Browsing by Author "Mosalaosi, Modisa."

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Characterization and modelling of the channel and noise for broadband indoor powerline communication (plc.) networks.
(2016) Mosalaosi, Modisa.; Afullo, Thomas Joachim Odhiambo.
Power Line Communication (PLC) is an interesting approach in establishing last mile broad band access especially in rural areas. PLC provides an already existing medium for broad band internet connectivity as well as monitoring and control functions for both industrial and indoor usage. PLC network is the most ubiquitous network in the world reaching every home. However, it presents a channel that is inherently hostile in nature when used for communication purposes. This hostility is due to the many problematic characteristics of the PLC from a data communications’ perspective. They include multipath propagation due to multiple reflections resulting from impedance mismatches and cable joints, as well as the various types of noise inherent in the channel. Apart from wireless technologies, current high data rate services such as high speed internet are provided through optical fibre links, Ethernet, and VDSL (very-high-bit-rate digital subscriber line) technology. The deployment of a wired network is costly and demands physical effort. The transmission of high frequency signals over power lines, known as power line communications (PLC), plays an important role in contributing towards global goals for broadband services inside the home and office. In this thesis we aim to contribute to this ideal by presenting a powerline channel modeling approach which describes a powerline network as a lattice structure. In a lattice structure, a signal propagates from one end into a network of boundaries (branches) through numerous paths characterized by different reflection/transmission properties. Due to theoretically infi nite number of reflections likely to be experienced by a propagating wave, we determine the optimum number of paths required for meaningful contribution towards the overall signal level at the receiver. The propagation parameters are obtained through measurements and other model parameters are derived from deterministic power system. It is observed that the notch positions in the transfer characteristics are associated with the branch lengths in the network. Short branches will result in fewer notches in a fixed bandwidth as compared to longer branches. Generally, the channel attenuation increase with network size in terms of number of branches. The proposed model compares well with experimental data. This work presents another alternative approach to model the transfer characteristics of power lines for broadband power line communication. The model is developed by considering the power line to be a two-wire transmission line and the theory of transverse electromagnetic (TEM) wave propagation. The characteristic impedance and attenuation constant of the power line v are determined through measurements. These parameters are used in model simplification and determination of other model parameters for typical indoor multi-tapped transmission line system. The transfer function of the PLC channel is determined by considering the branching sections as parallel resonant circuits (PRC) attached to the main line. The model is evaluated through comparison with measured transfer characteristics of known topologies and it is in good agreement with measurements. Apart from the harsh topology of power line networks, the presence of electrical appliances further aggravates the channel conditions by injecting various types of noises into the system. This thesis also discusses the process of estimating powerline communication (PLC) asynchronous impulsive noise volatility by studying the conditional variance of the noise time series residuals. In our approach, we use the Generalized Autoregressive Conditional Heteroskedastic (GARCH) models on the basis that in our observations, the noise time series residuals indicate heteroskedasticity. By per forming an ordinary least squares (OLS) regression of the noise data, the empirical results show that the conditional variance process is highly persistent in the residuals. The variance of the error terms are not uniform, in fact, the error terms are larger at some portions of the data than at other time instances. Thus, PLC impulsive noise often exhibit volatility clustering where the noise time series is comprised of periods of high volatility followed by periods of high volatility and periods of low volatility followed by periods of low volatility. The burstiness of PLC impulsive noise is therefore not spread randomly across the time period, but instead has a degree of autocorrelation. This provides evidence of time-varying conditional second order moment of the noise time series. Based on these properties, the noise time series data is said to suffer from heteroskedasticity. GARCH models addresses the deficiencies of common regression models such as Autoregressive Moving Average (ARMA) which models the conditional expectation of a process given the past, but regards the past conditional variances to be constant. In our approach, we predict the time-varying volatility by using past time-varying variances in the error terms of the noise data series. Subsequent variances are predicted as a weighted average of past squared residuals with declining weights that never completely diminish. The parameter estimates of the model indicates a high de gree of persistence in conditional volatility of impulsive noise which is a strong evidence of explosive volatility. Parameter estimation of linear regression models usually employs least squares (LS) and maximum likelihood (ML) estimators. While maximum likelihood remains one of the best estimators within the classical statistics paradigm to date, it is highly reliant vi on the assumption about the joint probability distribution of the data for optimal results. In our work, we use the Generalized Method of Moments (GMM) to address the deficien cies of LS/ML in order to estimate the underlying data generating process (DGP). We use GMM as a statistical technique that incorporate observed noise data with the information in population moment conditions to determine estimates of unknown parameters of the under lying model. Periodic impulsive noise (short-term) has been measured, deseasonalized and modeled using GMM. The numerical results show that the model captures the noise process accurately. Usually, the impulsive signals originates from connected loads in an electrical power network can often be characterized as cyclostationary processes. A cyclostationary process is described as a non-stationary process whose statistics exhibit periodic time varia tion, and therefore can be described by virtue of its periodic order. The focus of this chapter centres on the utilization of cyclic spectral analysis technique for identification and analysis of the second-order periodicity (SOP) of time sequences like those which are generated by electrical loads connected in the vicinity of a power line communications receiver. Analysis of cyclic spectrum generally incorporates determining the random features besides the pe riodicity of impulsive noise, through the determination of the spectral correlation density (SCD). Its effectiveness on identifying and analysing cyclostationary noise is substantiated in this work by processing data collected at indoor low voltage sites.
