Abstract
The estimation with high accuracy of the line-of-sight delay is a prerequisite for all global navigation satellite systems. The delay locked loops and their enhanced variants are the structures of choice for the commercial GNSS receivers, but their performance in severe multipath scenarios is still rather limited. The new satellite positioning system proposals specify higher code-epoch lengths compared to the traditional GPS signal and the use of a new modulation, the binary offset carrier (BOC) modulation, which triggers new challenges in the delay tracking stage. We propose and analyze here the use of feedforward delay estimation techniques in order to improve the accuracy of the delay estimation in severe multipath scenarios. First, we give an extensive review of feedforward delay estimation techniques for CDMA signals in fading channels, by taking into account the impact of BOC modulation. Second, we extend the techniques previously proposed by the authors in the context of wideband CDMA delay estimation (e.g., Teager-Kaiser and the projection onto convex sets) to the BOC-modulated signals. These techniques are presented as possible alternatives to the feedback tracking loops. A particular attention is on the scenarios with closely spaced paths. We also discuss how these feedforward techniques can be implemented via DSPs.
Highlights
AND MOTIVATIONApplications of GNSS are rapidly evolving
L=1 where Eb is the bit or symbol energy of the signal, fD is the Doppler shift introduced by the channel, L is the number of channel paths, αl,n(t) is the time-varying complex fading coefficient of the lth path during the nth code epoch, τl is the corresponding path delay, and η(·) is an additive noise component of double-sided wideband power spectral density Nw, which incorporates the additive white noise of the channel and the interference coming from the other satellites
Where JMF = [JMF(dminTs), . . . , JMF(dmaxTs)]T, dmin is the minimum delay in samples, and dmax is the maximum delay in samples, the sampling interval Ts is chosen sufficiently small to model fractional path delays4 (e.g., Ts = 0.05TBOC)
Summary
Due to the complexity and to the high sensitivity of the EKF algorithm to the initialization conditions, such as the error covariance matrices [8], the use of EKF estimators is not widespread in the today’s research community Since their complexity is directly related to the code epoch length (or, equivalently, the spreading factor), EKF estimators are clearly not suitable for Galileo and modernized GPS applications. After presenting the signal model in the presence of BOC modulation, we continue with a discussion regarding the advantages and drawbacks of feedback delay estimation algorithms in multipath propagation and we show that feedforward delay estimators may be used as viable alternatives, in order to attain good accuracy via simple implementation.
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