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We propose a noncoherent decoder-assisted (NDA) frame synchronizer, a two-stage, synchronization scheme based on the list-synchronization principle [P. Robertson, 1992], [M. Howlader and B. Woerner, 2001], for serially concatenated coded differential modulation (SCCD) system [K. R. Narayanan and G. L. Stuber, 1999] in packet transmission. For the first stage, we derive a suitable noncoherent maximum likelihood (SNML) rule, more effective for the noncoherent decoder's operating region of smaller channel phase offsets, to provide a reliable list of potential frame starting positions. For the second stage, we propose a hybrid sync method by exploiting the combined sign difference ratio (SDR) and sync word log-likelihood ratio (SW LLR) information [M. Howlader and B. Woerner, 2001] to obtain more reliable final decisions and reduce computational complexity. Performance and complexity of the proposed scheme are evaluated under constant and time-varying phase offsets in the AWGN channel via Monte Carlo simulation. [ABSTRACT FROM PUBLISHER]