Browsing by Author "Murad, Ridwan"

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    Application of Smart Antennas to Cognitive Radio Systems
    (Addis Ababa University, 2013-02) Murad, Ridwan; Mohammed, Abdo (Prof.); Eduard, Jorswieck (Prof.)
    This dissertation studies three interrelated topics: design of non-uniform antenna arrays using genetic algorithm (GA), application of smart antennas to cognitive radio (CR) and determination of the distributions of signal-to-interference ratio (SIR) in Rayleigh fading. The performance of a single-element antenna is limited. To obtain high directivity, narrow beamwidth, low sidelobes, point-to-point and preferred-coverage pattern characteristics, etc., antenna arrays are used. Nowadays, antenna arrays appear in wireless terminals and smart antennas, so robust and e cient array design is increasingly becoming necessary. In array design, it is frequently desirable to achieve both a narrow beamwidth and a low sidelobe level. A uniform array yields the smallest beamwidth and hence the highest directivity. It is followed, in order, by the Dolph-Chebyshev and Binomial arrays. In contrast, Binomial arrays usually possess the smallest sidelobes followed, in order, by the Dolph-Chebyshev and uniform arrays. In the dissertation, GA is used to design a non-uniform linear array that approximates the beamwidth of a uniform array and having smaller sidelobe level than the Dolph-Chebyshev array. The designed array generally exhibits the largest directivity as compared to the uniform, Binomial and DolphChebyshev arrays. The result can be used in applications where narrow beamwidth and low sidelobes levels are preferred such as in switched-beam smart antennas. The second topic addressed by the dissertation is application of smart antennas to cognitive radios. Conventional CRs exploit the licensed spectrum by opportunistically seeking the underutilized radio resource in time, frequency and space (geographic) domains. However, there are other dimensions that need to be explored further for spectrum opportunity. The angle dimension is a typical example. The literatures surveyed point to the fact that little work has been done in this dimension. The dissertation particulary investigates the performance of CRs equipped with smart antennas in exploiting the angle opportunity in Rayleigh fading channel. The proposed cognitive transmitter keeps the interference to the primary receiver below a given threshold while at the same time ensuring high enough signal-to-interference-plus-noise ratio (SINR) at the cognitive receiver. It was shown that the angle spectrum can be signi cantly exploited for spectrum opportunity using smart antennas. It was also shown that the angle spectral e ciency for Rayleigh channel is lower as compared to a non-fading channel with the same power levels and radio terminal locations. Moreover it has been shown that increasing the cognitive transmitter power beyond a certain level has negligible e ect on the angle spectral e ciency. The third topic of the dissertation extends the determination of the distribution of SIN in Rayleigh fading channel when N interferers are present. In fading channels the SINR and SIR are random variables and their distributions have to be known for a correct description of the receiver. The distributions of SIR in Rayleigh fading have been described by deriving a closed form expression for the probability density function, cumulative distribution and outage probabilities.
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    Performance Evaluation of Downlink Power Domain Non-orthogonal Multiple Access over Rayleigh Channel
    (Addis Ababa University, 2020-06) Hana, Getachew; Murad, Ridwan
    Abstract In the future demand or number of mobile user for wireless service become very large, eventually the number of channels assigned to each cell will be insufficient to support the required number of users. And as its obvious spectrum is a very scares and limited resource, so that our country or any other countries can’t use the resource as they want which direct the mobile communication unsustainable in system capacity/ efficiency and quality of user experience. In order to continue to guarantee the sustainability of mobile communication services over the following decade, new technology solutions that can answer to future challenges need to be recognized and developed. Non-orthogonal multiple access (NOMA) is one of radio access techniques for performance enhancement in next-generation cellular communications. It’s imperative to evaluate the performance of NOMA under Rayleigh fading channel. Multipath fading is a challenging channel for wireless communication and in the presence of Rayleigh fading channel it can cause drastic increase in either signal outage probability or average BER. Therefore this thesis uses outage probability and BER performance measurement matrix to evaluate downlink NOMA taking advantage of power domain user multiplexing scheme that utilizes superposition coding (SC) at the transmitter, successive interference cancellation (SIC) applied at the receiver and integrated with multiple-input multiple-output (MIMO) for future radio access. Implementation of MIMO over NOMA and evaluate the impacts of Rayleigh fading channel on NOMA is done through simulation using Matlab simulation. From the simulation result the performance of NOMA is better than OFDMA in terms of BER reduction and for higher SNR value NOMA achieve acceptable performance based on both matrixes, therefore it’s recommended to implement for future radio access.