Browsing by Author "Rezenom, Seare Haile."

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Cell search in frequency division : duplex WCDMA networks.
(2006) Rezenom, Seare Haile.; Broadhurst, Anthony D.
Wireless radio access technologies have been progressively evolving to meet the high data rate demands of consumers. The deployment and success of voice-based second generation networks were enabled through the use of the Global System for Mobile Communications (GSM) and the Interim Standard Code Division Multiple Access (lS-95 CDMA) networks. The rise of the high data rate third generation communication systems is realised by two potential wireless radio access networks, the Wideband Code Division Multiple Access (WCDMA) and the CDMA2000. These networks are based on the use of various types of codes to initiate, sustain and terminate the communication links. Moreover, different codes are used to separate the transmitting base stations. This dissertation focuses on base station identification aspects of the Frequency Division Duplex (FDD) WCDMA networks. Notwithstanding the ease of deployment of these networks, their asynchronous nature presents serious challenges to the designer of the receiver. One of the challenges is the identification of the base station identity by the receiver, a process called Cell Search. The receiver algorithms must therefore be robust to the hostile radio channel conditions, Doppler frequency shifts and the detrimental effects of carrier frequency offsets. The dissertation begins by discussing the structure and the generation of WCDMA base station data along with an examination of the effects of the carrier frequency offset. The various cell searching algorithms proposed in the literature are then discussed and a new algorithm that exploits the correlation length structure is proposed and the simulation results are presented. Another design challenge presented by WCDMA networks is the estimation of carrier frequency offset at the receiver. Carrier frequency offsets arise due to crystal oscillator inaccuracies at the receiver and their effect is realised when the voltage controlled oscillator at the receiver is not oscillating at the same carrier frequency as that of the transmitter. This leads to a decrease in the receiver acquisition performance. The carrier frequency offset has to be estimated and corrected before the decoding process can commence. There are different approaches in the literature to estimate and correct these offsets. The final part of the dissertation investigates the FFT based carrier frequency estimation techniques and presents a new method that reduces the estimation error.
Iterative graphical algorithms for phase noise channels.
(2020) Rezenom, Seare Haile.; Takawira, Fambirai.
This thesis proposes algorithms based on graphical models to detect signals and charac- terise the performance of communication systems in the presence of Wiener phase noise. The algorithms exploit properties of phase noise and consequently use graphical models to develop low complexity approaches of signal detection. The contributions are presented in the form of papers. The first paper investigates the effect of message scheduling on the performance of graphical algorithms. A serial message scheduling is proposed for Orthogonal Frequency Division Multiplexing (OFDM) systems in the presence of carrier frequency offset and phase noise. The algorithm is shown to have better convergence compared to non-serial scheduling algorithms. The second paper introduces a concept referred to as circular random variables which is based on exploiting the properties of phase noise. An iterative algorithm is proposed to detect Low Density Parity Check (LDPC) codes in the presence of Wiener phase noise. The proposed algorithm is shown to have similar performance as existing algorithms with very low complexity. The third paper extends the concept of circular variables to detect coherent optical OFDM signals in the presence of residual carrier frequency offset and Wiener phase noise. The proposed iterative algorithm shows a significant improvement in complexity compared to existing algorithms. The fourth paper proposes two methods based on minimising the free energy function of graphical models. The first method combines the Belief Propagation (BP) and the Uniformly Re-weighted BP (URWBP) algorithms. The second method combines the Mean Field (MF) and the URWBP algorithms. The proposed methods are used to detect LDPC codes in Wiener phase noise channels. The proposed methods show good balance between complexity and performance compared to existing methods. The last paper proposes parameter based computation of the information bounds of the Wiener phase noise channel. The proposed methods compute the information lower and upper bounds using parameters of the Gaussian probability density function. The results show that these methods achieve similar performance as existing methods with low complexity.