Power line communication (PLC) channel measurements and characterization.
(2014) Mosalaosi, Modisa.; Afullo, Thomas Joachim Odhiambo.
The potential of the power line to transport both power and communication signals simultaneously has been realized and practiced for over a century, dating back to the 1900’s. Since the key aspect of power line communications being its expansivity, its implementations were largely as a retrofit technology. This motivation of power line communication is typical for low-, medium-, and high voltage distribution networks. Beyond the “last mile” part, there’s an uprising appeal for intra-building networks currently targeted for home automation (smart homes/buildings) and in-building networking. The optimum use of the existing power line channels has been a focus area for researchers and designers, with the inherent channel hostility proving a serious drawback for high speed data communications. The low-voltage electrical network has unpredictable noise sources, moreover it has two other main disadvantages as a communication channel. The first short coming has to do with the unknown characteristics of the power cable and topology of the network, the second arises from the time-dependent fluctuation of the impedance level of the power line as the loads are switched into and out of the power line network in an unpredictable manner. These factors determine the behaviour of the power line channel when a high frequency signal is impressed on it. This study has shown that the behaviour of indoor power line channels can be captured using a multipath based model even with limited qualitative and/or quantitative knowledge of the network topology. This model is suitable for typical indoor power line channels where knowledge of the topology is near impossible. Some of the feed parameters are obtained through measurements. With sufficient adjustment of control parameters, this model was successfully validated using sample measured channels from the numerous measurements. Through noise measurements, this study has established that impulsive noise is the rifest in the frequency band of interest. The impulsive energy rises well above background noise, which translates to possible data “black outs”. The statistics of the components of this noise are presented. A model of sufficient simplicity is used to facilitate the qualitative description of the background noise through its power spectral density. Two descriptions are provided in terms of the worst and best case scenarios of the background noise occurrences. The model has a good macroscopic capture of the noise power spectral density, with narrow-band interference visible for the worst case noise. Due to the multipath nature of the power line channel, this study also presents the dispersive characteristics of the power line as a communication channel. The power delay profile is used to determine parameters such as first arrival delay, mean excess delay, root mean square delay spread and maximum delay spread. The statistics of these parameters are presented. Also, the coherence bandwidth of power line channels is studied and its relationship with the rms delay spread is developed. It is in view of this work that further research in power line communication and related topics shall be inspired.
Power line communication impedance profiling and matching for broadband applications.
(2018) Chelangat, Florence.; Afullo, Thomas Joachim Odhiambo.; Mosalaosi, Modisa.
