Abstract: A concatenated coding scheme of a low-density parity-check code and a watermark code is capable of correcting non-binary synchronization errors. In order to improve the capability of the watermark code identifying these errors, an improved scheme is proposed using not two but multiple signal subsets. Specifically, in the proposed scheme, each transmitted symbol includes have two watermark bits and thus separate the constellation into four subsets. In this way, the expanded constellation can be classified into four more distinguishing subsets and thus the synchronization identification is improved using more watermark bits. Simulation results reveal that when the additive noise is small, for example, the signal-to-noise ratio is 10 dB and the frame error rate is10^-3 , the synchronization error probability that the proposed scheme can correct is as much as 13%. Also, the proposed scheme is more suitable for non-linear channels.