Doctoral Degrees (Electronic Engineering)

Permanent URI for this collectionhttps://hdl.handle.net/10413/6867

Browse

Now showing 1 - 16 of 16

A semi-empirical formulation for determination of rain attenuation on terrestrial radio links.
(2010) Odedina, Modupe Olubunmi.; Afullo, Thomas Joachim Odhiambo.
Advances in today’s fast growing communication systems have resulted in congestion in the lower frequency bands and the need for higher capacity broadband services. This has made it inevitable for service providers to migrate to higher frequency bands so as to accommodate the ever increasing demands on radio communication systems. However, the reliability of such systems at these frequency bands tend to be severely degraded due to some natural atmospheric phenomena of which rain is the dominant factor. This is not to say that other factors have become unimportant; however, if attenuation by rain is so severe that a radio link is unavailable for use, then other factors become secondary. Therefore, it is paramount to establish a model capable of predicting the behaviour of these systems in the presence of rain. This study employs a semi-empirical approach for the formulation of rain attenuation models using the knowledge of rain rate, raindrop size distribution, and a signal level measurement recorded at 19.5 GHz on a horizontally polarized terrestrial radio link. The semi-empirical approach was developed by considering the scattering effect of an electromagnetic wave propagating through a medium containing raindrops. The complex forward scattering amplitudes for the raindrops are determined for all raindrop sizes at different frequencies, utilizing the Mie scattering theory on spherical dielectric raindrops. From these scattering amplitudes, the extinction cross-sections for the spherical raindrops are calculated. Applying the power-law regression to the real part of the calculated extinction cross-section, power-law coefficients are determined at different frequencies. The power-law model generated from the extinction crosssection is integrated over different raindrop-size distribution models to formulate theoretical rain attenuation models. The developed rain attenuation models are used with 0.01 R rain rate statistics determined for four locations in different rain climatic zones in South Africa to calculate the specific rain attenuation. From a horizontally polarized 6.73 km terrestrial line-of-sight link in Durban, South Africa,experimental rain attenuation measurements were recorded at 19.5 GHz. These rain attenuation measurements are compared with the results obtained from the developed attenuation models with the same propagation parameters to establish the most appropriate attenuation models that describe the behaviour of radio link performance in the presence of rain. For the purpose of validating the results, it is compared with the ITU-R rain attenuation model. This study also considers the characteristics and variations associated with rain attenuation for terrestrial communication systems. This is achieved by utilizing the ITU-R power-law rain attenuation model on 5-year rain rate data obtained from the four different climatic rain zones in South Africa to estimate the cumulative distributions of rain attenuation. From the raindrop size and 1-minute rain rate measurement recorded in Durban with a distrometer over six months, rain events over the six months are classified into drizzle, widespread, shower and thunderstorm rain types and the mean rain rate statistics determined for each class of rain. Drop-size distribution for all the rain types is estimated. This research has shown a statistical analysis of rain fade data and proposed an empirical rain attenuation model for South Africa localities. This work has also drawn out theoretical rain attenuation prediction models based on the assumption that the shapes of raindrops are spherical. The results predicted from these theoretical attenuation models have shown that it is not the raindrop shapes that determine the attenuation due to rain, but the raindrop size distribution and the rain rate content in the drops. This thesis also provides a good interpretation of cumulative rain attenuation distribution on seasonal and monthly basis. From these distributions, appropriate figures of fade margin are derived for various percentages of link availability in South Africa.
Alternative techniques for the improvement of energy efficiency in cognitive radio networks.
(2016) Orumwense, Efe Francis.; Srivastava, Viranjay Mohan.; Afullo, Thomas Joachim Odhiambo.
Abstract available in PDF file.
Channel characterization for broadband powerline communications.
(2014) Mulangu, Chrispin Tshikomba.; Afullo, Thomas Joachim Odhiambo.; Ijumba, Nelson Mutatina.
The main limiting factor in broadband powerline communications is the presence of impedance discontinuities in the wired channel. This phenomenon is present in both outdoor and indoor powerline communication (PLCs) channels. It has been established that the impedance of the electrical loads and line branching are the main causes of impedance discontinuities in PLC channel networks. Accurate knowledge of the expected impedances of the corresponding discontinuity points would be vital in order to characterize the channel for signal transmission. However, the PLC channel network topologies lead to different branching structures. Additionally, the existence of a myriad of electrical loads, whose noise and impedance vary with frequency, are a motivation for a rigorous design methodology in order to achieve a pragmatic channel model. In order to develop such a channel model, an approach similar to the one applied in radio propagation channel modeling is adopted, where specific attenuation determined at a point is used in predicting the attenuation for the entire power cable length. Therefore, the powerline is modeled with the assumption of a randomly spread multitude of scatterers in the vicinity of the channel with only a sufficient number of impedance discontinuity points. The line is considered as a single homogeneous element with its length divided into a grid of small areas with dimensions that range from 0.5 to 3 mm. Thus, each small area transmits an echo and the forward scattered response gets to the receiver. With this approach, point specific attenuation along the line is proposed and used to derive the channel transfer function. Measurement results show that both the analytical specific attenuation model developed in this work and the channel transfer function are feasible novel ideas in PLC channel network characterization. It is seen from the measurements that the signal attenuation is directly proportional to the number of branches, and this is in line with the findings of previous researchers. A comparison between the measured values and the simulation results of the frequency response shows a very good agreement. The agreement demonstrates applicability of the models in a practical enviroment. Thus we conclude that the models developed do not require knowledge either of the link topology or the cable models but requires an extensive measurement campaign.
