Browsing by Author "Assefa, Amare"

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    Digital Iq Imbalance Compensation Techniques for ofdm Receivers
    (Addis Ababa University, 2007-06) Assefa, Amare; Ayele, Hailu (PhD)
    With increasing demands for higher data rate and better services, more efficient modulation techniques such as orthogonal frequency division multiplexing (OFDM) are being adopted by many broadband wireless standards. However, the implementation of OFDM-based systems suffers from impairments such as in-phase and quadrature-phase (IQ) imbalances in the receiver front-end analog processing. IQ imbalance has been identified as a key front-end effect for OFDM systems. Such imbalances are caused by the analog processing of the radio frequency (RF) signal. The resulting IQ distortion limits the achievable operating SNR at the receiver and the achievable data rates. The IQ imbalances can severely limit the operating SNR and, consequently, the supported constellation sizes. This leads either to heavy front-end specifications and thus expensive front-end systems or large performance degradations. The direct-conversion receiver is a potential solution offering a flexible architecture to cover multiple RF standards while simplifying the system design and reducing overall costs. However, IQ imbalance has been identified as one of the most serious concerns in the practical implementation of the direct conversion receiver architecture. A very promising approach for coping with these analog impairments is to compensate them digitally. In this thesis, OFDM receiver with front-end analog impairments (such as IQ imbalance) is analyzed and descriptions of the physical sources of IQ imbalances are given. Based on the mathematical analysis an IQ imbalance model is developed. The development of model is based on the direct conversion receivers. Using the developed model, compensation algorithms in the digital domain is presented. Two alternative digital techniques are proposed and analyzed. The first one is based on Least Square (LS) method whereas the second method is based on Least Mean Square (LMS) method. Both compensation schemes use training to estimate the distortion parameters that model the IQ imbalances. The performance of both compensation methods is studied using computer simulation. The bit error rate (BER) verses SNR result indicates that the proposed methods can provide sufficient compensation performance with reasonable assumption.
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    Study on Conversion of Existing HVAC Lines to Hybrid HVAC/HVDC Transmission Line to Increase Transmission Capacity and Efficiency
    (Addis Ababa University, 2015-12) Assefa, Amare; Biru, Getachew (PhD)
    Ethiopia’s economy has been one of the fastest growing economy in the world with two digit figures in the last decade. Consequently, the rate of electric consumption has also been increasing very fast. A large number of new consumers has been recently connected to the grid or waiting to be connected. Infrastructure development boosted on different areas like, new country-wise railway developments, Addis Ababa light rail system, large new irrigation, new Industrial zone development and electricity export consumers. Gilgel gibe III hydropower project is one of mega project undertaken by the government of Ethiopia. The objective of the project is to enhance the hydropower generation capacity of the country through developing a plant with a capacity of 1870 MW. The project is expected to raise the present installed capacity from 2268 MW to 4138 MW. A total of 402km new 400 KV HVAC double circuit transmission line is constructed from Gibe III – Wolayita – Akaki II substation with a capacity of 1500MVA. In the near future the government is trying to finance for Gibee VI and Gibee V hydro power plant with combined capacity of 2100MW as part of a series of cascade dams on the Gibe River. The existing transmission line capacity cannot able to evacuate such power, therefore new transmission line construction is a must to evacuate power to load center, Addis Ababa. This thesis studies the technical and economic feasibility of converting existing 400KV HVAC line from Gibe III – Wolyita- Addis Ababa to Hybrid HVAC/HVDC line to upgrade the power transmission capacity. The new line will have a bipolar 500 KV HVDC as part of the whole transmission line In our study we have observed that we can increase the transmission efficiency from 1500MVA to 2.5GW. The investment cost for constructing new single circuit 400KV HVAC line about 855,978,477.20 USD. (17,958,428,451.66 Birr). The Hybrid HVAC/HVDC line costs 439,173,420.00 USD (9,200,683,149.00 Birr). This price reduction is about 416,805,057.20 USD (8,732,065,948.34 Birr). From Investment point of view, the total reduction of cost is about 49%. Considering the active power loss and corona loss of the two systems, the hybrid HVAC/HVDC line 97% efficient while HVAC system is 91%. The environmental effects like audible noise and radio interference as well as electric level is essentially under standard. It is found that the hybrid HVAC/HVDC line is found to be technically and economically feasible. Key words: Hybrid HVAC/HVDC, HVDC, HVAC, Radio interference, Audible noise, loss cost