PCM-FDM: System Capability and Performance Improvement on Waveform Equalization and Synchronization

To obtain wider waveform equalization range and higher synchronization accuracy, we have introduced: 1) a multilevel pilot-pulse equalization system and 2)an in-band in-phase pilot synchronization system into multilevel VSB-conversion PCM-FDM equipment. This equipment is used to transmit digital signals on existing analog lines. 1) Multilevel pilot pulses with a smaller number of levels than the information pulses are inserted into the information pulse train. Due to the larger eye opening of the received pilot pulse, the automatic equalizer can converge and equalize the distortion more quickly and easily. For example, let 2-level pilot pulses be inserted into every 32 16-level information pulses. As long as the eye pattern of the pilot pulse is open, it is possible to allow up to 15 times more distortion in the transmission line. Thus the transmission span can be greatly extended. There are various other advantages in using this pilot pulse, such as dc drift suppression, in-service error rate monitoring, etc. 2) A carrier pilot and a sample timing pilot for phase synchronization are superimposed on the carrier frequency and 1/2 Nyquist rate, respectively, within the transmission signal spectra. Thus no excess bandwidth for pilots is necessary, and the phase error between pilot and signal is smaller. In this case, the phase jitter of regenerated carrier and timing clock caused by the neighboring signal spectra can decrease if the pilots are coupled in the same phase with the neighboring signal. For example, phase jitter of less than 0.5° (which was experimentally 1/10 phase error of the quadrature coupling) could be realized easily, even if the signal-to-pilot ratio is about 20 dB.