Feedforward Delay Estimators in Adverse Multipath Propagation for Galileo and Modernized GPS Signals
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Feedforward Delay Estimators in Adverse Multipath Propagation for Galileo and Modernized GPS Signals Elena Simona Lohan, Abdelmonaem Lakhzouri, and Markku Renfors Institute of Communications Engineering, Tampere University of Technology, P.O. Box 553, Tampere 33101, Finland Received 31 May 2005; Revised 8 March 2006; Accepted 29 March 2006 The estimation with high accuracy of the line-of-sight delay is a prerequisite for all global navigation satellite systems. The delay locked loops and their enhanced variants are the structures of choice for the commercial GNSS receivers, but their performance in severe multipath scenarios is still rather limited. The new satellite positioning system proposals specify higher code-epoch lengths compared to the traditional GPS signal and the use of a new modulation, the binary offset carrier (BOC) modulation, which triggers new challenges in the delay tracking stage. We propose and analyze here the use of feedforward delay estimation techniques in order to improve the accuracy of the delay estimation in severe multipath scenarios. First, we give an extensive review of feedforward delay estimation techniques for CDMA signals in fading channels, by taking into account the impact of BOC modulation. Second, we extend the techniques previously proposed by the authors in the context of wideband CDMA delay estimation (e.g., Teager-Kaiser and the projection onto convex sets) to the BOC-modulated signals. These techniques are presented as possible alternatives to the feedback tracking loops. A particular attention is on the scenarios with closely spaced paths. We also discuss how these feedforward techniques can be implemented via DSPs. Copyright © 2006 Elena Simona Lohan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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BACKGROUND AND MOTIVATION
Applications of GNSS are rapidly evolving. A new European satellite system, Galileo, is currently in standardization process [1, 2]. Modernized GPS proposals have also been introduced recently [3–5]. Galileo signals, as well as GPS signals, are based on direct-sequence code division multiple access (DS-CDMA) technique. Spread spectrum systems are known to offer better frequency reuse, better multipath diversity, better narrowband interference rejection, and, potentially, better capacity compared to narrowband techniques [6]. On the other hand, code and frequency synchronization are fundamental prerequisites for a good performance of the receiver. These two tasks pose several problems in the presence of mobile wireless channels, due to the various adverse effects of the channel, such as the multipath propagation, the possibility of having the line-of-sight (LOS) component obstructed by closely spaced non-line-of-sight (NLOS) components, or even the absence of LOS, and the high level of noise (especially in indoor scenarios). Moreover, the fading statistics of the channel and the
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