Electrical Engineering

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A study of rain attenuation on terrestrial paths at millimetric wavelengths in South Africa.
(2006) Olubunmi, Fashuyi Modupe.; Afullo, Thomas Joachim Odhiambo.
Rain affects the design of any communication system that relies on the propagation of electromagnetic waves. Above a certain threshold of frequency, the attenuation due to rain becomes one of the most important limits to the performance of terrestrial line-of-sight (LOS) microwave links. Rain attenuation which is the dominant fading mechanism at these frequencies is based on nature which can vary from location-to-Iocation and from year-to year. In this dissertation, the ITU-R global prediction techniques for predicting the cumulative distribution of rain attenuation on terrestrial links are studied using a five-year rain rate data for twelve different geographical locations in the Republic of South Africa. The specific attenuation rR (dB/km) for both horizontal and vertical polarization is determined. The path attenuation (dB) exceeded for 0.01% of the time is estimated using the available existing models for the twelve different geographical locations on a I-minute integration time rain rate at 0.01% exceedance of the time averaged over a period of 5 years. A comparison study is done on these available rain attenuation mode'ls; The ITU-R model, Crane Global model, and the Moupfouma models at different frequencies and propagation path lengths based on the actual I-minute integration time rain rate exceeded at 0.01% of the time averaged over a period of 5 years for each geographical locations. Finally, from the actual signal attenuation measurements recorded in Durban over a period of 1 year at 19.5 GHz and a propagation path length of 6.73 km, a logarithmic attenuation model and power attenuation model is proposed for Durban, South Africa. Recommendation for future work is given in the concluding chapter for future improvement on this study. Radio communication designers will find the results obtain in the report useful.
Effective earth radius factor (the k-factor) distribution for southern Africa.
(2005) Kemi, Odedina Peter.; Afullo, Thomas Joachim Odhiambo.
Proper radio link design requires an accurate prediction of the effective earth radius factor (the kfactor) distribution, for the location where propagation is intended. Though a median value of k equals 4/3 is normally use for communication design purposes, in reality the true k-factor values differ, for different locations, globally. The effective earth radius factor distribution for Southern Africa was evaluated in the dissertation. The two Southern African countries chosen for the study are Botswana and Republic of South Africa. The dissertation reports in detail a study on the topic using three years radiosonde data obtain in Maun, Botswana and ten months radiosonde data collected in Durban, South Africa. An analytical model was proposed, which predicts the probability density function of the k-factor for the Southern Africa using the data from these two countries. Also a comparison of the data from the two countries was done in the analysis and reported in the write-up. The application ofthe work was also investigated and reported by simulating a radio link between Sherwood and Umlazi in Kwazulu-Natal Province of South Africa. The consequence of using inappropriate design value of k on link reliability was also investigated and reported. Recommendation for future work was given in the concluding chapter for future improvement on the study. Radio communication designers will find the results obtain in the report useful.
Microwave and millimetre radio wave propagation modelling for terrestrial line-of-sight links in Central Africa.
(2018) Djuma, Sumbiri.; Afullo, Thomas Joachim Odhiambo.
The rapid expansion of the global telecommunication has led to an exponential growth in the demand of wireless services. This has led to the migration to higher frequency bands in the microwave and millimeter wave spectrum. Research has shown that rainfall is the most dominant factor affecting the provision of network services in these bands. Rainfall attenuation is among the major factors often considered in the design of wireless networks operating at higher bands within microwave and millimeter wave spectrum. At tropical and equatorial locations, not only is the occurrence frequency of rainfall events of serious concern to terrestrial and satellite communication systems, but also the high intensity of rain rates and drop size distribution result in extreme fading of line of sight (LOS) system during such events. In this work, daily rainfall measurements from the Rwanda Meteorology Agency (Meteo Rwanda) are obtained for 60 locations within equatorial Rwanda (between latitudes of 1o2'S and 2o45'S and longitudes of 280 45'E and 30052'E), in Central Africa, to develop rain rate and rain attenuation maps for wireless radio links. From these long term annual rainfall measurements spanning a minimum of 10 years at these locations, rainfall rate statistics and drop size distribution result in extreme fading of line of sight (LOS) system during such events. In this work, daily rainfall measurements from the Rwanda Meteorology Agency (Meteo Rwanda) are obtained for 60 locations within equatorial Rwanda (between latitudes of 1o2'S and 2o45'S and longitudes of 280 45'E and 30052'E), in Central Africa, to develop rain rate and rain attenuation maps for wireless radio links. From these long term annual rainfall measurements spanning a minimum of 10 years at these locations, rainfall rate statistics estimated from appropriate models are applied to determine fade margin for radio link availabilities between 99% and 99.999%. Furthermore, specific attenuation estimates due to rainfall are proposed from International Telecommunication Union (ITU) recommendations at selected frequencies of the microwave and millimeter bands, for the design of wireless networks. Results obtained from this approach incorporating both rainfall rate zones and specific attenuation over Rwanda are presented as spatial contour maps representations for different ranges of link availability. Further, disdrometer data collected in Butare, Rwanda (20 35' 53.88” S and 290 44' 31.5” E) for a period of 32 months between 2012 and 2015 have been use to develop a suitable model on drop size distribution in the region. Rainfall data was classified into four different regimes, namely, drizzle, widespread, shower and thunderstorm. Different raindrop size distribution (DSD) models such as Lognormal, Gamma, Marshall-Palmer and Weibull distributions are selected and the method of moment technique is applied for estimating input DSD fit-parameters for those DSD models. From the results, it is observed that different models have varying performances as the rainfall regime varies from drizzle to widespread, shower and later as thunderstorm, except the Marshall- Palmer model which shows the inadequacy for the region. It is found that neither the Lognormal nor other models match perfectly wel I with the measured DSD, particularly at high rainfall rates. Therefore, a new rainfall DSD model or Central Africa is developed and found to be an improvement over the existing models. The Mie Scattering technique (spherical method) is employed to derive the scattering parameters. Therefore, the derived scattering parameters with DSD models are used for the estimation of rainfall attenuation in the region of Central Africa. Finally, the synthetic storm techniques (SST) is applied for comparison with other rainfall attenuation models.
