDTH

When Coding is used, bandwidth is increased, and less power is required to attain the same performance

requirements. This reduction in power noted

is equal to the decoding gain.

⁄ =

⁄ (

⁄ )

The reduction in the required

⁄ , which translates to an equal reduction in the required carrier power, is paid

for by an increase in the required bandwidth used on the satellite link.

6.4.2 CODING WITH CONSTANT BANDWIDTH

It is performed when a given bandwidth is allocated to a given satellite link. Coding is introduced without changing

the carrier bandwidth B, and therefore at a constant transmitted rate Rc. Therefore Rb must be reduced. If the

bandwidth is constant, the reduction in ⁄ is higher, as a result in reduction in the information rate. This

⁄ reduction can be used to combat temporary link degradation due to rain, at the expense of temporary

capacity reduction on the considered link.

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CHAPTER 7 SATELLITE LINK BUDGET

A satellite link budget is like a financial budget in which:

Signal power = Credit/Income

Noise power = Debit/Expense

A link budget is the basic tool of the satellite engineer. It is used to predict the performance of a satellite link at the

receive terminal by;

Computing the power gain/loses along the satellite link

Computing the impact of various impairments along the satellite link

The main goal of a link budget is to determine;

The Forward link budget : Given the power at the transmit terminal, predict the link performance at the

receive terminal

Reversed Link Budget: Determine the power at the transmit terminal required to achieve a desired link

performance at the receive terminal.

We will begin this section by looking at the configuration of a satellite links. The Links we are talking of here are;

Uplink from a transmit earth station to the satellite

Downlink from a satellite to a receive terminal earth station

End-to-End link from a transmit earth station through the satellite to a receive earth station.

We will then proceed to analyze the performance of each individual link and conclude with that of an overall (end-

to-end) link of a transparent satellite.

7.1 CONFIGURATION OF A LINK

The figure 7.1 represents the elements

participating in a link. The transmit

equipment consist of a transmitter Tx,

connected by a feeder to the transmit

antenna of gain GT in the direction of the

receiver. Power radiated by the transmit

equipment in the direction of the receive

equipment is PT

The performance of the transmit equipment

is measured by its effective isotropic

radiated power (EIRP), defined as

EIRP = (7.1)


61

On its way the radiated power suffers from path loss L.

The receiving equipment consists of a receiving antenna of gain GR in the direction of the transmit equipment. The

antenna is connected by a feeder to the receiver Rx. At the receiver input, the power of the modulated carrier is C

and all sources of noise in the link contribute to the system noise temperature T.

The system noise temperature T conditions the noise power spectral density N0, which is used to determine the

performance of the RF link at the input of the receiver ⁄

The performance of the receiving equipment is measured by its figure of merit, ⁄ , where G represents the

overall receiving equipment gain

The following section presents definitions of the relevant parameters that condition the link performance and

provide useful equations that help in calculating ⁄ .

7.2 ANTENNA PARAMETERS

7.2.1 ANTENNA GAINS

The gain of an antenna is the power radiated (or received) per unit solid angle by the antenna in a given direction to

the power radiated (or received) per unit solid angle by an Isotropic antenna fed with the same power. The gain of

the antenna is maximum, in the direction of maximum radiation (boresight) and has a value given by;

( ⁄ )

Where λ ⁄ and is the velocity of light and frequency of the electromagnetic wave. For an

antenna, with a circular aperture, or reflector of diameter D. The surface area

, but ,

where η is the antenna efficiency. Therefore (

)

(

⁄ )

Expressed in dBi (the gain relative to an isotropic antenna), the actual maximum antenna gain is;

(

) ( (

)

)

The efficiency η of the antenna is the product of several factors which take account of the spill-over loss, surface

impairments, ohmic and impedance mismatch losses.

7.2.2 RADIATION PATTERN AND ANGULAR BEAMWIDTH

The radiation pattern indicates the variation of gain with direction. Figure 7.2a and 7.2b show the radiation pattern

for a circular antenna in polar (7.2a) and Cartesian (7.2b) coordinates. The main lobe contains the direction of

maximum radiation. The side lobes should be kept to a minimum.

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62

The Angular beamwidth is the angle defined by the directions corresponding to a given gain fallout with respect to

the maximum gain. The 3dB beamwidth, indicated by , in figure 7.2a is often used.

The 3dB beamwidth corresponds to the angle in the directions in which the gain falls to half the maximum value. It

is related to the ratio ⁄ by a coefficient. The coefficient commonly used is , which leads to the expression;

( ⁄ ) ( ⁄ )

In the direction with respect to the boresight, the value of gain is given by

( ⁄ )

and is valid only when

⁄

Combining equation (7.3) and (7.5), we can obtain the maximum gain of an antenna as a function of

beamwidth (

)

(

⁄ )

If η=0.6 is considered, it gives

, where is in degrees.

Figure 7.3 shows the relationship between 3dB

beamwidth and maximum gain for three most

common values of antenna efficiency.

From figure 7.3 it can be seen that as the 3dB

beamwidth increases, the antenna gain drops for

each of the three efficiency values. The higher the

efficiency, the higher the antennae gains.


63

7.2.3 POLARIZATION

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

two components are orthogonal and perpendicular to the direction of propagation of the wave, as shown in figure

7.4 below. They both vary at the frequency of the wave. By convention, the polarization of a wave is defined by the

direction of the electric field component. This electric field component is not fixed in direction.

Polarization is characterized by;

Direction of rotation(with respect to direction of propagation); right- hand (clockwise) or left-

hand(counter clockwise)

Axial ratio(AR);

, ratio of the major and minor axes of the ellipse. When the ellipse is a circle

(axial ratio=1=0dB), polarization is said to be circular. When the ellipse reduce to one axis( infinite axial

ratio, the electric field maintains a fixed direction), polarization is said to be linear.

