Pole-Zero Decision Feedback Equalization with a Rapidly Converging Adaptive IIR Algorithm

A decision feedback equalizer (DFE) containing a feedback filter with both poles and zeroes i s 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, or IIR, filter that cancels the smoothly decaying tail of the impulse response. Modifications of an existing adaptive IIR algorithm, based on the Steiglitz-McBride identification scheme, are proposed to adapt the feedback filter. These new algorithms have comparable complexity to gradient-based adaptive IIR algorithms when the number of poles is small, but converge significantly faster. A measured subscriber loop impulse response is used to compare the performance of the adaptive pole-zero DFE, assuming a two-pole feedback filter, with a conventional DFE having the same number of coefficients. Results show that the pole-zero DFE offers a significant improvement in mean squared error (i.e., 4 dB at a signal-to-noise ratio of 25 dB) relative to the conventional DFE. Furthermore, the speed of convergence of the adaptive pole-zero DFE is comparable with that of the conventional DFE using the standard LMS adaptive algorithm.