Summary Frame detection is an important step in satellite‐based automatic identification system for its contributions in verifying the presence of automatic identification system signal before frame synchronization. In this paper, a constant false alarm rate frame detector is proposed, which exploits the feature implied in the training sequence, to realize frame detection in the presence of additive white Gaussian noise and frequency offset. False alarm probability is related with a threshold, which is independent of the signal and noise. For fixed false alarm probability, the relationship between detection performance and Eb/N0 is analyzed. Simulations prove that the proposed detector outperforms the detector based on cyclic autocorrelation when message collisions exist. Copyright © 2016 John Wiley & Sons, Ltd.

SUMMARYA non-coherent receiver for automatic identification system (AIS) signals is proposed in this paper. The proposed receiver is based on the Viterbi algorithm with cyclic redundancy check (CRC) trellis. It takes bit stuffing into consideration and is designed to simultaneously demodulate decode and correct the received messages. The complexity of the proposed receiver has been reduced with state-complexity reduction. Simulations prove that the proposed receiver outperforms those AIS receivers without CRC trellis error correction in terms of BER and packet error rate.

Summary In space‐based automatic identification system, message collision is inevitable. To separate collision messages in multichannel space‐based automatic identification system, an improved least squares constant modulus algorithm (LSCMA) is proposed in this paper. The algorithm initializes LSCMA by using the result of complex symmetric fast independent component analysis and reduces the dimension of the received signal by using the minimum description length algorithm. At the same time, a new metric named failure rate is proposed to measure the separation performance. The simulation shows that the performance loss caused by the delay, the frequency and phase offset, and the input signal‐to‐interference ratio is within an acceptable range. The proposed algorithm outperforms the original LSCMA and other algorithms in terms of red packet error rate and failure rate.

The frequency offset caused by the Doppler shift and the oscillators instability degrades the performance of satellite-based demodulators for automatic identification system (AIS) signals unless proper synchronization algorithms are used. A noncoherent demodulation algorithm (NDA) based on cyclic redundancy check (CRC) error correction is proposed in this paper, which is able to tolerate large frequency offsets, thus avoiding the use of an explicit frequency synchronizer. To reduce the complexity, the number of CRC register states for error correction can be limited. Simulation results of the proposed algorithm are presented and compared with other demodulation algorithms in the literature in terms of bit error rate (BER) and packet error rate (PER). KEYWORDSCRC error correction, complexity reduction, Doppler shift, NDA, satellite-based AIS, Viterbi algorithm INTRODUCTIONThe automatic identification system (AIS) is primarily developed for vessel collision avoidance and usually works within the range of very high frequency band. 1 Coverage of coastal stations is about 40 nautical miles, and the communication between a vessel and a coastal station is impossible if the vessel is outside the radio visibility. Therefore, it is difficult to obtain a real-time supervision of all ocean-going vessels. To ensure a global coverage, a good solution is to receive the AIS messages from a constellation of low Earth orbit satellites. 2 However, the AIS standard has not been designed for such a use, and several technical issues arise at the receiver in case of a reception from a low Earth orbit satellite. 3 Doppler shift, message collisions, path delay, and low signal-to-noise values are the main issues degrading the demodulation performance. 3Studies on improving the demodulation performance of AIS signals received by a satellite are still ongoing. Based on maximum likelihood estimation and Viterbi algorithm, an innovative coherent detector is proposed for satellite AIS signals in Gallardo and Sorger. 4 However, the performance of coherent detection relies on accurate parameter estimation and tracking. Especially when a message collision exists, it is difficult to recover and track the carrier phase, thus resulting into a degradation of the coherent detection performance. Noncoherent detection is an attractive alternative, because it is very robust to the oscillators' instability. In Burzigotti et al, 5 a scheme based on the noncoherent sequence detection algorithm of Colavolpe and Raheli 6 is proposed, using three zonal demodulators that process different (but overlapping) frequency bandwidths to increase the system diversity. Since in AIS the cyclic redundancy check (CRC) is used at the transmitter side, the receivers in previous studies 4,5 compare the checksum computed from the received data with the frame check sequence (FCS) to detect transmission error bits. Colavolpe et al 7 replaced the algorithms for synchronization, detection, and postprocessing in Burzigotti et al 5 with new ones obtaining a significant performance i...

The satellite-based automatic identification system (AIS) receiver has to encounter the frequency offset caused by the Doppler effect and the oscillator instability. This paper proposes a non-coherent sequence detection scheme for the satellite-based AIS signal transmitted over the white Gaussian noise channel. Based on the maximum likelihood estimation and a Viterbi decoder, the proposed scheme is capable of tolerating a frequency offset up to 5% of the symbol rate. The complexity of the proposed scheme is reduced by the state-complexity reduction, which is based on per-survivor processing. Simulation results prove that the proposed non-coherent sequence detection scheme has high robustness to frequency offset compared to the relative scheme when messages collision exists.