Inclination, of the ellipse

Two waves are in orthogonal polarization if their electric field defines identical ellipses in opposite direction. In

particular we can have;

Two orthogonal circular polarization described as right-hand circular(RHCP) and left-hand circular(LHCP)

polarizations

Two orthogonal linear polarization described as horizontal and vertical polarizations

Polarization enables an increase in capacity through frequency reuse. This must take into account the imperfection

of the antenna and possible depolarization of wave by transmission medium, which can lead to mutual

interference.

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Consider figure 7.5 below with two

orthogonal lineally polarized waves.

Amplitude of the wave A, transmitted

with vertical polarization ac

amplitude of wave B, transmitted

with horizontal linear polarization bc

=energy of signal B, found in A due to

depolarization

= energy of signal A, found in B due to

depolarization

The following can be defined

The Cross- Polarization Isolation:

⁄

⁄

(

⁄ ) Or (

⁄ )

The Cross-Polarization Discrimination

⁄ (

⁄ ) (when a single polarization is

transmitted)

7.3 RADIATED POWER

7.3.1 EFFECTIVE ISOTROPIC RADIATED POWER (EIRP)

It is the parameter that characterizes the performance of a transmit equipment and it is given by

To obtain EIRP, we consider the power radiated by an isotropic antenna fed from a radio-frequency source of

power PT, given by

⁄

In a direction where the value of the transmitted gain is , any antenna radiates a power per unit solid angle given

by

, the product is called the EIRP

7.3.2 POWER FLUX DENSITY

A surface area A situated at a distance R from the transmitting antenna, subtends a solid angle A/R2 at the

transmitting antenna as shown in figure 7.6. It receives a power equal to


65

(

) (

⁄ )

The magnitude

is called the power flux density expressed in ⁄

7.4 RECEIVED SIGNAL POWER

7.4.1 POWER CAPTURED BY THE RECEIVING ANTENNA AND FREE SPACE PATH LOSS

As shown in figure 7.7, a receiving antenna of effective aperture area located at a distance R from the

transmitting antenna receives power equal to;

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66

(

)

The effective area of an antenna is expressed as a function of its receiving gain according to equation (7.2)

(

)

Hence an expression for the received power (

)

(

) ( (

))

(

)

( ⁄ )

Where (

)

is called the free space loss and represents it is usually of the order of 200dB for an earth

station situated at an altitude of about 35786Km. It is loss linked to the distance that exists between the

transmitting equipment and the receiving equipment. It is not linked to any attenuation.

7.5 ADDITIONAL LOSSES

In practice, it is necessary to take into account additional losses due to various causes

Attenuation of the wave as they propagate through the atmosphere

Losses in transmitting and receiving equipment

Depointing losses

Polarization mismatch losses


67

7.5.1 ATTENUATION IN THE ATMOSPHERE

The attenuation of waves in the atmosphere, denoted by , is due to the presence of gaseous components in the

troposphere, water (rain, clouds, snow and ice) and ionosphere. The overall effect on the power of the received

carrier can be taken into account by replacing in equation (7.9) by the Path Loss, L, where

7.5.2 LOSSES IN THE TRANSMITTING AND RECEIVING EQUIPMENT

Figure (7.8) shows the losses in the terminal equipment. We have the following;

-The feeder loss between the transmitter and the antenna; to feed the antenna with power PT it is necessary

to provide a power at the output of the transmission amplifier such that;

Expressing the EIRP as a function of the power at the output of the transmission amplifier, we have;

-the feeder loss between the antenna and the receiver; has an impact on the power at the input of the

receiver, , such that it will be equal to

⁄

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7.5.3 DEPOINTING LOSSES

Figure 7.9 shows the geometry of the link in case of imperfect alignment between the transmitting and the

receiving antennas. The result is fallout in antenna gain with respect to the maximum gain in transmission and in

reception, called Depointing Loss. These Depointing losses are a function of a misalignment of angle of

transmission and reception . They are evaluated using equation (7.6);

(

⁄ )

(

⁄ )


69

7.5.4 LOSSES DUE TO POLARIZATION MISMATCH

When the receiving antenna is not oriented with the polarization of the received wave, a polarization mismatch

occurs. In a link with circular polarization the transmitted wave is circularly polarized only on the axis of the

antenna and becomes elliptical off this axis. Propagation through the atmosphere can also change circular into

elliptical polarization.

In linear polarization link, the wave can be subject to rotation of its plane of polarization as it propagates through

the atmosphere. Finally with linear polarization, the receiving antenna may not have its plane of polarization align

with that of the incident wave. If is the angle between the two planes, the polarization mismatch loss

(in dB) is . In a case where a circularly polarized antenna receives a linearly polarized wave,

will have a value of 3dB. Considering all sources of loss, the signal power at the receiver input will be;

(

⁄ ) (

⁄ ) (

⁄ )

7.5.5 CONCLUSION

Equations (7.9) and (7.14), which express the received power at the input to the receiver, are of the same form;

they are a product of three factors;

-EIRP, which characterizes the transmitting equipment

⁄

Which takes into account loss, between the transmit amplifier and the antenna. Reduction in gain LT due to

misalignment of the transmit antenna

-1/L, which characterizes the transmission medium

⁄

⁄

The path loss takes in to account free space attenuation and atmospheric attenuation

-The gain of the receiver, which characterizes the receiving equipment;

⁄

Which takes into account losses, between the antenna and the receiver, LR due to misalignment of receiver

antenna and, ,due to polarization mismatch.

