ION GNSS 2012
Session D3: GNSS Algorithms & Methods 1: Signal Processing

Title: Multipath Mitigation in SoL Environments Using a Combination of Squared Correlators
Author(s): E. Falletti, B. Motella, M. Troglia Gamba, Istituto Superiore Mario Boella, Italy; C. Facchinetti, Agenzia Spaziale Italiana, Italy
Date/Time: Thursday, September 20, 2012, 10:40 a.m.
Room: 204 (NCC)

The disruptive effects due to the presence of reflected signals (i.e., multipath) around the receiver antenna are well known and proved by a very wide GNSS literature. Numerous are the works tackling the problem and the presented solutions oriented to multipath mitigation. In addition, multipath robustness is one of the first signal characteristics taken into account in the process of signals definition. As stated in [1], many different performance characteristics have been considered during waveform optimization (i.e., Multiplexed Binary Offset Carrier, MOC, modulations for the new Galileo OS and GPS L1C signals), but the primary objective has been to improve tracking performance in multipath. The rejection of the multipath effects become an even more sensitive issue in Safety-of-Life (SoL) environments, when the signal reliability might result a crucial requirement. Two are the main categories in which the multipath mitigation techniques are typically classified: the antenna-based and the signal processing techniques. The former avoid the reflected signals to impinge the receiver antenna; the latter act at the tracking stage of the receiver and can be further distinguished in global multipath cancellers and methods for multipath estimation and excision. Among them, the first category of techniques seems to better face the need of having solutions that rapidly adapt themselves to changing multipath profiles. In fact they do not attempt to estimate the presence of reflected rays nor their parameters: they simply adopt specific Delay Lock Loop (DLL) discrimination functions robust with respect to the signal distortions. On the other hand, the principal architectures belonging to this category, e.g., NarrowTM correlator, High Resolution Correlator (HRC)/ StrobeTM correlator, EdgeTM correlator, are covered by patents. As pointed out in [2], the need of implementing patent-free solutions can become a strong motivation to find innovative discriminator structures. Looking at the question from this point of view, this paper presents a novel DLL architecture, referred to as Combination of Squared Correlators (CSC), belonging to the set of global multipath cancellers, that does not fall into the wide family of patented architectures. The CSC architectures combines at the same time novelty and implementation simplicity. In fact the architecture defines a novel discrimination rule within the DLL, but it still maintains a light implementation, employing four correlators per channel (i.e., early, late, very-early, and very-late), as in the case of the StrobeTM. The paper defines the CSC architecture, presenting its performance both theoretically and via simulation test. Open loop performance have been tested by the analysis of the S-curve (discrimination function) and the multipath error envelope; while the variance of estimated code phase and the statistics of the multipath-induced phase error are the parameters chosen to test the solution in closed loop conditions. The error produced on the pseudo-range has also been studied and compared with that obtained using different discriminator structured. Furthermore, within the wide scenario of new and modernized GNSS signals, the CSC DLL architecture has been tested with different modulations schemes. In fact, besides the signal bandwidth, which can be considered the key parameter of a signal structure with respect to the possible multipath error, chip rate and code length are other important characteristics related with the signal multipath parameters [3]. Moreover, being the multipath characteristics heavily correlated on the environment in the close vicinity of the GNSS antenna, the discriminator has been tested in specific simulated scenarios, namely urban/suburban and rural [4], appropriately tailored to SoL rail applications. All the results demonstrate that CSC gives performance just slightly worse than those given by the StrobeTM, maintaining a similar degree of complexity.

[1] G. W. Hain et al., "MBOC: The New Optimized Spreading Modulation Recommended for Galileo L1 OS and GPS L1C", Inside GNSS, May/June 2006 [2] H. Hurskainen et al., "Multiple Gate Delay Tracking Structures for GNSS Signals and Their Evaluation with Simulink, SystemC, and VHDL", Hindawi International Journal of Navigation and Observation, Volume 2008, Article ID 785695, doi:10.1155/2008/785695 [3] G. W. Hain et al., "The GALILEO Frequency Structure and Signal Design," Proceedings of the 14th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 11 - 14, 2001 [4] Lehner, A., Steingass, A., "A novel channel model for land mobile satellite navigation" Proceedings of the Institute of Navigation Conference ION GNSS 2005, Long Beach, USA, September 13-16, 2005.

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