Application of Turbo Techniques to GNSS Receivers
Background & Objectives
‘Turbo’ techniques represent a class of digital processing algorithms that involve iterative and collaborative processing between several receiver components that exchange probabilities or likelihood information. In various fields of data transmission (satellite, wireless, etc.), embodiments of this paradigm, such as turbo error correction, turbo equalisation, turbo synchronisation, turbo interference cancellation and turbo multi-user detection, prove to be very powerful in solving various problems such as noise, multipath, interference, code and carrier acquisition, and synchronisation.
They have brought outstanding performance gains, often approaching theoretical limits. The project team has an excellent background in iterative turbo algorithms and implementation in the communications fields (satellite and wireless). One major challenge of the project is to assess the applicability of these advanced techniques to GNSS receivers.
TGR will perform an algorithm study on how iterative turbo techniques can be applied in the Galileo receiver design in order to improve its performance, in terms of ‘time to first fix’ and the positioning precision in a severe multipath environment.
The TGR project considers the study of advanced signal processing techniques applied to GNSS receivers, particularly those applied to code synchronisation and tracking. A pre-study phase considered several techniques (both existing and original algorithm proposals have been considered) and selected two of them based upon their technical and industrial potentialities. The following two techniques were selected for deep algorithm refinement and performance characterisation:
- the turbo DLL receiver, achieving excellent multipath mitigation;
- the coarse self synchronisation (CSS), an original algorithm aiming at rapid code acquisition.
Intensive algorithm investigations and performance assessment have been carried out by the project partners. Receiver models have been developed and integrated in the Granada environment. A large number of simulations has been carried out to optimise both algorithms parameters.
The project aims at devising innovative algorithms inspired from the partners’ knowledge in turbo techniques to address the issues of ‘time to first fix’ and the positioning precision in a severe multipath environment. The implementation aspects are also part of the project – in the form of a detailed hardware architectural study – in order to lead to solutions that can be implemented in today’s receiver hardware technologies.
Work performed & results
The outcomes of the project are very positive in terms of innovation and performance results. The turbo DLL receiver has been significantly improved and augmented by an original scheduling algorithm reaching optimal performance without prior channel estimation (see figure 1), and adapts automatically to multipath channel conditions. An advanced receiver architecture has also been proposed. The coarse self synchronisation technique is an original algorithm proposed by project partners. It aims at fast acquisition of the code, and can be implemented with reduced (low-cost) hardware. The proposed algorithm has been simulated and its principles have been validated in several realistic conditions.