GPS Software Receiver Enhancements for Indoor Use

Bo Zheng and Gérard Lachapelle

Abstract:	Signal sensitivity and cross-correlation are two important issues for GPS signal processing indoor. This paper presents the implementation of an enhanced GPS software receiver for indoor applications. A dot product integration method has been fully investigated and implemented into a GPS software receiver previously developed by the PLAN group. This new method outperforms the conventional non-coherent integration method in terms of processing gain. Three cross-correlation detection and mitigation algorithms are also studied and implemented. Simulation results show that the cross-correlation, which is several dB stronger than the correlation sought, can be detected and mitigated in most cases. The signal sensitivity for an indoor signal is normally 10 dB to 30 dB lower than that of the line of sight (LOS) signal. In order to improve signal sensitivity, it is desirable to maximize the coherent integration period. However, the existence of navigation bit transition limits the coherent integration period to 20 ms, normally 10 ms without the external aiding, using the current GPS L1 signal. For assisted GPS, the coherent integration period can be extended to more than 20 ms to get a higher correlation gain. However, the long coherent integration time increases the number of frequency search cells, which in turn, increases the time to first fix. Meanwhile, the coherent integration time is also limited by the user dynamics and the quality of the front-end oscillator. Normally, non-coherent integration is required after the coherent integration to detect weak signals. The conventional non-coherent integration induces the well-known squaring loss through its nonlinear summation. A dot product integration method has been investigated and implemented in this paper, which is based on a modified non-coherent integration algorithm proposed by Choi et al. [2002]. Monte Carlo simulation results show that the dot product integration has a gain of up to 3 dB compared to the non-coherent integration. Several cross correlation mitigation techniques are fully investigated. Some of them can be practically implemented in a software receiver. An example is to search for strong satellites first then gradually increases the detection sensitivity as required. However, this method fails when a strong satellite appears later. This paper proposes a multi-peak detection method, which can distinguish between the cross-correlation of a strong signal and the autocorrelation of a weak signal. A strong signal cancellation method, proposed by Norman et al. [2005], is then implemented to subtract the strong signal interference and extract the weak signal correlation peak.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	1138 - 1142
Cite this article:	Zheng, Bo, Lachapelle, Gérard, "GPS Software Receiver Enhancements for Indoor Use," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 1138-1142.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In