Synchronization at low SNR: performance bounds and algorithms

This contribution compares several types of feed-forward synchronization algorithms when applied to powerful error-correcting codes. As such codes operate at very low SNR, conventional synchronization algorithms often prove to be unreliable. By exploiting the code properties we may improve the synchronization performance. We investigate a code-aided synchronizer that operates according the EM algorithm. This synchronizer is applied to the problem of carrier phase and timing synchronization. Lower bounds on the estimator performance are derived: we express the true Cramer-Rao Bound for the system as a function of the marginal posterior probabilities of the coded symbols. Various synchronization algorithms are compared in terms of their estimation error variance and their effect on the BER.