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(2005)ThesisIncreasing demand for wireless communication challenges the availability of limited radio resources, such as bandwidth and power etc. Limited resources cause a trade off with the quality of service. The work presented in this thesis is intended to develop algorithms those can be used to demodulate information with optimal amount of resources (Signal to Noise Ratio, Processing memory requirement, Computational complexity etc ). In the first part of the thesis a decision feedback sequence detection algorithm has been proposed that provides exactly the same bit error rate as in standard maximum likelihood sequence estimation but with 95% lowmomputational complexity. Besides that the proposed algorithm achieves 2 dB signal to noise ratio (SNR) gain over the existing decision feedback algorithms. The proposed algorithm is applicable in multiple input multiple output (MIMO) as well as single input single output (SISO) wireless communication systems. In the second part of the thesis an adaptive blind sequence detection algorithm has been proposed where a novel reference channel has been exploited. The problem of bit-shift ambiguity in blind sequence detection is completely eliminated exploiting the proposed algorithm. A 3 dB SNR gain is achieved against the existing blind sequence detection algorithms for the system without error correction code. The BER performance is highly scalable with the variation of segmentation window size. In the third part of the thesis, two different sequence detection algorithms have been proposed to track rapidly time varying channels. One of the algorithms, called extended window survivor processing (EWSP), requires lower computational complexity than that of Per-survivor processing (PSP) sequence detection process. The other algorithm, called bi-directional survivor processing reduces 17% of channel misacquisation than that of PSP. Consequently, both of these algorithms reduce the probability of error propagation in the detection process. In the final part of the thesis, the capacity and coverage of the UMTS urban network has been analyzed while the Repeaters are inserted. It has been found that the system capacity with repeaters is doubled in an environment with the propagation constant 3.7-3.8. As a by product, 10% increase in the cell coverage was also found.
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(2005)ThesisTurbo codes discovered in 1993 by Berrou et al. [1] were the first coding technique that approached the Shannon theoretical limit of information transmission to within 0.5 dB. Soon after this discovery, it was recognized that the same Turbo principle could be applied to a variety of detection/decoding problems such as equalization, multi-user detection, joint channel estimation and decoding etc. The computational complexity of such schemes remains a challenging issue since many Turbo detectors require computationally complex Maximum Likelihood (ML) detection in combination with channel decoding. Consequently, a class of low complexity Turbo detectors employing linear and decision feedback filtering instead of ML have been invented recently in order to solve this problem. This dissertation describes low complexity adaptive turbo detection methods for wireless communications, namely Turbo equalization and Turbo Multi-user detection. The adaptive turbo detectors are shown to outperform their conventional Minimum Mean Squared Error (MMSE) counterparts regarding the SNR-BER performance. For Turbo equalization the most remarkable improvement has been achieved for highly frequency-selective channels. For Turbo Multi-user detection most of the gain is obtained for overloaded DS-CDMA systems where the number of users exceeds the processing gain. A theoretical analysis of Turbo equalization provides a new set of MMSE coefficients. The proposed new detector is shown to outperform all turbo detectors of similar computational complexity. The second major contribution of this thesis is a proposed adaptive method for user ordering for Successive Decision Feedback multi-user Detectors (S-DFD). The method is shown to outperform all previously proposed ordering methods delivering significant improvement in SNR-BER characteristic. The analysis of S-DFD has revealed that a proper user ordering can significantly improve the performance of the system. The proposed ordering has also been implemented in the adaptive iterative S-DFD improving the BER performance especially after the first turbo iteration since this is the most critical part, which determines the SNR floor at which the Turbo effect starts.
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(2005)ThesisThis thesis presents an original contribution to the field of planar silica optical waveguide devices. An integrated collimating lens pair is proposed which enables an optical signal to travel in free space between two opposing planar waveguides with minimal optical loss. Each lens in the lens pair consists of a convexly shaped front face to focus the optical beam in the horizontal plane, and a parabolically graded refractive index profile to focus the beam in the vertical plane. The lens pair has significant advantage over alternative means of collimation because the lens fabrication can be integrated with the planar silica waveguide fabrication. The lens pair has application in micro-optical systems which require free-space propagation such as planar waveguide MEMS optical switches and on-chip optical interconnects. This work provides a rigorous theoretical analysis of the lens pair. A design methodology is presented to determine the lens pair parameters required for zero optical power loss for any pre-set propagation distance (excluding reflection loss). The sensitivity of the lens pair to fabrication errors is investigated by using the Beam Propagation Method (BPM). The propagation of an optical beam is simulated under conditions of etch mask angular and lateral misalignment, deviation of the parabolically graded index lens from the desired refractive index profile, poor dimensional control of the planar silica waveguides, and non-vertical etching of the front face of the lens. A compensation scheme is proposed to minimise the optical loss caused by the non-vertical front face etch and the effectiveness of this technique is verified through further BPM simulation. The lens pair can be fabricated using Plasma Enhanced Chemical Vapour Deposition (PECVD) and Reactive Ion Etching (RIE) techniques, provided state of the art processing facilities are available. Preliminary PECVD and RIE process characterisation work has been done, however the complete fabrication of the device is left for future work.
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(2005)Conference PaperAn improved algorithm is proposed to demodulate signals from a spatial-frequency multiplexed fibre-optic Fizeau strain sensor system. We have demonstrated that this algorithm, which includes wavelet signal-processing techniques, reduces noise and increases strain resolution.