Pole-zero decision feedback equalization for the digital subscriber loop

A decision feedback equalizer (DFE) containing a feedback filter with both poles and zeros is proposed for high-speed digital communications over the subscriber loop. The feedback filter is composed of two sections: a relatively short finite-impulse-response (FIR) filter that cancels the initial part of the channel impulse response, which may contain rapid variations due to bridge taps, and a pole-zero filter than cancels the smoothly decaying tail of the impulse response. It is shown that the feedback filter can be adapted using existing infinite-impulse-response (IIR) adaptation techniques. Numerical results using a simulated subscriber loop impulse response show that a feedback filter with two poles adapted via a simplified gradient algorithm can offer significant improvement in mean squared error (MSE) (i.e., 3 to 4 dB at a signal-to-noise ratio of 20 dB) relative to a conventional DFE of the same complexity.