Scielo RSS <![CDATA[SAIEE Africa Research Journal]]> vol. 109 num. 1 lang. en <![CDATA[SciELO Logo]]> <![CDATA[<b>Thermal instability analysis of a synchronous generator rotor using direct mapping</b>]]> This paper presents a direct and practical method for mapping the thermal behaviour of a synchronous generator. Since temperature variations can lead to rotor thermal instability which adversely affects the operation of the generating unit, a better understanding of this phenomenon is required. The two main methods of performing thermal instability testing - direct current injection and friction/windage - are found to be practiced internationally without preference. Infrared thermography is used here as a means of determining the thermal performance of the rotor under different testing scenarios. The experimental testing is conducted using a scaled setup of a balancing facility and a 600 MW generator rotor. The results obtained are presented in the form of surface temperature maps. The thermal distribution of the two different methods were found to differ substantially with the friction method exhibiting a uniform surface distribution while the current method exhibited areas of higher temperature concentration around the rotor pole faces. <![CDATA[<b>Detection of GSM And GSSK signals with soft-output demodulators</b>]]> A special case of conventional spatial modulation (SM) is demonstrated in space shift keying (SSK) modulation, where the amplitude and/or phase modulation symbols are eliminated from the transmission process so as to reduce system complexity. However, the spectral efficiencies of both schemes increase only logarithmically. Hence, large antenna arrays are required to achieve high spectral efficiencies. To reduce the transceiver overhead, while achieving improved spectral efficiencies in both SM and SSK, generalised SM (GSM) and generalised SSK (GSSK), respectively, were proposed. Typically, in coded channels, soft-output detection coupled with soft-input channel decoding yields significant signal-to-noise ratio (SNR) gain. Hence, in this paper, we propose soft-output maximum-likelihood (ML) detectors (SOMLDs) for the GSM and GSSK schemes, with an aim to further improve the error performance of the systems. Monte Carlo simulation results demonstrate that the error performance of the proposed SOMLD schemes closely match with that of their hard-decision ML detector counterparts in uncoded channels; while significant SNR gains are yielded in coded channels. <![CDATA[<b>Neural network fault diagnosis system for a diesel-electric locomotive's closed loop excitation control system</b>]]> In closed loop control systems fault isolation becomes extremely difficult in the case of feedbacks being oscillatory due to corrupted signals or malfunctions in actuators. This paper investigates and highlights the development of an off-line fault detection and isolation system for the isolation of faults, which cause oscillatory conditions on a General Electric (GE) Diesel-Electric Locomotive's excitation control system. The paper illustrates the use of artificial neural networks as a replacement to classical analytical models used for residual generation. The artificial neural network model's design is based on model-based dedicated observer theory to isolate sensor, as well as component faults, where observer theory is utilised to effectively select input-output data configurations for detection of sensor and component faults causing oscillations. Residual Evaluation is done with the use of a moving average filter incorporated with the simple thresholding technique. The results indicated 100% accuracy for the detection and isolation of the component or sensor responsible for causing excessive oscillation in the excitation control system. <![CDATA[<b>Energy efficient statistical cooperative spectrum sensing in cognitive radio networks</b>]]> Cooperative spectrum sensing (CSS) alleviates the problem of imperfect detection of primary users (PU)s in cognitive radio (CR) networks by exploiting spatial diversity of the different secondary users (SUs). The efficiency of CSS depends on the accuracy of the SUs in detecting the PU and accurate decision making at the fusion center (FC). This work exploits the higher order statistical (HOS) tests of the PU signal for blind detection by the SUs and combination of their decision statistics to make a global decision at the FC. To minimize energy, a two stage optimization paradigm is carried out, firstly by optimal iterative selection of SUs in the network using Lagrange criterion and secondly optimized fusion techniques achieved by Neyman Pearson. The probability of detecting the PU based on HOS and hard fusion schemes is investigated. The results indicate that the Omnibus HOS test based detection and optimized majority fusion rule greatly increases the probability of detecting the PU and reduces the overall system energy consumption. <![CDATA[<b>Enhanced congestion management for minimizing network performance degradation in OBS networks</b>]]> Current global data traffic is increasingly dominated by delay and loss intolerant IP traffic which generally displays a structural self-similarity. This has necessitated the introduction of optical burst switching (OBS) as a supporting optical backbone network switching technology. Due to the buffer-less nature of optical burst switched (OBS) networks, contention/congestion in the core network can quickly lead to degradation in overall network performance at moderate to high traffic levels due to heavy burst loses. Several approaches have been explored to address this problem, notably measures that would minimize burstification delays, congestion, blocking at the same time enhancing end-to-end throughput as well as rational and fair utilization of the links. The aim is to achieve a consistent quality of service (QoS). Noting that congestion minimization is key to a consistent QoS provisioning, in this paper we propose a congestion management approach called enhanced congestion management (ECM) that seeks to guarantee a consistent QoS as well as rational and fair use of available links. It is primarily a service differentiation based scheme that aims at congestion, blocking and latency minimization, by way of combining time averaged delay segmented burstification as well as random shortest path selection based deflection routing and wavelength assignment. Simulation results show that ECM can effectively minimise congestion and at the same time improve both throughput and effective utilization, under moderate to high network traffic conditions. Overall, we show that the approach guarantees a consistent QoS. <![CDATA[<b>SAM: a meta-heuristic algorithm for single machine scheduling problems</b>]]> The main contribution of this work is to investigate the hypothesis that the performance of the Simulated Annealing (SA) algorithm can be improved by combining it with other sampling methods in solving the single machine weighted earliness and tardiness scheduling problem. In this paper we present the formulation of our novel hybrid algorithm, SAM, and the main results. The algorithm SAM, which stands for Simulated Annealing with Metropolis-Hastings, is a two-step process. To initialise, the search space of possible feasible schedules is divided into a number of sections. In the first step Metropolis-Hastings sampling is performed over the sections in order to obtain characteristics of a likelihood function over the sections so that a section with a high likelihood of containing the optimal schedule is chosen for step two. In step two SA is run on the pruned search space to find a solution schedule. This relies on a novel way of visualising the search space in a geometric way as a wheel of indices. The results show that low deviation solutions can be obtained in significantly shorter runs with SAM than seen in the literature or required of the basic SA algorithm. We can achieve a 4.5 times reduction in required algorithm run time to achieve a less than 2% deviation from the optimum value. SAM even enables us to find the optimal solution in as few as 1000 iterations of SA in some cases. <![CDATA[<b>Computational modelling for dish-to-dish coupling investigations on MeerKAT telescope</b>]]> The MeerKAT telescope will form part of the Square Kilometre Array (SKA) which will have orders of magnitude greater sensitivity than existing radio telescopes. Radio Frequency Interference (RFI) poses a great threat to such sensitive equipment. We characterize electromagnetic (EM) signals on a single dish structure using a physical scale model, computational electromagnetic (CEM) modelling and field measurement. The CEM code and method of moments frequency domain numerical technique are utilised throughout the study. We successfully achieve CEM model verification through measurement of a physical scale model in an anechoic chamber. The validated model is further verified with full scale on-site measurements. A transfer function determined from an incident electric field and the associated induced currents on various cables of a single dish structure is used to compare simulation and measurement. We use our corroborated CEM model to investigate dish to dish coupling in order to predict whether the MeerKAT system would be at risk. With a low power radiation of -70 dBm from a neighbouring dish, we predict coupled power of less than -120 dBm to the nearest dish.