characterization of the equatorial f2-region...
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CHARACTERIZATION OF THE EQUATORIAL F2-REGION PLASMA
DRIFT USING DOPPLER INTERFEROMETRY
AT PARIT RAJA
ABDULL ZUBI BIN AHMAD
A thesis submitted in fulfilment of the requirement
for the award of the degree of
Doctor of Philosophy (Electrical)
Faculty of Electrical and Electronic Engineering
Universiti Tun Hussein Onn Malaysia
NOVEMBER 2013
v
ABSTRACT
The ionosphere has long been used as a medium for long-distance transmission
before the advent of satellites. There has now been a resurgence in its research after
events that have occured in Iraq and Aceh where, ground-based infrastructures are
destroyed from the effects of war or natural disasters. The bulk of research work in
this area of study which uses the ionosphere as a channel medium comes from
advanced countries representing the midlatitude regions of the world. It is therefore
the objective of this thesis to characterize the behaviour of the ionosphere which is
more representative of the equatorial regions. This work presents the measurements
of plasma drifts from the ionospheric F2-layer at the equatorial station of Parit Raja
(1° 52' N and 103° 48' E) using the technique of Doppler interferometry. Analysis is
carried out from data gathered during periods of low solar and geomagnetic activities
of 2005 with a view to statistically model or characterize its properties. As a result of
plasma drift, Doppler effects are observed on the reflected echoes due to scattering
from the irregular and non-uniform reflection layer. An approximation for the
ionospheric F2-layer as a non-selective flat Rayleigh channel is first developed using
an FIR filter that follows a Jakes’ Doppler channel response with the assumptions
that multipaths are not resolvable and appear as one at the receiver with uniform
Doppler rate. Since the actual ionosphere is non-flat with frequency-selectivity and
also exhibits time-variability, a more realistic modelling of the ionospheric structure
is needed. This is achieved by subdividing the operating bandwidth of the
ionospheric medium into an aggregate of narrowband and orthogonal subchannels,
where each is narrowband enough to possess flat Rayleigh fading. By adopting this
technique of multicarrier transmission, each subchannel can be considered as flat and
uniformly time-varying, which can be practically implemented using an IIR filter.
The effect of variable Doppler rates is addressed by employing the techniques of
upsampling and interpolation. The channel modelling is based on the Jakes Doppler
response for 35 KHz in bandwidth which is more appropriate for equatorial region as
opposed to ITU-R F.1487 that adopts the Gaussian spectral response with 10 KHz of
bandwidth for midlatitude regions. Empirical simulation of results for the developed
models using measured data and the performance measures have been reasonably
accurate in characterizing the channel as having a Jakes Doppler response with
Rayleigh fading within limits of +/-2 Hz Doppler shifts or +/-100 m/s plasma drifts.
vi
ABSTRAK
Penggunaan ionosfera bagi penghantaran isyarat secara jarak-jauh telah lama wujud
sebelum penggunaan satelit. Pengkajian mengenainya di dunia ketiga menjadi lebih
rancak susulan bencana di Iraq dan Aceh di mana berlakunya kerosakan besar-
besaran infrastruktur komunikasi atas bumi. Walaubagaimanapun, sebahagian besar
kerja-kerja penyelidikan yang dilakukan mengenai penggunaan ionosfera adalah
hasil daripada negara-negara maju yang terletak di latitud pertengahan dunia. Oleh
itu, satu pengamaatan baru diperlukan bagi tujuan yang sama bagi negara-negara
berdekatan khatulistiwa. Tesis ini membentangkan ukuran aliran plasma dari lapisan-
F2 ionosfera di stesen khatulistiwa Parit Raja (1°52' N dan 103°48' E) menggunakan
teknik interferometri Doppler. Analisis data yang dikumpulkan dilaksanakan semasa
tempoh aktiviti suria dan kuasa magnet bumi rendah bagi tahun 2005. Ini adalah
untuk menentukan struktur ionosfera bagi tujuan pemodelan secara statistik ke atas
ciri-cirinya yang mewakili keadaan tempatan negara ini. Disebabkan plasma yang
bergerak, kesan Doppler juga dirasakan pada gema pergi balik di tanah. Buat
permulaan, satu anggaran bagi lapisan-F2 ionosfera tempatan akan dilakukan dengan
tanggapan isyarat yang melalui saluran Rayleigh diajukan dengan andaian yang
pantulan berselerak tidak dapat diselesaikan dan kelihatan sebagai satu isyarat di
penerima. Ini dilakukan menggunakan tapisan FIR yang bertindak mengikuti saluran
Doppler Jakes. Secara praktiknya, saluran ionosfera juga mempamirkan
kepelbagaian masa dan pemilihan frekuensi. Untuk itu, satu keperluan bagi
menggabungkan faktor-faktor ini harus diambil kira di dalam peragaan sebenar.
