CRC-based detection algorithms for AIS signals received by satellite

This paper addresses the problem of demodulating signals transmitted in the automatic identification system (AIS). The main characteristics of such signals consist of two points: i) they are modulated using a trelliscoded modulation, more precisely a Gaussian minimum shift keying (GMSK) modulation; ii) they are submitted to a bit-stuffing procedure, which makes more difficult the detection of the transmitted information bits. This paper presents several demodulation algorithms developed in different contexts: mono-user and multi-user transmissions, and known/unknown phase shift. The proposed receiver uses the cyclic redundancy check (CRC) present in the AIS signals for error correction, and not for error detection only. Using this CRC, a particular Viterbi algorithm, based on a so-called extended trellis, is developed. This trellis is defined by extended states composed of a trellis-code state and a CRC state. Moreover, specific conditional transitions are defined in order to take into account the possible presence of stuffing bits. The algorithms proposed in the multi-user scenario present a small increase of computation complexity with respect to the mono-user algorithms. Some performance results are presented for several scenarios in the context of the automatic identification system and compared with those of existing techniques developed in similar scenarios. Copyright c © 0000 John Wiley & Sons, Ltd.