Assisted-GPS Based Snap-shot GPS Receiver with FFT-accelerated Collective Detection: Time Synchronisation and Search Space Analysis

In this paper, the architecture of the Assisted-GPS based snap-shot GPS receiver is introduced. Such a system would be suitable for emergency services, freeing up the processing burden of the client handset receiver while enhanced signal processing algorithms are used to process a “snap-shot” of transferred raw radio-frequency data at an A-GPS server. This system can be treated as a hybrid of advanced technologies that comprises the concepts of Assisted-GPS, snap-shot GPS receiver and Collective Detection. The implementation and time synchronisation issues of such a system are the main focus of this paper. Specifically, two implementations are described and contrasted by means of computational efficiency and implementation losses. Subsequent investigations then focus on the resulting position accuracy and precision due to non-ideal time synchronisation between the Base Station and the Mobile Station. The findings highlight the importance of considering time synchronisation errors (i.e. coarse time error) when evaluating the performance of Collective Detection. Also, the effects of the use of various spatial step sizes with non-zero coarse time errors are shown. The associated precisions are empirically characterised as a function of time synchronisation error and position-clock domain step size. Finally, two computationally efficient algorithms are proposed and analysed in this paper to compensate for the time synchronisation issues. One of the methods explored is shown to have improved convergence and is capable of resolving coarse time error to a high resolution for a relatively low increase in computational load. There is a trade-off between precision and computational load by way of step size selection, which is a parameter in the Collective Detection algorithm. To improve precision without increasing the instantaneous computational load, a simple averaging approach is applied to multiple processed snap-shots. This approach is especially applicable when Collective Detection is operating under limited computational resource or limited receiver-to-server network bandwidth. Unlike many other previous research contributions, in all investigations, the entire Common Clock Bias search range (i.e. 0-300km) is considered in the Collective Detection search space. In addition, live signals are used in real-life non-ideal scenarios for performance evaluations.