Multipath Mitigation Methods Based on Diversity Algorithms

S. Rougerie, G. Carrie, J. Israel, L. Ries, M. Monnerat, P. Thevenon

Abstract:	In Global Navigation Satellite Systems (GNSS) applications, multipath (MP) errors are still one of the major error sources in conventional receivers. The additional signal replicas due to reflections introduce a bias in Delay Lock Loops (DLL), which finally leads to a positioning error. Several techniques have been developed for multipath mitigation or estimation. The most popular approach is the Narrow Correlator Spacing [1], which reduces the Chips spacing between two correlators in order to cut down the impact of multipath on the DLL. However, this technique suffers from high sensitivity to noise, and can not mitigate short delay multipath (<0.1Chips). Based on the Maximum Likelihood (ML) estimation, Multipath Estimating Delay-Lock-Loop (MEDLL) [2] algorithm has also been proposed to estimate the delay and the power of all the paths by studying the shape of the cross correlation function. This approach shows better performance than the Narrow Correlator Spacing technique, but short delay multipath mitigation (<0.1Chips) is still an issue. More recently, the use of diversity algorithms has been proposed for multipath mitigation. The diversity can lie either on the spatial domain (antenna diversity) or on the frequency domain (frequency band diversity). In both cases, the algorithms principle consists in taking benefit from some redundant information seen with some difference on diverse channels. On the space domain, the redundant parameters are the time of arrival and the Doppler of the echoes and the diversity lies in the phase delays between antennas of an array. Actually, antenna arrays perform a spatial sampling of the wave front which makes possible the discrimination of the sources on the space domain (azimuth and elevation). As the time-delay domain and space domain can be assumed independent, we can expect to mitigate/estimate very short delay multipaths by using an antenna array. However, we don't want to increase too much the size, the complexity and the cost of the receivers and thus, we focus our study on small arrays with a small number of antennas: typically a square 2x2 array. Consequently, conventional beamforming (space Fast Fourier Transform) is not directive enough to assure the mitigation of the multipaths, and then this first class of solutions was rejected. In order to improve the resolution, adaptive beamformers have also been tested. However, the line of sight signal and the multipaths signals are strongly correlated and thus, classical adaptive algorithms are not able to discriminate the sources. Then, a new class of space-time-delay algorithms has been proposed in [3] and [4]. These algorithms are based on Space Alternating Generalized Expectation maximisation (SAGE) techniques [5] and they are applied at a post-correlation stage in order to reduce the computational burden and to be compatible with real-time processing. Based on theoretical studies and on numerical simulations, they proved to outperform single antenna algorithms in [3] and [4] in terms of pseudo-distance estimation in presence of multipaths. Recently, these results have been confirmed on real world data acquired with the experimental device presented in [6]. This experimental validation will be presented in this paper. On the frequency domain, the redundant parameters are the time of arrival of the echoes and the diversity is due to homothetic Doppler shifts and potentially to waveforms. Based on experimental power measurements, [7] assumes that multipaths on Galileo E5a and E5b bands are independent. Therefore, the outputs of correlators on both frequency bands can be considered as any independent statistical variables and averaged right before the discriminator computation. By contrast, [8] considers that purely deterministic relationships exist between the wideband channel parameters and the authors propose a particle filtering approach which takes benefit of this information redundancy to improve the estimation of the direct path propagation delays. Based on asymptotic methods and ray bouncing, the GNSS multifrequency urban channel was analysed from the lower part to the upper part of the GNSS L-Band [9], [10]. This paper will present the validation of the results in [9] and [10] based on exact electromagnetic FDTD tools (Finite Domain Time Domain) for canonical vegetation elements. Based on the resulting multifrequency channel model, this paper will also present the potential contribution of frequency diversity techniques in the GNSS multipath mitigation context. [1] A. J. Van Dierendonck, P. Fenton, and T. Ford, "Theory and Performance of Narrow Correlator Spacing in a GPS Receiver", Journal of the institute of navigation, vol.39, n°3, Fall 1992. [2] R. D. J. van Nee, J. Siereveld, P. C. Fenton, and B. R Townsend, "The Multipath Estimating Delay Lock Loop: Approaching Theoretical Accuracy Limits", IEEE Position, Location and Navigation Symposium, Las Vegas, Nevada, April U-15, 1994. [3] S. Rougerie, G. Carrie, L. Ries, F. Vincent, R. Pascaud, E. Corbel, M. Monnerat : “Multipath mitigation methods based on antenna array”, Proc. ION ITM 2011, San Diego, CA, January 2011. [4] S. Rougerie, G. Carrie, F. Vincent, L. Ries, M. Monnerat , "A New Multipath Mitigation Method for GNSS Receivers based on an Antenna Array", International Journal of Navigation and Observation, vol. 2012. Paper ID 804732, 11 pages, 2012. doi:10.1155/2012/804732 [5] Bernard H. Fleury, Martin Tschudin, Ralf Heddergott; Dirk Dahlhaus, Klaus Ingeman Perdensen, "Channel Parameters Estimation in Mobil Radio Environments Using SAGE algorithm", IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 17, NO. 3, MARCH 1999 [6] S. Rougerie, G. Carrie, L. Ries, F. Vincent, R. Pascaud, M. Monnerat : “A new sensor array receiver for multipath mitigation in GNSS”, Proc. European Navigation Conference 2010, Braunschweig, Germany, October 2010. [7] M. Musso, G. Gera, A. Cattoni, and C. S. Regazzoni, “GNSS multifrequency receivers in urban environment: Theoretical analysis,” in Proceedings of ION GNSS 2005, Long Beach Convention Center, CA, sep 2001, pp. 2661–2669. [8] N. I. Ziedan, “Multi-frequency combined processing for direct and multipath signals tracking based on particle filtering,” in Proceedings of ION GNSS 2011, Portland, OR, 2011, pp. 1090– 1101. [9] J. Israel, G. Carrié, M. Ait-Ighil, “A Wideband and Multifrequency GNSS Propagation Channel Simulation”, Proc. First CNES-ONERA Workshop on Earth-Space Propagation, 21 to 23 January 2013, ONERA, Toulouse, France. [10] J. Israel, G. Carrié, M. Ait-Ighil, “A Wideband and Multifrequency Propagation Channel Simulation”, Accepted for presentation at EuCAP 2013, Gothenburg, Sweden, April 2013.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1596 - 1605
Cite this article:	Rougerie, S., Carrie, G., Israel, J., Ries, L., Monnerat, M., Thevenon, P., "Multipath Mitigation Methods Based on Diversity Algorithms," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1596-1605.
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