Characteristics of rain at microwave and millimetric bands for terrestrial and satellite links attenuation in South Africa and surrounding islands.
(2010) Owolawi, Pius Adewale.; Afullo, Thomas Joachim Odhiambo.; Malinga, Sandile B.
The emergence of a vast range of communication devices running on different types of technology has made convergence of technology become the order of the day. This revolution observed in communications technology has resulted in a pressing need for larger bandwidth, higher data rate and better spectrum availability, and it has become important that these factors be addressed. As such, this has resulted in the current resurgence of interest to investigate higher electromagnetic spectrum space that can take care of these needs. For the past decade, microwave (3 GHz-30 GHz) and millimeter waves (30 GHz-300 GHz) have been used as the appropriate frequency ranges for applications with properties such as wide bandwidth, smaller components size, narrow beamwidths, frequency re-use, small antenna, and short deployment time. To optimize the use of these frequency ranges by communication systems, the three tiers of communication system elements - receiver, transmitter and transmission channel or medium must be properly designed and configured. However, if the transmitter and receiver meet the necessary requirements, the medium in which signals are transmitted often becomes an issue at this range of frequencies. The most significant factor that affects the transmission of signals at these bands is attenuation and scattering by rain, snow, water vapour and other gases in the atmosphere. Scattering and absorption by rain at microwave and millimeter bands is thus a main concern for system designers. This study presents results of research into the interaction of rainfall with microwave and millimeter wave propagation as a medium. The study of rainfall characteristics allows estimation of its scattered and attenuated effects in the presence of microwave and millimeter waves. The components of this work encompass rainfall rate integration time, cumulative distribution and modelling of rainfall rate and characteristics of rain drop size and its modelling. The effects of rain on microwave and millimeter wave signals, which result in rain attenuation, are based on rainfall rate variables such as rainfall rate cumulative distribution, raindrop size distribution, total scattering cross sections, rain drop shape, and rain drop terminal velocity. A regional rainfall rate conversion factor from five-minute rainfall data to one-minute integration time is developed using the existing conversion method and a newly developed hybrid method. Based on these conversion factor results from the hybrid method, the rainfall at five-minute integration time was converted to a one-minute equivalent to estimate its cumulative distributions. In addition, new rain zones based on ITU-R and Crane designations are suggested for the entire region of South Africa and the surrounding Islands. The results are compared with past research work done in the other regions. Rain attenuation is acutely influenced by rain drop size distribution (DSD). This study thus also investigates DSD models from previous research work. There are several DSD models commonly used to estimate rain attenuation. They are models which have their root from exponential, gamma, lognormal and Weibull distributions. Since DSD is dynamic and locationdependent, a simple raindrop size distribution model is developed for Durban using maximum likelihood estimation (MLE) method. The MLE method is applied to the three-parameter lognormal distribution in order to model DSD for Durban. Rain drop size depends on rainfall rate, drop diameter and rain drop velocity. Semi-empirical models of terminal velocity from previous studies are investigated in this work and proposed for the estimation of specific rain attenuation.
Clear-air radioclimatological modeling for terrestrial line of sight links in Southern Africa.
(2010) Kemi, Odedina Peter.; Afullo, Thomas Joachim Odhiambo.
This thesis has investigated radioclimatological study in a clear-air environment as applicable to terrestrial line of sight link design problems. Radioclimatological phenomena are adequately reviewed both for the precipitation effect and clear-air effect. The research focuses more on the clear-air effect of radioclimatological studies. Two Southern African countries chosen for case study in the report are Botswana and South Africa. To this end, radiosonde data gathered in Maun, Botswana and Durban, South Africa are used for model formulation and verification. The data used in the thesis ranges from three years to ten years in these two stations. Three to ten years of refractivity data gathered in Botswana and South Africa is used for the model formulation. On the other hand, eight months signal level measurement data recorded from the terrestrial line of sight link set up between Howard College and Westville Campuses of the University of KwaZulu-Natal, Durban South Africa is used for model verification. Though various radioclimatic parameters could affect radio signal propagation in the clear-air environment, this report focuses on two of these parameters. These two parameters are the geoclimatic factor and effective earth radius factor (k-factor). The first parameter is useful for multipath fading determination while the second parameter is very important for diffraction fading, modeling and characterization. The two countries chosen have different terrain and topographical structures; thus further underlying the choice for these two parameters. While Maun in Botswana is a gentle flat terrain, Durban in South Africa is characterized by hilly and mountainous terrain structure, which thus affects radioclimatological modeling in the two countries. Two analytical models have been proposed to solve clear-air radioclimatic problems in Southern Africa in the thesis. The first model is the fourth order polynomial analytical expression while the second model is the parabolic equation. The fourth order polynomial model was proposed after an extensive analysis of the eight month signal level measurement data gathered in Durban, South Africa. This model is able to predict the fade exceedance probabilities as a function of fade depth level. The result from the fourth order polynomial model is found to be comparable with other established multipath propagation model reviewed in the thesis. Availability of more measurement data in more location will be necessary in future to further refine this model. The second model proposed to solve clear-air propagation problem in the thesis is the modified parabolic equation. We chose this technique because of its strength and its simplistic adaptation to terrestrial line of sight link design problem. This adaptation is possible because, the parabolic equation can be modified to incorporate clear-air parameters. Hence this modification of the parabolic equation allows the possibility of a hybrid technique that incorporates both the statistical and mathematical procedures perfectly into one single process. As a result of this, most of the very important phenomena in clear-air propagation such as duct occurrence probabilities, diffraction fading and multipath fading is captured by this technique. The standard parabolic equation (SPE) is the unmodified parabolic equation which only accounts for free space propagation, while the modified parabolic equation (MPE) is the modified version of the parabolic equation. The MPE is classified into two in the thesis: the first modified parabolic equation (MPE1) and second modified parabolic equation (MPE2). The MPE1 is designed to incorporate the geoclimatic factor which is intended to study the multipath fading effect in the location of study. On the other hand, MPE2 is the modified parabolic equation designed to incorporate the effective earth radius factor (k-factor) intended to study the diffraction fading in the location of study. The results and analysis of the results after these modifications confirm our expectation. This result shows that signal loss is due primarily to diffraction fading in Durban while in Botswana, signal loss is due primarily to multipath. This confirms our expectation since a flatter terrain attracts signal loss due to multipath while hilly terrain attracts signal loss due to diffraction fading.