Untuk peragaan yang lebih realistik, pembahagiaan keseluruhan jalur frekuensi yang
melalui medium ionosfera terpilih ke dalam satu agregat jalur sempit dan saluran-
saluran ortogonal digunakan, di mana setiap saluran adalah jalur sempit dan
mewakili sepenuhnya Rayleigh memudar dalam julat frekuensinya. Dengan ini,
saluran ionosfera boleh dianggap sebagai tidak mempunyai kepelbagaian masa,
seperti kes di dalam peragaan pertama, kecuali sekarang teknik penghantaran
pelbagai karrier digunakan. Ini dilakukan menggunakan tapisan IIR berdasarkan
kepada ketumpatan kuasa spektra Jakes. Hasil dari kajian yang dilakukan dengan
menggunakan data yang dikumpulkan telah berjaya memodelkan lapisan ionosfera
tempatan sebagai mengikuti spektra Jakes yang mewakili saluran Rayleigh di dalam
lingkungan +/-2 Hz peralihan Doppler yang bersamaan +/-100 m/s kelajuan plasma.
vii
CONTENTS
CHAPTER TITLE PAGE
TITLE i
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES xiii
LIST OF FIGURES xiv
LIST OF ABBREVIATIONS
AND ACRONYMS
xxi
LIST OF SYMBOLS xxiii
LIST OF APPENDICES xxvi
CHAPTER I INTRODUCTION
1.1 Introduction
1.2 Background
1
2
1.3 Ionospheric Research in Malaysia
1.4 Problem Statements
4
5
1.5 Motivations for Using Ionospheric Channel 6
1.6 Aims and Objectives of Study
1.7 Scope of Study
6
7
viii
1.8 Importance and Contributions of Study
1.9 Thesis Outline
1.10 Summary
8
9
10
CHAPTER II LITERATURE REVIEW
2.1 Introduction
12
2.2 Characterizing the Ionospheric Medium 17
2.3 Ionospheric Doppler Interferometry
2.4 Plasma Drift (Convection) Measurements
24
32
2.5 Ionospheric Sounding using the CADI
Digital Ionosonde at UTHM
2.5.1 Features of CADI Digital
Ionosonde
2.5.2 Transmitter
2.5.3 Receiver
2.5.4 Frequency Synthesizer
2.5.5 Noise Suppression
2.5.6 CADI Operation
2.6 Summary
33
34
35
35
35
36
36
37
CHAPTER III METHODOLOGY
3.1 Introduction 39
3.2 Ionospheric Doppler Interferometry 42
3.3 Ionospheric Channel Characterization and
Modelling
47
ix
3.4 Characterizing the Parit Raja F2-Layer
Ionosphere as Flat Rayleigh Fading Channel
65
3.5 Scattering Model for Flat Fading Ionosphere
Channel
3.6 Simulation Model for UTHM Parit Raja
Ionospheric Channel
3.7 Use of Doppler FIR Filter to Approximate
the Channel Response
3.8 Modelling Frequency Selective Ionospheric
Structure using Multicarrier Techniques
3.9 Orthogonality and Minimum Frequency
Separation Between Subcarriers
3.10 Cyclic Prefix in Multicarrier Rayleigh
Channel Simulation
3.11 Use of Fourier Transformations in
Multicarrier Techniques
3.12 Overall Research Flowchart
3.13 Summary
66
71
76
84
89
93
97
98
104
CHAPTER IV RESULTS AND ANALYSIS
4.1 Introduction
4.1.1 Geomagnetic Data Analysis
105
106
4.1.2 Solar Flux and Sunspot Data
Analysis
4.2 Results Derived From Skymap Data of
Receiver Interferometer
115
121
x
4.2.1 Plasma Drift Direction Analysis
From Skymap Data
4.2.2 Doppler Shift from Plasma Drift
Analysis for 2005
4.2.3 Maximum Drift Distance Covered by
Drifting Plasma Based on Directions
of Motions
4.2.4 Drift Distance Covered by Moving
Plasma Based on Local Time
4.2.5 Maximum Drift Velocity Attained by
Moving Plasma
4.2.6 Virtual Height Variations for
Ionospheric Reflection Layer
122
134
147
150
152
159
4.3 Multipath Rayleigh Fading for Drifting
Ionospheric Plasma Layer
168
4.4 Reflected Echo Phase Angles and
Ionospheric Channel Model
176
4.5 Ionospheric Channel Model and Echo
Delay Time Variations
179
4.6 Characterizing the Ionospheric Plasma Drift
Behaviour at Parit Raja
4.6.1 Doppler Frequency Shift (Hz) of
Reflected Echoes
4.6.2 Phase Angle (φ) of Reflected Echoes
4.6.3 Path Loss Lp (dB) of Reflected
Echoes
4.6.4 Ionospheric Channel and
Propagation Path Loss Variations
4.6.5 Quarterly Ionospheric Propagation
Path Loss Variations
183
184
185
186
189
193
xi
4.6.6 Random Behaviours of Propagation
Path Loss Variations
4.6.7 Received Echo Signal Loss
Variations at Parit Raja
198
205
4.7 Verification of Results with Empirical
Simulation of Data
208
4.8 Characterizing the Parit Raja F2-Layer
Ionosphere as Frequency-Selective
Rayleigh Fading Channel
4.8.1 Time-Varying Channel Empirical
Simulation by Multicarrier
Technique at Parit Raja Station
4.8.2 Frequency Response of a Complex
Sinusoid in Multicarrier Transmission
4.8.3 BER Performance of BPSK
Modulation with Multicarrier
Transmission
4.8.4 Empirical Simulation of BER
Performance for Multicarrier in
AWGN at Parit Raja Station
4.8.5 Empirical Simulation of BER
Performance for Multicarrier in
Rayleigh Fading at Parit Raja Station
4.8.6 BER for BPSK in Rayleigh Fading for
Singlecarrier and Multicarrier
Modulations with Empirical Data
217
218
219
221
223
228
237
4.9 Level Crossing Rates as Statistical Measure
of Rayleigh Fading Waveform
242
xii
4.10 Discussions
4.11 Summary
253
261
CHAPTER V
CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions 263
5.2 Recommendations for Future Work
5.2.1 DQPSK Differential Modulation
Scheme to Increase Capacity
5.2.2 Performance Measure Improvement
using Channel Coding
5.2.3 Antenna Diversity for Improved
Reception
5.2.4 Signal Spreading for Improved Error
Performance
265
265
266
267
267
REFERENCES
268
APPENDICES 280
1
CHAPTER I
INTRODUCTION
1.1 Introduction
The ionosphere everywhere is a very dynamic and also disturbed region. It
contains irregularities which, on scales much greater than a wavelength may be
considered as providing a rough reflecting surface. As a result of this roughness,
signals associated with each skywave propagation mode may arrive at the receiver
over a range of azimuth and elevation angles.
