Code Acquisition for Next Generation Mobile Broadband Satellite Services A. Vanelli-Coralli * , G.E. Corazza * , C. Palestini * , R. Pedone * , M. Villanti * , H.J. Lee † , P. S. Kim † * University of Bologna, DEIS-ARCES, Viale Risorgimento, 2 - 40136 Bologna, Italy email: {avanelli, gecorazza, cpalestini, rpedone, mvillanti}@arces.unibo.it † Global Wireless Technology Research Group, ETRI, 161 Gajeong-dong Yuseong-gu Deajeon, Republic of Korea, 305-350 email: {hjlee, pskim}@etri.re.kr Abstract—This paper tackles the problem of time synchro- nization for the mobile extension of DVB-S2/RCS air interfaces. To increase robustness against other system interference, Direct Sequence Spread Spectrum (DS-SS) is selected, which calls for efficient code acquisition. Designed to operate jointly with frame synchronization, a novel high level control logic is proposed for code acquisition, along with robust detectors to cope with the large frequency uncertainty experienced at terminal start-up. The results show that the mean acquisition time is always limited to a few milliseconds, allowing fast acquisition even in the most challenging railway scenario. I. I NTRODUCTION In 2006 the Digital Video Broadcasting - Technical Module (DVB-TM) approved a new study mission aimed at extending the capabilities of the DVB-RCS (Return Channel via Satellite) standard to support broadband services to mobile collective terminals in aeronautical, maritime, and railway land mobile scenarios 1 [1][2]. The new standard is expected to be finalized by early 2008 and will be identified as DVB-RCS+M. Although the DVB-RCS group activities have been mainly aimed at the standardization of the satellite return link, the new study mission was also addressing the satellite forward link design in order to provide a full broadband mobile satellite system toolbox. On the ground that the DVB-S2/RCS pair [4][5] is widely accepted for fixed broadband satellite com- munication systems, the DVB-RCS+M adopts these standards as the starting baseline configuration for the mobile extension. DVB-RCS+M is designed for operation in Ku (11-14 GHz) and Ka-band (20-30 GHz). Indeed, this design choice allows to exploit the existing DVB-RCS and DVB-S2 technologies and to use small antennas, thus reducing the deployment and operational costs. However, the drawback is that specific interference countermeasures are needed, because these bands are allocated to Mobile Satellite System (MSS) applications with a lower priority (on a secondary basis) with respect to fixed satellite systems (FSS), thus imposing stricter constraints on the admissible interference level caused to other primary systems and and a lower protection from the FSS generated interference. The solution devised by the DVB-RCS+M group for interference mitigation is the use of an optional direct sequence spread spectrum (DS-SS) mode for the DVB-S2 waveform, with spreading factors up to 16 for the return link single channel per carrier option [3]. 1 The vehicular land mobile scenario is also addressed by the new standard but with a lower priority with respect to the railway. The adoption of DS-SS in the DVB-S2 waveform dictates the introduction of a code synchronization subsystem at the receiver side. In this paper, we report the original results of the design and performance assessment of the code synchro- nization subsystem that have been carried out by the authors in support of the adoption of DS-SS by the DVB-RCS+M ad-hoc group. The design described in the paper refers in particular to the DVB-RCS+M forward link (FL) in the most challenging railway scenario [2], characterized by periodic blockages and spreading factors up to 4. Code synchronization is accomplished jointly with frame acquisition in order to limit the impact on the receiver architecture. As in common practice, the code/frame epoch domain is discretized into a number of cells or hypotheses per chip, and acquisition is achieved through the detection of the spread DVB-S2 Start of Frame (SOF) [4] within the transmission flow. The novelty of the paper lies in the control logic that manages the overall acquisition subsystem and the introduction of a novel detector, which is particularly robust against the large frequency offset typical of FL transmissions. II. DS SPREADING IN FL DVB-RCS+M The DVB-RCS+M specifications foresee the adoption of the DVB-S2 waveform and frame structure for FL transmissions [4]. Accordingly, as depicted in fig.1 the physical layer frame (PLFRAME) consists of L F modulated symbols including the SOF of L SOF =26 symbols, the physical layer signalling (PLS) field of 64 symbols, and the information payload interlaced every 1440 symbols with a pilot field of 16 symbols. P #1 #16 #1 #16 P #S S O F PLS L=1440 symbols 26 64 90 symbols P=36 symbols Figure 1. DVB-S2 Physical Layer Frame (PLFRAME) structure The insertion of the SOF is required for frame synchro- nization purposes in the DVB-S2 receiver, while pilots are foreseen to ease the following estimation steps. Note that, while in the original DVB-S2 standard the pilot inclusion is optional, for mobile applications it becomes mandatory to enable efficient channel estimation in the very harsh scenarios This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the ICC 2008 proceedings. 978-1-4244-2075-9/08/$25.00 ©2008 IEEE