Design and analysis of metamaterial based microstrip patch antennas for wireless applications.
(2023) Ajewole, Bukola Doyinsola.; Kumar, Pradeep.; Afullo, Thomas Joachim Odhiambo.
Due to the tremendous growth of wireless communication applications, there is an enormous demand for more compact antennas with high speed, wider coverage, high gain, and multi-band properties. The microstrip patch antennas (MPAs) and multiple-input multiple-output (MIMO) antennas with high gain and multi-band properties are suitable to fulfil these requirements. MPAs have been found to possess unique qualities such as light weight, low profile, easy fabrication, and integration. However, the low gain, narrow bandwidth, and mutual coupling in the MIMO antennas limit the performance of MIMO systems. Several techniques have been studied and implemented over the years, but they are not without limitations. The utilization of artificial materials such as metamaterials has proven to be efficient in overcoming the limitations of MPAs. Due to the advancement in modern technology, it is necessary to study and use recently developed metamaterial structures. Metamaterials (MeTMs) are artificially engineered materials with electromagnetic properties that are not found in nature. MeTMs are used due to their electric and magnetic properties. The goal of this thesis is to design and investigate a novel metamaterial structure which can be integrated into the microstrip patch antennas for improving their performance. The design, simulation, and measurement of the metamaterial is carried out on the Computer Simulation Technology (CST) studio suite, Advance Design Systems (ADS) software, MATLAB, and the Rohde and Schwarz network analyzer etc. In this thesis, a novel I-shaped metamaterial (ISMeTM) structure is proposed, designed, and investigated. The proposed novel ISMeTM unit cell structure in this work has a characteristic shape that distinguishes it from earlier multi-band MeTMs in the literature. The structure's unit cell is designed to have an overall compact size of 10 mm × 10 mm. The structure generates transmission coefficients at 6.31 GHz, 7.79 GHz, 9.98 GHz, 10.82 GHz, 11.86 GHz, 13.36 GHz, and 15. 5 GHz. These frequency bands are ideal for multi-band satellite communication systems, C, X, and Ku-bands, and radar applications etc. The performance of the MPA is improved in this work, by integrating a novel square split ring resonator (SSRR) metamaterial. The performance of the proposed antenna is investigated and analyzed. The SSRR is designed to have a dimension of 25 x 21.4 x 1.6 mm2 which is the same dimension as the radiating patch of the MPA. The SSRR is etched over the antenna, and it operates at single operating frequency of 5.8 GHz with improved gain from 4.04 to 5.3 dBi. Further, the MPA with improved parameters for multiband wireless systems is designed, analyzed, fabricated, and measured. The proposed design utilizes the ISMeTM array as superstrate with the area of 70 x 70 mm2. The superstrate is etched over a rectangular MPA exhibiting multi-band properties. This antenna resonates at 6.31, 9.65, 11.45 GHz with increased bandwidth at 240 MHz, 850 MHz, and 1010 MHz. The overall gain of the antenna increases by 74.18%. The antenna is fabricated and measured. The simulated results and the measured results are found to be in good agreement. The mutual coupling and low gain problems in MIMO patch antennas is also addressed in this thesis. A 3 x 5-unit cell array of the ISMeTM is used as a superstrate over a two port MIMO patch antenna. The two port MIMO antenna with the superstrate provides triple-band operation and operates over three resonance frequencies at 6.31, 9.09, and 11.41 GHz. A mutual coupling reduction of 26 dB, 33 dB, and 22 dB for the first band, second band and third band, respectively is attained. In this thesis, a novel I-shaped metamaterial structure is introduced, which produces multiband operation. The presented metamaterial is suitable for various multiband wireless communication applications. The integration of a square split ring resonator metamaterial enhances the performance of the antenna. Using the I-shaped metamaterial a high gain multiband microstrip antenna is designed. The I-shaped metamaterial array is utilized to improve the performance of the MIMO antenna. Various antenna parameters confirm that the presented MIMO antenna is suitable for multiband wireless communications.
Determination of millimetric signal attenuation due to rain using rain rate and raindrop size distribution models for Southern Africa.
(2014) Malinga, Senzo Jerome.; Afullo, Thomas Joachim Odhiambo.
