An optical transport system that transmits data using relatively short FEC-encoded data frames. The corresponding modulated optical signals are decoded at an optical receiver using frame-based maximum likelihood sequence estimation that relies on data redundancy present in each FEC-encoded data frame for the determination of its source bits. In some embodiments, the modulated optical signals carrying the FEC-encoded data frames are generated using a polarization-division-multiplexed constellation. The FEC-coding rate and frame length can be adjusted without changing the constellation, which advantageously enables the optical transport system to dynamically adapt its transmission format to the changing link conditions in a manner that results in better overall receiver sensitivity than that achieved with comparable bit-rate-adaptation methods that rely on a constellation change rather than on a change of the FEC-coding rate.