A coded eight-phase-shift-keying (C8PSK) method is described that is characterized by a quaternary phase-shift-keyed signal embedded in the modulated sequence. This method is a double-trellis-coded modification of the well-known C8PSK and is referred to as MC8PSK. Five coded bits are generated from four user bits in a single coding step and then mapped to an 8PSK symbol followed by a QPSK symbol. Given comparable decoder or receiver complexity, the coding gains of C8PSK and MC8PSK are comparable. Advantages of MC8PSK concern carrier synchronization. With C8PSK, problems with carrier phase tracking are encountered. With MC8PSK, these can be avoided, since carrier phase control can be based on the embedded QPSK. For some codes, error-free decoding is achievable in any locking condition of a QPSK-dependent PLL (phase locked loop), making synchronization and decoding especially simple. The partitioning of the channel symbol set, the associated bit mapping, and the code design are treated in order to describe MC8PSK. Aspects of receiver synchronization are discussed for C8PSK and for MC8PSK, focusing on QPSK phase-detection principles. The implementation of a four-state MC8PSK modem for an information rate of 2.048 Mb/s is described, and experimental results demonstrating the very robust carrier synchronization are presented. >