The advantages offered by Super High Frequency (SHF) and Extremely High Frequency (EHF) bands such as large bandwidth, small antenna size, and easy installation or deployment have motivated the interest of researchers to study those factors that prevent optimum utilization of these bands. Under precipitation conditions, factors such as clouds, hail, fog, snow, ice crystals and rain degrade link performance. Rain fade, however, remains the dominant factor in the signal loss or signal fading over satellite and terrestrial links especially in the tropical and sub-tropical regions within which South Africa falls. At millimetre-wave frequencies the signal wavelength approaches the size of the raindrops, adversely impacting on radio links through signal scattering and absorption. In this work factors that may hinder the effective use of the super high frequency and extremely high frequency bands in the Southern African region are investigated. Rainfall constitutes the most serious impairment to short wavelength signal propagation in the region under study. In order to quantify the degree of impairment that may arise as a result of signal propagation through rain, the raindrops scattering amplitude functions were calculated by assuming the falling raindrops to be oblate spheroidal in shape. A comparison is made between the performance of the models that assume raindrops to be oblate spheroidal and those that assume them to be spherical. Raindrops sizes are measured using the Joss-Waldvogel RD-80 Distrometer. The study then proposes various expressions for models of raindrops size distributions for four types of rainfall in the Southern Africa region. Rainfall rates in the provinces in South Africa are measured and the result of the cumulative distribution of the rainfall rates is presented. Using the information obtained from the above, an extensive calculation of specific attenuation and phase shift in the region of Southern Africa is carried out. The results obtained are compared with the ITU-R and those obtained from earlier campaigns in the West African sub region. Finally, this work also attempts to determine and characterize the scattering process and micro-physical properties of raindrops for sub-tropical regions like South Africa. Data collected through a raindrop size measurement campaign in Durban is used to compare and validate the developed models.
The impact of visibility range and atmospheric turbulence on free space optical link performance in South Africa.
(2022) Layioye, Okikiade Adewale.; Afullo, Thomas Joachim Odhiambo.; Owolawi, Pius Adewale.
In the recent years, the development of 5G and Massive Internet of Things (MIoT) technologies are fast increasing regularly. The high demand for a back-up and complimentary link to the existing conventional transmission systems (such as RF technology) especially for the “last-mile” phenomenon has increased significantly. Therefore, this has brought about a persistent requirement for a better and free spectrum availability with a higher data transfer rate and larger bandwidth, such as Free Space Optics (FSO) technology using very high frequency (194 𝑇𝐻𝑧−545 𝑇𝐻𝑧) transmission system. There is currently unavailable comprehensive information that would enable the design of FSO networks for various regions of South Africa based on the impact of certain weather parameters such as visibility range (mainly in terms of fog and haze) and atmospheric turbulence (in terms of Refractive Index Structure Parameter (RISP)) on FSO link performance. The components of the first part of this work include Visibility Range Distribution (VRD) modeling using suitable probability density function (PDF) models, and prediction of the expected optical attenuation due to scattering and its cumulative distribution and modeling. The VRD modelling performed in this work, proposed various location-based PDF models, and it was suggested that the Generalized Pareto distribution model best suited the distributions of visibility in all the cities. The result of this work showed that the optical attenuation due to scattering within the coastal and near-coastal areas could reach as high as 169 𝑑𝐵/𝑘𝑚 or more, while in the non-coastal areas it varies between 34 𝑑𝐵/𝑘𝑚 and 169 𝑑𝐵/𝑘𝑚, which suggests significant atmospheric effects on the FSO link, mostly during the winter period. The BER performance analysis was performed and suitable mitigating techniques (such as 4 × 4 MIMO with BPSK and L-PPM schemes) were suggested in this work. The general two-term exponential distribution model provided a good fit to the cumulative distribution of the atmospheric attenuation due to scattering for all the locations. In order to ascertain how atmospheric variables contribute or affect the visibility range, which in turn determines the level of attenuation due to scattering, a time series prediction of visibility using Artificial Neural Network (ANN) technique was investigated, where an average reliability of about 83 % was achieved for all the stations considered. This suggests that climatic parameters highly correlate to visibility when they are all combined together, and this gave significant predictions which will enable FSO officials to develop and maintain a strategic plan for the future years. The modules of the second part of this work encompass the determination of the Atmospheric Turbulence Level (ATL) for each of the locations in terms of RISP (𝐶𝑛2) and its equivalent scintillation index, and then the estimation of the optical attenuation due to scintillation. The cumulative distributions of the optical attenuation due to scintillation and its modeling were also carried out. This research work has been able to achieve the prediction of the ground turbulence strength (through the US-Army Research Laboratory (US-ARL) Model) in terms of RISP using climatic data. In an attempt to provide a more reliable study into the atmospheric turbulence strength within South Africa, this work explores the characteristic behavior of several meteorological variables and other thermodynamic properties such as inner and outer characteristic scales, Monin-Obhukov length, potential temperature gradient, bulk wind shear and so on. According to the predicted RISP from meteorological variables (such as temperature, relative humidity, pressure, wind speed, water vapour, and altitude), location-based and general attenuation due to scintillation models were developed for South Africa to estimate the optical attenuation. The attenuation due to scintillation results show that the summer and autumn seasons have higher ATL, where January, February and December have the highest mean RISP across all the locations under study. Also, the comparison of the monthly averages of the estimated attenuations revealed that at 850 nm more atmospheric turbulence with specific attenuations between 21.04 𝑑𝐵/𝑘𝑚 and 24.45 𝑑𝐵/𝑘𝑚 were observed in the coastal and near-coastal areas than in the non-coastal areas. The study proposes the two-term Sum of Sine distribution model for the cumulative distribution of the optical attenuation based on scintillation, which should be adopted for South Africa. The obtained results in this work for the contributions of scattering and turbulence to the optical link, and the design of the link budget will serve as the major criteria parameters to further compare the outcomes of these results with that of the available terrestrial FSO systems and other conventional transmission systems like RF systems.