Furthermore, ionospheric drift velocities and movements close to the F2-layer
reflection points often impose large Doppler spreads and Doppler shifts onto the
signal. This time-varying behaviour will have an impact on the characterization of
the local structure at Parit Raja as a medium for HF transmissions.
Evident also is a variation in the bearing of multipath echoes with Doppler
frequency. This is attributed to the reflections from the bottomside ionospheric
irregularities drifting across the reflection points. When the drift of the plasma is
such as to shorten the path of the received signal, positive Doppler shifts are
imposed.
2
On the other hand, negative Doppler shifts are imposed when the plasma drift
is such as to lengthen the path. A good understanding of these plasma drift
mechanisms as well as the methods used to analyse their effects are therefore
important for the purposes of modem designs and ionospheric propagation
predictions. Apart from these, it is important to fully understand the behaviour and
limitations of many types of HF communication systems available today so as to
facilitate the design of new systems that are representative of local conditions.
1.2 Background
The upper layers of the earth’s atmosphere are ionized (ie. electrons are
detached from the atmospheric gas atoms) mainly due to ultraviolet radiation from
the sun. A state of equilibrium is reached where the free electron density is
maintained almost constant with the ionization rate being balanced by the
recombination rate of electrons with positive ions.
As the ionization rate is dependent also upon solar radiation, the electron
density will vary between day and night times. Because of this, fluctuations in the
concentration of electron densities that occur during the day have been observed to
happen (Salwa et. al. 2000).
In equatorial region like Malaysia, the total electron content (TEC) has its
variations based on time of day (diurnal) as well as the solar activity. It is also
proportional to the time delay of the wave signal being transmitted as it traverses the
ionospheric plasma. Even when the solar is relatively quiet, there is the possibility of
a bigger spread of TEC. This spread can be explained by the fact that the occasional
occurrences of solar flares tend to push the TEC higher (Zain, A.F.M. et al., 2000).
TEC is important as signal propagation via the ionosphere depends on its level.
3
Previous works in the field of ionospheric simulation models have been
proposed and experimentally confirmed by Watterson, et. al., (1970), which is the
most widely used approach for HF simulation purposes. However this model is based
on the typical mid-latitude regions of the world where, the upper atmospheric
weather and solar conditions as well as the earth’s rotational speeds are not similar or
representative enough of equatorial regions like Malaysia.
Channel characterizations which are used to explain the observed statistical
nature and fading of signal propagations via a time-varying multipath channel are
generally based on scattering. Among them, Clarke’s (1968) and Jakes (1974)
models are widely used. It is to be shown in the thesis that propagation via the
ionospheric channel at Parit Raja follows similar behaviour where the incident HF
signal wavefronts from the transmitting digital ionosonde are scattered by the non-
uniform and irregular bottomside plasma reflecting layer.
The isotropic scattering results in random and (equiprobable) reflections of
returned echopaths forming a composite multipath signal arriving at the station’s
receiver interferometer which is to be processed and analyzed. By invoking the
central limit theorem (CLT) it is possible to show that the baseband signal received
from the multipath fading ionospheric channel is approximately a complex Gaussian
random process (i.e. stochastic) when the number of reflected echoes is large.
By the adoption of such an approach, the statistical characteristics of the
multipath echoes arriving at the receiver can similarly be deduced from scattering
imposed by the drifting or time-varying plasma reflectors. Data measurement carried
out on the bottomside ionosphere F2-layer is based on a 5.9 MHz/6 MHz ground-
based transmit digisonde with a 50-foot delta antenna and a coherent phase receiver
employing fast Fourier transform (FFT) for processing.
4
1.3 Ionospheric Research in Malaysia
Early equatorial ionospheric research work in this country was carried out by
Abdullah M. and Zain, A.F.M. (1999) and Salwa H. et al. (2000) which focused
principally on the investigation of ionospheric total electron content (TEC) using
GPS receivers from the Jabatan Ukur dan Pemetaan Malaysia (JUPEM) GPS
stations. However, these pioneering research work on the total electron content in
Malaysia was mainly carried out using GPS data.
Successive research work in equatorial ionospheric activity in this country
involves the determination of ionogram critical frequencies of operation at UTHM’s
Wireless and Radio Science (WARAS) station by Abdullah S. (2011) using a digital
ionosonde beginning with the year 2005.
The present research work however, concerns the measurement of the local
equatorial F2-layer ionospheric drifts or motions at Parit Raja station and analyzing
its behaviour in relation to drift velocity, Doppler shifts in frequency, returned
echoes angles of arrival as well as the roles played by the earth’s geomagnetic fields
and the sun’s solar activity in influencing the ionospheric structure. This is done with
a view to study and characterize its behaviour which is representative of the local
equatorial conditions at Parit Raja and the surrounding areas. Subsequently,
appropriate statistical models using filters will be developed and performance
measures quantified by empirical simulation in order to gauge the suitability of the
models for use in modem modulation designs for HF transmissions representing the
equatorial regions.