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7.6 NOISE POWER SPECTRAL DENSITY AT THE RECEIVER INPUT

7.6.1 ORIGIN OF NOISE

Noise consists of all unwanted contributions whose power adds to the wanted carrier power. It reduces the ability

of the receiver to reproduce correctly the information content of the received wanted carrier.

As seen in chapter 4, noise can originate from;

Thermal source(noise emitted by natural sources of radiation situated around the receiver antenna and

noise generated by components of the receiving equipment)

Interfering sources from neighboring systems

7.6.2 NOISE CHARACTERIZATION

The equivalent noise power captured by a receiver with equivalent noise bandwidth , is given by

Where N0 is the noise power spectral density

7.6.3 NOISE TEMPERATURE OF A NOISE SOURCE

The noise temperature of a noise source of noise power spectral density N0 is given by

⁄

Where k the Boltzmann’s constant = 1.379x10-23

= -228.6dBW/HzK

7.6.4 NOISE FIGURE

If the reference temperature at the input of an element is T0=290K, also if the element has a gain G, a bandwidth B

and is driven by a source of noise temperature T0. The total power at the output is . The noise power

originating from the source is . The noise figure is thus

⁄

The noise figure is usually quoted in decibel (dB): (

⁄ )


71

7.6.5 EFFECTIVE INPUT NOISE TEMPERATURE OF AN ATTENUATOR

An attenuator has passive components, all at temperature which is generally the ambient temperature. If

is the attenuation caused by the attenuator, then the effective input noise temperature of the attenuator is;

7.6.6 EFFECTIVE INPUT NOISE TEMPERATURE OF CASCADED ELEMENTS

Consider a chain of N elements in cascade, each element j having a power gain and effective

input noise temperature

The overall effective input noise temperature is

⁄

⁄

⁄

The noise figure will be

7.6.7 EFFECTIVE INPUT NOISE TEMPERATURE OF A RECEIVER

Figure (7.10) shows the arrangement of a receiver. By using equation (7.19), the effective input noise temperature

of the receiver can be express as

⁄

⁄

Example for a low noise amplifier(LNA); ,

Mixer; ,

IF amplifier; =30dB

Hence;

It can be seen that the high gain of the LNA limits the noise temperature of the receiver to that of the LNA,

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7.6.8 ANTENNA NOISE TEMPERATURE

An antenna picks up noise from the radiating bodies within the radiation pattern of the antenna. The noise output

from an antenna is a function of the direction in which the antenna is pointing, it’s radiation pattern and the state

of the surrounding environment

The antenna is assumed to be a noise source characterized by a noise temperature called the noise temperature of

the antenna . Two cases are considered

A satellite antenna (uplink)

An earth station antenna (downlink)

7.6.8 NOISE TEMPERATURE OF A SATELLITE ANTENNA

As seen in chapter 4, noise is captured by this antenna from the earth and from outer space. The earth is the major

contributor. For a beamwidth of 17.5 , the antenna noise temperature depends on the frequency and orbital

position of the satellite. For a smaller beamwidth (spot beam), it depends on the frequency and the area covered.

For a preliminary design, the value 290K can be taken as a conservative value.

7.6.9 NOISE TEMPERATURE OF AN EARTH STATION ANTENNA (DOWNLINK)

It comes from the sky and noise due to radiation from the earth. This is shown in figure (7.11a) and (7.11b), for

clear sky and rain attenuation conditions


73

In clear sky

In the presence of rain

⁄

(

⁄ )

7.7 SYSTEM NOISE TEMPERATURE

Consider the receiving equipment shown in figure (7.12) below. It consists of an antenna connected to a receiver.

The connection (feeder) is a lossy one and at a thermodynamic temperature TF(which is closed to T0=290K). It

introduces an attenuation , which corresponds to a gain

⁄ and is less than 1.

The effective input noise temperature of the receiver is .

The noise temperature may be determine at two points as follows

At the antenna output before the feeder losses, temperature T1;

At the receiver input, after the feeder losses,

temperature T2

The noise temperature T1 at the antenna output is the sum of

the noise temperature of the antenna and the noise

temperature of the subsystem, consisting of the feeder and

receiver in cascade. The noise temperature of the feeder is given

by equation (7.18). From equation (7.21), the noise temperature

of the sub system is

⁄ , adding the

contribution of the antenna, this becomes

⁄

Now consider the receiver input. This noise factor must be

attenuated by a factor . Replacing by ⁄ ,

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74

the noise temperature Ts at the input of the receiver will be;

⁄

⁄ (

⁄ )

The noise temperature T2 which takes into account the noise generated by the antenna and the feeder together

with the receiver noise, is called the system noise temperature T at the receiver input.

System Noise Temperature Example

Consider the receiving system of figure (7.21) with the following values.

-Attenuation noise temperature: ; thermodynamic temperature of the feeder; ; effective

input noise temperature of the receiver ;

The system noise temperature at the receiver input will be calculated for two cases: (1) no feeder loss between the

antenna and the receiver and (2) feeder loss . Using equation (7.25)

⁄ (

⁄ )

Case (1): T=50K+290(1-1)K+50K = 100K

Case (2): T= ⁄

⁄ =149.3K or around 150K.

Notice the influence of the feeder loss; it reduces the antenna noise but makes its own contribution to the noise

and this finally causes an increase in system noise temperature.

The contribution of attenuation the noise can quickly be estimated using the following rule: every attenuation of

0.1dB upstream of the receiver makes a contribution to the system noise temperature of ( ⁄ )=6.6K

or around 7K. to realize a receiving system with a low noise temperature, it is imperative to avoid losses upstream

of the receiver.

