Pre- and Post-Correlation Methods for Acquisition of New GNSS Signals with Secondary Code
Jiri Svaton, Frantisek Vejrazka, Czech Technical University in Prague, Czech Republic
The correlation of the GNSS signal implemented into a block processing by help of a FFT allows using and evolving methods that has never been used before in the traditional continual processing. Our contribution focuses on utilization of block pre-correlation processing for an acquisition of the modern GNSS signals. This method is combined with a parallel in code search (PCS) algorithm.
The acquisition of primary code is an important part of signal processing. It acquires a signal parameters prior to transition to a steady state in which it is tracking signal parameters in FLL and DLL locked loops with a limited lock-in range. The long-period modern codes use handover from a basic signal to decrease requirements for massive processing in case of direct acquisition. When this basic signal is not available or is jammed the acquisition of more complex signal could be problematic. The fast acquisition is crucial for modern receivers with high demands to receiver robustness working in weak and high dynamic signals environments. It allows fast transition to tracking state and reacquisition without additional delay. Serial methods for a direct acquisition fail because of time demands due longer and longer code periods and requirements on time of coherent averaging of weak signals.
A parallel in code search (PCS) algorithm has been used for a long time in previous years. It is based on faster computation of correlation function by a Fourier transform which FFT implementation has lower algorithmically complexity. Although algorithm described above substantially speed up an acquisition problem is still a processing of code with long period, because it requires a long FFT to preserve required resolution. To build sizable FFT blocks in HW is problematic due its limited resources. Solutions based on processing of signal in blocks and their combination in post-correlation stage are suggested in our contribution. Its essential part is devoted to pre-correlation processing. A method of Pre-Correlation Averaging (PCA, or called block averaging in any sources) is used for extending of coherent averaging time. We utilize it for an acquisition of secondary code of new GNSS signals. PCA method allows using a longer coherent time without additional costs in correlation stage and without boost of the FFT unit size. PCA averages samples of consecutives signal periods and averages them together period-by-period coherently.
Long coherent time realized in pre-correlation stage is then used for a detection of secondary code shift with a help of specific manner in data storing and detection algorithm in post-correlation stage. Methods for acquisition of secondary code in post-correlation were presented. PCA was presented too, but without these contexts. Our contribution is considering it with a processing in pre-correlation stage by help of PCA to reduce requirements for a FFT implementation to an available platform of receiver. Usage of this method is complicated by a cyclical property of correlation function realized by help of Fourier transform. Realization has a similar problems as bit sign change and has some other implementation drawbacks, for example due to implementation of Doppler removal by cyclic spectra shift. Addition losses are calculated and new type of detection is offered to speed up an acquisition of modern signal in hardware with limited resources. Methods could be extended and used in the same manner for a faster bit pre-synchronization by adoption of known method (e.g. half/full-bit) for standard signals (e.g. GPS C/A) to reach higher speed and receiver robustness. Methods are implemented in combination with methods for an acquisition of long primary code. Proposed methods are designed for a HW FPGA receiver based on modern SoC platform, to utilize a direct acquisition of modern signal. Contribution focuses on advanced acquisition engine core with three 4K FFT units. Methods for processing of Galileo E1, B1 BeiDou and L1C GPS in acquisition stage will be presented for this target platform.