ABSTRACT
The line of sight communication between an orbiting LEO satellite and
its Ground Station is time constrained by duration of visibility and
number of visible passes in a day. In Equatorial region like Nigeria, the
average duration is short and number of passes is very few. In most
cases, the satellite size is small and therefore power generated on-board
is also small, consequently the downlink budget is power constrained.
Transmission bandwidth is also constrained by Regulations and need for
cost effective RF design. High resolution remote sensing LEO satellite
that acquires large data to be downloaded to a Ground station requires
high capacity downlink at minimum power that guarantee BER of 10-6.
Application of high level MPSK for high data rate requires more power to
reduce transmission errors. Convolution or Block codes when used for
Error Correction adds overhead bits to the detriment of the link capacity
and without significant coding gain for a power limited downlink. Trellis
Coded Modulation (TCM) developed in 1982 by Ungerboeck et el,
though still adds overhead bits, but generates significant coding gain
that can be extended up to 6dB.This Thesis seeks to optimise LEO
Satellite Downlink that is Time, Power and Bandwidth constrained,
through trade-off between the coding gains of TCM for bandwidth
efficient high level MPSK schemes.
TABLE OF CONTENTS
TABLE OF CONTENTSvi
LIST OF TABLESix
LIST OF FIGURES
xi
LIST OF ACRONYMS
xiv
ABSTRACTxvi
CHAPTER ONE
GENERAL INTRODUCTION
1.0 Introduction1
1.1 Motivation
11.2 Problem Statement 2
1.3 Objective of the Study 2
1.4 Significance of the Study21.5Scope of Study and Methodology
2
CHAPTER TWO
LITERATURE REVIEW
2.0 Introduction
4
2.1 Low Earth Orbit and Ground Track
4
2.2 Geometry of LEO Satellite Visibility
8
2.3 Downlink Budget Analysis of a LEO Satellite
12
2.4 Basic Digital Modulations for SatelliteCommunications16
2.5 Probability of Error in BPSK and QPSK Schemes
20
2.6 Error Control and Coding Gain
25
2.7 Spectral Efficiency in Power and Bandwidth Limited Link
26
2.8 Reviews on Past Publications on Optimisation of LEO satellite
Downlink 29
7
2.8.1 Adaptive Variable Data Rate: An IEEE Paper by M.A Matar
29
2.8.2 Optimisation Through Hybrid-ARQ: An IEEE Paper from
University of Surrey 31
2.8.3 Adaptive Communication System for Implementation on Board a
Future Algerian LEO Satellite, a 2007 IEEE Paper
34
CHAPTER THREE
TRELLIS CODED MODULATION AND THE CONCEPTUAL MODEL
3.0 Introduction 37
3.1 Euclidean Distance in MPSK Schemes 38
3.2 Convolution Coding Technique 39
3.2.1 Trellis Diagram 41
3.2.2 Coding Gain in Convolution Coding 43
3.3 Set Partition Theory
46
3.4 Trellis Cod The ed Modulation (TCM) Scheme 48
3.4.1 Implementation of TCM/8-PSK 50
3.4.1 Coding Gain in Trellis Coded Modulation 54
3.4.3 High Gain TCM/8-PSK Schemes 56
3.5 The Conceptual Model: Integrated Multi-State TCM 64
3.5.1 Design of Multiple State Encoders 66
CHAPTER FOUR
COMPUTER SIMULATIONFOR CODING GAIN
4.0 Introduction 89
4.1 MATLAB Bit Error Rate Analysis Tool (bertool) 90
4.2 Simulations of MPSK Theoretical Links 93
4.3 MATLAB Template for Downlink Mode
100
4.4 Building MATLAB Downlink Block Model
102
4.4.1 Settings for Monte Carlo Simulation 105
8
4.5 Simulations of MPSK Downlink Block Models
106
4.6 Simulations of TCM Block Downlink
115
4.7 Discrete Simulations of Multi-State TCM/8-PSK Encoders
120
4.8 Modification and Simulation of the Conceptual Model
127
4.9 Data ThroughputMeasurements for Optimisation 129
CHAPTER FIVE
ANALYSIS OF SIMULATION RESULTS
5.0 Introduction
133
5.1 Performance of Convolution coded MPSK on Theoretical Links
134
5.2 Performance of Convolution coded MPSK on Downlink Block
Models
135
5.3 Performance of TCM/MPSK on Downlink Block Models
137
5.4 Discrete Performance of Multi-StateTCM/8-PSK Encoders139
5.5 Performance of Multiplexed Multi-State TCM/8-PSK Encoders 140
5.6 Data Throughput Comparison and Optimisation on LEO Downlink
Block Models
141
CHAPTER SIX
CONCLUSION AND RECOMMENDATION
144
REFERENCES146
APPENDICES
9
A. Pulse Shaping and Nyquist Filters148
B. Analysis on High Power Amplifier 155
CHAPTER ONE
GENERAL INTRODUCTION
1.0 Introduction
Aremote sensing satellite is launched into Low Earth Orbit (LEO) to
acquire data from the Earth surface. When viewed from the Earth,the
satellite in LEO does not appear stationaryas in the case of a satellite in
Geostationary orbit (GEO), instead it is observed to rise from horizon
and travelalong its orbituntil it set in the other side ofhorizon.