7.7.1 CONCLUSION

At the receiver input, all sources of noise in the link contribute to the system noise temperature T. These sources

include noise captured by the antenna and generated by the feeder, which can actually be measured at the receiver

input, plus the noise generated downstream in the receiver, which is modeled as a fictitious source of noise at the

receiver input, treating the receiver as noiseless.

The noise superimposed on the received carrier power has a power spectral density given by;

, where

k is the Boltzmann constant (k=1.379x10-2

J/K = -228.6dBJ/K)


75

7.8 INDIVIDUAL LINK PERFORMANCE

The link performance is evaluated as a ratio of the received carrier power C, to the noise power spectral density, N0

and is quoted as the ⁄ ratio, express in hertz. One can evaluate the link performance using other ratios

besides ⁄ , for instance

⁄ represents the carrier power over the system noise temperature expressed in units of watts

per kelvin (W/K), it is given by ⁄ ⁄ , where k is the Boltzmann constant

⁄ represents carrier power over the noise power; it is dimensionless, it is given by

⁄ ( ⁄ )

⁄ , where is the noise bandwidth

7.8.1 CARRIER TO NOISE POWER SPECTRAL DENSITY RATIO AT THE RECEIVER INPUT

The power received at the receiver input, as given by equation (7.14), is that of the carrier. Hence

The noise power spectral density at the same point is , where T is given by equation (7.25)

Hence

⁄ [(

⁄ ) ( ⁄ ) (

⁄ )] [(

⁄ (

⁄ ) )]

This expression can be interpreted as follows:

⁄

( ⁄ ) (

⁄ ) ( ⁄ )

⁄ Can also be express as a function of the power flux density ;

⁄ (

⁄ ) (

⁄ ) ( ⁄ )

Where

⁄

Finally it can be verified that evaluation of ⁄ is independent of the point chosen in the receiving chain as long as

the carrier power and noise power spectral are calculated at the same point.

Equation (7.27) for C/N0 introduces three factors;

EIRP, which characterizes the transmitting equipment

1/L, which characterizes the transmission medium

The composite receiving gain/noise temperature, which characterizes the receiving equipment; it is called

the figure of merit, or G/T, of the receiving equipment.

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76

By examining equation (7.26), it can be seen that the figure of merit G/T of the receiving equipment is a

function of the antenna noise temperature and the effective input noise temperature of the

receiver. These magnitudes will now be quantified.

In conclusion, equation (7.26) boils down to;

⁄ ( ⁄ )( ⁄ )( ⁄ )

7.8.2 CLEAR SKY CONDITION

Figure (7.13) shows the geometry of the link. It is assumed that the transmitting earth station is on the edge of the

3dB beamwidth coverage of the satellite receiving antenna.

The data used are given below;

-Frequency; =14GHz

For the earth station(ES);

o Transmitting amplifier power;

o Loss between the amplifier and antenna;

o Antenna diameter; D=4m

o Antenna efficiency;

o Maximum pointing error;

Earth station – satellite distance; R= 40,000Km

Atmospheric attenuation; (typical value at this frequency for elevation angle 10 )

For the satellite(SL)

o Receiving beam half power angular width;


o Receiver noise figure; F=3dB

o Loss between antenna and receiver;

o Thermodynamic temperature of the connection;

o Antenna noise temperature;

To calculate the EIRP of the earth station;

⁄


77

With =100W=20dBW, (

⁄ )

(

⁄ )

( ⁄ )

(

⁄ )

(

⁄ )

,

Hence

To calculate the upward path loss (U);

With (

)

(

)

, Hence

To calculate the figure of merit G/T of the satellite (SL);

( ⁄ )

(

⁄ )

[

⁄ (

⁄ ) ]

With (

)

(

⁄ )

( ⁄ )

(

⁄ )

, Since the earth station is at the edge of the 3dB coverage area,

⁄ ,

Assume , Given

Hence ( ⁄ )

[ ⁄ (

⁄ ) ]

Notice that when the thermodynamic temperature of the feeder between the antenna and the satellite receiver is

close to the antenna noise temperature, which is the case in practice, the uplink system noise temperature at the

receiver input is . It is therefore needlessly costly to install a receiver with a low

noise figure on board the satellite

To calculate the ratio ⁄ for the uplink;

( ⁄ )

(

⁄ ) ( ⁄ )

( ⁄ )

Hence: 71.7dBW – 207.7dB + 6.6dBK-1

+ 228.6dBJ/K =99.2dBHz.

Figure (7.14) shows the path of the signal in uplink and the power at various points

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78

7.8.2.1 CLEAR SKY DOWNLINK PERFORMANCE

Figure (7.15a) shows the geometry of the downlink. It is assumed that the receiving earth station is located on the

edge of the 3dB coverage area of the satellite receiving antenna. The data are as follows;

Frequency,

For the satellite (SL)

o Transmitting amplifier power;

o Loss between amplifier and antenna;

o Transmitting beam half power angular width;


o Earth station- satellite distance; R=40,000Km

o Atmospheric attenuation ; (typical attenuation at this frequency for an elevation of

10 )

For the earth station(ES);

o Receiver noise figure; F=1dB

o Loss between antenna and receiver;

o Thermodynamic temperature of the feeder;

o Antenna diameter; D=4m



79

o Maximum pointing error;

o Ground noise temperature;

To calculate the EIRP of the satellite

⁄

With , (

)

(

⁄ )

( ⁄ )

(

⁄ )

, since the station is at the edge,

⁄ , ,

Hence;

To calculate the downlink path loss (D);

With (

)

(

)

,

Hence;

To calculate the figure of merit G/T of the earth station in the satellite direction;

( ⁄ )

(

⁄ )

is the downlink system noise temperature at the input given by [

⁄ (

⁄ ) ]

And (

)

(

)

(

)

