Code Tracking Algorithms for Mitigating Multipath Effects in Fading Channels for Satellite-Based Positioning

Abstract

The ever-increasing public interest in location and positioning services has originated a demand for higher performance global navigation satellite systems (GNSSs). In order to achieve this incremental performance, the estimation of line-of-sight (LOS) delay with high accuracy is a prerequisite for all GNSSs. The delay lock loops (DLLs) and their enhanced variants (i.e., feedback code tracking loops) are the structures of choice for the commercial GNSS receivers, but their performance in severe multipath scenarios is still rather limited. In addition, the new satellite positioning system proposals specify the use of a new modulation, the binary offset carrier (BOC) modulation, which triggers a new challenge in the code tracking stage. Therefore, in order to meet this emerging challenge and to improve the accuracy of the delay estimation in severe multipath scenarios, this paper analyzes feedback as well as feedforward code tracking algorithms and proposes the peak tracking (PT) methods, which are combinations of both feedback and feedforward structures and utilize the inherent advantages of both structures. We propose and analyze here two variants of PT algorithm: PT with second-order differentiation (Diff2), and PT with Teager Kaiser (TK) operator, which will be denoted herein as PT(Diff2) and PT(TK), respectively. In addition to the proposal of the PT methods, the authors propose also an improved early-late-slope (IELS) multipath elimination technique which is shown to provide very good mean-time-to-loselock (MTLL) performance. An implementation of a noncoherent multipath estimating delay locked loop (MEDLL) structure is also presented. We also incorporate here an extensive review of the existing feedback and feedforward delay estimation algorithms for direct sequence code division multiple access (DS-CDMA) signals in satellite fading channels, by taking into account the impact of binary phase shift keying (BPSK) as well as the newly proposed BOC modulation, more specifically, sine-BOC(1,1) (SinBOC(1,1)), selected for Galileo open service (OS) signal. The state-of-art algorithms are compared, via simulations, with the proposed algorithms. The main focus in the performance comparison of the algorithms is on the closely spaced multipath scenario, since this situation is the most challenging for estimating LOS component with high accuracy in positioning applications.

Multipath Mitigation Techniques for Satellite-Based Positioning Applications

“…However, simulation results performed in Irsigler & Eissfeller (2003) showed that ELS is outperformed by HRC with respect to Multipath Error Envelopes (MEEs), for both BPSK and SinBOC(1,1) modulated signals. An Improved ELS (IELS) technique was proposed by the Author in Bhuiyan et al (2008), which introduced two enhancements to the basic ELS approach. The first enhancement was the adaptation of random spacing between the early and the late correlator pairs, while the later one was the utilization of feedforward information in order to determine the most appropriate peak on which the IELS technique should be applied.…”

“…The first enhancement was the adaptation of random spacing between the early and the late correlator pairs, while the later one was the utilization of feedforward information in order to determine the most appropriate peak on which the IELS technique should be applied. It was shown in Bhuiyan et al (2008) that IELS performed better than nEML only in good C/N 0 for BPSK and SinBOC(1,1) modulated signals in case of short-delay multipath, but still had poorer performance than HRC.…”

“…Moreover, MEDLL has stimulated the design of different maximum likelihood based implementations for multipath mitigation. One such variant is the non-coherent MEDLL, developed by the authors, as described in Bhuiyan et al (2008). The classical MEDLL is based on a maximum likelihood search, which is computationally extensive.…”

“…The authors implemented a non-coherent version of MEDLL that reduces the search space by incorporating a phase search unit, based on the statistical distribution of multipath phases. It was shown in Bhuiyan et al (2008) that the performance of this suggested approach depends on the number of random phases considered; meaning that the larger the number is, the better the performance will be. But this will also increase the processing burden significantly.…”

“…But this will also increase the processing burden significantly. The results reported in Bhuiyan et al (2008), show that the non-coherent MEDLL provides very good performance in terms of RMSE, but has a rather poor Mean-Time-to-Lose-Lock (MTLL) as compared to the conventional DLL techniques. …”