For the additive white Gaussian noise channel, we consider the problem of frame synchronization for coded systems. We present an algorithm that takes advantage of soft information provided by a soft decoder to produce an enhanced estimate of the frame boundary. To reduce complexity, a companion algorithm is introduced that is a hybrid of the optimal uncoded frame synchronizer introduced by Massey and the list synchronizer introduced by Robertson. The high-complexity coded maximum-likelihood frame synchronizer used by Robertson will accordingly be replaced by our algorithm, which operates on decoder-provided soft decisions. The algorithm begins by obtaining a list of high-probability starting positions via the log-likelihood function of the optimal uncoded frame synchronizer. Then, a test /spl delta/ is used to decide if the decision of the optimal uncoded frame synchronizer is sufficient, or whether list synchronization is required. If the test chooses in favor of using the optimal uncoded synchronizer, the estimate is obtained with relative ease. Otherwise, list synchronization is performed, and statistics provided by the decoder are used to resolve the frame boundary. Monte Carlo simulations demonstrate that the frame-synchronization-error rate (the probability of the synchronizer making an error) achieves the lower bound for signal-to-noise ratio values exceeding 1 dB.

Channel coding is an integral part of most data communication systems. However, most existing carrier synchronization algorithms do not take advantage of the redundancy introduced in the data stream by the code and operate at an uncoded SNR which can be significantly lower than the coded SNR. Thus, whereas it used to be the case that if detection could be achieved reliably, so could synchronization, this may not be true any more for heavily coded systems. We introduce an approximate maximum likelihood estimator that incorporates the code structure in obtaining phase estimates. Simulation results show the performance of the introduced algorithm is significantly better than those which do not make use of the code structure.