(

⁄ )

(

⁄ )

, ,

, with and , for which

, ,

Hence ⁄ (

⁄ )

( ⁄ )

To calculate the ( ⁄ )

(

⁄ ) ( ⁄ )

( ⁄ )

Hence ( ⁄ )

Figure 7.15b shows the clear sky downlink power variation

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80

7.9 LINK PERFORMANCE UNDER RAIN CONDITIONS

7.9.1 UPLINK PERFORMANCE

In the presence of rain, propagation attenuation is greater due to the attenuation caused by rain in the

atmosphere. This is in addition to the attenuation due to gases in the atmosphere (0.3dB). A typical value of

attenuation due to rain for an earth station situated in the temperate climate (for example Europe) can be

considered to be Such an attenuation would not be exceeded, at a frequency of 14GHz, for more

than 0.01% of an average year. This gives

Hence

Referring to the example of section 7.8.2, the uplink performance under rain conditions becomes

( ⁄ )

The ratio ( ⁄ )

for the uplink would be greater than the value calculated this way for 99.99% of an average year.


81

7.9.2 DOWNLINK PERFORMANCE

Referring now to the example of section 7.8.2.1, is taken as a typical value of the attenuation due to

rain for an earth station in the temperate climate (for example in Europe), which will not be exceeded at a typical

frequency of 12GHz, for more than 0.01% of an average year. This is .

Hence, . The antenna noise temperature is given by

⁄ (

⁄ )

Taking , ⁄ (

⁄ )

⁄ (

⁄ )

Hence( ⁄ )

,

To calculate the ratio ( ⁄ )

(

⁄ ) ( ⁄ )

( ⁄ )

Hence ( ⁄ )

= 84.7dBHz

The ( ⁄ )

ratio for the downlink would be greater than the value calculated in this way for 99.99% of an average

year.

7.9.3 CONCLUSION

The quality of a link between a transmitter and a receiver can be characterized by the ratio of the carrier power to

the noise power spectral density ⁄ . This is a function of the transmitter EIRP, the receiver figure of merit G/T

and the properties of the transmission medium. In a satellite link between two stations, two links must be

considered- the uplink, characterized by the ratio ( ⁄ )

, and the downlink, characterize by the ratio (

⁄ )

.

The propagation conditions in the atmosphere affect the uplink and the downlink differently; rain reduces the value

of the ratio ( ⁄ )

by decreasing the received power , while it reduces the value of(

⁄ )

, by reducing the

value of the received power and increases the downlink system noise temperature. Denoting the resulting

degradation by ( ⁄ ) gives

( ⁄ )

( ⁄ )

( ⁄ )

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82

7.10 OVERALL LINK PERFORMANCE WITH A TRANSPARENT SATELLITE

In this section we will discuss the station-to-station link performance that is a link involving one uplink and one

downlink via a transparent satellite. Up to now the noise on the uplink and downlink has been considered to be

thermal noise only

In practice one has to account for interference noise originating from other carriers in the considered frequency

band and intermodulation noise resulting from multi-carrier operation of non-linear amplifiers.

We will first discuss overall link performance without interference or intermodulation. Then overall link

performance is discussed considering interference and finally intermodulation.

The following notations are used;

( ⁄ )

is the uplink carrier power to noise power spectral density ratio (Hz) at the satellite receiver

input, considering no other noise contributions than the uplink system thermal noise temperature .

( ⁄ )

is the downlink carrier power to noise power spectral density ratio(Hz) at the input of the earth

station receiver, considering no other noise contributions than the downlink system thermal noise

temperature .

( ⁄ )

Carrier power to interference noise power spectral density ratio (Hz) at the input of the

considered receiver.

( ⁄ )

Carrier power to intermodulation noise power spectral density ratio (Hz) at the output of the

considered non-linear amplifier.

( ⁄ )

Overall carrier power to noise power spectral density ratio (Hz) at the earth station receiver

input.

7.10.1 CHARACTERISTICS OF THE SATELLITE CHANNEL

Figure (7.16) shows a transparent payload, the overall bandwidth is split into several sub bands, amplified by a

dedicated power amplifier. The amplifying chain associated with each sub-band is called a satellite channel, or

transponder. The satellite channel amplifies one or several carriers. Here are some notations;

carrier power at the satellite receiver input, at saturation it is denoted

is the power at the input of the satellite channel amplifier (i=input, n=number of carriers)

power at the output of the satellite channel amplifier ( o=output, n=number of carriers)

single carrier operation of a satellite channel amplifier

power at the input to the satellite channel amplifier at saturation in single carrier operation

power at the output of the satellite channel amplifier at saturation in single carrier operation

mode


83

Saturation refers to the operation of a satellite channel amplifier to produce maximum output power in

single carrier operation mode. The Operator provides characteristics values of a satellite channel in terms

of flux density at saturation, , and EIRP at saturation, .

7.10.2 SATELLITE POWER FLUX DENSITY AT SATURATION

The power flux density is provided by the transmit station and considered at the satellite receive antenna.

The nominal value of power flux density to drive the satellite channel amplifier at saturation is given by

( ⁄ )

is the front end gain from the input of the satellite receiver to the input of the satellite channel amplifier; is

the loss from the output of the satellite receive antenna to the input of the satellite receiver and is the

satellite receive antenna maximum gain.

The formula assumes that the transmit station is located at the center of the satellite receive coverage.

