Communication Engineering

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http://etd.aau.edu.et/handle/12345678/191

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Browsing Communication Engineering by Author "Adugna, Eneyew(PhD)"

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    Convex Combination of Adaptive Linear Finite Impulse Response and Nonlinear Volterra Filters for Acoustic Echo Cancellation
    (Addis Ababa University, 2011-10) Labena, Tagel; Adugna, Eneyew(PhD)
    We all know how much joy and freedom hands-free communication system has brought to our daily life. But associated with this is the problem of acoustic echo which degrades the quality of communication. A number of researches have been conducted in order to eliminate acoustic echo. Most of the acoustic echo cancellers are made on the assumptions of the linearity of the echo. However, this assumption is no longer valid when the nonlinearity along the echo path is taken into account. This led to the invention of Nonlinear Acoustic Echo Cancellation (NLAEC) techniques such as Volterra filters. However, in some situations, NLAEC is inferior to a linear Acoustic Echo Cancellation (AEC), especially when the channel generates a negligible (or no) nonlinear echo. In general, the ratio of the linear to nonlinear echo signal power is unknown a priori, and will vary over time, thus making it difficult to know if NLAEC would improve or degrade the cancellation. Therefore, in this thesis a convex combination of the linear and the nonlinear Volterra filters is studied. Simulation has been carried out and the results indicate the combination scheme follows the best contributing filter regardless of the nature of the echo. Another important problem of AEC is the problem of double talk (DT) which occurs when both parties talk at the same time. If DT is not controlled it results in diverging of the adaptive filter coefficients and as a result audible echoes pass without being cancelled. In order to alleviate this problem most DT controllers have been studied based on correlation of the available signals. Since correlation is valid only for linear relationship; this assumption doesn’t hold when the nonlinearity along the echo path is considered. Therefore, in this thesis mutual information (MI) based DT detection has been studied together with AEC. Simulation is carried out by introducing a DT region and the results obtained indicate performance improvement of 97.06% in terms of algorithm misalignment error (AME). Key words: Acoustic echo, In general, Convex Combination, Volterra, adaptive filters
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    Handoff Initiation and Performance Analysis in Cdma Cellular Systems
    (Addis Ababa University, 2007-02) Sori, Negassa; Adugna, Eneyew(PhD)
    Mobile terminals allow users to access services while on the move. This unique feature has driven the rapid growth in the mobile network industry, changing it from a new technology into a massive industry within less than two decades. Handoff is the essential functionality for dealing with the mobility of the mobile users. Compared with the conventional hard handoff employed in the GSM mobile networks, the soft handoff used in IS-95 CDMA and being proposed for 3G has better performance on both link and system level. In this thesis, an in-depth study of the soft handoff effects on the uplink direction of IS-95 CDMA networks is carried out, leading to optimize soft handoff for capacity under perfect power control approach. We analyze the performance of different handoff algorithms on the forward link or downlink of a CDMA cellular system. Unlike the reverse link, soft handoffs on the forward link requires additional resources such as CDMA codes and transmit power and also causes additional interference. If handoff requests can be processed and completed instantaneously, transmission from the base station with the best link to the user would achieve a significant fraction of the macro diversity gain without utilizing additional resources. However, in practical systems, there is a nonzero handoff completion delay and soft handoff provides the required robustness to delays, although it comes at the expense of additional network resources. Thus, there is a tradeoff between the extent of soft handoff required and the handoff execution delay. We present an analytical framework to study this tradeoff and also discuss simulation results simulated with the help of Matlab. For this, handoff dropping probability is minimized up to 0.1%. Markov concept is applied to describe the system’s statistic behavior in steady state. System performances such as blocking and dropping probabilities and channel efficiency are also determined
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    Performance Evaluation of Receivers for Ultra-Wideband Wireless Communication Systems
    (Addis Ababa University, 2011-07) Yenieneh, Welelaw; Adugna, Eneyew(PhD)
    Ultra Wide Band (UWB) is a promising technology for short range broadband wireless data communication, sensor networks, radar imaging and target identification, location finding and positioning applications. This technology is the best choice for short range wireless communication systems since it is cost-effective, power-efficient and has extremely higher data rates. Any wireless system that has a fractional bandwidth greater than or equal to 20% or a total bandwidth larger than or equal to 500MHz enters in the UWB definition. The Federal Communications Commission (FCC) has allocated 7.5GHz of spectrum for unlicensed use of ultra-wideband (UWB) devices in the 3.1 to 10.6 GHz frequency band and limits the maximum power spectral density (PSD) of the transmitted signal to -41.3dBm/MHz. The main subject of this thesis is to evaluate bit error rate (BER) performance of RAKE and adaptive MMSE receivers for UWB wireless communication systems. First, bit error rate (BER) performance of RAKE receiver on standard IEEE 802.15.3a channel model has been simulated. Here, the BER performance of RAKE receiver for different RAKE types, number of RAKE fingers, combining techniques, length of bit repetition codes, transmitted signal formats and channel types has been evaluated and compared for single user scenario. The simulation results show that an ideal ARAKE receiver using TH-BPSK-UWB transmitted signal format, MRC technique and applying bit repetition coder has the best performance. Then, bit error rate (BER) performance of adaptive MMSE receiver using LMS, NLMS and RLS adaptive algorithms has been evaluated and analyzed on standard IEEE 802.15.3a channel model for different number of users and the simulation results illustrate that adaptive MMSE receiver using RLS algorithm has the best BER performance. Finally, BER performance SRAKE receiver with 5 and 10 fingers has been compared with BER performance adaptive MMSE receiver using the above three adaptive algorithms for single user scenario. The simulation result describes that adaptive MMSE receiver using all the above three algorithms has by far the best BER performance compared to SRAKE receiver with 5 and 10 fingers. All the above simulations have been conducted using MATLAB software. Key Words: UWB, RAKE Receiver, Adaptive MMSE Receiver, LMS, NLMS, RLS, Modulation, Multiple Access, Spread Spectrum, WPAN