Passband Equalization of Differentially Phase-Modulated Data Signals

A new approach to the automatic equalization of differentially phase-modulated data signals passing through a linear distorting medium is presented. The equalizer, which is of the transversal filter type, operates in the frequency passband and contains two sets of taps — in-phase and quadrature. A tap-rotation property of the equalizer is used to establish an absolute phase reference at the equalizer output, and once this crucial step is taken, the passband equalizer output can be used to automatically (as well as adaptively) adjust the tap weights so as to minimize a mean-square distortion function. The resulting algorithm requires correlating an error signal with the tap voltages, and thus it is possible to use a structure similar to that employed when equalizing baseband PAM. The generality of the approach makes it applicable to the equalization of any double-sideband data signal.