Variable dwell time verification strategies for CDMA acquisition with application to GPS signals

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O'Mahony, Niamh M.
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University College Cork
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This thesis explores the question of how decisions should be made regarding the presence or absence of GPS signals at the acquisition stage. The goal of the investigation is to determine the best choice of decision-making strategy, such that signal synchronisation can be achieved in the minimum time. The synchronisation process involves two steps: coarse synchronisation (acquisition) and fine synchronisation (tracking). The treatment begins with the cell-level decision-making strategy, where a 'cell' describes a particular estimate of the synchronisation parameters. The optimal sequential probability ratio test, which has largely been ignored for the GPS problem, is shown to outperform the computationally simple single dwell detector, even in the presence of adverse signal effects. A new approximation for the distribution of the probability ratio in the GPS case is presented and shown to significantly reduce the analytical complexity of the strategy. Sub-optimal strategies are investigated at a cell level to find a suitable trade-off between performance and computational load, with a particular focus on the optimisation of coincidence detector and up-down counter strategies. A novel up-down counter strategy is presented and shown to reduce the mean dwell time relative to that of the traditional ‘Tong’ detector. The impact of unknown power levels and other environmental effects is investigated for each strategy and the dwell time variance, a metric which is often neglected, is considered for comparison of strategies. The cell-level investigation leads to the recommendation that, if dwell time variance is not of concern for receiver design, then the novel up-down counter is an excellent choice of strategy for achieving the goal outlined above, whilst the upper limit on the dwell time of the coincidence detector can be attractive if significant variance is not acceptable. Proceeding from cell-level analysis to a view of the acquisition system as a whole, a novel hybrid serial/parallel acquisition system is proposed and shown to provide a substantial reduction in overall mean acquisition time, relative to that of a fully serial search, but with no significant increase in implementational complexity. All of the theoretical results derived in the thesis are verified by both Monte Carlo simulations and by tests on GPS signals collected from operational satellites.
Global Positioning System
O'Mahony, N. M., 2011. Variable dwell time verification strategies for CDMA acquisition with application to GPS signals. PhD Thesis, University College Cork.