Power line communication(PLC) is a wired communication technology that has recently re- ceived a lot of attention due to its attractive prospects towards home and /or neighborhood network applications as well as smart grid technologies. It allows establishing digital com- munications without any additional wiring requirements. Effectively, one’s home and/or neighborhood wiring contributes into a smart grid to deploy various data services. It is well known that the power grid is one of the most pervasive infrastructure built to provide electricity to customers, therefore, utilizing this infrastructure for digital communications will only result in an ubiquitous telecommunications network. It is common practice to use wires to establish a physical connection in many telecommunications channels, but most electronic devices already have a pair of wires connected to the power lines. Therefore, these wires can be used to simultaneously establish digital communications. Thus, power line communications can be used as an alternative solution to more established technologies such as wireless, coaxial and optical communications. As a promising technology, PLC has attracted a lot of research and has become an active area of research which continues to evolve over time. Notwithstanding its advantages, PLC has issues, namely, severe noise at low frequencies and varying characteristic impedance. This is primarily because the power line channel was not originally designed to be used for communications, thus, it remains a harsh channel. Other challenges arise from the fact that there are different wiring practices around the world, unpredictable loading characteristics as well as differential- and common-mode characteristic impedance. As a result, there is a considerable amount of noise signal attenuation during data transmission. Loss of signal can be addressed by increasing the power at the transmitter, noise reduction and/or reducing channel attenuation to improve the signal-to-noise ratio. However, PLC modems are subject to legislation that impose a limit with regards to the signal levels in the lines. Power lines are good radiators at high frequencies which makes them behave like large antennas with the ability to intercept other radiations in the same frequency range. The radiated signal is proportional to the currents in the line, thus, increasing line currents will not solve the problem but would rather lead to violation of electromagnetic compatibility (EMC) regulations. In this work, an alternative solution is provided which seeks to address the issue of signal attenuation caused by the changing input impedance of a typical power line channel. The deleterious effects of noise are not considered since this work focuses on broadband PLC in the 1–30 MHz frequency range. The objective of this work was to design and build an impedance adaptive coupler to mitigate effects of channel attenuation caused by varying impedance. In this way, the propagating signal will “see” a uniform impedance and as a result the data output will be improved. The work was facilitated by measuring several impedance profiles of PLC channels in the band of interest. Typically, the network topology of PLC networks is not known and the building architectural blueprints are not always readily available. To overcome this issue,this work was performed on power line test-beds designed to mimic varied typical PLC network topologies. Moreover, there is an additional benefit in that it is possible to relate the output impedance profile to the network topology. The channel input impedance characteristics were determined in a deterministic manner by considering a power line network as a cascade of parallel resonant circuits and applying transmission line theory to develop the model. The model was validated by measurements with good agreement over the frequency range was considered. Several measurements were then used to determine the minimum, average and maximum input impedance that a signal will experience as it traverses the channel. It was found that, regardless of the network size (in terms of number of branches), the average input impedance is 354 ± 1.1 % Ω in the 1-30 MHz frequency band. Due to the unpredictable nature of the input impedance of the power line network, an impedance adaptive bidirectional coupler for broadband power line communications was designed. The impedance matching is achieved by using typical L-section matching networks in the 1–30 MHz band. The matching section of the coupler has the characteristics of a lowpass filter while the coupling section is a highpass filter, effectively forming a bandpass network. The simulated transfer characteristics of the designed coupler performs very well for impedances starting around 150 Ω and the performance improves a great deal as the impedance increases. The coupler can still be improved to accommodate much lower input impedances (as low as 50 Ω). However, based on the measured results of input impedance, it was observed that the power line channel impedance is statistically higher than 200 Ω most of the time which makes the presented design acceptable.
Rain Attenuation Modelling and Prediction for Optical Wireless Communication Systems in Durban, South Africa.
(2021) Buthelezi, Sabelo Qiniso.; Afullo, Thomas Joachim Odhiambo.; Mosalaosi, Modisa.
The continuous demand for more reliable wireless communication systems with extremely high data rates has accelerated various aspects of research topics to be able to meet future needs. One of the most crucial topics in the field of communication is free-space optics (also known as optical wireless communication). It is well-known that the performance of any optical wireless communication system is strongly influenced by the atmospheric conditions in a given environment. In foggy, rainy, and clear weather conditions, optical signals are known to be attenuated due to scattering. The received signal is diminished in the presence of snow, rain, or even haze. Rain and clear weather conditions will be the focus of this research as there is hardly snow or haze in South Africa, especially Durban since it is a subtropical region. In this research work, rain attenuation modelling and prediction will be done using an empirical method based on the relationship between the observed attenuation distribution and the related observed rain intensity distribution at a 30 second integration period. A disdrometer is used to obtain the rain intensity, and a power meter is used to log the received signal power level every 30 seconds to evaluate the influence of rain on the signal transmitted. The International Telecommunication Union (ITU-R) recommends targeting for 99.99 % system availability; as a result, the rainfall rate (R0.01) in the research region must be estimated for 0.01 percent of the time. The rain intensity and raindrop size distribution (DSD) modelling is then performed from the empirical method, obtaining R0.01 for Durban for all months throughout the experiment period. Using the disdrometer diameter ranges, the spherical droplet assumption is used to estimate the scattering parameters for frequencies between 2 GHz and 1000 GHz. The relationship between the received signal level and the intensity of rain for a particular weather condition at a specific time is then obtained. Transceivers with a fixed length of 7 meters between them, due to shortage of material such as the fiber cables to link the transceivers to the computer for data monitoring and logging, and for accurate alignment, were used to conduct these experiments. This relationship is compared against the French model at a wavelength of 850 nm. The main results obtained from this work reveal that there are extremely high attenuation values compared to the French model, which thus calls for further investigation to provide the optimum model that can accurately predict these effects for reliable optical wireless communications in Durban, South Africa.