5
1.4 Problem Statements
The world has experienced two world wars previously before satellites were
put into orbit. The only means of establishing long-distance and beyond-line-of-sight
(BLOS) communications then were by using high frquency or HF transmissions via
the ionospheric channel, especially for the military and diplomatic services.
Even though satellite transmissions were subsequently deployed with state-
of-the-art technologies, it is now considered insecure to countries at war (and their
enemies) since it is vulnerable to sabotage (jamming). In addition, satellites move
along predictable orbits and can be shot down by missiles as proven by the Chinese
military and the US Navy a few years ago.
It is therefore desirable for commanders of ground forces or infantry platoons
and warships to be able to communicate more securely via the ionosphere using HF
as an alternative method and rely on it to make strategic decisions. With the security
aspects of satellite transmission and also its networks being potentially compromised,
it now becomes imperative for field commanders to consider more use of adhoc and
mobile HF transmissions to complement satellite links to counter enemy forces.
This is most suited for a country like Malaysia where the land area and sea coverage
are quite extensive that range from the Peninsula right up to Sabah and Sarawak.
However, the standard ionospheric model adopted by ITU R-1487 uses the
Gaussian Doppler channel model with 68% of confidence interval and represents
only the midlatitude regions of the world where advanced countries are located.
Since the equatorial regions of the world have higher ionospheric drifts and Doppler
shifts and are also not affected by equinox and solstice, it is to be shown in this thesis
that the Jakes Doppler channel model with 95% of confidence interval is more
representative of the local equatorial Parit Raja station.
6
1.5 Motivations for Using the Ionospheric Channel
The ionospheric medium is a time-varying channel with an inherent narrow
bandwidth. The transmission channel is subjected to unpredictable variations which
occur as a product of time of day, seasons, and such occurrences as geomagnetic and
sunspot activities. Variations in the ionospheric plasma reflection layer also result in
multipath propagation and reception at the ground receiver.
The ionospheric radio channel has also been recognized as a useful and
economical medium for signal transmission over long distances. It is considered as a
‘gift of nature’ whose properties depend much on solar and geomagnetic activities.
The increasing availability of high speed digital signal processing (DSP) techniques
in recent decades as well as events that occurred in Iraq and Aceh have resulted in a
resurgence and more renewed interest in research work for HF communications
within the 3 MHz to 30 MHz range.
Hence, HF transmission via the ionospheric medium has seen increased
applications in situations where normal communications are difficult or unreliable.
The reason is that the ionospheric channel offers a relatively inexpensive and long
range communications link without the need for a third party equipment like a
satellite, whose operation can be compromised as well as being prone to sabotage.
1.6 Aims and Objectives of Study
The purpose of this study is to investigate and characterize statistically the
dynamics of the ionosphere at UTHM’s Parit Raja station under different conditions
of Doppler shifts due to the changing plasma drift velocities as well as planetary
effects such as the geomagnetic and solar activities.
7
Specifically, the objectives of this study are:-
i. to analyse the distributions of the ionospheric channel statistical parameters
like the autocorrelation function, scattering function, Doppler power spectral
density, probability density functions of the phase angles of arrival as well as
the amplitude fading due to random echo cancellations
ii. to implement a model for statistical characterization of the behaviour of the
local ionospheric reflection layer using a finite impulse response (FIR) filter
for flat channel response with uniform Doppler rate and infinite impulse
response (IIR) filter for non-flat (frequency-selective) channel response with
rapidly variable Doppler rates
iii. to carry out performance measures in order to gauge the validity of the
characterized model in terms of its bit error rates (BER) versus signal-to-
noise ratios (SNR) for future development of HF modem modulation methods
representative of local equatorial conditions like Malaysia
1.7 Scope of Study
This study will only focus on the behaviour of ionospheric plasma drifts and
the associated Doppler shifts in frequencies and the angles of arrivals of reflected
echowaves at this equatorial station of Parit Raja (latitude 1° 52' N and longitude
103° 48' E) representing the local F2-region with critical frequencies ranging from
5.9MHz to 6MHz. Only ionospheric plasma convection with pronounced cloud of
scattering points and moving with bulk flow will be considered.
8
In addition to that, only the bottomside of the local F2-region ionosphere will
be probed using a digital ionosonde employing Doppler interferometric reception
technique. Near topside or deep probing are not considered in this research work.
This research topic can be justified by the increasing interest in propagation
that affects navigation (eg. global positioning satellite system or GPS), surveillance
using over-the-horizon (OTH) radar as well as non-satellite-based remote military
communications in the high-frequency band (HF) for the army ground forces as well
as the navy in our country. These help complement the existing satellite links.
In this thesis, Matlab programmes are written to model the ionospheric
channel medium and empirically simulate its properties using FIR and IIR filters. It
also facilitates the graphical plotting of results in order to reflect the characterization
of the ionosphere as well as to validate its performance measures in terms of the bit
error rate (BER) versus the signal-to-noise ratios (SNR) which is important for
modem design.
1.8 Importance and Contributions of Study
HF can be used to establish adhoc remote military networks without much
expensive satellite equipments. It is cost-effective for beyond-line-of-sight (BLOS)
transmission and can support sources of information that are vital to modern military
communications like voice and data as well as the newer traffic types that utilize
asynchronous transfer mode (ATM) for narrowband services.
Apart from non-satellite-based skywave communications for the military
ground forces (eg. West Malaysia to East Malaysia), this research is also relevant for
9
rural telecommunication services where long-distance repeaterless transmission is a
necessity without third-party involvement (like the use of an intermediate satellite
link) or in times of disasters (eg. earthquakes and tsunamis) when terrestrial
communication infrastructures are destroyed.
