IEEE TRANSACTIONS ON BROADCASTING, VOL. 53, NO. 3, SEPTEMBER 2007 685 On Pilot-Symbol-Assisted Carrier Synchronization for DVB-S2 Systems Alan Barbieri and Giulio Colavolpe, Member, IEEE Abstract—We consider the problem of carrier synchroniza- tion in future 2nd-generation satellite digital video broadcasting (DVB-S2) receivers. In this scenario, this task is made harder by the complexity constraints, related to the use of consumer-grade equipment. Making use of the distributed pilot symbols of the DVB-S2 standard, low-complexity techniques for fine frequency estimation and for detection in the presence of a strong phase noise, typical of consumer-grade equipment, will be proposed. The performance of the described algorithms will be analysed in detail through computer simulations. Index Terms—Carrier synchronization, DVB-S2 systems, itera- tive detection and decoding, phase noise, satellite communications. I. INTRODUCTION I N FUTURE 2nd-generation satellite digital video broad- casting (DVB-S2) systems [1], carrier synchronization is a hard task. First of all, at the very low operating signal-to-noise ratio (SNR) of some of the modulation and coding (MODCOD) formats, in particular those based on the quaternary phase shift keying (QPSK) modulation and the low-density parity-check (LDPC) codes with the lowest rates, frequency estimation is not sufficiently accurate and can be degraded by the occurrence of outliers. On the other hand, for those MODCODs working at high SNR values, namely those based on amplitude phase shift keying (APSK) signals and the highest code rates, the main problem is represented by the phase noise which is particu- larly strong, due to the use of consumer-grade equipment and possible low signaling rates. The phase noise also limits the accuracy of any frequency estimator for high SNR values [2]. Hence, it is particularly difficult to find a single low-complexity solution for carrier synchronization that could be adopted for all MODCODs and all signaling rates. In this semi-tutorial paper, we report the solution designed in the context of the “Study of enhanced digital transmission tech- niques for broadband satellite digital transmissions (BSDT),” funded by the European Space Agency [3]. A coarse frequency synchronization is preliminary accomplished through an auto- matic frequency control (AFC) loop [3]. Although this block Manuscript received July 28, 2006; revised May 18, 2007. This work is part of the “Study of enhanced digital transmission techniques for broadband satellite digital transmissions (BSDT)” supported in part by the European Space Agency, ESA-ESTEC, Noordwijk, The Netherlands, under Contract 19370. This paper was presented in part at the 9th International Workshop on Signal Processing for Space Communications (SPSC 2006), ESTEC, Noordwijk, The Netherlands, September 2006. A. Barbieri is with the Dipartimento di Ingegneria dell’Informazione, Univer- sita di Parma, I-43100 Parma, Italy and also with the Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA 90089-2560 USA (e-mail: barbieri@tlc.unipr.it). G. Colavolpe is with the Dipartimento di Ingegneria dell’Informazione, Uni- versita di Parma, I-43100 Parma, Italy (e-mail: giulio@unipr.it). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBC.2007.903603 is not necessary for the highest signaling rates, since the un- compensated frequency offset normalized to the symbol rate is low enough to guarantee that frame and timing synchroniza- tion can be successfully performed, for lower signaling rates it is practically unavoidable. From the design point of view, this coarse AFC loop does not represent a concern. In fact, a clas- sical first-order loop, with an error signal generated according to the delay-and-multiply algorithm [4], is sufficient to guarantee the required performance [3]. We would like to simply men- tion that it is necessary to adaptively compensate the amplitude distortions on the received signal, mainly due to the low-noise block and the coaxial cable at the consumer side, since they would produce a bias in the coarse frequency estimate of the AFC loop [5]. In addition, in order to avoid an increase of the already strong phase noise, due to the phase jitter of the AFC loop, the receiver can adopt the following technique. The output of the AFC loop, at the beginning of each codeword, is used to derotate the entire codeword before the further process of fine frequency estimation and compensation and detection/de- coding in the presence of phase noise, that will be described in this paper. In other words, although the AFC is still running, we use its output frozen at the beginning of each codeword. In this way, each codeword is not only affected by a constant frequency error equal to the instantaneous frequency error of the AFC loop at the beginning of the codeword, but also by the entire Doppler rate (and also by the received phase noise). However, it can be shown that the amount of this Doppler rate does not affect the performance of the algorithm we propose for joint detection and decoding in the presence of phase noise and therefore can be ignored [3]. The fine frequency estimation and the problem of detection and decoding in the presence of phase noise deserve a greater attention [6], [7] and in this paper we will focus on them. The solution we report is a merging of both new and already pro- posed techniques customized for this scenario—in this sense this is a semi-tutorial paper. In particular, after the description in Section II of the system model, in Section III we will consider the frequency estimation of the residual frequency offset after the coarse AFC loop. This residual frequency offset will be as- sumed constant over a frame and, due to the coherence time of the AFC loop, independent frame by frame [3]. The low-com- plexity technique that will be described makes use of distributed pilot symbols, as in the DVB-S2 standard, and the presence of the decoder. In fact, by using an autocorrelation-based estima- tion algorithm already proposed in the literature [4], [8], cus- tomized for the pilot symbol allocation at hand, a set of mul- tiple estimates is identified leaving to the decoder the selection of the final estimate. The more challenging problem, that is the detection and decoding in the presence of phase noise, will be faced in Section IV. The described solution is based on an itera- tive algorithm already proposed by the authors in [9]. This algo- rithm will be briefly reviewed in this paper. Since the algorithm works with a serial schedule, which is not suited to a parallel 0018-9316/$25.00 © 2007 IEEE