Modeling and estimation of scattering attenuation and scintillation effects on optical wireless communication systems in South Africa.
(2021) Kolawole, Olabamidele Olanrewaju.; Afullo, Thomas Joachim Odhiambo.; Mosalaosi, Mosalaosi.
Optical wireless communication (OWC) is a viable complementary solution for next-generation communication networks saddled with meeting the great demands of high data rates and fast internet connectivity. Its numerous advantages include: high data throughput; secure transmission; license-free spectrum; relative low cost of deployment; flexible network connectivity; etc. However, OWC system performance is severely degraded by atmospheric conditions such as fog and scintillation. Most of the proposed FSOC and hybrid FSOC systems in the literature are limited in their capacity to predict the extent to which atmospheric disturbances will impact on the performance of FSOC links in each location where they are to be deployed. This is because of the complexities involved in accessing and analyzing the information on the unique meteorological and climatic characteristics of the locations of interest prior to FSOC link deployment. This important information is necessary for determining the fade margin required by FSOC systems to withstand atmospheric disturbances in various locations of deployment. The effects of other atmospheric conditions such as gas absorption, molecular scattering, and aerosol absorption on the transmission wavelengths of interest (850 and 1550 nm) are negligible, and as such, were not considered in this study. This research, therefore, focuses on the investigation and modeling of scattering attenuation and irradiance fluctuations based on the unique climatic peculiarities of nine major cities in each of the provinces of South Africa where OWC links are to be deployed. These cities are Bloemfontein, Cape Town, Durban, Johannesburg, Kimberley, Mafikeng, Mbombela, Polokwane, and Port Elizabeth. Meteorological data of visibility, wind speed, relative humidity, temperature, fractional sunshine, and atmospheric pressure from 1st January 2010 till 30th June 2018, for each of the locations of interest, are statistically processed and used in the investigation, estimation, and modeling of atmospheric phenomena affecting the performance of OWC signals. To achieve this, visibility modeling and prediction for OWC systems are performed using regression analysis. The results obtained show that various simple and multiple linear regression models reliably forecast visibility from other meteorological parameters considered in this study. The model's selection may be influenced not only by its performance but also by the parameters' availability. While caution is taken to avoid model over-specification, multiple linear regression models are preferable over simple regression models. The significance of the results obtained is the validated alternatives the simple and multiple linear regression models provide while saving costs and avoiding the complexities of measuring FSO visibility in the investigated locations. The relationship between atmospheric visibility and aerosol scattering attenuation has been established by various aerosol scattering models based on the Mie scattering theory. This is made possible because the radii of aerosol particles in the atmosphere are approximately equal to the infra-red wavelengths of optical signals. Thus, the cumulative distribution of visibility and aerosol scattering attenuations based on the Ijaz fog and Kim models for transmission wavelengths of 850 and 1550 nm in nine cities in South Africa are presented. The Ijaz fog and Kim models are also used in computing the probabilities of exceedance, deceedance, and encountering of different aerosol scattering attenuations for 850 and 1550 nm. The impact of these specific attenuations on free space optical communication (FSOC) link performance are investigated for all the various locations of interest. The results show that during foggy weather, the optical signals transmitted at 1550 nm encounter more scattering attenuation than those transmitted at the 850 nm wavelength. The reverse is the case during clear weather periods. Modeling of the minimum required visibility cumulative distribution functions (CDF) during foggy and clear weather conditions for both optical wavelengths is also presented. These CDFs are employed in evaluating the FSOC link availabilities in various cities in South Africa.
Modeling of raindrop size distribution and critical diameters for rainfall attenuation over microwave links in Southern Africa.
(2014) Adetan, Oluwumi.; Afullo, Thomas Joachim Odhiambo.
The inability of service providers to constantly meet the design target of 99.99 % availability of the line-of-sight (LOS) microwave links has caused concern among both the operators and consumers. The non-availability of the links is predominantly due to propagation impairments along the propagation link. These propagation effects include cloud, snow, fog, gas attenuation, rain and atmospheric scintillation. Various studies have shown high vulnerability of radio communication systems operating at microwave (3-30 GHz) and millimeter wave (30-300 GHz) to rainfall attenuation especially in the tropical regions characterized by heavy rainfall and relatively large rain drops when compared to the temperate regions. In order to understand the effects of attenuation due to rain on communication systems in any locality (region), a good knowledge of the raindrop size distribution (DSD) and the rainfall rate estimates is necessary for accurate prediction and estimation of the rainfall attenuation. For this study, experimental raindrop size measurements gathered over a period of three years, using the Joss-Waldvogel RD-80 disdrometer installed at the roof top of the Electrical, Electronic and Computer Engineering building, University of Kwa-Zulu Natal, Durban, a subtropical location in South Africa, is analysed. Disdrometer measurements, sampled at one-minute rate over a period of nine months from Butare, an equatorial site in Rwanda, is also analysed for the purpose of comparison. The estimated R0.01 values for Durban and Butare are employed for the purpose of analysis. Based on the statistical analysis of the measured data samples, DSD parameters are proposed from the negative exponential, modified gamma, Weibull and the lognormal models. The DSD models are compared to models from other countries within and outside the region. The Mie scattering approximation at temperature of 20oC for spherical raindrop shape is adopted for the estimation of the scattering functions. The study further investigates the influence of critical raindrop diameters on the specific rain attenuation for the annual, seasonal and various rainfall regimes in southern Africa. This is achieved analytically by integrating the total rainfall attenuation over all the raindrop sizes and observing the differential change in the attenuation over a given range of drop size diameters. The peak diameter at which the specific rainfall attenuation is maxima is determined for different rainfall regimes. Finally, the cross-polarisation discrimination (XPD) due to rain over Durban is computed at two elevation angles. The results of this study will be helpful for the proper design and allocation of adequate fade margins to achieve the expected quality of service (QoS) in a radio communication system operating in the Southern Africa region.
