CONTROL ALGORITHM IN QAM MODEMS

摘要

A method and apparatus for automated acquisition of a QAM signal. The method employs an embedded state machine progressing from an initial state (14; State = 1) to a final state (30; State = 9). The method comprises the following steps: (A) Performing an automatic gain control (AGC) operation on the incoming QAM signal to maintain a steady amplitude of the QAM signal; (B) Performing a symbol timing recovery of the input QAM signal by adjusting a sampling clock of the symbol timing recovery loop; (C) Performing a blind equalization of the QAM signal without carrier lock to minimize a dispersion error of the received QAM signal constellation as compared with an error-free QAM signal constellation by adjusting a set of coefficients of the equalizer; (D) Performing a carrier recovery of the QAM signal to eliminate a residual carrier frequency error and to eliminate a phase error from the acquired QAM signal; and (E) Performing a decision directed equalization (DDE) of the QAM signal by updating a set of coefficients of the equalizer by using a decision based algorithm.