In practice, the flux density to be provided from a given earth station to drive the satellite channel amplifier to

saturation depends on the location of the transmit earth station within the satellite coverage and the polarization

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84

mismatch of the satellite receiving antenna with respect to the uplink carrier polarization. If the receive satellite

antenna gain in the direction of the transmit earth station experiences a gain fallout , from the maximum gain

and polarization mismatch of , then the actual flux density is

7.10.3 SATELLITE EIRP AT SATURATION

The satellite EIRP at saturation and at boresight, relates to the satellite channel amplifier output power

at saturation, and is given by

Where is the loss from the output of the power amplifier to the transmit antenna and is the transmit

antenna maximum gain

In practice, the , which conditions the available carrier power at a given earth station receiver input is

reduced by the transmit antenna gain fallout , when the earth station is not located at the center of the satellite

transmit antenna coverage.

7.10.4 SATELLITE REPEATER GAIN

The satellite repeater gain, , is the gain from the satellite repeater input to the satellite channel amplifier

output. At saturation it is called

Where the satellite channel amplifier is gain and is the gain from the receiver input to the satellite channel

amplifier input.

7.10.5 INPUT AND OUTPUT BACK-OFF

In practice, the satellite channel amplifier is not always operated at saturation and it is convenient to determine the

operating point Q of the satellite channel amplifier. The point Q is determined by the input power and the

output power . It is also convenient to normalize these quantities with respect to and

respectively. Below are definitions of input back-off and output back-off.

⁄


85

⁄

We will leave out the subscript Q for the operating power from now.

7.10.6 CARRIER POWER AT THE SATELLITE RECEIVER INPUT

The carrier power at the satellite receiver input required to drive the satellite channel amplifier to operate at the

considered operating point Q is given by

Expressing carrier in terms of satellite channel amplifier output power, we have

With being the satellite channel amplifier gain at saturation, can be expressed as

Where

, is the carrier power at the satellite receiver input to drive the satellite

channel amplifier at saturation. Can also be expressed as a function of ;

Note that input back-off can also be expressed as a ratio of the power flux density required to operate the

satellite channel amplifier at the considered operation point to the power flux density at saturation

7.10.7 EXPRESSION FOR ( ⁄ )

WITHOUT INTERFERENCE FROM OTHER SYSTEMS OR

INTERMODULATION

The power of the carrier received at the input of the earth station receiver is . The noise at the input of the earth

station receiver correspond to the sum of the following

The downlink system thermal noise considered in isolation ( , given by equation(7.25), which

defines the ratio ⁄ for the downlink (

⁄ )

can be calculated as )

The uplink noise retransmitted by the satellite

Hence ( ⁄ )

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86

Where

⁄ is the total power gain between the satellite receiver input and the earth station

receiver input. G takes into account the satellite repeater gain from the input to the satellite receiver to the

output of the satellite channel amplifier; the gain

⁄ of the satellite transmit antenna including the gain

fallout and the loss from the output of the power amplifier to the transmit antenna; the downlink path loss

and the receiving station composite gain

⁄ . This gives

( ⁄ )

⁄

⁄

⁄

In the above expression, the term represents the carrier power at the satellite receiver input. Hence

⁄ ( ⁄ )

, finally ( ⁄ )

( ⁄ )

( ⁄ )

In this expression;

( ⁄ )

⁄

⁄

⁄ ( ⁄ )

( ⁄ )

(

⁄ ) ( ⁄ )

( ⁄ ) ( ⁄ )

( ⁄ )

and (

⁄ )

are the values of ⁄ for the uplink and downlink when the satellite channel operates

at saturation. represents the downlink attenuation and ( ⁄ )

, the figure of merit of the earth station in the

satellite direction.

7.10.8 EXPRESSION FOR ( ⁄ )

TAKING ACCOUNT OF INTERFERENCE AND

INTERMODULATION

Intermodulation and interference where explained in chapter four. When both effects are taking in to account, we

have

( ⁄ )

( ⁄ )

( ⁄ )

( ⁄ )

( ⁄ )


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CHAPTER 8 OPTIMIZATION

In chapter four we saw the different types of noise that affect a communications link. In chapter five we saw the

atmospheric impairments on a communication link. Chapter 6 presented the different modulation techniques used

to transmit information in satellite communications link and how this techniques together with channel coding help

improve the performance of a satellite communications link. Chapter 7 presented means of evaluation satellite

communications link performance.

This chapter focuses on the various means of optimizing the performance of a fixed satellite link. Some of them can

be applied to mobile satellite link but our focus on fixed end- to-end link.

By optimization we mean providing network with improve reliability and high capacity service. There are basically

two groups of techniques; Power restoral techniques and Signal modification techniques.

Most of these techniques play on the link margin to ensure availability of service.

Before looking at these techniques, let us talk a little on link margin.

8.1 LINK MARGIN

All satellite links are design to function at a certain annual availability. The closer to 100% we demand of our link

availability, the more link margin we need to meet this demand.

Design specifies a value of ⁄ greater or equal to (

⁄ )

during a given percent of time, equal to

(100-p%). For example, 99.99% of time implies p=0.01%. As seen in chapter 7, the attenuation due to rain

causes a reduction of the ratio ⁄ given by

( ⁄ )

(

⁄ )

for uplink and

( ⁄ )

(

⁄ )

( ⁄ ) for the downlink

( ⁄ ) ( ⁄ )

( ⁄ )

Represents a reduction (in dB) of the figure of merit due to increase of noise

temperature

For a successful design (system), one must have a ( ⁄ )

(

⁄ )

This can be achieve by including a margin in the clear sky link budget with defined as

( ⁄ )

(

⁄ )

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88

8.2 POWER RESTORAL TECHNIQUES

These techniques optimize the link without touching the basic signal format. They include;

Beam diversity

Power control

Site diversity

8.2.1 BEAM DIVERSITY

The receive power density on the satellite downlink can be increased during path attenuation by switching to a

satellite antenna with a narrower beamwidth. The narrower beamwidth correspond to a higher antenna gain,

concentrating the power onto a smaller area on the earth surface, resulting in higher EIRP at the ground terminal

undergoing the path attenuation. This is shown in figure (8.1) below

The increase in EIRP can be very significant as displayed in figure (8.2) .

