Year

2014

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

This thesis conducted an investigation into how to select the promising spreading sequences over a Multiple Input Multiple Output (MIMO) system. The Multiple Access Interference (MAI) effects, associated with spreading sequences pairs over Multiple Output Space-Time-Spreading (MIMO-STS) systems were analysed.

In order to evaluate the spreading sequences over the MIMO-STS system, a test bed developed by Software Defined Radios (SDR) was needed. Towards this end, a SDR test bed was developed to evaluate the selected spreading code, in an attempt to mitigate against the MAI effects over 2×1 MIMO-STS systems.

Results are first provided for MIMO-STS system and SISO system in the presence of MAI when using Gold codes, orthogonal Gold codes, original Walsh codes and modified Walsh codes. The results of the SDR test bed inferred that the “pairs” of spreading sequences are a crucial factor in order to select the most promising spreading code for MIMO systems. Motivated by the differences between Single Input Single Output (SISO) and MIMO systems, a new criterion for selecting promising spreading sequences in particular for MIMO-STS systems was proposed. In this study, more promising spreading sequences sets for MIMO systems were located by employing the proposed selection criteria, by selecting sequences with low cross-correlation in terms of pairs of sequences, rather than single sequences. The thesis concluded that the choice of spreading sequences pairs in a MIMO-STS system, in the presence of MAI sources, is an important factor to be considered.

From this point onwards, some of the recently proposed spreading families were evaluated by the proposed criterion. The SDR test bed results revealed that, for some of the latest proposed spreading families, with modified or novel construction of the classic spreading codes, improved transmission properties for asynchronous transmissions can be achieved, when using the spreading sequences pairs selection criterion. This indicated that those viable constructions can be considered in any further combinatorial designs of spreading codes for MIMO-STS systems, in order to mitigate against MAI effects. The thesis finally provides some further considerations for future related work.

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