HYBRID SEARCH SINGLE AND DOUBLE-DWELL ACQUISITION ARCHITECTURES FOR GALILEO SIGNAL Abdelmonaem Lakhzouri, Elena Simona Lohan, Markku Renfors Institute of Communications Engineering, Tampere University of Technology P.O.Box 553, FIN-33101, Tampere, Finland, {Abdelmonaem.Lakhzouri, Elena-simona, Markku.Renfors}@tut.fi ABSTRACT GPS and Galileo receivers must perform two dimensional searches, in time and in frequency, in order to acquire the correct code phase and correct Doppler frequency. Accurate and fast acquisition plays an important role in achieving reli- able satellite-based positioning. When the code length is high (e.g., Galileo and modernized GPS), the searching strategy is of utmost concern for fast signal acquisition. In this pa- per, we first introduce a double-dwell structure with hybrid search strategy for fast acquisition of CDMA signals with long spreading factors. We also derive its Mean Acquisi- tion Time (MAT) and we compare its performance with that of single-dwell hybrid-search structures. Typically, double- dwell architectures have been preferred to single-dwell archi- tectures. However, comparisons between the two structures are still hard to find in the existing literature, especially for hybrid-search approaches. We will show here that single- dwell architectures may be still better than double-dwell ar- chitectures under certain conditions. We will also discuss the influence of various parameters, such as the penalty factors and the target detection and false alarm probabilities, on the proposed acquisition structure. The simulation results were carried out for Galileo signals. I. I NTRODUCTION Since the new proposals of the European satellite system Galileo [1], [2] and the new GPS civil signals in L2 and L5 bands [3], the satellite-based positioning has drawn an in- creasing attention in the research community. The reason for this increased attention is that, together with these new pro- posals, new features related to the physical layer design have been specified, such as much longer codes compared with tra- ditional GPS signal (e.g., spreading factors of 10230 chips or higher, instead of 1023 chips) and a new modulation type (the so-called Binary Offset Carrier, BOC, modulation) [4]. These new features triggered new challenges in the code-frequency acquisition process, which is a fundamental pre-requisite for reliable satellite-based positioning. One challenge is the fact that the acquisition time is increased considerably in the pres- This work was carried out in the project ”Advanced Techniques for Mobile Positioning” funded by the National Technology Agency of Fin- land (Tekes), and also supported in part by Tampere Graduate School in Information Science and Engineering (TISE) Finland. ence of CDMA codes with long spreading factors. The tradi- tional serial acquisition [5] clearly fails to provide a sufficient performance, and using fully parallel structure will be pro- hibitive due to the high number of correlators needed. There- fore, hybrid strategy seems to give the best tradeoff between complexity and performance [6], [7], [8], [9], [10]. In hy- brid structures the search space is divided into several time- frequency windows, and each window contains several time- frequency bins or cells. Very few papers considered the situa- tion when we have several correct bins per window [6]; how- ever, this situation is very encountered in practice due to the width of the autocorrelation function (ACF). For example, for a single-path channel, the width of the main lobe of the ACF is 2 chips and if a time-bin length of 0.5 chips is assumed, we have up to 4 correct bins per window (a correct bin is a bin where the reference code is aligned with less than one chip error to the incoming signal [5]). Besides the search strat- egy, the acquisition architecture is generally built as a single- dwell [2], [8] or a double-dwell structure [6], [7], [9]. The double-dwell structure consists of the so-called search mode with short dwell time, followed by a verification mode with a longer dwell time. Typically, double-dwell strategy is con- sidered to be better than single-dwell strategy (in terms of MAT) [5], [9], but no justification has been generally given regarding this assumption. In this paper, we propose first, a novel hybrid-search double-dwell architecture for acquiring the Doppler and code shifts in a CDMA system and we derive its mean acquisition time. In deriving the mean acquisition time, we also take into consideration the percentage of false alarms in a correct window, which is different from previous approaches. Our proposed architecture performs the coher- ent correlation in the frequency domain (instead of the time domain), in order to reduce the complexity. Second, we com- pare the single-dwell and double-dwell structures in terms of mean acquisition time. We show that, contrary of what has been reported in the literature so far, the double-dwell strategy is not always better than a single-dwell strategy and we give the conditions under which a double-dwell strategy is always the best choice. Discussion about the number of correlators needed will be also presented. II. ACQUISITION MODEL - DOUBLE- DWELL STRATEGY The proposed double-dwell architecture is shown in Fig. 1. The search mode has a dwell time of τ 1 = N c N nc msec (if the