For example the use of the metropolitan spot beam antenna in place of CONUS antenna will provide 24.1dB of

additional EIRP.


89

8.3 POWER CONTROL

The objective of power control is to vary the transmit power in direct proportion to the attenuation on the link, so

that the received power stays constant during severe fade. We can have uplink power control and downlink power

control.

8.3.1 UPLINK POWER CONTROL

Provide a direct means of restoring the link the uplink signal during rain attenuation events. Two types of power

control can be implemented, closed loop and open loop power control systems

8.3.1.1 CLOSED LOOP

In a closed loop system, the transmit power level is adjusted directly as the detected received signal level at the

satellite, returned via a telemetry link back to the ground, varies with time. Control rages of up to 20dB are possible

and response time can be nearly continuous if the telemetered received signal level is available on a continuous

basis. This is shown below in figure 8.3a.

8.3.1.2 OPEN LOOP

In an open loop power control system, the transmit power level is adjusted by operation on a radio frequency

control signal that itself undergoes path attenuation and is used to infer the attenuation experience on the uplink.

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90

This control signal is called Beacon and is sometimes at the same frequency as the uplink. The system is shown in

figure 8.3b

8.4 SITE DIVERSITY

It describes the use of geographically separate ground terminals in a space

communication link to overcome the effect of downlink path attenuation during

intense rain period. It improves overall link performance by taking advantage of

the limited size and extent of intense rain cells. This is shown in figure (8.4)


91

8.5 SIGNAL MODIFICATION TECHNIQUES

This involves optimization techniques in which the basic format of the signal is modified.

Space segment costs are typically the most significant operating expense for any satellite-based service, having

direct impact on the viability and profitability of the service. A satellite transponder having finite resources in terms

of bandwidth and power, the transponder leasing costs are determined by bandwidth and power used. For optimal

utilization, a satellite circuit should be design to use similar share of transponder bandwidth and transponder

power.

The traditional approach to balancing a satellite circuit involves trade-off between modulation and coding. A lower

order modulation requires less transponder power at the expense of more bandwidth. Conversely a higher order

modulation reduces required bandwidth, but at a significant increase in power.

Some of the new dimension optimization techniques of satellite communications are; DoubleTalk carrier-in-carrier

(CnC), Adaptive Coding and Modulation (ACM)

8.5.1 OPTIMIZATION BY DOUBLETALK CARRIER-IN-CARRIER

This innovative technology provides a significant improvement in bandwidth and power utilization, beyond what is

possible with traditional with forward error correction (FEC) and modulation alone, allowing users to achieve

unprecedented savings. When combined with advanced modulation and FEC, it allows for multi-dimensional

optimization

o Reducing Operational Expenses(OPEX)

Occupied bandwidth and transponder power

o Reducing Capital Expenditure (CAPEX)

BUC/HPA/ size and antenna size

o Increasing throughput without using additional transponder resources

o Increasing link availability (margin) without using additional transponder resources

o Or a combination to meet different objectives

DoubleTalk Carrier-in-carrier bandwidth compression is based on patented “Adaptive Cancellation” technology that

allows the transmit and receive carriers of a duplex link to share the same transponder space. Figure 8.5a shows

the typical full duplex satellite link, where the two carriers are adjacent to each other. Figure 8.5b shows the

DoubleTalk carrier-in-carrier where the two carriers are overlapping, thus sharing the same spectrum.

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92

DoubleTalk carrier-in-carrier is complementary to all advancement in technology, including advanced FEC and

modulation techniques. As these technologies approach theoretical limits of power and bandwidth efficiencies,

DoubleTalk carrier- in-carrier utilizing advanced signal processing techniques provides a new dimension in

bandwidth and power efficiency.

DoubleTalk carrier-in-carrier allow users to achieve spectral efficiency (bps/Hz) that cannot be achieved with

modulation and FEC alone, example when used with 16-QAM, it approaches the bandwidth efficiency of 256-QAM

(8bps/Hz).

As DoubleTalk carrier-in-carrier allows equivalent spectral efficiency using a lower order modulation and/or FEC

code, it can simultaneously reduce CAPEX by allowing the use of a smaller BUC/HPA and/or antenna

As DoubleTalk carrier-in-

carrier can be used to save

transponder bandwidth

and/or transponder power, it

has been successfully

deployed in bandwidth-

limited as well as power-

limited scenarios.


93

8.5.6 DOUBLE TALK CARRIER-IN-CARRIER CANCELLATION PROCESS

In traditional full duplex satellite connection between two sites, separate satellite channels are allocated to each

direction. If both directions transmitted on the same channel, each side would normally find it impossible to extract

the desired signal from the aggregate, due to interference resulting from its local oscillator. However since this

interference is produced locally, it is possible to estimate and remove its influence prior to demodulation of the

data transmitted from the remote location.

DoubleTalk carrier-in-carrier achieves state-of-art performance by combining the latest signal processing

technology. It continually estimates and tracks all parameter difference between the local uplink signal and its

image within the downlink signal. Through advanced adaptive filtering and phase locked loop implementation, it

dynamically compensates for this difference by appropriately adjusting the delay, frequency, phase and amplitude

of the sampled uplink signal. The result is excellent cancellation performance.

For the Double Talk carrier-in-carrier it is necessary to provide each demodulator with a copy of its local modulator

output. Figure 8.7 shows the actual movement of signals in this network.

