Optimisation of Data Throughput for Leo Satellite Downlink Using Trellis Coded Modulation.
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
TITLE PAGE i
CERTIFICATION AND APPROVAL ii
DECLARATION iii
DEDICATION iv
ACKNOWLEDGEMENT v
TABLE OF CONTENTS vi
LIST OF TABLES ix
LIST OF FIGURES xi
LIST OF ACRONYMS xiv
ABSTRACT xvi
CHAPTER ONE GENERAL INTRODUCTION
- Introduction1
- Motivation 1
- Problem Statement 2
- Objective of the Study 2
- Significance of the 5Scope of Study and Methodology 2
CHAPTER TWO LITERATURE REVIEW
- Low Earth Orbit and Ground Track 4
- Geometry of LEO Satellite Visibility 8
- Downlink Budget Analysis of a LEO Satellite 12
- Basic Digital Modulations for SatelliteCommunications16
- Probability of Error in BPSK and QPSK Schemes 20
- Error Control and Coding Gain 25
- Spectral Efficiency in Power and Bandwidth Limited Link 26
- Reviews on Past Publications on Optimisation of LEO satellite Downlink 29
- Adaptive Variable Data Rate: An IEEE Paper by A Matar 29
- Optimisation Through Hybrid-ARQ: An IEEE Paper from University of Surrey 31
- 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
3.5.1 Design of Multiple State Encoders 66
CHAPTER FOUR COMPUTER SIMULATION FOR CODING GAIN
- Introduction 89
- MATLAB Bit Error Rate Analysis Tool (bertool) 90
- Simulations of MPSK Theoretical Links 93
- MATLAB Template for Downlink Mode 100
- Building MATLAB Downlink Block Model 102
- Settings for Monte Carlo Simulation 105
- Simulations of MPSK Downlink Block Models 106
- Simulations of TCM Block Downlink 115
- Discrete Simulations of Multi-State TCM/8-PSK Encoders 120
- Modification and Simulation of the Conceptual Model 127
- Data ThroughputMeasurements for Optimisation 129
CHAPTER FIVE ANALYSIS OF SIMULATION RESULTS
- Performance of Convolution coded MPSK on Theoretical Links 134
- Performance of Convolution coded MPSK on Downlink Block Models 135
- Performance of TCM/MPSK on Downlink Block Models 137
- Discrete Performance of Multi-StateTCM/8-PSK Encoders139
- Performance of Multiplexed Multi-State TCM/8-PSK Encoders 140
- Data Throughput Comparison and Optimisation on LEO Downlink Block Models 141
CHAPTER SIX CONCLUSION AND RECOMMENDATION
REFERENCES146
APPENDICES
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.
REFERENCES
James R. Wertz and Wiley J. Carson, Space Mission Analysis and Design, Third Edition, Published by Microcosm Press TorrenceCalifornia 1999.
Denis Roddy, Satellite Communications, Third Edition, Published by MacGraw Hill Companies, Inc . 2001.
Timothy Pratt, Charles Bostian and Jeremy Allnutt, Satellite Communications,Published by John Wiley & Sons Inc.2003.
JohnG. Proakis, Digital Communications, Published by McGraw Hills Companies Inc.2001.
Peter Fortescue and John Stark, Spacecraft Systems Engineering Second Edition, Published by Wiley and Sons. 2001.
John G. Proakis and MasoudSalehi, Communication Systems Engineering,Published by Pearson Prentice Hall. 2004.
A Matar, Throughput /Channel Capacity Improvement Issues for LEO Satellite Based Remote Sensing Systems, IEEE Publication 2003.