Computer Engineering

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Browsing Computer Engineering by Author "Abdo, Mohammed (Prof.)"

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    SNR Enhancement of Energy Detector Algorithm Using Adaptive Wiener Filter In Cognitive Radio
    (Addis Ababa University, 2016-08) Benyam, Hailegnaw; Abdo, Mohammed (Prof.)
    The underutilization of existing radio spectrum and scarcity of available channel become a challenge for modern era communication system. Services such as high speed data communication requires more bandwidth and wide spectrum range. The rising demand in wireless services and applications bring channel constraint on the existing frequency spectrum allocation scheme. To mitigate such a problem the idea of cognitive radio technique came into picture. Cognitive radio can be defined as a radio system that can adaptively and dynamically allow user(s) to use the spectrum when the primary user is absent. The detection and decisions of the primary signal can be facilitated by spectrum sensing technique. Among those techniques, energy detector spectrum sensing is a transmitter based spectrum sensing technique which detects the presence of primary user in cognitive radio system. Detection of primary signal is made by comparing the received signal energy to that of pre-determined energy threshold value. However the performance indicator matrices show that the energy detector algorithm has poor performance when the received SNR value is too low. This thesis work attempts to analyze and compare the performance of energy detector when adaptive Weiner filter is inserted on the front end of conventional energy detector. Based on the performances matrices value, evaluation of the systems (conventional and enhanced energy detector) response will be done. The performance matrices parameters include: Receiver operating curve (ROC), Complement Receiver Operating Curve (CROC), threshold value vs false alarm probability and Detection probability Vs SNR value. The results are presented for AWGN and Rayleigh flat fading channel model. Accordingly focus is made on how the conventional energy detector behaves when the adaptive Wiener filter is inserted on the front end. Finally simulation results show that, insertion of adaptive Wiener filter in the front end of conventional energy detector has improved all the performance matrices considered in thesis work. Keywords: Cognitive radio, primary user, spectrum sensing, energy detector, performance matrices (receiver operating curve, complementary receiver operating curve, threshold value, false alarm and detection probability), adaptive Weiner filter.
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    Study and Review on the Design and Performance Aspects of III-V Semiconductor Solar Cells (Case Study: Comparison on GaAs, Ge, and Si substrate)
    (Addis Ababa University, 2016-11) Behailu, Netsanet; Abdo, Mohammed (Prof.)
    This thesis focuses on studying and reviewing the design and performance aspects of III-V Solar cells; and make a comparison among the solar cells on GaAs, Ge, and Si substrate mainly based on efficiency and cost. The performances of 1-junction GaAs and 2-junction InGaP/GaAs solar cells on GaAs substrate is reviewed and the latest efficiencies are found to be 28.8% and 34.1% respectively. The performance of 3-Junction solar cell on Ge substrate is also reviewed and the latest efficiency is 44.4%. Finally, the performance of 1-Junction, 2-Junction and 3-Junction III-V solar cells on Si substrate is reviewed and the recent efficiencies are 26%, 29.2% and 33.5% respectively. Regarding the cost, one of the most significant cost contributors for III-V solar cells is the cost of the starting substrate. A recent study has revealed that transitioning of a 4” Ge or GaAs substrate to 8” Si substrate would correlate to about 60% reduction in cost for multijunction solar cells. In general, the review shows that the efficiencies of III-V solar cells on Si substrate are continually increasing and recently, the efficiencies of 1-Junction and 2-Junction III-V solar cells on Si substrate become comparable to the one on GaAs substrate. The study indicates that the successful integration of III-V solar cells on Si substrate offers a great promise for lowering the cost of solar energy by combining the high efficiency merits of the III-V materials with the low-cost and abundance of the Si substrate.