Multiscaling analysis and modelling of bursty impulsive noise in broadband power line communication channels.
(2017) Asiyo, Mike Omondi.; Afullo, Thomas Joachim Odhiambo.
Power line communication (PLC) networks have the potential to offer broadband application services to homes and small offices cheaply since no additional wiring is required for it implementation. However, like other communication systems, it has its own challenges and the understanding of its channel characteristics is key to its optimal performance evaluation and deployment. Multipath propagation due to impedance mismatch and bursty impulsive noise are the important challenges that must be understood and their effects minimized for optimal system performance. Noise in power line communication networks is non-Gaussian and as such cannot be modelled as the convenient additive white Gaussian noise. The noise is known to be impulsive and in most cases, occurs in bursts. Therefore, it can be referred as bursty impulsive noise. Due to unique nature of this noise in power line channels, modulation and decoding schemes optimized for Gaussian channels may not necessarily work well in PLC systems. Recently developed noise models though take into consideration memory inherent in PLC noise, models capturing both long range correlations and multiscaling behaviour are not yet available in the literature. Furthermore, even though it is known that PLC noise has memory (i.e., it is correlated), the statistical properties of it is not well documented in the literature and will be the focus of this thesis. In this thesis, multiscaling behaviour of PLC noise is investigated. Both fractal and multifractal analysis methods are employed on noise data collected in three different scenarios (small offices, stand-alone apartment and University electronic laboratory) and their characteristics analysed. Multifractal analysis is employed since it is able to characterize both the strengths and frequency of occurrence of bursts in power line noise. Specifically, the contributions in this thesis are as follows: Firstly, empirical evidence is provided that PLC noise clearly manifests long range correlations behaviour. This is achieved by calculating the Hurst parameter (which is a measure of self similarity) in data from the above scenarios. Various methods employed to estimate this Hurst parameter reveal that in all the scenarios, long range dependence is evidenced. Secondly, multifractal detrended fluctuation analysis (MDFA) and multifractal detrending moving average (MDMA) analysis have been used to investigate the temporal correlations and scaling behaviour of power line channel noise measured from the three different scenarios mentioned earlier. Empirical results show that power line noise clearly manifests both long-range correlation and multifractal scaling behaviour with different strengths depending on the environments where they were captured. From the estimated singularity spectrum which is left truncated, it is evident from the two methods used that power line noise is sensitive to small fluctuations and is characterized by large scaling exponents. Multifractal analysis of the reshuffled time series noise reveal that the multifractal nature of PLC noise is as a result of long range correlation inherent in the noise and not from the heavy tailed distributions in it. Thirdly, we propose a multiplicative cascade model for PLC noise that is able to reproduce the empirical findings concerning the PLC noise time series: its local scaling behaviour and long range correlations. Model parameters are derived from the shape of multifractal spectrum of the PLC time series noise collected from measurement campaigns. Since in the recent past, the main challenge in PLC systems has been on how to model bursty impulsive PLC noise, the proposed model will be very useful in evaluating system performance of PLC networks in the presence of the bursty impulsive noise inherent in PLC networks. Moreover, bursts of different frequencies and strengths can be modelled by this proposed model and hence their effects on system performance evaluated. This will also open up investigations into designing modulation and decoding schemes that are optimal in systems prone to bursty impulsive noise.
Novel improvements of empirical wireless channel models and proposals of machine-learning-based path loss prediction models for future communication networks.
(2022) Elmezughi, Mohamed Khalifa Mohamed.; Afullo, Thomas Joachim Odhiambo.
