Code tracking algorithms for mitigating multipath effects in fading channels for satellite-based positioning

Abstract

The ever-increasing public interest in locationand positioning services has originated a demand for higherperformance global navigation satellite systems (GNSSs). Inorder to achieve this incremental performance, the estimationof line-of-sight (LOS) delay with high accuracy is a prerequisitefor all GNSSs. The delay lock loops (DLLs) andtheir enhanced variants (i.e., feedback code tracking loops) arethe structures of choice for the commercial GNSS receivers,but their performance in severe multipath scenarios is stillrather limited. In addition, the new satellite positioning systemproposals specify the use of a new modulation, the binary offsetcarrier (BOC) modulation, which triggers a new challenge in thecode tracking stage. Therefore, in order to meet this emergingchallenge and to improve the accuracy of the delay estimationin severe multipath scenarios, this paper analyzes feedback aswell as feedforward code tracking algorithms and proposes thepeak tracking (PT) methods, which are combinations of bothfeedback and feedforward structures and utilize the inherentadvantages of both structures. We propose and analyze here twovariants of PT algorithm: PT with second-order differentiation(Diff2), and PT with Teager Kaiser (TK) operator, which willbe denoted herein as PT(Diff2) and PT(TK), respectively. Inaddition to the proposal of the PT methods, the authors proposealso an improved early-late-slope (IELS) multipath eliminationtechnique which is shown to provide very good mean-time-to-lose-lock (MTLL) performance. An implementation of a noncoherentmultipath estimating delay locked loop (MEDLL)structure is also presented. We also incorporate here an extensivereview of the existing feedback and feedforward delay estimationalgorithms for direct sequence code division multiple access(DS-CDMA) signals in satellite fading channels, by taking intoaccount the impact of binary phase shift keying (BPSK) as wellas 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 performancecomparison of the algorithms is on the closely spaced multipathscenario, since this situation is the most challenging for estimatingLOS component with high accuracy in positioning applications.