Telemetry and acquired data on the satellite are communicated to
Earth or Ground Station via Downlink, while command and control signal
from the Ground Station are communicated to the satellite via Uplink.
Both the Downlink and Uplinkare established onlywithin line of
sight.Because of that antenna dish of the Ground Station tracks LEO
satellite from horizon to horizon. The number of time a LEO satellite
passes over a Ground Station are few in a day and its durationsusually
last few minutes depending on altitude of the orbit and latitude of the
Ground Station .
Selection of orbit altitude is determined by satellite mission and Ground
stations located in Equatorial region haveshorter duration and less
number of satellite passes in a day than those in high latitude. There are
various forms of low earth orbit orientation, but in general all have limited
duration of visibility and number of passes in a day. [1]
In most remote sensing missions, LEO satellites are designed to be
small in size which limits on-board power generation, consequently the
downlink and other satellite subsystems are power limited. Also most
LEOsatellites are designed to be cost effective, for that reason wide
bandwidth are avoided in the communication subsystem.
In a Downlink digital communicationdata bits are subjected to error
control or channel coding before modulation. This is necessary in low
power link and noisy channel to improve reliability of received data.
However, coding in general requiresoverhead bits, which reduces data
throughput of the link or requires additional bandwidth.To conserve
bandwidth at improveddata throughput some high level digital
modulations with good spectral (bandwidth) efficiencyare used, but this
also requires more transmission power.
1.1 Motivation
When Nigeria launched its first remote sensing satellite Nigeriasat-1, in
September, 2003,it was launched into Low Earth Orbit of altitude 686Km
and inclination of 98.19°, with mission objective to obtain optical image
of the Earth surface at 32m resolution. A Ground station for the Satellite
was built in Abuja, which experiences two or three communication
passes daily.Many of the daily passes are too short for communication
18
purpose, the maximum duration was 10 minutes, which occurred at 5
days intervals.
A low power, high rate transmitter of 8Mbps was used to download
image taken from any part of the globe. Optimisation of downlink
capacity for LEO Satellite is therefore important in Nigeria andEquatorial
region
1.2Problem Definition
The downlink of Nigeriasat-1 is constrained in power, bandwidth, and
communication time. Daily data throughput on the downlink
transmissions was not adequate to download the whole contents of the
image recorder, the contents have to be fragmented and downloaded
over many days.
In LEO satellite communications convolution coding and Multi-Phase
Shift Keying (MPSK) are two preferred techniques used for error control
coding and digital modulation respectively [1, 11]. Convolution coding
has maximum gain of 3 dB which is not sufficient for the problem stated.
Trellis Coded Modulation (TCM) is a technique that applies coding and
modulation differently from conventional communication systems. It
combined convolution coding and MPSK concurrently to yield high
coding gain, while maintaining high spectral efficiency of MPSK.
1.3 Objective of the Study
This research work seeks to harness communication time, coding gain
and spectral efficiency in digital communications for optimisation of data
throughputby:
· Evaluationof orbital factors that restrict visibility of LEO satellite
to maximise communication time in Equatorial region.
· Evaluationand simulation of the limitation ofconvolution codes
in LEO satellite downlink.
· Evaluation and simulation ofthe power constrain for using
higher MPSK in LEO satellite downlink.
· Evaluation and Simulation of thepotentials of TCM/MPSK for
optimisation of data throughput in LEO satellite downlink.
1.4 Significance of the Study
Technological trend in remote sensing satellite is moving toward using
high resolution sensor or camera thisimplied increased downlink data
size.[1] Optimisation of downlink data throughputtherefore is imperative
in the communication subsystem offuture remote sensing satellites,
especially for Earth Stations in regions that have few and short
communication passes.
19
It is also expected that results of this research thesiswouldbe useful
inother wireless terrestrial transmission that are constrained in power,
bandwidth and communication time.
1.5Scope of the Studyand Methodology
The Shannon–Hartley Theorem stated that for error free communication
the capacity of a transmission linkcan be increased by increasing its
signal-to-noise ratio (SNR) and/or bandwidth. In LEO satellite downlink
both the transmission power and bandwidth are limited. This work will
seek apparent power and bandwidth from TCM/MPSK to increase the
capacity of the downlink.
This research workwill confine itself to the analytical method and
MATLAB simulations of various downlink modelsto harness coding gain
and spectral efficiency for the optimisation.The approach is similar to
some past researches on optimisationofLEOsatellite downlink in which
modulations, error control codings and other communication protocols
were harnessed. [7, 8, 9,10]
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