Modelling of broadband powerline communication channels.
(SAIEE Publications., 2011) Mulangu, Chrispin Tshikomba.; Afullo, Thomas Joachim Odhiambo.; Ijumba, Nelson Mutatina.
This paper develops a new PLC model and investigates the impact of the load, line length, and diameter of the transmission line on the channel transfer function over the frequency range of 1-20 M Hz. The results show that with 42 degrees of freedom, the proposed model leads to an average RM SE value of approximately 5.2 dB. With the same conditions, the Phillips model leads to an average RMSE value of approximately 1.62 dB.
Performance evaluation of FSO communication systems over weak atmospheric turbulence channel for eastern coast of South Africa.
(2017) Ogunmodede, Henry Ayodeji.; Afullo, Thomas Joachim Odhiambo.; Owolawi, Pius Adewale.
Free space optical (FSO) communication, otherwise known as optical wireless communication (OWC), is an established line-of-sight telecommunication technique which utilises an optical signal carrier to propagate modulated signals in the form of a light wave (visible or infrared) over the atmospheric medium. It has numerous advantages, including ease of deployment, large bandwidth, cost effective, full duplex high data rate throughput, protocol independence, highly secured data rate transmission, unregulated frequency spectrum, limited electromagnetic interference, and minimum amount of power consumption. With all the inherent advantages in FSO systems, the technology is impaired by atmospheric turbulence. Atmospheric turbulence occurs due to the persistent random changes of the refractive index as a result of variations in atmospheric temperature and pressure. This results in fluctuations in the irradiance of the laser (simply referred to as scintillation), which may lead to attenuation of optical signals in the FSO communication system. Thus, atmospheric attenuation and turbulent conditions have negative effects on the performance and ease of deployment of FSO communication systems. In this dissertation, we examine the performance of FSO systems over weak atmospheric turbulence channel for the eastern coast of South Africa. We evaluate the feasibility of the FSO link and how to improve the reliability by estimating the link margin, probability of attenuation exceedance, power scintillation index, overall power loss due to attenuation and turbulence, link budget estimate for different link lengths and wavelengths. The FSO system availability estimated for the eastern coast of South Africa is above 99% for link distances ranging from 1 km-4 km at 850 nm, 950 nm and 1550 nm. It is also observed that the FSO link availability increases with corresponding increase in wavelengths. Adopting the Kim model to estimate the atmospheric attenuation at 850 nm wavelength, the attenuation due to scattering contributes 9.47% to the absolute atmospheric losses while the atmospheric turbulence loss contributes 90.53% to the overall power loss at a link range of 4 km. Using the Ferdinandov model for a link range of 4 km at 950 nm wavelength, the attenuation due to scattering contributes 8.81% to the total power loss while the atmospheric turbulence loss contributes 91.19% to the overall power loss. It is observed that the attainable link distance increases with increase in atmospheric visibility status. The FSO system availability reduces with increase in the propagation link distance. Furthermore, it is found that the fading loss from scintillation effects strongly depends on the power scintillation index. An increase in the power scintillation index, causes an increase in the fading loss. Thus, the power scintillation index also increases per unit increase in transmission link length and refractive index. The compensation margin for such atmospheric fading loss increases with decrease in accessible FSO system bound probability. Therefore, for a highly reliable FSO system link, extra margin must be incorporated to compensate for fading loss caused by scintillation.
WiMAX performance analysis in a selected rural area in South Africa.
(2006) Naidoo, Deshree.; Afullo, Thomas Joachim Odhiambo.
Bringing broadband access to rural and remote communities has been a very difficult task due to the high cost of installation and complete lack of infrastructure in certain areas. Options currently under serious consideration to enable this access have included GSM, CDMA, Power line telecommunications, and DSL technologies. However, with the addition of the IEEE 802.16 standard - commonly referred to as WiMAX - with its various advantages, another serious option is now being considered. WiMAX is standard developed to provide wireless broadband access to metropolitan area networks. It is capable of providing LOS and NLOS coverage in the range of 350 km with data rates up to 75 Mbps. This research therefore looks at the WiMAX standard MAC and PHY layers with special focus on the PHY layer properties enabling provision ofNLOS coverage (WirelessMAN OFDM PHY). The research evaluates the performance of WiMAX in a rural environment in terms of coverage and traffic analysis. Other aspects include multipath fading and channel estimation. The traffic analysis is carried out using traffic projection formulated over a period often years for Nkandla, a typical rural area in South Africa. This is done to evaluate the sustainability of the designed base stations over the ten-year period with the growing traffic.

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Browsing Electrical Engineering by Author "Afullo, Thomas Joachim Odhiambo."