Path loss is the primary factor that determines the overall coverage of networks. Therefore, designing reliable wireless communication systems requires accurate path loss prediction models. Future wireless mobile systems will rely mainly on the super-high frequency (SHF) and the millimeter-wave (mmWave) frequency bands due to the massively available bandwidths that will meet projected users’ demands, such as the needs of the fifth-generation (5G) wireless systems and other high-speed multimedia services. However, these bands are more sensitive and exhibit a different propagation behavior compared to the frequency bands below 6 GHz. Hence, improving the existing models and developing new models are vital for characterizing the wireless communication channel in both indoor and outdoor environments for future SHF and mmWave services. This dissertation proposes new path loss and LOS probability models and efficiently improves the well-known close-in (CI) free space reference distance model and the floating-intercept (FI) model. Real measured data was taken for both line-of-sight (LOS) and non-line-of-sight (NLOS) communication scenarios in a typical indoor corridor environment at three selected frequencies within the SHF band, namely 14 GHz, 18 GHz, and 22 GHz. The research finding of this work reveals that the proposed models have better performance in terms of their accuracy in fitting real measured data collected from measurement campaigns. In addition, this research studies the impact of the angle of arrival and the antenna heights on the current and improved CI and FI models. The results show that the proposed improved models provide better stability and sensitivity to the change of these parameters. Furthermore, the mean square error between the models and their improved versions was presented as another proof of the superiority of the proposed improvement. Moreover, this research shows that shadow fading’s standard deviation can have a notable reduction in both the LOS and NLOS scenarios (especially in the NLOS), which means higher precision in predicting the path loss compared to the existing standard models. After that, the dissertation presents investigations on high-ordering the dependency of the standard CI path loss model on the distance between the transmitting and the receiving antennas at the logarithmic scale. Two improved models are provided and discussed: second-order CI and third-order CI models. The main results reveal that the proposed two models outperform the standard CI model and notable reductions in the shadow fading’s standard deviation values as the model’s order increases, which means that more precision is provided. This part of the dissertation also provides a trade-off study between the model’s accuracy and simplicity.
Queueing theory approach to rain fade analysis at microwave and millimeter bands in tropical Africa.
(2014) Alonge, Akintunde Ayodeji.; Afullo, Thomas Joachim Odhiambo.
With an overwhelming demand of larger bandwidth required for high capacity data with content-rich services ranging from high-speed video streaming to multimedia content, there is a continuous need to migrate to higher microwave bands, particularly beyond the regular Ku and Ka bands (between 11 - 40 GHz). The presence of precipitation at these microwave and millimeter bands (3-300 GHz) generally induce rain fade, which is a constraint to network providers intending to achieve optimal service delivery, at acceptable signal to noise ratios (SNRs). In practice, fade countermeasures – static or dynamic – are necessary to combat the consequences of chronic fluctuations of rainfall resulting in signal deterioration and impairment over communication links. However, the implementation of dynamic fade countermeasures is systematically tied upon the available Channel State Information (CSI), which is often timevariant relative to the occurrence of precipitation events. Time-variation of rainfall events are perceptible in measurable rainfall microstructural parameters which vary intensely in space and time. These spatio-temporal variations yield the generation of observable random patterns of signal attenuation during rain events, often in a stochastic manner. To this end, researchers have emphasized on understanding the underlying behaviour of generic rainfall microstructural parameters such as rainfall rate, rainfall Drop Size Distribution (DSD) and radar reflectivity. Therefore, the investigation of these stochastic properties of rainfall processes is primary in the determination of recognisable patterns of rainfall rate and other microstructures. This thesis introduces the queueing theory approach via the Markov Chain technique to investigate the time-varying characteristics of the rainfall process from distrometer data in subtropical and equatorial Africa. Rainfall data obtained from these two climatic locations, at one minute integration time, were processed from sites in Durban, South Africa and Butare, Rwanda, over a specified measurement period. Initial investigation and comparison of rainfall microstructures undertaken at both sites clearly show key differences in their probability distribution profiles at Stratiform-Convective (SC) bounds. The underlying queue discipline of rainfall spikes and their queue metrics are determined and appraised for system performance using rainfall time series database. The results show rain spike generation processes vividly exhibit a First-Come, First- Served (FCFS) semi-Markovian distributed traffic of M/Ek/s discipline, with a varying degree of servers, for different rainfall regimes. Comparison of queue statistics results over different rainfall regimes at the two locations reveal significant differences in their queue metrics and performances. The knowledge obtained from the queue statistics and SC probability analysis are further employed in the determination and classification of rainfall cells, rainfall growth models and path attenuation prediction. The results are compared and validated with data collected from a 6.73km, 19.5 GHz terrestrial link in Durban.
Radio wave propagation modeling under precipitation and clear-air at microwave and millimetric bands over wireless links in the horn of Africa.
(2017) Diba, Feyisa Debo.; Afullo, Thomas Joachim Odhiambo.
Abstract available in PDF file.
Rainfall attenuation prediction model for dynamic rain fade mitigation technique considering millimeter wave communication link.
(2018) Nabangala, Mary.; Alonge, Akintunde Ayodeji.; Afullo, Thomas Joachim Odhiambo.
