Assignment of Spreading Codes in Code Division Multiple Access Radio Systems

  • Samuel Oluwasesan Sanusi

    Student thesis: Doctoral Thesis


    This thesis is concerned with spreading code construction and assignment in Direct Sequence Code Division Multiple Access radio systems. Particular emphasis is given to Quasi-Synchronous Code Division Multiple Access. The frequency spectrum required for these systems depends on the length of the spreading code used. Frequency spectrum is an important and expensive resource of a radio system. Short spreading codes are desirable but can lead to an inadequate number of codewords, requiring undesirable codeword re-use. It is shown that careful assignment of spreading codes can effectively minimise interference, manage the spreading codewords available and ultimately ensure that the spectrum is used efficiently.

    Interference in Direct Sequence Code Division Multiple Access systems depends on the correlations of the spreading code used. Spreading codes are designed to have low correlation. It is possible to achieve lower correlations (or even zero correlations) within a window in a quasi-synchronous system. The problem of limited availability of codewords can be overcome using a scrambling code, but the lower inter-cell correlations are lost.

    As an alternative, it is shown that lower interference can be achieved for a given length of spreading code if assignment techniques which take account of both correlations and codeword re-use are adopted. This is particularly true in a quasi-synchronous system. An interference measure based on the signal-to-interference ratio is adopted, similar to that recently proposed for frequency assignment algorithms. The interference component of this measure depends on the aperiodic correlations of the code used.

    The codes compared for assignment include well known Gold, Kasami and Kerdock codes as well as modifications of Lin Chang Simplex and Loosely Synchronous codes. These last two codes are designed for quasi-synchronous operation. The modifications double the number of codewords, at a cost of allowing some higher correlations. Tabu search and simulated annealing algorithms of the type used for frequency assignment are adapted for spreading code assignment. They are able to avoid or minimise interference arising from these higher correlations.

    The modified Lin Chang Simplex and (in particular) Loosely Synchronous codes are shown to present better system performances (in terms of higher signal-to-interference ratios and greater network coverage) than the combination of Walsh Hadamard code and Gold code used in third generation mobile telephone systems. They are also better than the other codes considered. This is true for both mean square and peak planning and is particularly true for the most demanding networks considered.
    Date of Award2006
    Original languageEnglish
    SupervisorDerek Smith (Supervisor) & Stephanie Perkins (Supervisor)

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