1. A frequency-domain echo canceller comprising: an error processing unit configured to transform a time-domain (TD) error signal into a frequency-domain (FD) error signal; a gradient constraint unit configured to process at least a portion of a full weight vector; a least-mean-square (LMS) adaptation unit, coupled with the error processing unit and an output of the gradient constraint unit, configured to compute filter coefficients; a FD multiplier configured to multiply a FD input signal and the full weight vector to generate a FD output signal; and an output processing unit configured to transform the FD output signal to a TD output signal.