Hence, it is considered important to analyze and alternatively characterize the
ionospheric plasma drift behaviour at this local equatorial station based on another
Doppler model instead, i.e., the Jakes Doppler channel response, as opposed to the
conventional Gaussian Doppler channel model adopted by ITU R-1487 for
midlatitude regions of the world. This will better predict in a more accurate manner
the extent of spectrum spreading that occur in the Doppler channel and hence, the
behaviour of the equatorial ionospheric structure, caused by the moving plasma
reflectors on the incident HF propagated signals. The effects of employing a model
that is designed for the midlatitude region which does not represent the actual
conditions of the equatorial region may degrade the effectiveness of equipments
requiring precise timing and signal frequency information like those used in
navigation and positioning systems.
1.9 Thesis Outline
Chapter two provides a literature review of the current research work on the
subject and deals with the typical ionospheric structure and its characteristics as well
as the techniques used for probing the bottomside ionospheric reflection layer.
Chapter three presents the research methodology employed during the
research work and includes equipment setup at both the transmitter and receiver sides
as well as the data measurement and time analysis and frequency analysis methods
by fast Fourier transform (FFT) being used. The use of finite impulse response (FIR)
10
and infinite impulse response (IIR) filters with upsampling and interpolation to
simulate the behaviour of the local equatorial ionospheric channel medium for
different Doppler rates are also included.
Chapter four lays down the research results obtained from the empirical
simulation of measured data being analyzed together with an attempt to characterize
the Doppler channel for HF transmissions that is more representative enough of the
equatorial ionospheric structure at Parit Raja. Performance measures are also carried
out in order to validate and gauge the effectiveness of the modelled ionospheric
medium using filters.
Finally, chapter five list down the conclusions and recommendations for
future work in this ‘forgotten’ but important area of research.
1.10 Summary
This chapter provides a background introduction to the ionosphere and its
potential use as a medium for long-distance transmission in this country as well as
the problems associated with its use. It is considered important after events that
occurred in Iraq and Aceh where ground infrastructures are destroyed due to war and
natural disasters which makes long-distance transmission almost impossible.
The realization of this problem led to the motivation to investigate the
behaviour and characteristics of the local equatorial ionosphere to be potentially used
as an alternative medium for high-frequency (HF) skywave communications, in the
event that such disasters do occur in this country. The standard channel model that is
currently being adopted is based on the conditions relevant to midlatitude regions of
11
the world which is not representative of the local conditions of Parit Raja. A good
understanding of HF transmission mechanisms and its limitations over the equatorial
ionospheric channel as well as the techniques used to analyse its effects are therefore
important for the purpose of ionospheric propagation. This will then help to facilitate
the design of new HF ionospheric modems that reflects more on the equatorial
conditions of this country for use in military as well as in civil defence applications.
The next chapter will discuss further the characteristics of the ionosphere
together with the techniques being used to probe and measure its drift or convection
properties using a digital ionosonde or digisonde.
CHAPTER II
LITERATURE REVIEW
2.1 Introduction
As the main area of research for this research study is equatorial ionospheric
plasma drift measurement, analysis and modelling using Doppler interferometry,
therefore this chapter discusses the characteristics of the ionospheric reflection layer
as well as the ionospheric sounding technique that is being employed to probe the
properties of the ionospheric medium. This will then form the foundation for channel
characterizations which will be used to explain the observed statistical nature and
properties of the multipath signal propagations via a time-varying ionosphere.
Basically, the ionosphere is the region of ionised plasma as a result of solar
activity. Its height starts at about 50 km and can reach as high as 1000 km. The use
of pulse sounding technique for ionospheric investigation has long been practised by
radio propagation researchers and is widespread. Sounding to probe the behaviour of
the bottomside ionosphere is thus important for scientific purposes, frequency
prediction purposes, and for real-time assistance to HF communication systems.
Pulse transmitters are used in conjunction with suitable receivers to vertically
sound the bottomside of the ionospheric layer, or to obliquely sound long-distance
ionospheric paths between points on the earth's surface. By employing precise timing
13
units for the generation of the pulse repetition rate, as well as for frequency control,
the transmitter and receiver are maintained in synchronism as they are swept across a
portion of the high frequency spectrum (1 to 20 MHz) as carried out at the Wireless
and Radio Science Centre (WARAS) in UTHM.
Generally, the ionosphere is structured into three main layers namely, the D-
layer, the E-layer and the F-layer (which is further subdivided into the F1 and F2
layers). Figure 2.1 shows a simplified view of the layers in the ionosphere over the
period of one day (Poole, 2006):-
Figure 2.1 Simplified view of the ionospheric layers
(Source: Poole, Adrio Communications Ltd UK, 2006)
14
Figure 2.2 shows a typical vertical profile of the ionosphere (Davies,1990):
Figure 2.2 Vertical profile of the Earth’s ionosphere
(Source: Davies, “Ionospheric Radio” Peter Peregrinus Ltd.,1990)
Typically, ionospheric activity is subjected to regular diurnal, seasonal, and
solar cycle variations together with the irregular geomagnetic activity caused by solar
storms (ITU-R 1487, 2000). Usually diurnal electron density peaks at approximately
two hours past local noon, and during solar variations that results from increased
sunspot activity associated with 11 years of solar cycles (Joanna et. al., 2010).
The D-layer ranges in height from about 50 km to 90 km. The primary
source of ionization in this layer is solar radiation. At night time, electrons attached
to the atoms and molecules form negative ions that cause the D-layer to disappear.
But during the daytime, due to solar radiation, the electrons tend to detach
15
themselves from the ions causing the D-layer electrons to reappear again.
