Acquisition analysis for Galileo BOC modulated Signals: theory and simulation

This paper is about the performance of a Galileo acquisition stage where the noise reduction and signal preservation is achieved by means of non-coherent summations. In fact, the presence of a secondary code, planned for many Galileo modulations and which further modulates the primary pseudo-random sequence, does not easily allow a coherent integration on more than one code period. Moreover, coherent integration is made much more complex by the longer codes employed for Galileo and by the use of the Binary Offset Carrier (BOC) modulations. In a mass-market receiver, where the complexity burdens are extremely important, these problems must be taken into account. A very easy solution is to combine the correlator outputs in a non-coherent process after the squaring operation in the acquisition block with an effective noise reduction without increasing dramatically the processing and memory capabilities of the receiver hardware. An analytical model of the acquisition block, starting from the well known statistical analysis carried out for GPS has been extended to Galileo. A complete digital acquisition system is considered and parametric expressions, depending on the number of terms coherently and non-coherently integrated, on the impact of the sampling frequency, code time and Doppler step resolution have been derived for the threshold settings and detection probability. The derived model for the noncoherent acquisition is validated by means of Monte Carlo simulations. Finally the effects of additional impairments over the correlation function have been considered: an incomplete code delay recovery and slight mistakes in the Doppler frequency estimation, which can potentially reduce the acquisition performance, have been taken into account.