Complexity Considerations for Unambiguous Acquisition of Galileo Signals

In order to obtain a higher spectral separation from the BPSK-like signals, i.e. GPS C/A code, the signals proposed for future Galileo and GPS M-code signals are processed using split-spectrum type modulations, such as Binary Offset Carrier (BOC) modulation. These BOC modulations create deep fades (ambiguities) in the envelope of the Autocorrelation Function (ACF) of signal, and therefore the acquisition and tracking of these signals pose new challenges. To overcome these problems, two approaches have been recently proposed in literature, referred either as ”sideband techniques” (Betz, Fishman & al.) or ”BPSK-like” techniques (Martin, Heiries & al.). These methods allow the use of a higher search step in time domain, but employ an modified reference PRN code at receiver, which lead to an increase in implementation complexity. Moreover, the BPSK-like method does not work for odd BOC modulation orders. In this paper we present an extension of BPSK-like method, which provides a significantly lower complexity in the correlation part, and it works for both even and odd, sine and cosine BOC modulation orders. This technique is compared with the existing sideband methods in terms of performance and implementation complexity. As a benchmark, we also keep the ambiguous BOC processing. For a further decrease in implementation complexity, we investigate the effect of different IIR and FIR filtering structures used for the side-band selection in the receiver. We use here an interpolated FIR filter structure which provides a lower computational complexity than a direct form FIR filter and has similar performance with the others filters. The analysis is done in the presence of realistic multipath fading channels and the signals are modeled according to the current proposals for Galileo system Open Service (OS). 1 Background and motivation Future space-borne navigation receivers will operate with both GPS and Galileo navigation signals [1]. In order to provide a better spectral separation with existing GPS C/A code signals, the sine and cosine Binary Offset Carrier (BOC) modulations have been selected, for both Galileo and modernized GPS [2],[3]. A BOC(m,n) modulated signal is created by a square carrier modulation, where the signal is multiplied by a rectangular sub-carrier at sub-carrier frequency. The modulation parameters satisfy the relationships m = fsc fref and n = fc fref where fref=1.023 MHz is the reference frequency, fsc is the sub-carrier frequency and fc is the code rate. Thus the BOC modulation splits the signal spectrum into two symmetrical components, around the carrier frequency fcarrier. A generic characterization of the signal at baseband is given by the BOCmodulation orderNBOC = 2fsc fc . While allowing a better usage of available bandwidth for different GNSS signals, the BOC modulation brings new challenges into the acquisition process, due to the deep fades (ambiguities) that appear into the ±1 chips interval around the maximum peak of ACF envelope. These ambiguities lead to an increased number of timing hypotheses in order to detect correctly the signal, thus the necessary step to search a given time-uncertainty window ∆tbin should be small enough in order to find the main lobe of ACF. Therefore the computational complexity of the acquisition process is increased, the computational load being inversely proportional with the step time bin ∆tbin [6]. Additionally, the tracking of BOC signals has to cope with more false lock points than traditional BPSK modulated signals. On the other hand, tracking accuracy increases if BOC modulation is used instead of BPSK modulation, because of the decrease in the width of the main lobe of the envelope of the ACF. PROCEEDINGS OF THE 3rd WORKSHOP ON POSITIONING, NAVIGATION AND COMMUNICATION (WPNC’06)