Consequently, the D-layer electrons are present only by day time at an altitude of
about 60 to 70 km, but not at night time, causing a distinct diurnal variation in the
electron density. The significant gas molecules found in this layer are nitrogen (N2),
oxygen (O2) and nitric oxide (NO), (ITU-R P.531-4, 1997).
The E-layer exists at a distance from the earth’s surface of about 100 to
140km. Oxygen forms most of the chemical compound of this layer. The behaviour
of the E-layer almost entirely depends on the level of solar activity and the zenith
angle of the sun. The E-layer is only present during the daytime. At night the E-layer
rapidly disappears because the primary source of ionization is no longer present. The
primary source of ionization is the sun's X-ray emissions resulting in electron
densities showing distinct solar-cycle, seasonal and daily variations. The upper
portion of the E-layer is also called the sporadic E-layer as it can come and go
unpredictably. It is characterized by small, thin clouds of intense ionization, which
can support reflection of radio waves, rarely up to 225 MHz. Sporadic-E events may
last for just a few minutes to several hours. Sporadic E propagation makes amateur
radio possible, as propagation paths that are generally unreachable can now open up.
The F-layer on the other hand is the most important layer in so far as HF
communications is concerned. It is divided into two sublayers, namely F1 and F2.
The distance from the earth’s surface to the F-layer varies depending on their layers
(F1 or F2). The F1-layer extends from 140 km to 250 km with an average height of
about 200 km. The most representative chemical elements present in the F1-layer is
nitrogen and the layer can only be observed during the daytime since the electron
densities are primarily controlled by the zenith angle of the sun.
The F2-layer is present during the daytime and night time. Its average height
ranges from 250 km to about 350 km. It is also the most important ionospheric layer
from the point of view of long distance HF propagation. The most representative
chemical elements present in F2-layer is oxygen. In addition, the global spatial
16
distribution of its critical frequency (foF2) reveals a strong geomagnetic dependence
(manifested by the planetary, Kp index) as well as solar sunspot numbers (SSN). The
short-term relationship of the equatorial peak electron density and the solar short-
wavelength irradiance has been examined using foF2 observations from Jicamarca,
Peru and recent solar irradiance measurements from satellites (Wang et. al., 2007).
The results indicated that soft X-rays show similar or higher correlation with foF2 at
short timescales (27 days or less) than extreme ultraviolet (EUV) does, although the
EUV correlation is higher for longer periods.
After sunset and during night fall, the two F1 and F2 layers combine together
to form the F-layer. In practice, the F1-layer absorbs and attenuates some of the HF
waves propagated through it but most of the transmission passes through where they
are refracted back by the F2-layer to the earth’s surface. Hence, the F2-layer can be
most suited as a channel medium for adhoc, beyond line-of-sight, long distance,
skywave communications to complement the existing satellite links for military
transmissions as well as in times of disasters when infrastructures are destroyed.
Existence of the F3 layer is a common feature within ±10° of the magnetic
equatorial ionosphere in the daytime (Zhao et. al, 2011). It occurs mainly during the
morning-noon period due to the combined effect of the upward E×B drift and the
neutral wind that provides upward plasma drifts above the F2 layer. The F3 layer
occurrence rate is higher in summer and decreases with increasing solar activity.
Figure 2.3 shows the world divided into different ionospheric regions based
on their latitudes by Ionospheric Prediction Service (IPS), Australian Bureau of
Meteorology (2000). These are the low-latitude (0o - 30
o), mid-latitude (30
o – 60
o)
and high-latitude (60o
– 90o) regions. Parit Raja sits at 1
oN, 103
oE of the equatorial
region.
17
Figure 2.3 Divisions of ionospheric regions of the world
(Source: IPS, Australian Bureau of Meteorology, 2000)
Practically, as far as communication is concerned, regions of the ionosphere,
D, E, F and top-side ionization have been identified as contributing to the
concentration of the total electron content (TEC) at the ionospheric layers. In each of
this region, the ionized medium is neither homogeneous in space nor stationary in
time and affects the propagation of HF radiowaves through it (Warrington, 2009).
2.2 Characterizing the Ionospheric Medium
The effects of HF signal propagation via the ionosphere depends not only on
the plasma drift, but also on the TEC level (Warrington, 2009). Apart from the
background ionization, there is the presence of highly dynamic, small-scale non-
stationary structures known as irregularities. Both the background ionization and
irregularities degrade radiowaves during propagation. In addition, the background
ionization and irregularities cause the refractive index of the ionospheric structure to
become frequency dependent, i.e. the medium is dispersive (Davies, 1990).
Parit Raja
18
Many models have been proposed to predict the electron density distribution
of the ionosphere at low geomagnetic latitudes or the equatorial regions. Some of
them are theoretical (Hunt, 1973), some are partly theoretical, partly observational
(Somayajulu, 1979) and the rest are purely empirical (Bent, 1975, Rajaram, 1977).
The Chapman layer model on the other hand, is used to describe the electron density
in terms of production rate versus altitude (Guillermo et. al. 2007). It depicts an
exponential increase and decrease of electron density with increasing altitude.
Models are usually expressed as computer programs. The model may be
based on basic physics of the interactions of the ions and electrons with the neutral
atmosphere and sunlight, or it may be a statistical description based on a large
number of observations or a combination of physics and observations. One of the
most widely used models is known as the IRI or the International Reference
Ionosphere (Bilitza, 2001) which is based on geophysical data and specifies certain
important parameters like electron density, electron and ion temperature and
also ionic composition. Radio propagation depends uniquely on electron density.