The interference cancellation algorithm uses the composite signal and local copy of S1 to estimate the necessary

parameters of scaling, delay offset and frequency offset.

DoubleTalk carrier-in-carrier can only be used for full duplex link where the transmitting earth station is able to

receive itself. Maximum savings is generally achieved when the original link is symmetric in data rate.

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94

8.6 ADAPTIVE CODING AND MODULATION (ACM)

Adaptive coding and modulation is a statistical, non-static advantage that enables dynamic changes in user

throughput. Benefit and value vary over time and are not guaranteed, but are predictable.

ACM turns fade margin into increased link capacity- gains of 100% or more are possible, compared to

traditional constant coding and modulation (CCM). This is accomplished by automatically adapting the

modulation type and FEC code rate to give highest possible throughput.

ACM maximizes throughput regardless of Link conditions (noise or other impairments, clear sky, rain fade,

etc.). Initial setup is easy, and then requires no further human intervention.

With a CCM system, severe rain fading can cause the total loss of the link, and zero throughput. ACM

keeps the Link up(with lower throughput)-and can yield much higher system availability

It is currently used for IP traffic only.

All satellite links are design to function at a certain annual availability. The closer to 100% we demand of our link

availability, the more link margin we need to meet this demand. Figure 8.8a below is a graph of availability vs. link

margin of a Ku-Band link from Germany to Nigeria. A change in guaranteed annual availability from 99.8% to 99.6%

(as little as 0.2% per year) equates to 17.5 hours per year(365Days*24Hours/day*0.02=17.5Hours).

In this link, it can be seen that this 17.5hours/year demands or saves 2.5dB of link margin. This means that

someone who requires 99.8% availability instead of 99.6% would need an additional 2.5dB link margin for the

entire year. Conversely, deciding to run this link with 99.6% would save 2.5dB of link margin for the entire year.


95

Different links have different link margin requirements. Consider the C-Band link between Italy and China with

different link availability characteristics. From figure 8.8b you can clearly see that the same change from 99.6%

availability to 99.8% availability requires a mere 0.35dB of additional link margin.

Because ACM converts link margin into additional user throughput, it can be clearly seen that the greater the link

margin, the greater the benefit of ACM. As link margin is reduced, so too is ACM. I t can also be stated that as

guaranteed availability is increased, link margin will also need to be increased. Conversely as the guaranteed

availability is reduced, link margin will also need to be reduced and the value of ACM will therefore be reduced.

8.6.1 ACM BACKGROUND

The primary function of ACM is to optimize throughput in a wireless data link, by adapting the modulation order

used and the forward error correction(FEC) code rate(which both directly affects spectral efficiency), according to

the noise conditions (or other impairments) on the link.

The implicit in this concept is that the symbol rate (and power) of the wireless communication system must remain

constant. This ensures that the bandwidth allocated for a particular link is never exceeded. Given that the symbol

rate does not change, if modulation and coding are changed, the data rate must therefore be modified.

This is expressed in the simple equation: symbol rate = bit rate/(modulation order*code rate)

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96

Rearranging we have; bit rate = symbol rate*modulation order*code rate

Therefore in changing to a higher order modulation or code rate, the bit rate is increased, and in changing to a

lower order modulation or code rate, the bit rate is reduced.

8.6.2 REQUIREMENTS FOR ACM

a. A modulator and FEC encoder that can instantaneously, when commanded, change either modulation type

(order) or FEC encoder rate, or both. This need to be accomplished without the corruption of data

anywhere in the path. Block FEC codes are considered to be the most practical in achieving the required

synchronization. Recently, a specific nomenclature has emerged to describe a combination of modulation

type and code rate-namely, ModCod. The modulator is required to send the value of the ModCod at the

start of each code block to signal to the demodulator/decoder how to configure the correct modulation

type and FEC code rate.

b. A receiver that is capable of demodulating and decoding the signal transmitted by a) without any prior

knowledge of when a change has taken place, but based purely on the value of the ModCod seen at the

start of each FEC block. Again this need to be accomplished without the corruption of data anywhere in

the path.

c. The receiver in b) need to derive an estimation of the link quality (in terms of

⁄ , , etc.) and then

communicates this estimate, via a return channel, to the modulator in a)

d. The modulator in a) need to be able to process the link metric form the demodulator in b), and then,

based upon a predetermined algorithm, adapt the data rate and change the ModCod sent to the receiver

at the distant end. Thus,

the data rate on the link

can be maximized, given

the current link noise

conditions

A generic example of ACM

over satellite is shown in

figure 8.9a and 8.9b below.


97

9.0 GENERAL CONCLUSION

Satellite communications as we have seen is highly affected by propagation impairments at the atmosphere, non-

linearity of the satellite channel, thermal noise and interference. We saw that the traditional way of overcoming

these effects is by increasing the link margin, during fade conditions.

The Power restoral technique which we looked at tries to maintain the link in presence of fade conditions by

increasing the

⁄ , to the required value. Some of these techniques can be costly in CAPEX; installing a new site

(site diversity), multiple antennas onboard the satellite (beam diversity) for example.

Advances in modulation, coding gain, fade adaptation and carrier cancelling technologies can provide substantial

saving in bandwidth, improve capacity, improve reliability, or all three while maintaining contracted service level

agreements (SLAs). These as we have seen can be realize using DoubleTalk carrier-in-carrier and Adaptive coding

and modulation.

The second technology; Adaptive Coding and Modulation help us to maintain our link in all conditions and greatly

increase throughput in clear sky conditions.

BIBLIOGRAPHIC REFERENCES

1- Satellite communications systems by 5th

edition by Gerard Maral and Michel Bosquet

2- Satellite communications systems engineering by Louis J. IPPOLITO

satellite communications link optimization

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