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.
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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
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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.
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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]
This material content is developed to serve as a GUIDE for students to conduct academic research
OPTIMISATION OF DATA THROUGHPUT FOR LEO SATELLITE DOWNLINK USING TRELLIS CODED MODULATION.
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