Mathematically, the ionospheric model is a description of the ionosphere as a
function of location, altitude, day of year, phase of the sunspot cycle and
geomagnetic activity.
Apart from these, the IRI empirical TEC models are also available for low
and mid-geomagnatic latitudes. These IRI models provide monthly averages of the
electron density, electron temperature, ion temperature, and ion composition in the
altitude range from 50 km to 2000 km. The major data sources are the worldwide
network of ionosondes, the powerful incoherent scatter radars (ISR), the
International Satellites for Ionospheric Studies (ISIS) and Alouette topside sounders,
as well as in-situ instruments on several satellites and rockets.
The IRI is actually an international project sponsored by the Committee on
Space Research (COSPAR) and the International Union of Radio
19
Science (URSI). The major data sources are the worldwide network of
digital ionosondes, the powerful incoherent scatter radars (Jicamarca, Arecibo,
Millstone Hill, Malvern, St. Santin), the ISIS and Alouette topside sounders, as well
as in situ instruments on several satellites and rockets. IRI is updated yearly. IRI is
more accurate in describing the variation of the electron density from bottom of the
ionosphere to the altitude of maximum density than in describing the total electron
content (TEC). Since 1999 this model is "International Standard" for the terrestrial
ionosphere (standard TS16457).
All these models essentially use three different parameters, namely hmF2
(height above ground at which maximum electron density occurs), NmF2 (maximum
electron density in the F2 region), and Ym (semi-thickness of the bottomside
ionosphere) to generate the electron density profiles. Generally for the F2-layer, the
variations in the F2-layer peak electron density, its height, and the F2-layer thickness
parameters are greater during the daytimes compared to at nignt times (Lee and
Reinisch, 2012). The parameters also vary as functions of (i) geomagnetic latitude
(ii) hour of the day (iii) season (equinox, winter, and summer) and (iv) solar activity,
(IRI 2008, Retrieved 2011-11-08).
Most of the models also describe the ways of deriving the hmF2, NmF2, and
Ym. Measured parameters like M(3000)F2 (maximum usable frequency over distance
of 3000 km on the ground using reflection of the EM waves from the F2 layer of the
ionosphere), foF2 (critical frequency of the F2-region) are generally used to derive
these parameters. The choice of a particular model is based on how good it accounts
for the observed TEC and considerations of computational simplicity, availability
and reliability of the basic parameters used to generate the model.
Since the ionospheric F-layer is the most important region in defining HF
communications, where the transmitting signal is scattered as a result of the irregular
and time-varying electron density layer, it is therefore the region where the research
is focused upon. This layer is separated into the lower altitude F1 and higher altitude
20
F2 layers during the daytime. As a result of the increased ionization production in the
F2-layer, it supports HF communications longer during the night when the F1-layer
dissipates and recombines with the F2-layer to form the F-layer. Observations of the
F3-layer at equatorial region during 2005 have been made by (Zain et. al., 2007) and
evidence of an equatorial plasma fountain enhancement caused by the magnetic field
disturbance resulting in the F3 layer, formed with peak densities NmF3 exceeding
NmF2 was also made by Paznukhov et. al. (2007). The day-to-day variations of the
F3-layer has also been predicted and studied by Balan et. al. (2000).
Theoretical studies (Lin et. al., 2009) of new plasma structures in the low-
latitude ionosphere during major magnetic storms using model simulations for an
intense magnetic storm has indicated the creation of a low-latitude electron density
arch aligned along the geomagnetic field created by strong uplift of the F2 layer that
is driven by the penetration electric field. In a vertical profile of electron density, the
daytime elevated ionospheric layer can appear distinctly from the recreated F2 layer,
in which case the elevated layer becomes the F3 layer.
However, this research work concerns mainly on the F2-layer due to its
relatively strong presence in both the day and night times making it very useful for
longhaul skywave communications for many applications. Instead of studying its
TEC, which can be used to investigate long-term morphology of the key F2-layer
ionospheric characteristics useful for driving the International Reference Ionosphere
(Krause et. al., 2008), the main focus now is more on analyzing and characterizing
its time-varying structural behaviour statistically with a view to applying modem
modulation methods that are more suitably representative of the equatorial conditions
at Parit Raja and the surrounding regions. This will then help in using the local
channel medium for propagation for over-the-horizon HF signals more effectively.
The current practice of using HF modems with Gaussian Doppler channel
response (ITU R-1487) designed by advanced countries from midlatitude regions of
the world can then be re-examined so as to avoid its ineffectiveness and inaccuracy
21
of results (Khalifa et. al., 2008). In order to better reflect the channel behaviours for
countries in the equatorial region like Malaysia which has very dissimilar
ionospheric properties than midlatitude regions, a new channel characterization
based on the Jakes Doppler channel response will be adopted in this thesis using
finite impulse response (FIR) and infinite impulse response (IIR) filters.
In addition, reliable and cost-effective HF modem design for the local
equatorial ionospheric channel also requires a thorough characterization of the time
and space variability (i.e. drift) of the local channel medium. This then requires the
measurement, analysis and simulation of the channel Doppler response as well as
other performance measures to be carried out in order to estimate the channel
characteristics right across the available system bandwidth. Consequently, empirical
simulation of results using measured data gathered from the digital ionosonde system
at WARAS will be carried out with performance measures to gauge the model’s
effectiveness for different conditions of Doppler rates at this station.
