Code Phase Multipath Mitigation by Exploiting the Frequency Diversity in Galileo E5 AltBOC

N.C. Shivaramaiah

Abstract:	It is well known that multipath affects the performance of Global Navigation Satellite Systems (GNSS). Mitigation of multipath has been a research focus for several decades and the problem is still persistent (Ward 2006). The research community is trying to combat the multipath fading in two major ways. First, the signal structure design. Reduction of the effects of multipath has been one of the main design criteria for the newly proposed signals in the GNSS modernization process (Hein 2006). Second, the receiver design. A number of techniques that exist focus on different stages of the receiver signal processing chain to resolve the effect of multipath on the measurements (Irsigler 2003). The Galileo E5 signal, transmitted at a carrier frequency of 1191.795 MHz, is the most sophisticated signal among all the modernized GNSS signals. A special modulation belonging to the class of offset-carrier modulations known as Constant-Envelope AltBOC is used for the E5 signal. With a sub-carrier frequency of 15x1.023 MHz and code chipping rate of 10x1.023 MHz represented as AltBOC (15,10), the E5 signal offers unprecedented performance with code tracking jitter less than 5cm even at signal strength of 35 dB-Hz (Sleewaegaen 2004). Due to the code chipping rate and higher signal bandwidth, the AltBOC (15,10) also helps in eliminating the long-range multipath effects on code phase measurements. However with the standard Delay Locked Loop (DLL) and code discriminator architectures, the short-range multipath effects still remain. Multipath mitigation via the enhanced receiver design methodologies can be broadly categorized as follows. First, using the Receive Time Diversity (or tapped delay) techniques and the maximum likelihood principle, the multipath parameters viz. the attenuation, the multipath delay and multipath phase are estimated. Often, the receive time diversity has been achieved by employing multiple correlator arms spaced in time. The Multipath Estimating DLL (MEDLL) (Townsend 1995), Pulse Aperture Correlator (PAC), and the Vision Correlator belong to this category. The second category is the Receive Spatial Diversity schemes. A familiar method is to use multiple antennas in a predefined pattern with associated processing techniques (Moelker 1997, Ray et.al. 1999). The third category is the Receive Antenna Design itself (Brown and Mathews 2005, Counselman 1997, Kunysz 2003). This can include antennas with low gain below horizontal and the use of choke rings. The fourth category is the Receiver Signal Processing by virtue of tracking loop design, S-curve shaping, filtering and so on. A number of techniques exist in this category such as the {Narrow, High Resolution, Strobe, Gated, Shaping} correlators (McGraw and Braasch 1999, Garin 1997&2005, Veitsel et.al. 1998, Nunes and Sousa 2004, Pany et.al. 2005, Bhuiyan et.al., 2007). Peak tracking, Adaptive and Particle filtering, are also popular. A detailed comparison of most of these techniques is provided in Irsigler (2003), and Braasch (2001). Each of these techniques has its own advantages, disadvantages and pertinence. The categorization of some of the techniques belonging to receive time diversity or receiver signal processing could be arguable. Nevertheless, the aforementioned techniques do not exploit all of the properties of the signal that is transmitted. In this paper, the author introduces a method which exploits the Frequency Diversity feature of the Galileo E5 AltBOC signal. In wireless communication systems other than ranging systems, frequency diversity has been used to address the problem of multipath fading, but in a different context. In these systems, transmitting and receiving multiple frequencies effectively carrying the same information are used to combine the energies in multipath channels via some special techniques such as Maximal Ratio Combining (MRC) and Equal Gain Combining (EGC). The main aim of these techniques is to increase the channel capacity by reducing the Inter Symbol Interference (ISI) (Goldsmith 2005, Tse and Vishwanath 2005, Proakis 2004). However the focus of a ranging system is to estimate the code delay and carrier phase of the direct signal excluding all of the superimposed multipath components at the receiving antenna. This paper is organized as follows. Section II presents the motivation for this work. In section III, the standard Galileo E5 AltBOC signal tracking architectures (with their performance) are discussed. In section IV, the proposed method is described with the theory and the associated tracking loop architecture. Section V presents the simulation and test results. Conclusions and details of future work are provided in Section VI. Two appendices provide the mathematical derivations that are necessary for the paper.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	3219 - 3233
Cite this article:	Shivaramaiah, N.C., "Code Phase Multipath Mitigation by Exploiting the Frequency Diversity in Galileo E5 AltBOC," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 3219-3233.
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