REDUCED-COMPLEXITY TIME-DOMAIN CORRELATION FOR ACQUISITION AND TRACKING OF BOC-MODULATED SIGNALS * Abdelmonaem Lakhzouri, Elena Simona Lohan, and Markku Renfors Institute of Communications Engineering, Tampere University of Technology P.O. Box 553, FIN-33101, Finland, Emails: {abdelmonaem.lakhzouri,elena-simona.lohan, markku.renfors}@tut.fi Abstract Recent proposals for the modernized GPS and Galileo receivers suggested the use of a new modulation type, namely the Binary-Offset-Carrier (BOC) modulation, in order to get a better spectral efficiency and reduced interference com- pared to the traditional BPSK modulation. The acquisition and tracking of the code phase delay and Doppler shift is typically based on the correlation between the received signal and a reference code. The correlation can be performed in time-domain (TD) or in frequency-domain (FD). In this paper, we present a novel reduced-complexity TD correlation method. We compare its performance with two other TD correlation methods and with the traditional FD correlation method. Our new method takes into account the properties of the BOC waveform and re-uses the previously computed correlation values in such a manner to ensure the lowest number of additions and multiplications. The comparison will be carried out both in acquisition and tracking modes if long pseudorandom codes are employed, such as the 10230-chip length sequences in Galileo. 1 I NTRODUCTION CDMA receivers, such as Galileo and GPS receivers, spend a significant time in baseband processing for acquisition and tracking [1], [2]. This is due to the large number of required operations (e.g., additions and multiplications) to perform the correlation between the incoming signal and the replica code at the receiver with different candidate delays and Doppler shifts. There are two main methods to perform this correlation: either in Time Domain (TD) [3], or in Frequency Domain (FD) [2], [4]. In TD methods, a correlation value is computed for each code phase, usually in half-chip increments, until the full length of the pseudorandom (PRN) code is covered. The FD correlators employ FFT in order to search all the possible code phases in only one step. In both approaches, the procedure is repeated for all the possible Doppler shifts. The time-frequency search space depends on the receiver processing mode: in the acquisition mode (i.e., at cold start), the full PRN code length should be searched (i.e, 10230 chips for Galileo and modernized GPS receivers [5], [6]), while in the assisted-acquisition and in the tracking modes, only few tens to few hundreds of chips are usually enough. Recent proposals for Galileo and modernized GPS receivers suggested the use of a new modulation type, namely the Binary Offset Modulation (BOC) [6], [7] which triggers new challenges in the acquisition process, since the correlation waveforms are changed in the presence of BOC modulation. The features and properties of BOC modulated code sequences are still not well-understood in the context of fast acquisition algorithms. The goal of this paper is to analyze the TD and FD correlators for acquisition and tracking of BOC modulated PRN codes and to propose a new reduced-complexity TD correlation method which takes into account the properties of BOC signals. The sequel of this paper is organized as follows. In Section 2, the signal model for acquisition and tracking is described. The new TD correlation technique is presented in Section 3. Then, in Section 4, generic TD and FD correlation techniques are presented. Simulation results are shown in Section 5, and Section 6 gives the conclusions. * This work was carried out in the project ”Advanced Techniques for Mobile Positioning” funded by the National Technology Agency of Finland (TEKES). 1