Multipath Mitigation

Galileo navigation system is based on spread spectrum technique. A pre-requisite for any spread-spectrum wave form is the code synchronisation of transmitter and receiver waveforms. The code-synchronisation is usually accomplished in two steps:
1.      Code acquisition stage
2.      Code tracking stage 

The main challenges in the code tracking of Galileo OS signals come from the presence of Binary Offset Carrier (BOC(1,1)) modulation and the presence of multipath fading channels. BOC modulation introduces false tracking points in the tracking algorithms. The presence of multipaths (especially closely spaced echoes) introduces a bias error in Line-of-Sight (LOS) delay tracking. Therefore, multipath, small CNRs and fading are considered to be the sources of errors in the navigation tracking and position estimation. Among them multipath, especially short multipath delay is the main source of error.  

A perfect frequency synchronisation cannot be assumed in the presence of a dynamic scenario for tracking receivers as the Doppler effects and fading translate the carrier frequency. The impact of slow/fast (multiplicative) fading of the multipath channel on the necessary high-resolution sub-chip synchronisation is to be considered. The efficient synchronization on sub-chip level is an important problem in navigation and pure chip-level synchronization is not sufficient.

In general, a number of parameters are to be optimised such as the number and spacing of correlators, discriminator function shape, Increased delay estimation accuracy at the expense of higher complexity and adaptive structures (e.g., adaptation to channel conditions)

The main objective of GREAT project under this work package is the investigation of efficient multipath mitigation (MM) and tracking algorithms for Galileo BOC signals which includes the following key tasks:

• Evaluation of different possible correlators
• Optimising several parameters for the delay tracking architecture
• Study of combined channel estimations and delay tracking units
• Performance analyses for various existing delay tracking structures considered for SinBOC (1, 1) type of signals
• Investigation of tracking loops (timing delay feedback)
• Timing error detection and timing error compensation
• Doppler fading and frequency error compensation
• WCDMA code and channel tracking

Performance criteria is based on Multipath Error Envelopes, Root Mean Square errors for LOS delay estimators, Mean time to loose lock in tracking process (e.g., convergence to a wrong peak/false lock point) and distance with respect to Cramer-Rao Bound.  

Advanced multipath mitigation algorithms can reduce the multipath in complex urban scenarios where the probability is more for the number of multipaths and this multipath mitigation can also lead to greater accuracy and better positioning in complex scenarios.

Figure 1: Illustration of an S curve with and without multipaths.