To deliver modern day broadband services to both fixed and mobile devices, ultra-high speed wireless networks are required. Innovative services such as the Internet-of-Things (IoT) can be facilitated by the deployment of next generation telecommunication networks such as 5G technologies. The deployment of 5G technologies is envisioned as a catalyst in the alleviation of spectrum congestion experienced by current technologies. With their improved network speed, capacity and reduced communication latency, 5G technologies are expected to enhance telecommunication networks for next generation services. These technologies, in addition to using current Long Term Evolution (LTE) frequency range (600 MHz to 6 GHz), will also utilize millimetre wave bands in the range 24-86 GHz. However, these high frequencies are susceptible to signal loss under rain storms. At such high frequencies, the size of the rain drop is comparable to the wavelength of the operating signal frequency, resulting in energy loss in the form of absorption and scattering by water droplets. This study investigates the effect of intense rain storms on link performance to accurately determine and apply dynamic rain fade mitigation techniques such as the use of a combination of modulation schemes to maintain link connectivity during a rain event. The backpropagation neural network (BPNN) model is employed in this study to predict the state of the link for decision making in employment of dynamic rain fade mitigation. This prediction model was tested on all rainfall regimes including intense rain storms and initial results are encouraging. Further on, the prediction model has been tested on a rainfall event rainfall data collected over Butare (2.6078° S, 29.7368° E), Rwanda, and the results demonstrate the portability of the proposed prediction model to other regions. The evolution of R0.01 (rain rate exceeded for 0.01% of the time in an average year) parameter due to intense rain storms over the region of study is examined and detailed analysis shows that this parameter is double the proposed ITU-R value of 60 mm/h. Moreover, an investigation on the largest rain drop size present in each rain storm is carried out for different storm magnitudes. The study goes further to examine the frequency of occurrence of rain storms using the Markov chain approach. Results of this approach show that rain spikes with maximum rain rates from 150 mm/h and above (intense storms) are experienced in the region of study with probability of occurrence of 11.42%. Additionally, rain spike service times for various rain storm magnitudes are analyzed using the queueing theory technique. From this approach, a model is developed for estimation of rain cell diameter that can be useful for site diversity as a dynamic rain fade mitigation strategy. Finally, the study further investigates second-order rain fade statistics at different attenuation thresholds.
Semi-empirical modelling of subtropical rain attenuation on earth-satellite microwave links.
(2018) Afolayan, Babajide Olugbenga.; Afullo, Thomas Joachim Odhiambo.; Alonge, Akintunde Ayodeji.
The exponential rise in demand for high fidelity content on multiple platforms has in recent years made increased use of the higher echelons of radio communication frequency inevitable. At these high frequencies, wavelength becomes small enough to compare with the size of rain drops and in some cases smaller than drop size. This implies that the impairment due to rain, which already usually forms the most severe form of impairment at higher radio frequency bands, will become even more acute and require rigorous parameterization. This thesis investigates both by rigorous measurements and by theoretical approaches, the attenuation effect of rainfall in a subtropical climate (Durban, South Africa) on a microwave earth-satellite link operating at 12.6 GHz. The link was set up and the received signal level monitored via spectrum analyser sweeps conducted every minute. A Joss-Waldvogel impact disdrometer was installed such that its diaphragm is located a few meters away from the link’s receive antenna. From such a location, all precipitation recorded by the disdrometer are assumed to have some effect on the link. The monthly variation in the received signal during clear air was investigated by taking into consideration the average monthly values of temperature, relative humidity and atmospheric pressure. By employing multiple regression, a linear expression was obtained that can be used to predict the change in received signal level in clear air over the link given the values of these three atmospheric parameters. The attenuation due to the rain events was extracted from the data by carrying out an even-by-event matching of rain rate spikes with the corresponding drop observed in the received signal level at and around the time of the precipitation. The average monthly received signal level during clear air was extracted from the spectrum analyser data and used as the base channel power to which the received signal during rain in the particular month is compared. The difference between the two is stored as the attenuation due to rain in that instant of measurement time. The attenuation data thus accumulated were entered into a computer algorithm and a regression fitting procedure carried out to deduce an empirical set of logarithmic and power law models that relate the total path attenuation to rain rate. The models were then validated by a largely favourable comparison with four existing models, one of which is the in-force ITU-recommended model for slant path attenuation estimations. Random number properties of rain attenuation statistics obtained from the measurement model were exploited to develop a Markov chain approach by which seasonal and annual slant path rain attenuation time series can be generated. By investigating the nature of the probability distributions of the seasonal and annual measured path attenuation statistics, which was found to be lognormal, the state probability matrix necessary for implementing a Markov chain prediction model for future patterns of rain attenuation on a similar link was obtained as the lognormal probability density function. The state transition probability vector for each time period was developed by extracting the fade slope statistics of the measured attenuation. The discrete-time Gaussian distributed fade slope PDF forms the basis for the state transition probability matrix. With these, Markov-generated time series of seasonal and annual slant path attenuation for up to five iterations were obtained. The results make useful data that can be used for long-term planning for rain fade mitigation in a subtropical climate easier to generate without the expense of measurements. The theoretical approach called the Synthetic Storm Technique was also applied to investigate the nature of slant path rain attenuation in Durban. Based on the rainfall pattern captured by the disdrometer, SST approximations for the four seasons of the subtropical year and for years of rain data collection were carried out. The results were compared with the values generated from the measurement model. It reveals that the two models exhibit significant agreement because in a majority of the cases, the A0.01 values obtained are very close. Comparison of the performance of SST as a theoretical model with that of the ITU-recommended method also reveals that the ITU performs slightly better as an alternative to measurement than the SST model. It was observed that during certain precipitation events, the satellite link registers significant attenuation levels several minutes before the disdrometer records any precipitation on the ground. This anomaly was investigated in this work and a few conclusions drawn. By proceeding on the assumption that the observed delay was due to the migrating rain cell interacting with the satellite beam several minutes before reaching the receive antenna, it was demonstrated that the time of delay between precipitation and attenuation is related to the rain height during that particular rain event. A simple mathematical analysis is presented that enables the rain height to be estimated from the delay time. The results obtained range between 1.4 km to 6.7 km which is similarity to rain height values obtained by the ITU model which range from 1.36 km to 6.36 km.

Browse

Browsing Doctoral Degrees (Electronic Engineering) by Author "Afullo, Thomas Joachim Odhiambo."