It is also known that the bottomside ionospheric layer is irregular and
stochastic in nature (Chuo et. al., 2011). Due to the random scattering of returned
echoes at the ground receiver, the central limit theorem (Barany, et. al., 2007) can be
invoked in order to allow the Doppler channel impulse response (CIR) to be
modelled as a Gaussian random process irrespective of the distribution of the echo
components if there is a sufficiently large number of scatterers. For no dominant
component to the scatterer, then such a process will have zero mean with phase
evenly distributed between 0 and 2π radians. The multipath envelope of the received
echoes should then follow the standard Rayleigh distribution (Yeh et. al., 2011,
Proakis, 2001 and Sklar, 1997).
The only internationally accepted channel model for narrowband HF
communications is by Watterson (1970), which is based on a 10-minute of data from
mid-latitude regions of the world. This model represents the Gaussian-scatter HF
ionospheric channel whereby it assumes that the HF channel is non-stationary (i.e.
22
variable) in both time and frequency, but if considered over narrow bandwidths and
sufficiently short times (about 10 mins), can be represented by a stationary model.
As a result of the inadequacy of the above model (which suits midlatitude
regions) to represent countries at the equatorial regions, there is a likely requirement
to extend and improve this model for the equatorial regions by adding modifications
to include signal distortions due to Doppler shifts and spread, together with time
delay spread. A statistical characterization of the F2-layer behaviour is thus
necessary at Parit Raja which incorporates also ionospheric drifts during the mainly
quiet periods of solar and geomagnetic activities that are generally representative of
local conditions. This is the area of study that is being pursued in this research work.
The ionospheric drift information and the directional structure of the received
signals can be derived from the delay and Doppler spread characteristics collected
from the digisonde at WARAS. The received signal characteristics (time delay,
Doppler shift, and phase angles) will be parameterized in terms of the local time of
day (LT), drift velocity, etc. which will then be used in determining a more realistic
HF channel model. This is to better represent the ionospheric structure at the local
F2-layer for ionospheric communications via HF modems in our country (eg. for
non-satellite based military ground forces and navy).
From a system design viewpoint, the impact of ionospheric effects on long
distance transmission can be summarized as follows (ITU-R P.531-4 (1997):-
(i) when propagating through the ionosphere, the linearly polarized wave from the
ground station will suffer a gradual rotation of its plane of polarization (Faraday
rotation) due to the presence of geomagnetic fields and the anisotropy of the plasma
medium
(ii) the rotation of the signal carrier‘s polarization and time delay of the signal, as
well as the change in the angles of arrival (AOA) due to random multipath
23
reflections are caused by the total electron content accumulated along the
transmission path penetrating the ionosphere
(iii) localized non-uniform irregularities at the bottomside ionospheric layer, further
cause excess and random rotations and time delays, which can only be described in
statistical terms. The presence of charged particles in the ionosphere is the cause that
slows down the propagation of ionospheric radio signals along the path.
(iv) the rotations and time delays relating to electron density are non-linearly
frequency dependent, and since localized ionospheric irregularities are also in motion
or drift that consequently cause Doppler effects, it further results in dispersion in
time and frequency of the signal carriers. When long-distance, trans-ionospheric HF
skywave signals occupy a significant bandwidth, the propagation delay (being a
function of frequency) introduces this dispersion.
(v) the localized ionospheric irregularities also act like convergent and divergent lens
which focus and defocus the incident radio waves. Small-scale irregular structures in
the ionization density mainly through the mechanisms of forward scattering and
diffraction, cause scintillation phenomena in which the returned echowaves envelope
at the receiver is replaced by one which is fluctuating in amplitude, phase and
apparent direction of arrival.
(vi) as a result of the relatively stationary refractive-index irregularities moving
horizontally past the ionospheric radiowave path, the spatial and temporal power
spectra are related by the drift velocity, and consequently the Doppler shifts in
frequency.
The actual relationships describing the channel property can be characterized
by a Doppler frequency response to represent its behaviour in the frequency domain
as well as inferring the F2-layer drifts that give rise to Doppler shifts in frequency
using ground-based digital ionosonde measurements (Adebesin et. al., 2013). Hence,
there is a need for modelling to be carried out in order to characterize the behaviour
of the ionospheric structure for future development of modem modulation methods
24
that are representative of local conditions (Cannon, 2007). Since ionospheric effects
influence the design and performance of over-the-horizon, HF skywave systems,
empirical simulation using measured digisonde data at the local Parit Raja station
will be presented which also allows the prediction of the ionospheric propagation
parameters needed in planning such systems.
Due to the complex nature of ionospheric physics affecting propagation of an
electromagnetic (EM) wave and its linear and nonlinear interactions with the
ionospheric layer (Eliasson, 2006), system parameters affected by ionospheric effects
as mentioned above cannot at all times be adequately summarized in simple
analytical formulas. Relevant data being gathered from the local station by the digital
ionosonde and presented in the form of tables and graphical plots, together with
further descriptive or qualifying statements, are (for all practical purposes) the best
way to present the effects (Kouba et. al., 2006).
2.3 Ionospheric Doppler Interferometry
This section discusses the specific area of ionospheric drift measurement
technique being employed using Doppler interferometry, since the collection of data
to determine the characteristics of the ionospheric reflection layer for this research
study revolves around its use.
The ionosphere everywhere is a very dynamic and also disturbed region
containing irregularities which, on scales much greater than a wavelength may be
considered as providing a rough reflecting surface (Bianchi, 2005). As a result of this
roughness, signals associated with each propagation mode may arrive at the receiver
over a range of angles in both azimuth and elevation angles. Furthermore,
ionospheric drifts and movements close to the reflection points often impose a
certain amount of Doppler shifts and spreads onto the signal (Reinisch, 2005).
268
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