Abstract: A 16 QAM communication system is provided wherein the proper arrangement and polarity of the four pulse trains on the receive side is determined by adding frame sync bits of a specific modulating character on the send side and reordering the receive side pulse trains in dependence upon the bits appearing in selected receive side pulse trains during the frame sync slots. The relation between the receive pulse trains and the transmit pulse trains is determined by the phase ambiguity between the transmit and receive carriers. By modulating the transmit carrier to have a specific amplitude during the frame sync slot, detecting this time at the receiver, and determining the bit values in certain of the pulse trains at the same time, the phase ambiguity can be determined and the pulse trains corrected.

Abstract: A channel monitoring system for a carrier wave differential phase modulation PCM transmission system is disclosed. Channel monitoring is accomplished with only one parity bit for one monitoring section of a PCM signal by making use of the correlation in the occurrence of errors due to an error in phase in the transmission path. The PCM signal is monitored at alternate code time slots with the parity check system so that a single code error to an adjacent phase in one parity monitoring section can be detected as a single bit error. Therefore, an error in one parity monitoring section can be reliably detected with the one parity bit so long as the error occurs only once in one monitoring section.